Mitigation of Impact Damage with Self-Healing and Anti-Sloshing Materials in Aerospace Fuel Tanks

aerospace

Article Mitigation of Impact Damage with Self-Healing and Anti-Sloshing Materials in Aerospace Fuel Tanks

Gerardus Janszen , Gabriele Capezzera, Antonio Mattia Grande and Luca Di Landro * Department of Aerospace Science and Technology, Politecnico di Milano, Via La Masa 34, 20156 Milano, Italy; [email protected] (G.J.); [email protected] (G.C.); [email protected] (A.M.G.) * Correspondence: [email protected]

Received: 29 November 2018; Accepted: 8 February 2019; Published: 12 February 2019

Abstract: In this study, self-healing and anti-sloshing materials are investigated for mitigation of impact-induced damage. The integration of these systems, for the prevention of fire or explosion due to impact or bullet damage, may significantly improve the safety of aerospace fuel tanks. Leakage, after bullet penetration or debris impact, may be prevented or at least limited if the container’s walls are made by materials with self-healing capabilities. The aim of this work is to define the self-healing behavior of the EMAA ionomer (poly-Ethylene-MethAcrylic Acid copolymer), with reference to the energy dissipation mechanisms involved during damage and autonomic healing. An experimental investigation on the healing capacity of the material when perforated by bullets shot at medium velocity (250 m/s−450 m/s) was carried out. In these tests, the influence of friction, temperature, and multiple impacts on the healing process was examined and discussed. Moreover, the material response in operating conditions similar to those encountered in actual aeronautical applications, that is, in presence of pressurized fluid and anti-sloshing material (Explosafe®) was tested. Results show that the presence of the liquid increases the self-healing capabilities, which are, however, slightly affected by pressurization and internal anti-sloshing filler; the contribution in terms of sloshing reduction remains relevant.

Keywords: fuel tanks; anti-sloshing ﬁller; self-healing; impact

1. Introduction Self-healing materials are becoming more and more attractive as a consequence of the increasing interest in the “damage management” concept, which is being adopted in the maintenance and repair programs of advanced and complex systems such as aircraft and aerospace vehicles. This design strategy is based on the idea that proper control of the damage extent and propagation can reduce human intervention and, in general, increase the reliability and duration of a damaged component. The ability of some materials to autonomously repair damages well with no or limited external intervention responds to these requirements. At present, several polymeric materials have been developed which, under certain conditions, can fully or partially restore their functionality as a consequence of autonomic repair. The mechanisms that lead to the healing process are, however, very diverse for the different polymeric systems and produce varying mending efﬁciencies [1–5]. The material studied in this work is an ethylene-methylacrylate copolymer-based ionomer (Surlyn®8940 by Du Pont de Nemours), which has demonstrated its remarkable self-healing capabilities when punctured in ﬁrearms shootings; thanks to these capabilities, a number of prospected applications in safety and space components have been proposed [6–8]. This polymer is intrinsically self-healing, in that the autonomic repair is activated by the damage itself. The healing mechanism is based on the strain and friction energy dissipation, leading to rapid melting of the material during the

Aerospace 2019, 6, 14; doi:10.3390/aerospace6020014 www.mdpi.com/journal/aerospace Aerospace 2019, 6, 14 2 of 17 Aerospace 2019, 6, x FOR PEER REVIEW 2 of 17 perforationand final material phase, followed welding by and the holesolidification, closure caused which by occur the strain immