Damage by Improvised Incendiary Devices on Carbon Fiber-Reinforced Polymer Matrix Composites

Article Damage by Improvised Incendiary Devices on Carbon Fiber-Reinforced Polymer Matrix Composites

Sebastian Eibl

Bundeswehr Research Institute for Materials, Fuels and Lubricants, Institutsweg 1, D-85435 Erding, Germany; [email protected]

Abstract: This study focuses on short-term thermal degradation of polymer matrix composites by one-sided impact of improvised incendiary devices (IID). Specimens of two commercial composites HexPly® 8552/IM7 and M18-1/G939 with various thicknesses (1–8 mm) are systematically investigated as well as sandwich structures thereof, applying various amounts of fire accelerant predominantly in laboratory scale fire tests. Results of preceding large-scale fire tests with IIDs justify the chosen conditions for the laboratory-scale fire tests. The aim is to correlate the amount of fire accelerant with heat damage and residual mechanical strength. Thermal damage is characterized visually and by ultrasonic testing, infrared spectroscopy, and residual interlaminar shear strength. Matrix degradation and combustion only contribute to the overall amount of released heat by the fire accelerant for thin and especially vertically aligned panels as tested by a cone calorimeter (without electrical heating), but not for horizontally orientated and thicker panels. Degradation processes are discussed in detail. Protective effects are observed for typical coatings, a copper mesh applied for protection against lightning strike, combinations thereof as well as an intumescent coating. Especially sandwich structures are prone to severe damage by assaults with IID, such as Molotov cocktails.

Citation: Eibl, S. Damage by Keywords: carbon fiber reinforced polymer; improvised incendiary device; fire accelerant Improvised Incendiary Devices on Carbon Fiber-Reinforced Polymer Matrix Composites. J. Compos. Sci. 2021, 5, 72. https://doi.org/ 10.3390/jcs5030072 1. Introduction Improvised incendiary devices (IID) have been used for a long time especially by Academic Editor: terrorists, as they are cheap and easy to prepare [1–4]. They represent a severe danger Francesco Tornabene in particular for materials with a low thermal stability or combustible materials such as polymers. Carbon fiber-reinforced polymers (CFRP) are widely used in defense industry Received: 22 January 2021 for vehicles and aircraft threatened by this type of assault. However, there is not sufficient Accepted: 26 February 2021 knowledge about the effect of these fire accelerants on CFRP, as these data are often Published: 5 March 2021 classified and not published. Many investigations on thermal damage of CFRP typically focus on accidentally overheating due to pipe bursts, malfunctions in electric equipment, Publisher’s Note: MDPI stays neutral during repair, engine overheating or impingement of engine exhaust, lightning strikes or with regard to jurisdictional claims in fires etc., [5]. Knowledge about the relation between input of thermal energy, degradation published maps and institutional affil- of the polymer matrix, and the loss of mechanical strength of composite materials is iations. of wide interest, especially in aircraft industry. Thermal degradation of composites is typically associated with softening and decomposition of the matrix material as well as the development of cracks accompanied by the degradation of the interphase between fiber and matrix. Matrix cracking and delamination have been shown to occur after thermal Copyright: © 2021 by the author. exposure by many authors [6–11]. Rapid heating beyond the temperature necessary for Licensee MDPI, Basel, Switzerland. a degradation of the polymer matrix (typically approx. 300 to 400 ◦C) up to ignition This article is an open access article temperature is achieved for example by irradiation experiments. Heat fluxes of 35 kW/m2 distributed under the terms and are characteristic for developing, and 50 kW/m2 for fully developed fires [6]. conditions of the Creative Commons Thermal degradation effects are typically described by residual strength and by non- Attribution (CC BY) license (https:// destructive testing methods such as ultrasonic testing and infrared spectroscopy [12]. creativecommons.org/licenses/by/ Infrared spectroscopy (IR) is widely used to pursue polymer matrix decomposition [13,14], 4.0/).

J. Compos. Sci. 2021, 5, 72. https://doi.org/10.3390/jcs5030072 https://www.mdpi.com/journal/jcs J. Compos. Sci. 2021, 5, 72 2 of 24

especially in a range of incipient heat damage, where visual and ultrasonic inspection is not sensitive enough. In previous studies [15–17] an empirical correlation was established between the degradation of the polymer matrix traced by infrared spectroscopy and the residual mechanical strength for the investigated CFRP HexPly® 8552/IM7 and M18- 1/G939. Also chemometric techniques were proposed to separately determine temperature and duration of a thermal pre-load [18]. This study focuses on short-term thermal degradation of CFRP by the influence of liquid fire accelerants. One-sided impact of typical improvised fuel mixtures is predominantly investigated in laboratory-scale fire tests. The amount of fire accelerant is chosen to be comparable to a realistic scenario when the ratio of liquid to CFRP surface is considered [19]. Therefore, large-scale fire tests with IIDs are carried out to justify the chosen conditions for the laboratory-scale fire tests. Under these conditions no thermal equilibrium throughout the composite is reached, but high temperature gradients occur. Temperature at the samples’ surface is in the range of the ignition temperature of the resin matrix. A systematic variation of the amount of fire accelerant, material thickness, monolithic, and sandwich structures as well as horizontal and vertical alignment of samples is carried out, in order to investigate the influence of these basic parameters on the damage of the CFRP. Two CFRP materials common in defense and aviation industry (HexPly® 8552/IM7 and M18-1/G939) are investigated. Reaction-to-fire properties of these materials have already been characterized for a forced combustion by cone calorimetry [20,21]. Additionally, the influence of a typical coating, an integrated copper mesh usually applied for the protection against lightning strike, and combinations thereof are considered, as well as the protection potential by an intumescent coating. With respect to the intumescent coating, differences of protection efficiency is compared for heat impact by irradiation and improvised fire accelerants. It is the goal of this work to assess how CFRP structures used in defense industry are prone to failure due to assaults by improvised incendiary devices. A deep insight into degradation mechanisms is gained. The aim is to correlate the amount of applied thermal energy with heat damage, to assess detectability of the damage and to predict residual mechanical strength.

2. Material All tests are performed with carbon ﬁber-reinforced epoxy systems HexPly® 8552/IM7 (unidirectional prepreg) and HexPly® M18-1/G939 (fabric prepreg) from Hexcel Compos- ites GmbH, Stade, Germany (subsequently assigned as 8552/IM7 and M18-1/G939). Both matrix systems of the CFRP “8552” and “M18-1” consist of aromatic epoxy resins (29 wt% and 36 wt%), which are toughened with the temperature-resistant thermoplastics polyethersulfone (6 wt% in “8552”) and polyetherimide (6 wt% in “M18-1”), respectively [22,23]. The “M18-1” matrix additionally contains zinc borate (1.6 wt%) and magnesium hydroxide (1.6 wt%) as ﬂame retardants [21]. The prepared 8552/IM7 and M18-1/G939 laminates consist of 1, 2, 4, 6, and 8 mm thick quasi-isotropic lay-ups:

8552/IM7: [(45/90/135/0)]S; [(45/90/135/0)2]S; [(45/90/135/0)4]S; [(45/90/135/0)6]S; [(45/90/135/0)8]S; and M18-1/G939: [(+45/−45)(90/0)(−45/+45)(0/90)]S; [(+45/−45)(90/0)(−45/+45)(0/90)]2S; [(+45/−45)(90/0)(−45/+45)(0/90)]3S, etc. They are cured in an autoclave according to the manufacturer’s recommended conditions [22]. Ultrasonic C-scans are performed to ensure that the test laminates are free of delaminations and voids. The laminates are cut into 100 mm × 100 mm panels for laboratory experiments in a cone calorimeter (without electrical heating) using a water- cooled diamond wheel saw and dried at 70 ◦C. Large-scale experiments are performed with 330 mm × 330 mm M18-1/G939 panels (see Figure1). An overview of all prepared samples and experimental conditions is given in Tables1 and2. J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 3 of 26

delaminations and voids. The laminates are cut into 100 mm × 100 mm panels for laboratory experiments in a cone calorimeter (without electrical heating) using a water-cooled diamond wheel saw and dried at 70 °C. Large-scale experiments are performed with 330 J. Compos. Sci. 2021, 5, 72 3 of 24 mm × 330 mm M18-1/G939 panels (see Figure 1). An overview of all prepared samples and experimental conditions is given in Tables 1 and 2.

Figure 1. Steel sample holder with a 4545°◦ installed 330 mm × 330330 mm mm carbon carbon fiber fiber-reinforced-reinforced polymers ( (CFRP)CFRP) panel used for thethe firefire tests,tests, and and images images of of a high-speeda high-speed camera camera representing representing the the application application of an of improvised an improvised incendiary incendiary devices devices (IID). (IID). As improvised incendiary device (IID) a fire accelerant mixture of petrol (10 vol.%) readilyAs ignitingimprovised and incendiary diesel fuel device (90 vol.%) (IID) a for fire sustained accelerant combustion mixture of ispetrol chosen (10 forvol.%) the readilylaboratory igniting experiments and diesel in fuel the (90 cone vol.%) calorimeter. for sustained Viscous combustion and sticky is chosen mixtures for arethe ob-la- boratorytained, when experiments polystyrene in the foam cone andcalorimeter. toluene Viscous are added and (mass sticky ratio: mixtures 3:1:4:2 are forobtained, diesel whenfuel:petrol:toluene:polystyrene polystyrene foam and toluene foam), are as added used for (mass the ratio: preceding 3:1:4:2 more for diesel realistic fuel:petrol:tol- IID fire tests uene:(see Figurepolystyrene1). However, foam), thin as used films for of polystyrenethe preceding residues more realistic on the samples IID fire after tests the (see combus- Figure 1).tion However, experiments thin lead films to of less polystyrene reproducible residues results. on Therefore, the samples all laboratory after the experimentscombustion ex- are perimentsconducted lead without to less thickener, reproducible even thoughresults. thickenersTherefore, are all typicallaboratory for Molotovexperiments cocktails. are con- By ductedpreceding without large-scale thickener, experiments even though with 330 thickeners mm × 330 are mm typical large for M18-1/G939 Molotov cocktails. samples (see By precedingFigure1), it large is proven-scale that experiments the amount with of fire330 accelerantmm × 330 relatedmm large to theM18 surface-1/G939 area samples of a CFRP (see 2 Fig(~1000–~3000ure 1), it is g/m proven) and that the the duration amount of of impact fire accelerant are comparable related for to laboratory the surface experiments area of a CFRPand real (~1000 assaults–~3000 [19 ].g/m Samples2) and usedthe duration for the large-scale of impact fireare testscomparable are summarized for laboratory in Table ex-1. 2 perimentsAdditionally, and larger real assaults amounts [19]. (>30 Samples g/0.01 used m ) for are the applied large- forscale thick fire samples tests are insummarized laboratory inexperiments Table 1. Additi (see Tableonally,1). larger amounts (>30 g/0.01 m2) are applied for thick samples in laboratorA two-componenty experiments (see polyesterurethane Table 1). is used as the top coat based on aliphatic di- and tri-isocyanatesA two-component “Decklack polyesterurethane 472-22, 754H is + used Hardener as the 400” top [coat24] withbased grey on coloraliphatic typical di- andfor militarytri-isocyanates aircraft: “Decklack “FS 35237” 472- (Federal22, 754H Standard+ Hardener 595C, 400” Paint [24] with Spec) grey with color semi-gloss typical (reflectometer values: 20 ± 5 at 60◦, EN ISO 2813 [25]). It is provided by Mankiewicz for military aircraft: “FS 35237” (Federal Standard 595C, Paint Spec) with semi-gloss (re- GmbH (Hamburg, Germany). The resulting coating is 50 ± 3 µm thick, and its properties flectometer values: 20 ± 5 at 60°, EN ISO 2813 [25]). It is provided by Mankiewicz GmbH are specified in [26]. (Hamburg, Germany). The resulting coating is 50 ± 3 µm thick, and its properties are specified in [26]. Table 1. Summary of all investigated samples and experimental conditions of realistic, large-scale IID fire tests with 330 mm A water-dilutable two component intumescent coating is used (2K - Brandschutz- × 330 mm M18-1/G939 panels. Selected test results are additionally given. lacksystem K1 + K2 by AISCO Chemieprodukte GmbH, Freiburg, Germany) with a pro- ◦ prietary but typical composition consistingMax. predominantly Temperature of [ C](poly) phosphoricNorm. acid (K1, Thick-ness Vol. Fire Flame Tilt Angle Mass Loss 16% P) and melamine (K2, 26% N). About 2 g (dryAt mass) Side coating is squeegeedResid. on eachIR **[-] 100 [mm] Accelerant [L] Time [s] [◦] [%] mm × 100 mm cured test panel resulting inFront a film thickness Back of 300 ILSSµm which [%] typically 2 0.5forms 222 an expanded 45 foam◦ of 4 cm0 in irradiation ~690 experiments 213 [27]. 100 94 2 2 0.5For 308 some samples hori-zontal a copper ~11 mesh (~120 g/m ~800; ~0.05 mm 335 wire diamete ~18r) * is additionally ~21 4 1applied 585 at the surface hori-zontal of 8552/IM7 0 samples before ~610 curing. 296 - - 4 1 265 hori-zontal 0 ~590 235 - - 6 1 400 hori-zontal ~3 ~900 234 - - * Samples taken from the center of the panel fail in various plies. Value therefore does not represent realistic interlaminar shear strength −1 −1 (ILSS). ** Intensity ratio of IR bands at 1510 cm and 1780 cm (I −1 /I −1 , see text). 1510 cm 1780 cm J. Compos. Sci. 2021, 5, 72 4 of 24

Table 2. Summary of all investigated samples and experimental conditions for laboratory tests. For each condition, the minimum residual interlaminar shear strength is given (ILSS is related to the initial values: 71 N/mm2 for 8552/IM7 and 65 N/mm2 for M18-1/G939; standard deviation is ~10% of the given value).

CFRP Thickness ILSS in %For Mass Fire Accelerant [g] Remark Material [mm] 5 10 15 20 25 30 40 50 75 1 - 32 0 0 0 - 0 0 - - 2 - 68 44 7 0 0 - - - - 2 vertically aligned 82 45 0 0 - 0 0 - - 2 coated 75 54 42 28 25 - - - - 2 copper mesh - 62 - 25 - 11 8 - - 2 copper m. + coat - 59 - 14 - 11 7 - - intumesc. 2 - 72 - 65 - 61 25 - - coating 4 - - 72 - 59 45 0 0 - - 8552/IM7 4 coated - 92 - 80 - 54 56 - - 4 copper mesh - 96 - 85 - 73 42 - - 4 copper m. + coat - 61 - 42 - 35 20 - - intumesc. 4 - 83 - 76 - 70 68 - - coating 6 - - - 86 - 92 82 - - - 6 coated - - - 92 - 82 46 28 - 6 copper mesh - 101 - 96 - 82 86 - - 6 copper m.+ coat - 54 - 41 - 41 37 - - intumesc. 6 - 90 - 79 - 70 77 - - coating 8 - - 77 87 83 - 79 76 77 68 1 - 57 0 0 0 - 0 0 - - 2 - 54 31 0 0 - 0 - - 2 vertically aligned 83 0 0 0 - 0 0 0 - M18-1/ 2 coated 80 37 28 23 - 22 - - - G939 4 - - 71 54 23 22 - 8 - - 6 - - - 85 82 83 85 71 77 - 8 - - 91 83 82 - 72 72 63 0 16.5 sandwich - 6 * 3 * 2 * - - - - - * related to initial force at break (825 N) in a four point bending test; - no test performed; 0% indicate samples, for which plies already fall apart before ILSS testing.

A water-dilutable two component intumescent coating is used (2K - Brandschut- zlacksystem K1 + K2 by AISCO Chemieprodukte GmbH, Freiburg, Germany) with a proprietary but typical composition consisting predominantly of (poly) phosphoric acid (K1, 16% P) and melamine (K2, 26% N). About 2 g (dry mass) coating is squeegeed on each 100 mm × 100 mm cured test panel resulting in a ﬁlm thickness of 300 µm which typically forms an expanded foam of 4 cm in irradiation experiments [27]. For some samples a copper mesh (~120 g/m2; ~0.05 mm wire diameter) is additionally applied at the surface of 8552/IM7 samples before curing. Selected 4 mm thick 8552/IM7 specimens are equipped with eight thermocouples (type K) prior to curing under the 1st (~0.125 mm), 8th (~1 mm), 12th (1.5 mm), 16th (~2 mm), 21st (~2.6 mm), 26th, (~3.25 mm), and 31st (~3.9 mm) ply, in order to trace the temperature rise in the bulk material during combustion; for M18-1/G939 underneath the 3rd (~0.75 mm), 5th (~1.25 mm), 7th (1.75 mm), 9th (~2.25 mm), 11th (~2.75 mm), 13th, (~3.25 mm), and 15th (~3.75 mm) ply. The thermocouples are located close to the center of the panels. Sandwich specimens are prepared by two 1 mm thick quasi-isotropic M18-1/G939 laminates bonded to a 14.7 mm thick nomex® honeycomb core resulting in 16.5 mm thick panels. The components are pressed (1000 N/m2, 12 h) prior to curing. J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 5 of 26

8 - - 77 87 83 - 79 76 77 68 1 - 57 0 0 0 - 0 0 - - 2 - 54 31 0 0 - 0 - - 2 vertically aligned 83 0 0 0 - 0 0 0 - M18-1/ G939 2 coated 80 37 28 23 - 22 - - - 4 - - 71 54 23 22 - 8 - - 6 - - - 85 82 83 85 71 77 - 8 - - 91 83 82 - 72 72 63 0

J. Compos. Sci. 2021, 5, 72 16.5 sandwich - 6 * 3 * 2 * - - - - 5 of 24- * related to initial force at break (825 N) in a four point bending test; - no test performed; 0% indicate samples, for which plies already fall apart before ILSS testing.

3. Experimental3. Experimental For realisticFor IIDrealistic experiments, IID experiments, 330 mm 330× 330 mm mm × 330 M18-1/G939 mm M18-1/G939 panels panels are placed are placed in in the the centercenter of an of inclinable an inclinable 120 cm 120× cm90 cm× 90 steel cm steel panel panel as part as ofpart a specially of a specially designed designed test test ap- apparatusparatus (see Figure (see Fig1).ure To 1). a glass To a bottle,glass bottle, containing containing at maximum at maximum 1 liter 1 liter of the of liquidthe liquid IID, a IID, a burningburning textile textile wick wick is attached.is attached. The The bottle bottle is guidedis guided along along its its way way to beto be shattered shattered above above thethe CFRP CFRP panel panel in orderin order to provideto provide reproducible reproducible test test conditions. conditions. The The tests tests are ﬁlmedare filmed with with a high-speeda high-speed camera camera Optronis Optronis CamRecord CamRecord CR1000 CR1000 at 1000at 1000 frames frames per per second. second. Test Test condi- conditionstions are are summarized summarized in Table in Tab1. le 1. For laboratoryFor laboratory scale experiments, scale experiments, 100 mm ×100100 mm mm × 100 samples mm samples are put in are a speciallyput in a specially designed,designed, quadratic quadratic steel holder steel (inner holder frame: (inner 98 frame: mm × 9898 mm mm) × allowing98 mm) allowing a horizontal a horizontal alignmentalignment of the panels of the and panels the application and the application of the liquid of the ﬁre liquid accelerant fire accelerant (Figure2). (Figure 2).

A B

Figure 2. Sample Figureholder with 2. Sample a horizontally holder with installed a horizontally CFRP panel installed (A) used CFRP for panel the fire (A) tests used in for a thecone fire cal testsorimeter in a cone (B). (For reference measurements of the fire accelerant, the CFRP panel is substituted by a steel plate. Vertically aligned CFRP calorimeter (B). (For reference measurements of the fire accelerant, the CFRP panel is substituted panels are positioned at the right edge of the frame with a horizontal steel plate in the sample holder.). by a steel plate. Vertically aligned CFRP panels are positioned at the right edge of the frame with a horizontal steel plate in the sample holder). The CFRP panel and the frame screws are fixed tight with a tape sealing in between Theto CFRP prevent panel the and leaking the frame of liquids. screws areFor fixedreference tight experiments with a tapesealing a steel inplate between is used. After to preventignition the leaking of the of liquid liquids. fire Foraccelerant reference with experiments a lighter, the a steel combustion plate is used. is investigated After by a ignition ofcon thee liquidcalorimeter fire accelerant (Fire Testing with Technology a lighter, the UK, combustion according is investigatedto ISO 5660 by[28]). a cone No external calorimeterirradiation (Fire Testing by the Technology cone heater UK, is accordingapplied. Temperatures to ISO 5660 [28 are]). Norecorded external by irradia-type-K thermo- tion by thecouples cone heaterfixed on is applied.the centers Temperatures of the front side are recordedand at three by type-Kpositions thermocouples on the backside of the fixed on thepanels centers with of a temperature the front side-resistant and at three tape positions(see Figure on 3). the Vertically backside aligned of the panelscomposite sam- with a temperature-resistantples are put at one edge tape of (see the Figure sample3). holder, Vertically which aligned is filled composite with the samples fire accelerant are on a put at onehorizontal edge of the steel sample plate. holder, All fire which tests is are filled repeated with the once fire accelerantfor each co onndition a horizontal and results are steel plate.averaged. All fire tests are repeated once for each condition and results are averaged. The thermallyThe thermally degraded degraded samples aresamples weighed are weighed and cut intoand testcut specimensinto test specimens for inter- for interlaminar shearlaminar strength shear strength (ILSS) testing. (ILSS) Sizetesting. and Size position and position of the specimens of the specimens are shown are in shown in Figure3. ILSS-testingFigure 3. ILSS is- chosentesting becauseis chosen small because specimens small allowspecimen to traces allow the inhomogeneousto trace the inhomogene- damage throughoutous damage the throughout panel. ILSS-tests the panel. for ILSS the- specimentests for the with specimen a thickness with of a 2thickness mm are of 2 mm performed in accordance to DIN EN 2563 [29]. For the specimen with a nominal thickness of 4 mm, the distance of the supports is set to 20 mm and for the specimen with a nominal thickness of 6 mm to 30 mm. The radii of loading nose and supports for the specimen with a nominal thickness of 4 mm and 6 mm are increased to 5 mm and 10 mm, respectively.

Specimens are tested with the thermally loaded side facing down. On sandwich panels (100 mm × 400 mm), the ﬁre accelerant is applied on an area of 98 mm × 98 mm at the center. Three 16.5 mm thick test specimen (27 mm × 394.6 mm) are cut from a panel and investigated by a four-point bending test according to DIN 53293 [30] with the thermally loaded side facing up. Ultrasonic scanning is performed according to EN 45000 with a USPC 3010-HFUS 2000 by DR. HILLGER equipped with a Panametrics 20 MHz sensor head with a resolution of 2 µm. Changes in the composition of the polymer matrix due to the thermal load are analyzed by micro attenuated total reﬂection Fourier transform infrared spectroscopy (µ-ATR-FTIR). Spectra are recorded with a Bruker Tensor 27 spectrometer and a Harrick J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 6 of 26

are performed in accordance to DIN EN 2563 [29]. For the specimen with a nominal thickness of 4 mm, the distance of the supports is set to 20 mm and for the specimen with a nominal thickness of 6 mm to 30 mm. The radii of loading nose and supports for the J. Compos. Sci. 2021, 5, 72 6 of 24 specimen with a nominal thickness of 4 mm and 6 mm are increased to 5 mm and 10 mm, respectively. Specimens are tested with the thermally loaded side facing down. On sandwich panels (100 mm × 400 mm), the fire accelerant is applied on an area of 98 mm × 98 ATRmm at cell the equipped center. Three with a16.5 silicon mm crystalthick test (diameter: specimen 0.1 (27 mm) mm on × the394.6 specimens’ mm) are cut surface from at a positionspanel and given investigated in Figure by3 .a Threefour-point spectra bending are recorded test according for every to DIN spot 53293 and the [30] received with the bandthermally intensities loaded are side averaged. facing up.

FigureFigure 3.3. SizeSize andand positionposition ofof thethe ILSS-specimensILSS-specimens cutcut fromfrom thethe thermallythermally loadedloaded panels:panels: o:o: represents thethe positionspositions forfor thethe IRIR measurementsmeasurements (sample(sample backside),backside), ••:: positionspositions ofof thermocouplesthermocouples (sample(sample backside).backside).

TemperaturesUltrasonic scanning are typically is performed recorded according with thermocouples to EN 45000 with (type a K)USPC attached 3010- toHFUS the front2000 by and DR. back HILLGER side of the equipped samples with using a Panametrics a picolog TC-08 20 MHz data sensor logger byhead pico with technology, a resolu- Germany.tion of 2 µm. Changes in the composition of the polymer matrix due to the thermal load are analyzed by micro attenuated total reflection Fourier transform infrared spectroscopy 4.(µ Results-ATR-FTIR). and DiscussionSpectra are recorded with a Bruker Tensor 27 spectrometer and a Harrick 4.1.ATR Preceding cell equipped Large-Scale with IIDa silicon Fire Tests crystal (diameter: 0.1 mm) on the specimens’ surface at positionsFigure given1 shows in Fig theure setup 3. Three and thespectra beginning are recorded of a large-scale for every fire spot test and with the a received 45 ◦ tilt angleband intensities of the CFRP are sample averaged. by high-speed camera images. Even though special care was takenTemperatures to reproducibly are perform typically these recorded experiments, with thermocouples it can be clearly (type seen, K) that attached the amount to the offront fire and accelerant back side applied of the persampl areaes isusing hard a to picolog control. TC Bursting-08 data oflogger the glassby pico bottle technology, spreads theGermany. fire accelerant randomly and variable amounts of the burning fire accelerant do not reach the CFRP. In order to improve the reproducibility of the experiments, most of them are4. Results carried and out discussion with horizontally aligned CFRP panels (see Table1). A comparison of the ◦ results4.1. Preceding obtained Large for-Scale the horizontally IID Fire Tests and 45 aligned panels, both 2 mm thick and 0.5 l of fire accelerant are applied, shows, that the horizontally aligned CFRP panel is severely damaged,Figure but 1 theshows other the one setup is not. and Residual the beginning strength of is a reduced large-scale for the fire horizontally test with aaligned 45° tilt panelangle (~18%of the comparedCFRP sample to initial by high value),-speed the matrixcamera is images. significantly Even degradedthough special as indicated care was by infraredtaken to spectroscopyreproducibly and perform pronounced these experiments, delaminations it can are observedbe clearly by seen, ultrasonic that the testing. amount A possibleof fire accelerant explanation applied is that per for area a 45 ◦isorientation hard to control. of the panelBursting a significant of the glass portion bottle of spreads the fire accelerantthe fire accelerant might be random lost by dripping.ly and variable However, amounts for a horizontalof the burning alignment fire accelerant of the panel do itnot is expectedreach the thatCFRP. not In the order hot to upper improve part the ofthe reproducibility flame impinges of the the experiments, CFRP panel most and thermalof them damageare carried might out notwith be horizontally pronounced. aligned CFRP panels (see Table 1). A comparison of the resultsHowever, obtained still for nothe basic horizontally correlations and of45° the aligned parameters panels, are both observed 2 mm thick for horizontally and 0.5 l of appliedfire accelerant panels. are For applied, example, shows, the overall that the burning horizontally time differs aligned significantly CFRP panel for is repeatedseverely experimentsdamaged, but with the 4 other mm thick one ispanels not. (265Residual s and strength 585 s). Figure is reduced4 shows for the the temperature horizontally developmentaligned panel for(~18 a% horizontally compared to aligned initial 4value), mm thick the matrix panel duringis significantly the test. degraded Front side as temperaturesindicated by infrared reach 600 spectroscopy◦C and significantly and pronounced differ atdelaminations various places are and observed test durations. by ultra- Backsidesonic testing. temperatures A possible are explanation considered is reliable,that for a whereas 45° orientation thermocouples of the panel at the a signifi front sidecant may detach from the CFRP panel. Test durations determined by the presence of visually observable flames are not significant, as small remaining flames without considerable heat impact may falsify them. In Figure4 it can be seen, that after ~200 s the temperature

maximum at the samples backside is reached, even though the ﬁre accelerant does not extinguish until 585 s (see Table1). J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 7 of 26

portion of the fire accelerant might be lost by dripping. However, for a horizontal alignment of the panel it is expected that not the hot upper part of the flame impinges the CFRP panel and thermal damage might not be pronounced. However, still no basic correlations of the parameters are observed for horizontally applied panels. For example, the overall burning time differs significantly for repeated experiments with 4 mm thick panels (265 s and 585 s). Figure 4 shows the temperature development for a horizontally aligned 4 mm thick panel during the test. Front side temperatures reach 600 °C and significantly differ at various places and test durations. Back- side temperatures are considered reliable, whereas thermocouples at the front side may detach from the CFRP panel. Test durations determined by the presence of visually observable flames are not significant, as small remaining flames without considerable heat impact may falsify them. In Figure 4 it can be seen, that after ~200 s the temperature max- J. Compos. Sci. 2021, 5, 72 imum at the samples backside is reached, even though the fire accelerant does7 ofnot 24 extinguish until 585 s (see Table 1).

FigureFigure 4. Temperature 4. Temperature recordings recordings for a large-scale for a large IID-scale ﬁretest IID withfire test a 4 mmwith thick a 4 mm horizontally thick horizontally aligned M18-1/G939aligned panel M18- (see1/G939 line panel 3 in Table (see 1line). 3 in Table 1).

The damageThe damage seems seems to be dependentto be dependent on sample on sample thickness. thickness. Even thoughEven though one liter one of liter of fire accelerantfire accelerant is equally is equally used for used the for 4 and the 64 mmand 6 thick mm samples,thick samples, the damages the damages significantly significantly differ.differ. The 6 mmThe thick6 mm sample thick sample exhibits exhibits a thermal a thermal damage damage indicated indicated by its mass by lossits mass of ~3% loss of ~ instead of 0% for the 4 mm thick samples. Additionally it reaches higher temperatures at 3% instead of 0% for the◦ 4 mm thick samples.◦ Additionally it reaches higher temperatures the samples’at the samples front side:’ front ~900 side:C instead~900 °C ofinstead 600 C. of 600 °C. In summary, this type of experiment shows, that reproducible results are difficult In summary, this type of experiment shows, that reproducible results are difficult to to obtain. However, they proof that IID can destroy CFRP structures. For a systematic obtain. However, they proof that IID can destroy CFRP structures. For a systematic inves- investigation, laboratory scale experiments are performed and described in the following. tigation, laboratory scale experiments are performed and described in the following. These laboratory experiments show similar amounts of applied fire accelerants per area, and These laboratory experiments show similar amounts of applied fire accelerants per area, similar developed temperatures and durations of burning. They are preferably carried out and similar developed temperatures and durations of burning. They are preferably car- in horizontal alignment of the CFRP samples without dripping of burning fire accelerant. ried out in horizontal alignment of the CFRP samples without dripping of burning fire In order to avoid residues after the test by polystyrene in the fire accelerant, which falsify accelerant. In order to avoid residues after the test by polystyrene in the fire accelerant, the determined mass loss of the CFRP, laboratory tests are performed without thickener. which falsify the determined mass loss of the CFRP, laboratory tests are performed with- This type of test guarantees reproducible results and simulates conditions of an IID out thickener. assault with respect to a comparable way of heat impact in worst case. This type of test guarantees reproducible results and simulates conditions of an IID 4.2. Laboratoryassault with Tests: respect Heat Release, to a comparable Temperature way Rise of and heat Mass impact Loss in Due worst to Combustion case. of Fire Accelerant4.2. Laboratory Tests: Heat Release, Temperature Rise and Mass Loss Due to Combustion of Fire FigureAccelerant5 shows representative heat release rate (HRR) curves obtained for the combustion of various amounts of fire accelerant on CFRP panels in a cone calorimeter (see Figure2 ). They reach similar peak heat release rates, and the total heat release nearly lineary increases depending on the amount of fire accelerant (see also Figure6). The burning time of the fire accelerant and therefore the duration of the conducted experiments

also lineary increase by approx. 180 s per 10 g of applied fire accelerant in average. The averaged determined effective heat of combustion for all reference experiments is 42 kJ/g, which corresponds to the fuel load of hydrocarbons in the fire accelerant [31] and indicates a complete combustion of the fire accelerant during the experiment. For each amount of fire accelerant, the HRR curves are nearly identical for the various investigated horizontally aligned CFRP materials and the inert metal plate used as reference. J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 8 of 26

Figure 5 shows representative heat release rate (HRR) curves obtained for the combustion of various amounts of fire accelerant on CFRP panels in a cone calorimeter (see Figure 2). They reach similar peak heat release rates, and the total heat release nearly lineary increases depending on the amount of fire accelerant (see also Figure 6). The burning time of the fire accelerant and therefore the duration of the conducted experiments also lineary increase by approx. 180 s per 10 g of applied fire accelerant in average. The averaged determined effective heat of combustion for all reference experiments is 42 kJ/g, which corresponds to the fuel load of hydrocarbons in the fire accelerant [31] and indicates a complete combustion of the fire accelerant during the experiment. For each amount of fire accelerant, the HRR curves are nearly identical for the various investigated horizontally aligned CFRP materials and the inert metal plate used as reference.

J. Compos. Sci. 2021, 5, 72 8 of 24

J. Compos. Sci. 2021, 5, x FOR PEERFigure REVIEW 5. Exemplary heat release rate (HRR) curves for the combustion of various amounts of ﬁre9 of 26 Figure 5. Exemplary heat release rate (HRR) curves for the combustion of various amounts of fire accelerantaccelerant on CFRP on samples. CFRP samples. The HRR The curve HRR of curve the forced of the combustion forced combustion of a 3 mm of a thick 3 mm CFRP thick at CFRP an at an 2 externalexternal heat ﬂux heat of 60 flux kW/m of 60 kW/mis added.2 is added.

FigureFigure 6. Total 6. heat Total release heat release (THR) for(THR) various for various amounts amounts of fire accelerants of fire accelerants on all types on all of types horizontally of horizon- alignedtally 8552/IM7 aligned samples 8552/IM7 (Line samples corresponds (Line corresponds to reference to material: reference fire material: accelerant fire on accelerant steel panel). on steel panel). Comparing the total heat releases for the combustion of the fire accelerants on various CFRP substratesComparing with that the of total an inertheat metalreleases plate for (reference) the combustion in Figure of 6 the exhibits fire accelerants that in on generalvarious no significant CFRP substrates increase ofwith the that total of heat an inert release metal is observed plate (reference) due to a in contribution Figure 6 exhibits by a combustionthat in general of the no CFRP significant matrix increase or the coating. of the total Only heat thin release samples is observed up to 2mm due to a partlycontribution indicate higher by a total comb heatustion releases of the thanCFRP the matrix reference or the experiments. coating. Only However, thin samples the up to contribution2 mm partly is low. indicate For some higher experiments, total heat releases especially than with the coatedreference 4 mmexperiments. samples, However, the total heatthe contribution releases are significantelyis low. For some lower experiments, than expected especially for the with combustion coated 4 mm of the samples, fire the accelerants.total heat For thesereleases experiments are significantely it is assumed lower that than little expected amounts for ofthe the combustion fire accelerants of the fire were lostaccelerants. by leaking For through these experiments the sample holder.it is assumed that little amounts of the fire accelerants were lost by leaking through the sample holder. For comparison reasons a typical HRR curve of the forced combustion of a 3 mm thick 8552/IM7 CFRP panel at an external heat flux of 60 kW/m2 is added. For the forced combustion of this CFRP, a residue of 75% related to the initial mass is formed, and the total heat release is 25 MJ/m2 [20], for comparable 2 mm thick M18-1/G939 panels, a heat release of 25 MJ/m2 and a residue of 73% is determined [21]. The effective heat of combustion (related to mass of combusted material) of both CFRP is ~22 kJ/g, which is typical for a combination of epoxy resin/polyethersulfon and epoxy resin/polyetherimide, respectively. The residue dominantely contains fibers and char. A detailed characterization of reaction-to-fire properties and the forced combustion of the two CFRP are reported in [20,21]. In summary, the obtained heat release measurements are sensitive enough to measure a contribution by a combustion of the CFRP samples with a detection limit of about 5 MJ/m2, which, however, typically does not occur. Therefore, the conducted experiments dominantely focus on the heat impact of the fire accelerant on the CFRP and do not force a significant, exothermic combustion of horizontally aligned, thick CFRP panels.

J. Compos. Sci. 2021, 5, 72 9 of 24

For comparison reasons a typical HRR curve of the forced combustion of a 3 mm thick 8552/IM7 CFRP panel at an external heat flux of 60 kW/m2 is added. For the forced combustion of this CFRP, a residue of 75% related to the initial mass is formed, and the total heat release is 25 MJ/m2 [20], for comparable 2 mm thick M18-1/G939 panels, a heat release of 25 MJ/m2 and a residue of 73% is determined [21]. The effective heat of combustion (related to mass of combusted material) of both CFRP is ~22 kJ/g, which is typical for a combination of epoxy resin/polyethersulfon and epoxy resin/polyetherimide, respectively. The residue dominantely contains fibers and char. A detailed characterization of reaction-to-fire properties and the forced combustion of the two CFRP are reported in [20,21]. In summary, the obtained heat release measurements are sensitive enough to measure a contribution by a combustion of the CFRP samples with a detection limit of about 5 MJ/m2, which, however, typically does not occur. Therefore, the conducted experiments dominantely focus on the heat impact of the fire accelerant on the CFRP and do not force a significant, exothermic combustion of horizontally aligned, thick CFRP panels. The observed mass loss is below 1% for 4 to 8 mm thick samples and increases very slightly with the applied amount of fire accelerant (data not depicted). However, for 2 mm thick panels, a degradation of the polymer matrix close to the surface provides a higher ratio of the overall sample mass and the mass loss for pure 8552/IM7 linearly increases up to 5% for 30 g fire accelerant. The highest mass loss of 7.5% for horizontally aligned panels is reached by a 1 mm thick M18-1/G939 sample after an impact of 30 g fire accelerant. In summary, the determined mass loss increases with the amount of fire accelerant, and decreases with sample thickness, which was also observed for irradiation experiments [17], but it is too low causing an additional significant heat release by a pronounced exothermic combustion of resin matrix for samples thicker than 2 mm (see above). Temperature necessary for ignition of 8552/IM7 was observed to be in the range of 400 ◦C at the surface of the irradiated side by cone calorimetry [20]. A determination of the temperature is however inaccurate by thermocouples at the front surface of the CFRP panel during combustion of the fire accellerant, because it may detach. Therefore, the temperature trace at the front side of a 4 mm thick panel with 20 g of applied fire accellerant (Figure7A) shows a wide variation for the reached maximum temperature, which reaches 500 ◦C instead of 390 ◦C in repeated experiments. In contrast, the temperature traces recorded at the backside are accurate and indicate higher temperatures at the center (275 ◦C) of the specimen compared to the areas close to its edges (218 ◦C). Additionally it takes slightly longer to reach the maximum temperatures at the edge of the sample compared to the center (400 s instead of 370 s). Both observations are to be expected for the formation of a cone shaped flame above the panel (see Figure2) and the heat conduction by the metal frame of the sample holder. Time to reach maximum backside temperature increases strongly with the increasing amounts of fire accelerant, as its combustion takes longer (see also Figure5). However, these times increase only slightly with increasing panel thickness (Figure7B). For example, maximum temperature is reached after 132 s when 10 g of fire accelerant is applied on a 2 mm panel, and after only 209 s when a 6 mm panel is investigated, but after 730 s when 40 g are applied on a 2 mm thick panel. The higher heat capacities of thicker samples and the longer duration of heat transfer are responsible for these observations, respectively. Also maximum backside temperatures increase with the amount of fire accelerant. For example, a temperature of 279 ◦C is reached for 10 g of fire accelerant, and 321 ◦C for 40 g on 2 mm thick samples. For 6 mm thick samples, these temperatures are lower: 195 ◦C for 10 g and 269 ◦C for 40 g, respectively. The temperature differences between 2 and 6 mm thick samples, for a certain amount of fire accelerant, is more pronounced for small amounts of fire accelerant (10 g: 84 ◦C; 40 g: 52 ◦C), indicating especially low backside temperatures for thicker samples and therefore more pronounced temperature gradients. In Figure7B, a remarkable unsteady temperature increase is observed for 2 mm thick samples and masses of fire accelerants more than 20 g. An occurring intermediate temperature drop J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 10 of 26

The observed mass loss is below 1% for 4 to 8 mm thick samples and increases very slightly with the applied amount of fire accelerant (data not depicted). However, for 2 mm thick panels, a degradation of the polymer matrix close to the surface provides a higher ratio of the overall sample mass and the mass loss for pure 8552/IM7 linearly increases up to 5% for 30 g fire accelerant. The highest mass loss of 7.5% for horizontally aligned panels is reached by a 1 mm thick M18-1/G939 sample after an impact of 30 g fire accelerant. In summary, the determined mass loss increases with the amount of fire accelerant, and decreases with sample thickness, which was also observed for irradiation experiments [17], but it is too low causing an additional significant heat release by a pronounced exothermic combustion of resin matrix for samples thicker than 2 mm (see above). Temperature necessary for ignition of 8552/IM7 was observed to be in the range of 400 °C at the surface of the irradiated side by cone calorimetry [20]. A determination of the temperature is however inaccurate by thermocouples at the front surface of the CFRP panel during combustion of the fire accellerant, because it may detach. Therefore, the temperature trace at the front side of a 4 mm thick panel with 20 g of applied fire J. Compos. Sci. 2021, 5, 72 accellerant (Figure 7A) shows a wide variation for the reached maximum10 of temper 24 ature, which reaches 500 °C instead of 390 °C in repeated experiments. In contrast, the temperature traces recorded at the backside are accurate and indicate higher temperatures at the center (275 °C) of the specimen compared to the areas close to its edges (218 °C). is correlated to a pronounced matrix degradation and is typically indicated by large areas Additionally it takes slightly longer to reach the maximum temperatures at the edge of of free carbon ﬁbers in pictures of the panels’ surface after the experiment. The reasons are the sample compared to the center (400 s instead of 370 s). Both observations are to be a combination of pronounced delaminations with a retarded heat transfer to the samples expected for the formation of a cone shaped flame above the panel (see Figure 2) and the backside and the endothermic pyrolysis of the matrix (see below). heat conduction by the metal frame of the sample holder.

Figure 7. ExemplaryFigure temperature 7. Exemplary rises temperature on 8552/IM7 rises CFRP on 8552/IM7 panels (with CFRP integrated panels (with copper integrated mesh and copper applied mesh coating). and (A) at various positionsapplied on a coating). 4 mm thick (A) panel at various for 20 positions g of fire onaccelerant a 4 mm (exact thick panelpositions for 20see g Figure of fire 3 accelerant (B) at the (exactcenter of the backside of various thick panels and amounts of fire accelerant. positions see Figure3( B) at the center of the backside of various thick panels and amounts of fire accelerant. Time to reach maximum backside temperature increases strongly with the increasing Figureamounts8 shows theof fire temperatures accelerant, inas various its combust depthsion inside takes 4longer mm thick (see panelsalso Fig duringure 5). However, the combustionthese oftimes 40 g increase fire accelerant. only slightly Temperatures with increasing nearly linearly panel thickness increase (Fig throughouture 7B). For exam- the intact sampleple, maximum with only temperature a slight gradient is reached into the after depth 132 untils when a rapid 10 g temperatureof fire accelerant rise is applied occurs afteron 640 a s2 formm 8552/IM7. panel, and At after the sameonly time209 s of when the rapid a 6 mm temperature panel is investigated, increase in areas but after 730 s close to thewhen surface, 40 g a are temperature applied on drop a 2 mm occurs thick underneath. panel. The higher This phenomenon heat capacities is of typical thicker samples for the formationand the of longer delaminations. duration Delaminations of heat transfer of theare topresponsible plies provide for these a barrier observations, for the respec- heat transfertively. into Also the bulk maximum material backside and promote temperatures the degradation increase with of the the top amount plies [of32 ].fire A accelerant. rapid temperatureFor example, increase a temperature also indicates of 279 that °C the is pyrolysisreached for zone 10 g may of fire have accelerant, reached and the 321 °C for depth of the40 thermocouple g on 2 mm thick [20]. samples. After 640 For s, the 6 mm degradation thick samples, of the polymerthese temperatures matrix reaches are lower: 195 a depth of 1 mm. Underneath the first ply (0.1 mm) a temperature of 310 ◦C is reached. For depths from 1 to 2 mm a shift of ~40 s for the beginning rapid temperature increase is observed, which corresponds to an average migration velocity of the pyrolysis zone of ~25 µm/s. For the forced combustion of 4 mm thick QI 8552/IM7 samples at 60 kW/m2, a similar migration velocity of the pyrolysis zone of 35 ± 3 µm/s was reported [20]. Matrix degradation is less pronounced in areas deeper than 2 mm, since the temperature rises are lower and only maximum temperatures of 300 ◦C are reached. With a maximum temperature of 495 ◦C at a depth of 0.1 mm, degradation of the matrix close to the surface is evident. The contribution of the matrix degradation to the total heat release during the application of the fire accelerant is not significant. Therefore, matrix degradation is considered to have predominantly pyrolysis instead of self-sustaining oxidative and exothermic character. As mass loss is ~1% only, no significant contribution to the overall heat release is measured and temperature rises inside the CFRP material are mainly induced by the burning fire accelerant. J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 11 of 26

°C for 10 g and 269 °C for 40 g, respectively. The temperature differences between 2 and 6 mm thick samples, for a certain amount of fire accelerant, is more pronounced for small amounts of fire accelerant (10g: 84 °C; 40 g: 52 °C), indicating especially low backside temperatures for thicker samples and therefore more pronounced temperature gradients. In Figure 7B, a remarkable unsteady temperature increase is observed for 2 mm thick samples and masses of fire accelerants more than 20 g. An occurring intermediate temperature drop is correlated to a pronounced matrix degradation and is typically indicated by large areas of free carbon fibers in pictures of the panels’ surface after the experiment. The reasons are a combination of pronounced delaminations with a retarded heat transfer to the samples backside and the endothermic pyrolysis of the matrix (see below). Figure 8 shows the temperatures in various depths inside 4 mm thick panels during the combustion of 40 g fire accelerant. Temperatures nearly linearly increase throughout the intact sample with only a slight gradient into the depth until a rapid temperature rise occurs after 640 s for 8552/IM7. At the same time of the rapid temperature increase in areas close to the surface, a temperature drop occurs underneath. This phenomenon is typical for the formation of delaminations. Delaminations of the top plies provide a barrier for the heat transfer into the bulk material and promote the degradation of the top plies [32]. A rapid temperature increase also indicates that the pyrolysis zone may have reached the depth of the thermocouple [20]. After 640 s, the degradation of the polymer matrix reaches a depth of 1 mm. Underneath the first ply (0.1 mm) a temperature of 310 °C is reached. For depths from 1 to 2 mm a shift of ~40 s for the beginning rapid temperature increase is J. Compos. Sci. 2021, 5, 72 observed, which corresponds to an average migration velocity of the pyrolysis zone 11of of~25 24 µm/s. For the forced combustion of 4 mm thick QI 8552/IM7 samples at 60 kW/m2, a similar migration velocity of the pyrolysis zone of 35 ± 3 µm/s was reported [20].

Figure 8. ExemplaryExemplary temperature temperature rises rises in in various various depth depth of of4 mm 4 mm thick thick 8552/IM7 8552/IM7 and and M18 M18-1/G939-1/G939 CFRP CFRP panels panels with withinte- integratedgrated thermocouples thermocouples during during application application of 40 of g 40 fire g ﬁreaccelerant. accelerant.

MatrixAfter flame degradation out, the surface is less cools pronounced fastest, as in indicated areas deeper by the thanlowest 2 temperatures mm, since the of temperaturethe thermocouple rises are in a lower depth and of 0.1 only mm. maximum temperatures of 300 °C are reached. With a maximumSimilar effectstemperature are observed of 495 °C for at M18-1/G939. a depth of 0.1 However, mm, degradation effects by of delaminations the matrix close are toless the pronounced surface is forevident. the fiber The fabric contribution reinforcement of the matrix [21], and degradation M18-1/G939 to thehas totala slightly heat releasehigher heatduring capacity the application than 8552/IM7 of the [fire17], accelerant explaining is the not longer significant duration. Therefore, (~720 s)matrix until degradationa rapid temperature is considered increase to occurs. have predominantly The presence of pyrolysis the flame instead retardants of self may-sustaining play an oxidativeadditional and role. exothermic character. As mass loss is ~1% only, no significant contribution to the overall heat release is measured and temperature rises inside the CFRP material are mainly4.3. Non-Destructive induced by the Testing burning after fire Thermal accelerant. Loading 4.3.1.After Visual flame Inspection out, the surface cools fastest, as indicated by the lowest temperatures of the thermocoupleFigure9 shows in representative a depth of 0.1 cross mm. sections and photographs of the treated front side of selected M18-1/G939 samples after the impact of various amounts of fire accelerant. For 2 mm thick samples a very fast transition from nearly intact material (after application of 5 g of fire accelerant with only slightly darker areas in the center of the panel) to severely

damaged samples is observed. After application of 10 g of fire accelerant, delaminations can be found throughout the whole thickness of the sample even if matrix degradation at the surface of the treated side is hardly visible. This transition is not accompanied by a significant increase in mass loss (~0.5 %). Therefore, thermally induced strain is responsible for this step of damage. With higher amounts of fire accelerant (15 g), carbon fibers are set free at the treated side after the matrix is decomposed and the number of delaminations increases. For 30 g of fire accelerant, the matrix is degraded also in deeper areas of the panel. Therefore, mass loss increases significantly from 1.2% to 5% and a large area of free carbon fibers at the treated surface are observed. Formed delaminations close to the treated side provide a protection for intact material underneath and the panels significantly expand. These effects are more pronounced for thicker samples. For 6 mm thick samples the major portion of the cross section remains intact and delaminations are visible only close to the treated side. For a comparable damage between 2 mm and 6 mm thick samples, far higher amounts of fire accelerant are necessary for the thick samples. Whereas for 2 mm thick samples, large areas show uncovered carbon fibers at the surface for 15 g of fire accelerant, 6 mm thick samples start to exhibit a comparable matrix degradation not until 30 g. Again, the higher heat capacities of thicker samples are responsible for the retarded damage initiation. J. Compos. Sci. 2021, 5, 72 12 of 24 J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 13 of 26

mfa Δm Cross Section Front Side Image US C-Scan

5 g ~0.4% 2 mm 6 cm

10 g ~0.5%

2 2 mm

15 g ~1.2%

30 g ~5.0%

15 g ~0.5% 6 mm

30 g ~0.5% 6 6 mm

50 g ~1.1%

Figure 9. Cross sections (hot sideFigure on 9. top), Cross photographs sections (hot side and on ultrasonic top), photographs C-scans and of 2 ult andrasonic 6 mm C- thickscans of M18-1/G939 2 and 6 mm thick panels M18-1/G939 panels after impact of various amounts of fire accelerant. Mass loss (Δm) is addition- after impact of various amounts of ﬁre accelerant. Mass loss (∆m) is additionally given. ally given.

4.3.2. Ultrasonic Testing 4.3.2. Ultrasonic Testing As a routine non-destructive technique, ultrasonic C-scans are performed for all investigated specimen (see exemplarily Figure9). For a moderate thermal load after application of 5 g ﬁre accelerant, no damage is detected on 2 mm thick M18-1/G939

samples. Beginning from 10 g, occurring delaminations are responsible for increasingly damaged areas in the C-scans, even if matrix degradation is hardly observed at the surface. J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 14 of 26

As a routine non-destructive technique, ultrasonic C-scans are performed for all investigated specimen (see exemplarily Figure 9). For a moderate thermal load after appli- J. Compos. Sci. 2021, 5, 72 cation of 5 g fire accelerant, no damage is detected on 2 mm thick M18-1/G93913 samples. of 24 Beginning from 10 g, occurring delaminations are responsible for increasingly damaged areas in the C-scans, even if matrix degradation is hardly observed at the surface. For the For6 the mm 6 mm thick thick sample, sample, more more than than 15 15 g gof of fire fire accelerant accelerant is is necessary necessary to detect aa damagedamage by by ultrasonicultrasonic testing, testing, as as thicker thicker samples samples are are less less damaged damaged (see (see above). above). The The ratio ra oftio damaged of damaged areaarea in the in ultrasonicthe ultrasonic C-scans C-scans correlates correlates with with the amountthe amount of applied of applied fire accelerantfire accelerant (data (data notnot depicted). depicted). Similar Similar data data are obtainedare obtained for 8552/IM7for 8552/IM7 samples. samples. In summary,In summary, ultrasonic ultrasonic testing testing is a suitableis a suitable technique technique to identify to identify thermal thermal damage damage by by improvisedimprovised fire fire accelerants. accelerants. Only Only for moderatefor moderate damages damages it is it not is not sensitive sensitive enough. enough. Here, Here, infraredinfrared spectroscopy spectroscopy provides provides a more a more sensitive sensitive alternative. alternative.

4.3.3.4.3.3. Infrared Infrared Spectroscopy Spectroscopy AnAn IR-spectroscopic IR-spectroscopic analysis analysis of the of the8552 8552 matrix matrix system system shows shows intensive intensive bands bands at at − − 15101510 cm cm1 and−1 and at 1486 at 1486 cm cm1,− which1, which are are attributed attributed to epoxyto epoxy resin resin (EP) (EP) and and polyethersulfone polyethersulfone (PES),(PES), respectively respectively [23, 33[23,33].]. Figure Figure 10 depicts 10 depicts that that thermal thermal degradation degradation of the of the EP resinEP resin is is preferredpreferred over over the thermoplasticthe thermoplastic toughener toughener PES, PES, as the as EPthe resinEP resin is thermally is thermal lessly less resistant resistant −1 andand indicated indicated by a by loss a loss of intensity of intensity for the for bandthe band at 1510 at 1510 cm cm, recorded−1, recorded in the in hotterthe hotter center center of aof specimen a specimen (spectrum (spectrum at 50 at mm) 50 mm) compared compared to the to edges.the edges.

FigureFigure 10. ATR-FTIR-spectra10. ATR-FTIR-spectra recorded recorded along along the the middle middle of theof the backside backside (given (given distance distanc frome from the the edgeedge in mm, in mm, see Figuresee Fig3ure) of 3 2) of mm 2 mm thick thick 8552/IM7 8552/IM7 (left) (left) and and M18-1/G939 M18-1/G939 (right) (right) specimens specimens with with −1 −1 −1 markedmarked characteristic characteristic bands bands for EP for (1510 EP (1510 cm− 1cm), PES), PES (1486 (1486 cm− cm1), and), and PEI PEI (1720 (1720 cm cm−1) after) after impact impact of 10 g fire accelerant (spectra are vertically shifted). of 10 g ﬁre accelerant (spectra are vertically shifted).

An IR-spectroscopicAn IR-spectroscopic analysis analysis of M18-1/G939 of M18-1/G939 is based is based on a on band a band at 1720 at 1720 cm− 1cm, which−1, which is characteristicis characteristic for polyetherimidefor polyetherimide (PEI) (PEI) as toughener as toughener [34]. [34]. Again, Again, the band the band at 1510 at 1510 cm− 1cm−1 originatesoriginates from from the the epoxy epoxy resin. resin. With With increasing increasing thermal thermal degradation degradation a decrease a decreas ofe the of the epoxyepoxy band band with with respect respect to to the the polyetherimide polyetherimide band band can can be be observed, observed, similarsimilar toto the be- behaviorhavior of of the the 8552 8552 matrix. matrix. For For both both materials, materials, oxidation oxidation of theof the polymer polymer matrix matrix is indicated is indicated by surfaceby surface carbonyl carbonyl species species with with a broad a broad band band in the in rangethe range of 1650 of 1650 cm− cm1 [35−1 ,[35,36].36]. ThermalThermal degradation degradation of the of polymerthe polymer matrix matrix is characterized is characterized by theby intensitythe intensity ratio ratio of of IR-bandsIR-bands at 1510 at 1510 cm− 1cm(EP)−1 (EP) and and at 1486 at 1486 cm− 1cm(PES)−1 (PES) [18] [18] for 8552/IM7, for 8552/IM7, as well as well as 1510 as 1510 cm−1 cm-1 (EP)(EP) and and 1720 1720 cm− cm1 (PEI)−1 (PEI) for for M18-1/G939. M18-1/G939. FigureFig 11ure exemplarily 11 exemplarily shows shows the matrixthe matrix degradation degradation by means by means of profiles of profiles of this of this band band intensityintensity ratio ratio through through the middlethe middle (see (see Figure Fig2ure) of 2 the) of specimens’ the specimens backside,’ backside, dependent dependent on the amount of fire accelerant and sample thickness. For 8552/IM7 the intensity ratio is slightly above 1 for an intact sample and decreases with increasing thermal damage. With increasing amounts of fire accelerant this ratio decreases especially for the 2 mm specimens and a pronounced difference between the hotter centers of the specimen (=50 mm position) and the colder edges is observed. For a certain amount of fire accelerant, matrix J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 15 of 26

J. Compos. Sci. 2021, 5, 72 on the amount of fire accelerant and sample thickness. For 8552/IM7 the intensity ratio is 14 of 24 slightly above 1 for an intact sample and decreases with increasing thermal damage. With increasing amounts of fire accelerant this ratio decreases especially for the 2 mm specimens and a pronounced difference between the hotter centers of the specimen (= 50 mm degradationposition) and is the more colder pronounced edges is observed. for thinner For a certain samples. amount For of example, fire accelerant, to reach matrix a matrix degradationdegradation indicated is more pronounced by a band for intensity thinner ratiosamples. of roughly For example, 0.8 at to the reach center a matrix of the deg- samples backside,radation 10indicated g of ﬁre by accelerant a band intensity is necessary ratio of forroughly the 2 0.8 mm at thickthe center specimen, of the samples whereas 30 g areba necessaryckside, 10 forg of thefire 4accelerant mm thick is specimen.necessary for As the the 2 mm reached thick maximumspecimen, whereas temperatures 30 g are are necessary for the 4 mm thick specimen. As the reached maximum temperatures are lower for thicker samples (see Figure7), matrix degradation is less pronounced at the lower for thicker samples (see Figure 7), matrix degradation is less pronounced at the samples backside. No spectra are obtained at the sample’s front side of the panels due to samples backside. No spectra are obtained at the sample’s front side of the panels due to contaminationcontamination of of the the surfacesurface by by residues residues of of combustion. combustion.

FigureFigure 11. Representative 11. Representative distribution distribution of of the the intensity intensity ratios ratios ofof bandsbands characteristic for for EP EP (1510 (1510 cm cm−1) a−nd1) andPES (1486 PES (1486cm−1) cm−1) after impactafter impact of various of various amounts amounts of fire of accelerantsfire accelerants on 2,on 4, 2, and 4, and 6 mm 6 mm thick thick 8552/IM7 8552/IM7 samples samples (with (with integrated integrated copper copper mesh), mesh), recorded at the backsides (horizontally aligned). recorded at the backsides (horizontally aligned). The same observations are made for the M18-1/G939 samples by means of the IR The same observations are made for the M18-1/G939 samples by means of the IR band band intensity ratio I1510 cm−1 / I1780 cm−1 (data not depicted). For a comparison of the efficiency intensityof various ratio protective I1510 cm− measures,1 /I1780 cm −specimen1 (data not at the depicted). center of For the a panel comparison with lowest of the band efficiency in- of varioustensity protective ratios are selected measures, (see specimen below). at the center of the panel with lowest band intensity ratios are selected (see below). 4.4. Residual Interlaminar Shear Strength (ILSS) 4.4. ResidualResidual Interlaminar ILSS of composite Shear Strength samples (ILSS) with an integrated copper mesh after impact of various amounts of fire accelerant is exemplarily shown in Figure 12. According to matrix degradationResidual measured ILSS of composite by IR spectroscopy, samples withILSS anshows integrated lower values copper in the mesh center after of impactthe of varioussamples, amounts compared of fire to the accelerant edges and is exemplarilyits decrease is shown more inpronounced Figure 12 .for According thin samples to matrix degradationwith increasing measured amounts by of IR fire spectroscopy, accelerant. For ILSS 6 mm shows thick samples lower values only three in the test center speci- of the samples,men can compared be gained tofrom the the edges panel, and and its measurin decreaseg is the more lateral pronounced distribution for of thinthe damage samples with increasingis limited. amounts Thick samples of fire show accelerant. a less pronounced For 6 mm and thick continuous samples onlydecrease three of testresidual specimen canstrength be gained compared from to the the panel, thin ones. and Whereas measuring the ILSS the decreases lateral distribution only by approx. of the 20% damage for is limited.the application Thick samples of 40 g of showfire accelerant a less pronounced for 6 mm samples, and continuousthe 2 mm specimen decrease does of not residual strengthretain a compared significant toresidual the thin ILSS. ones. In this Whereas case, samples the ILSS with decreases residual onlyILSS below by approx. roughly 20% for the application of 40 g of fire accelerant for 6 mm samples, the 2 mm specimen does not retain a significant residual ILSS. In this case, samples with residual ILSS below roughly 2 30 N/mm tend to fail in multiple plies, as pronounced delaminations occur throughout the thickness (see Figure9). Samples with plies falling apart before testing are attributed to a residual strength of 0 N/mm2. In Table2 it is obvious, that 1 mm thick panels lose their mechanical integrity for very low amounts of fire accelerant. The 8 mm thick panels withstand typical amounts of fire accelerants in an arson attack, as unrealistically high amounts are necessary for a severe damage. For a comparison of the efficiency of various J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 16 of 26

30 N/mm2 tend to fail in multiple plies, as pronounced delaminations occur throughout the thickness (see Figure 9). Samples with plies falling apart before testing are attributed to a residual strength of 0 N/mm2. In Table 2 it is obvious, that 1 mm thick panels lose their J. Compos. Sci. 2021, 5, 72 mechanical integrity for very low amounts of fire accelerant. The 8 mm thick panels15 of with- 24 stand typical amounts of fire accelerants in an arson attack, as unrealistically high amounts are necessary for a severe damage. For a comparison of the efficiency of various protectiveprotective measures, measures, specimen specimen at theat the center center of of the the panel panel with with the the lowest lowest residual residual strength strength areare selected selected (see (see below below and and Table Tab2le). 2)

FigureFigure 12. 12.Representative Representative distribution distribution of of interlaminar interlaminar shear shear strength strength (ILSS) (ILSS) after after impact impact of of various various amounts amounts of of ﬁre fire accelerantsaccelerants on on 2, 4,2, and4, and 6 mm 6 mm thick thick 8552/IM7 8552/IM7 samples samples (with (with integrated integrated copper copper mesh, mesh, horizontally horizontally aligned). aligned).

4.5.4.5. Influence Influence of Horizontalof Horizontal and and Vertical Vertical Alignment Alignment of theof the Panels Panels AA vertical vertical alignment alignment of of the the sample sample at at the the edge edge of of the the sample sample holder holder leads leads to to different different areasareas of of thermal thermal damage damage comparedcompared toto the horizontally horizontally aligned aligned panel. panel. In Inan anupright upright posi- position,tion, the the top top part part of a of 2 mm a 2 mmthick thick 8552/IM7 8552/IM7 panel panelis visually is visually more damaged, more damaged, as shown as in shownFigure in 13, Figure as the 13 flame, as the of the flame burning of the fire burning accelerant fire accelerantmay be higher may than be higher the panel than (10 the cm), paneland (10hotter cm), areas and hotterof the flame areas ofhave the impact flame haveon the impact panel. on Therefore, the panel. thermal Therefore, damage thermal is not damagedistributed is not symmetrically distributed symmetrically over the panel, over but the larger panel, at but the larger top compared at the top to compared the horizon- to thetally horizontally aligned panels, aligned as panels, also indicated as also indicatedby ultrasonic by ultrasonic testing. Correspondingly, testing. Correspondingly, residual in- residualterlaminar interlaminar shear strength shear strength decreases decreases continuously continuously and rapidly and rapidly from bottom from bottom to top to for topmasses for masses of fire of accelerant fire accelerant higher higher than than15 g (see 15 g Fig (seeure Figure 13). Minimum13). Minimum residual residual ILSS ILSSis nearly is nearlysimilar similar for a forhorizontal a horizontal and vertical and vertical alignment alignment of the of8552/IM7 the 8552/IM7 panels, panels, when the when center the of centerthe samples of the samples is considered is considered (see Tab (seele 2). Table For2 M18). For-1/G939, M18-1/G939, 10 g of 10fire g accelerant of fire accelerant is sufficient is 2 sufficientto reduce to reducethe residual the residual strength strength to 0 N/mm to 0 N/mm2 for verticallyfor vertically aligned aligned panels, panels, whereas whereas horizon- horizontallytally aligned aligned panels panels retain retaina residual a residual strength strength of 31%. of However 31%. However in the top in the part, top residual part, residualILSS is ILSSsignificantly is significantly lower for lower vertically for vertically aligned panels. aligned The panels. reason The for reasonthese observations for these observationsis a more pronounced is a more pronounced matrix degradation matrixdegradation and combustion, and combustion,as indicated by as the indicated heat release by the heat release rate curves in Figure 14. Vertically aligned samples show a prolonged heat rate curves in Figure 14. Vertically aligned samples show a prolonged heat release com- release compared to the horizontally aligned. In contrast to horizontally aligned samples, pared to the horizontally aligned. In contrast to horizontally aligned samples, shown in shown in Figure6, an additional contribution to the total heat release by the combustion of Figure 6, an additional contribution to the total heat release by the combustion of the ma- the matrix is indicated, as seen Figure 14 (inset A). A maximum contribution of approx. trix is indicated, as seen Figure 14 (inset A). A maximum contribution of approx. 30 MJ/m2 30 MJ/m2 for the 8552/IM7 sample investigated with 40 g fire accelerant is in the range of for the 8552/IM7 sample investigated with 40 g fire accelerant is in the range of the total the total heat release for the forced combustion of the CFRP (see above). Correspondingly, heat release for the forced combustion of the CFRP (see above). Correspondingly, mass mass loss linearly increases with increasing amounts of fire accelerant and is higher for loss linearly increases with increasing amounts of fire accelerant and is higher for verti- vertically than for horizontally aligned samples (see inset B in Figure 14). For example, a cally than for horizontally aligned samples (see inset B in Figure 14). For example, a 2 mm 2 mm thick M18-1/G939 sample reaches a mass loss of 17.5% for the application of 50 g of fire accelerant. For a forced combustion, a typical mass loss of 25% is observed (see above). Thus, these samples investigated in a vertical alignment are partly combusted and do not retain a significant residual strength, as it is also observed for samples after a forced combustion. In the upright position of the panels, the barrier effects of the carbon fiber for combustion are less pronounced. Matrix degradation and combustion are favored parallel to the fibers (wick effect) [37], and access of air is less hindered. J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 17 of 26 J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 17 of 26

thickthick M18M18--1/G9391/G939 samplesample reachesreaches aa massmass lossloss ofof 17.5%17.5% forfor thethe applicationapplication ofof 5050 gg ofof firefire accelerant.accelerant. ForFor aa forcedforced combustion,combustion, aa typicaltypical massmass lossloss ofof 25%25% isis observedobserved (see(see above)above).. Thus,Thus, thesethese samplessamples investigatedinvestigated inin aa verticalvertical alignmentalignment areare partlypartly combustedcombusted andand dodo notnot retainretain aa significantsignificant residualresidual strength,strength, asas itit isis alsoalso observedobserved forfor samplessamples afterafter aa forcedforced com-combustion. In the upright position of the panels, the barrier effects of the carbon fiber for J. Compos. Sci. 2021, 5, 72 bustion. In the upright position of the panels, the barrier effects of the carbon fiber16 of for 24 combustioncombustion are are less less pronounced. pronounced. Matrix Matrix degradation degradation and and combustioncombustion are are favored favored parallel parallel toto thethe fibersfibers (wick(wick effect)effect) [37],[37], andand accessaccess ofof airair isis lessless hindered.hindered.

FigureFigure 13.13. RepresentativeRepresentative distributiondistribution ofof interlaminarinterlaminar shearshear (ILS)(ILS) strengthstrength andaandnd correspondingcorresponding ultrasonicultrasonic scansscans afterafter thethe impact of various amounts of fire accelerants on vertically aligned 2 mm thick 8552/IM7 samples. impactimpact ofof variousvarious amountsamounts ofof ﬁre fire accelerants accelerants on on vertically vertically aligned aligned 2 2 mm mm thick thick 8552/IM7 8552/IM7 samples.samples.

Figure 14. Heat release rates (HRR) and total heat release (THR, inset A) for 2 mm thick horizontally and vertically aligned FigureFigure 14. 14. Heat Heat release release rates rates (HRR) (HRR) and and total total heat heat release release (THR, (THR, inset inset A) A) for for 2 2 mm mm thick thick horizontally horizontally and and vertica verticallylly aligned aligned 8552/IM7 samples as well as mass loss (inset B) for various amounts of ﬁre accelerants. Line in inset A corresponds to the 8552/IM78552/IM7 samplessamples asas wellwell asas massmass lossloss (inset(inset B)B) forfor variousvarious amountsamounts ofof firefire accelerants.accelerants. LineLine inin insetinset AA correspondscorresponds toto thethe referencereference materialmaterial material withoutwithout without CFRP CFRP (steel (steel panel).panel). (HRR (HRR curves curves of of reference referencereference measurements measurements correspond correspond toto those those of of horizontally horizontallyhorizontally alignedaligned CFRPCFRP panels).panels).

4.6. Sandwich Samples A 16.5 mm thick sandwich sample with 1 mm thick M18-1/G939 CFRP skins is investigated applying up to 20 g of ﬁre accelerant in an area of 98 mm × 98 mm at the center of the panel. Figure 15 shows the force at break in a four point bending test, which dramatically decreases even for low amounts of ﬁre accelerant. For 10 g, a residual force of 6% is observed, when compared to the initial force. Corresponding to the weakest residual mechanical performance of thin monolithic panels (see Table2), sandwich panels J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 18 of 26

4.6. Sandwich Samples A 16.5 mm thick sandwich sample with 1 mm thick M18-1/G939 CFRP skins is investigated applying up to 20 g of fire accelerant in an area of 98 mm × 98 mm at the center of J. Compos. Sci. 2021, 5, 72 17 of 24 the panel. Figure 15 shows the force at break in a four point bending test, which dramatically decreases even for low amounts of fire accelerant. For 10 g, a residual force of 6% is observed, when compared to the initial force. Corresponding to the weakest residual mechanical performance of thin monolithic panels (see Table 2), sandwich panels with thin with thin CFRP skins are prone to severe damage by improvised ﬁre accelerants. Failure CFRP skins are prone to severe damage by improvised fire accelerants. Failure mode is moderepresented is represented by the loss by of the adhesion loss of of adhesion the thermally of the loaded thermally CFRP skin loaded to the CFRP honeycomb skin to the honeycombcore and its core deformation and its deformation during destructive during testing destructive (sketch testing in Figure (sketch 15). Also in Figurethe honey- 15). Also thecomb honeycomb core is thermally core is thermally damaged damaged as indicated as indicatedby the color by change the color from change orange from to black orange to blackclose close to the to upper the upper side. side.

Figure 15. Force at break in a four-point bending test of sandwich panels made of the material Figure 15. Force at break in a four-point bending test of sandwich panels made of the material M18-1/G939M18-1/G939 after after impactimpact of of various various amounts amounts of fire of accelerant. ﬁre accelerant. Each data Each point data represents point representsthe av- the averageerage of three tested tested specimen specimen at position at position 1, 2, 3 1, (A 2,) 3Sketch (A) Sketch (B) represents (B) represents typical failure typical mode failure mode withwith the the thermally thermally treated treated side side onon top.

4.7.4.7. Evaluation Evaluation of of Protective Protective MeasuresMeasures InIn order order to to represent represent thethe realisticrealistic conditions conditions of ofa vehicle a vehicle or aircraft, or aircraft, samples samples with with variousvarious coatings coatings and and protective protective layers are are prepared. prepared. A typical A typical top topcoat coatfor military for military aircraft, aircraft, a coppera copper mesh mesh used used for forprotection protection against against lightning lightning strike, strike, a combination a combination thereof thereof,, and a and a typicaltypical commercial commercial intumescent intumescent coating are are additionally additionally applied. applied. Figure 16 illustrates a selection of samples after application of 40 g fire accelerant on Figure 16 illustrates a selection of samples after application of 40 g fire accelerant on 2 mm thick samples. The intumescent coating forms a char residue, which is ~0.5 cm thick 2 mmafter thick the experiment samples. The (Fig intumescenture 16A). The coating formed formschar can a charbe easily residue, removed which (Fig isure ~0.5 16B), cm thick afterbut the it experimentprovides an effective(Figure 16 protectionA). The formedfor the CFRP char can material, be easily as can removed be seen (Figure by the 16B), butcorresponding it provides an ultrasonic effective C protection-scan (Figure for 16C). the No CFRP delaminations material, asare canobserved be seen in contrast by the corre- spondingto the samples ultrasonic with a C-scan copper (Figuremesh or 16anC). additional No delaminations top coat (Figure are 16D observed–G). Even in though contrast to theresidues samples of with the top a copper coat in meshterms orof anthe additionalsilicate filler top remain, coat (Figureno pronounced 16D–G). protective Even though residueseffect is of indicated the top by coat the in ultrasonic terms of C the-scan. silicate The copper filler remain,mesh alone no expectedly pronounced shows protective a effectsimilar is indicated damage. by the ultrasonic C-scan. The copper mesh alone expectedly shows a similar damage. Figure 17A shows the minimum interlaminar shear strength (ILSS) after impact of various amounts of fire accelerants on 2, 4, and 6 mm thick 8552/IM7 samples with various protective measures. The minimum residual strength after impact is typically reached at the center of the panel. For the 2 mm thick samples, a significant influence of the various protective measures is observed. Whereas the sole CFRP and the coated CFRP decrease rapidly and similarly in residual ILSS, samples with integrated copper mesh, with and without coating, retain a residual ILSS after impact of up to 40 g fire accelerant. However, samples are severely degraded and fail within several plies during mechanical testing. The third group of data, indicated by lines in Figure 17A, represent 2 mm thick samples

protected by an intumescent coating. The intumescent coating provides the best protection (see above). A comparison for the impact of 20 g ﬁre accelerant, shows exemplarily no residual strength for coated or uncoated CFRP, 15% residual strength for the samples containing a copper mesh and 60% for the sample protected with an intumescent coating. J. Compos. Sci. 2021, 5J., Compos. 72 Sci. 2021, 5, x FOR PEER REVIEW 18 of 24 19 of 26

Figure 16. Figure2 mm thick 16. 8552/IM72 mm thick samples 8552/IM7 after impact samples of 40 after g fire impact accelerant: of 40 (A g) CFRP fire accelerant: with intumescent (A) CFRP coating, with (B) after removing theintumescent residues of coating, the intumescent (B) after removingcoating, (C the) correspondin residues ofg the ultrasonic intumescent C-scan, coating, (D) CFRP (C) correspondingwith copper mesh, (E) correspondingultrasonic ultrasonic C-scan, C-scan, (D) ( CFRPF) CFRP with with copper copper mesh, mesh (E and) corresponding coating, (G): corresponding ultrasonic C-scan, ultrasonic (F) CFRP C-scan. with copper mesh and coating, (G): corresponding ultrasonic C-scan. Figure 17A shows the minimum interlaminar shear strength (ILSS) after impact of Samples thickervarious than amount 2 mms of retain fire accelerants higher residual on 2, 4, strength, and 6 mm which thick is8552/IM7 highest samples for 6 mm with various samples withinprotective this comparison. measures. For The example, minimum the residual pure CFRP strength shows after no impact residual is strengthtypically reached at for the 2 mm thickthe center sample of exposed the panel. to For 20 gthe fire 2 accelerant,mm thick samples, 59% for a the significant 4 mm thick, influence and 92% of the various for the 6 mm thickprotective samples measures (see also is Tableobserved.2). For Whereas thicker the samples, sole CFRP the influence and the coated of various CFRP decrease protective measuresrapidly is and less similarly significant in residual compared ILSS, to thesamples 2 mm with samples, integrated because copper the highmesh, with and standard deviationswithout for coating, the determined retain a residual residual ILSS strength after impact prohibit of aup reliable to 40 g ranking fire accelerant. of the However, protective measuressamples for are these severely samples. degraded and fail within several plies during mechanical testing. AccordingThe to third the observed group of residualdata, indicated ILSS, theby lines recorded in Fig maximumure 17A, represent temperatures 2 mm thick at samples protected by an intumescent coating. The intumescent coating provides the best the center of the samples’ backside are shown in Figure 17B. Uncoated CFRP samples protection (see above). A comparison for the impact of 20 g fire accelerant, shows exhibit the highest temperatures. As expected, temperatures increase with the amount exemplarily no residual strength for coated or uncoated CFRP, 15% residual strength for of fire accelerant up to 366 ◦C on 2 mm thick samples for 25 g thereof. All protective the samples containing a copper mesh and 60% for the sample protected with an measures lead to a reduction of these temperatures, especially for increasing amounts intumescent coating. of fire accelerant. For example, 20 g of fire accelerant leads to a temperature of 340 ◦C on pure CFRP, to 320 ◦C in the presence of an additional copper mesh, to 315 ◦C for a copper mesh and a top coat, to 300 ◦C for a top coat, and to 290 ◦C for an intumescent coating. Best protection by means of low temperatures is provided by an intumescent coating. For samples thicker than 2 mm, the backside temperatures are in general lower. For the sole CFRP they are 307 ◦C for 4 mm thick samples and 270 ◦C for the 6 mm samples, when applying 25 g fire accelerant (see above). Again, the application of a copper mesh and/or a coating reduces the backside temperatures, but the protective effect diminishes with increasing sample thickness and a comparable differentiation of the efficiency of the various protective measures is less significant. Only the application of an intumescent coating results in significantly lower backside temperatures. However, even for 6 mm thick samples all other protective measures lead to slightly lower temperatures compared to the pure CFRP. Figure 17C provides the corresponding indication of matrix degradation at the samples´ backside by means of infrared spectroscopy. For conditions, where low residual strength and high backside temperatures were observed, a pronounced matrix degradation is determined by a low intensity ratio of bands characteristic for epoxy resin and polyethersulfone. In contrast to ultrasonic testing, infrared spectroscopy allows a sensitive quantification of moderate thermal damage by characterizing matrix degradation at the panels surface. Even for samples with 5 g of applied fire accelerant, for which a thermal damage is typically not detected by ultrasonic testing (see Figures9 and 13), infrared spectroscopy is able to measure a matrix degradation accompanied by a reduction of the band intensity ratio I −1 /I −1 down to 71% for a 2 mm thick 8552/IM7 sample. 1510 cm 1486 cm J. Compos. Sci. 2021, 5, 72 19 of 24 J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 20 of 26

Figure 17. ResultsFigure for 2, 17. 4, Resultsand 6 mm for thick 2, 4, and8552/IM7 6 mm samples thick 8552/IM7 with various samples protective with various measures protective after impact measures of various amounts of fire accelerants:after impact ( ofA) various Minimum amounts interlaminar of ﬁre accelerants: shear strength (A) Minimum(ILSS); (B) interlaminarmaximum temperatures shear strength at (ILSS);the center of −1 −1 the backside; (C)( BMinimum) maximum intensity temperatures ratios of at bands the center characteristic of the backside; for EP (1510 (C) Minimumcm ) and PES intensity (1486 ratioscm ). of bands characteristic for EP (1510 cm−1) and PES (1486 cm−1). Samples thicker than 2 mm retain higher residual strength, which is highest for 6 mm Similarsamples observations within for this residual comparison. strength, For as example, well as backside the pure temperatures CFRP shows and no residual matrix strength degradationfor as the explanation 2 mm thick thereof sample are exposed made for to the 20 CFRPg fire M18-1/G939accelerant, 59% (see for Table the2 4). mm thick, and 92% for the 6 mm thick samples (see also Table 2). For thicker samples, the influence of 4.8. Empiricalvarious Correlation protective of Temperature, measures Matrixis less Degradation,significant compared and Residual to Strengththe 2 mm samples, because Receivedthe high data standardfor destructive deviations and non-destructive for the determined testing residual are correlated strength inprohibit order a reliable to provideranking tools for of athe prediction protective of measures residual strengthfor these bysamples. non-destructive methods such

J. Compos. Sci. 2021, 5, 72 20 of 24

as ultrasonic testing and infrared spectroscopy, for example, in case of failure analysis, as well as in order to provide a deep insight in the thermal degradation mechanism of CFRP during impact of improvised incendiary devises. Figure 18 shows correlations of matrix degradation traced by infrared spectroscopy (Figure 18A) and residual ILS strength (Figure 18B) with the maximum occurring backside temperatures for 8552/IM7. For increasing temperatures, matrix degradation increases, as expected and indicated by a lower intensity ratio of bands characteristic for epoxy resin and polyethersulfone

(I1510 cm−1 /I1486 cm−1 ). Total of 2 mm thick samples (black filled data points) indicate the highest occurring temperatures at the right side of the diagram. The 6 mm thick samples (open data points) show the lowest matrix degradation and corresponding temperatures at the left and top part of the diagram. CFRP without the application of protective measures (large squares in Figure 18A) slightly deviate from the correlation of the rest of the data. For a certain temperature, pure CFRP show a more pronounced thermal damage, as damage is lower with the application of a coating. A linear correlation can be used to estimate occurring temperatures for various thick materials, when infrared spectroscopic data are available. However, prediction is not supposed to be precise, as the correlation coefficient is low (R2 = 0.63). Correspondingly, residual strength diminishes with increasing backside temperatures (Figure 18B). Again, 2 mm thick panels reach higher temperatures and lower residual strength compared to the thicker panels. The linear correlation is slightly better (R2 = 0.69) compared to the infrared spectroscopic data. For failure analysis and repair, it is most important to predict the residual strength with a rapid non-destructive technique. Figure 18C presents residual ILSS dependent on the ratio of damaged to undamaged area in an ultrasonic C-scan. Hereby, larger damaged areas in principal correspond to lower minimum residual ILSS at the center of the samples, even if delamination depth is not considered, but is decisive for the damage [38]. The ratio is characteristic for the damage of the 100 mm × 100 mm samples fixed in the sample holder, but it is not representative and applicable to assess a damage such as of an aircraft component. However, ultrasonic testing is proven to be able to quantify severe damage by improvised incendiary devices, even if the spread of data is high and prediction of residual strength is limited. As samples with applied intumescent coatings do not show a damage in the ultrasonic C-scan, data are not contained in Figure 18C. Infrared spectroscopy is more sensitive, and it is able to precisely detect the thermal damage. Figure 18D presents the correlation of matrix degradation characterized by IR- spectroscopy and ILSS. For the pure CFRP, significant differences are observed for various thick panels. The 2 mm thick samples show the most pronounced drop in the band intensity

ratio I1510 cm−1 /I1486 cm−1 , and therefore the most pronounced matrix degradation, when residual strength diminishes (16D1). This drop is increasingly ﬂatter for 4 and 6 mm thick panels, as expected due to the lower backside temperatures of thick samples (see above). However, the formation of delaminations and the corresponding depth is decisive for residual ILSS [38]. As 4 and 6 mm thick samples retain areas without delaminations (see Figure9), the drop of residual ILSS is less steep than for 2 mm thick samples, which are typically completely delaminated. A differentiation of panel thickness is not as clear for the samples additionally containing a copper mesh and or a top coat. Here, a continuous drop in the intensity ratio is observed for all sample thicknesses (Figure 18(D2)). Of course, thin panels show the lowest residual strength and most pronounced matrix degradation (lowest

I1510 cm−1 /I1486 cm−1 -ratio). Similar continuous correlations are also found for samples with a top coat and a copper mesh as well as for samples with a top coat only (data not depicted). Samples with an intumescent coating, in general retain high residual strength and therefore its drop are less pronounced with increasing matrix degradation. Only, when

matrix degradation exceeds a certain limit value (I1510 cm−1 /I1486 cm−1 falls below 0.7), a breakdown in residual strength may occur (Figure 18(D3)). This limit value accompanied by the beginning formation of delaminations was already observed for corresponding irradiation experiments [38]. J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 22 of 26

J. Compos. Sci. 2021, 5, 72 21 of 24 compared to the thicker panels. The linear correlation is slightly better (R2 = 0.69) compared to the infrared spectroscopic data.

Figure 18. Correlation of data for 8552/IM7 panels after impact of ﬁre accelerant:(A) matrix degradation characterized by −1 −1 IR-spectroscopy (min. I1510 cm /I1486 cm ) with max. backside temperature; (B) residual ILS-strength with max. backside temperature; (C) ratio of damaged area in ultrasonic C-scan and minimum ILSS; (D) minimum ILSS and matrix degradation characterized by IR-spectroscopy.

In summary, temperature, matrix degradation, and residual strength after the impact of incendiary devices can be empirically correlated and non-destructive methods such as infrared spectroscopy and ultrasonic testing open a way to estimate the residual strength. J. Compos. Sci. 2021, 5, 72 22 of 24

5. Conclusions This work focuses on short-term thermal degradation of composite materials induced by one-sided impact of improvised incendiary devices (IIDs). The aim is to correlate the heat damage of the polymer matrix induced by the IID to the residual strength for various material thicknesses and panel alignments as well as to characterize the degradation mechanism. Various protective measures are assessed. As large-scale fire tests with IIDs do not provide reproducible results, for a systematic study, dominant laboratory tests are performed with better controllable test conditions. Test results proof, that reached temperatures and test durations are comparable to the large-scale tests. In general, CFRP are prone to severe damage by IIDs: Especially thin panels and vertically aligned panels rapidly degrade and loose mechanical strength. Sandwich structures containing thin CFRP parts lose mechanical integrity at comparably low heat impact by IIDs. Higher amounts of fire accelerant lead to longer durations of heat impact and therefore increase thermal damage. High temperature gradients and thermal strain occur inside the CFRP materials. Cross sections for thicker panels exhibit limited areas with delaminations close to the surface, whereas thinner samples are damaged throughout the panel thickness. Increasing panel thickness results in a delayed thermal degradation due to increasing heat capacity. Decreasing strengths can be explained based on formed delaminations and matrix degradation. For vertically orientated panels, resin matrix combusts, because the access of air is not hindered and the carbon fibers may act as wick. For horizontally aligned panels, the combustion of the fire accelerant does not lead to a significant contribution to the heat release by the matrix. Under these laboratory test conditions, the access of air is limited, and a preferably pyrolytic degradation of the matrix is accompanied by comparably low mass losses (<5%). Therefore, a simulation of heat impact on CFRP by IIDs is not precise for example when typical irradiation experiments in a cone calorimeter are conducted. The introduction of energy into the CFRP material and the access of air significantly differ. Even if the conducted laboratory scale tests better characterize the impact of IID on CFRP, they also can not represent the conditions of an arson attack in complete. But important information is gained, how certain conditions such as horizontal and vertical orientation of a CFRP component influence its thermal damage. A cone calorimeter in combination with the new sample holder is a valuable tool for this purpose, because it is not used in a typical way to determine reaction-to-fire properties by irradiation and forced combustion, as it is proven, that irradiation may not represent conditions of a heat impact by IID. All investigated protective layers such as typical coatings, an integrated copper mesh, and an intumescent coating improve the performance of CFRP during an IID assault for 2 mm thick samples. However, the best protection is achieved by the intumescent coating. Even though, there is potential for improving the performance of an intumescent coating, as formed carbon residues may be combusted within the burning liquid fire accelerant. Non-destructive testing methods, such as ultrasonic testing and infrared spectroscopy, have been proven to be valuable tools for identifying thermal damage of CFRP by IIDs. A prediction of occurring temperatures during an IID assault or residual strength afterwards is possible with a correlation to these data. For moderate thermal damage, only infrared spectroscopy is sensitive enough. The presented results are obtained for high performance CFRP commonly used in defense industry. These epoxy-based materials do not easily ignite or are flame retarded. Both investigated CFRP materials behave similarly even if their composition differs, because of deviating thermoplastic tougheners or contained flame retardants. Composites with a polymer matrix, which easily ignites and continuously burns, are expected to behave J. Compos. Sci. 2021, 5, 72 23 of 24

signiﬁcantly differently and damage induced by IIDs is expected to be even severe. Con- ditions of the laboratory tests with limited oxygen access to horizontally aligned panels, may better ﬁt to the behavior of the investigated high performance CFRP during a real IID assault.

Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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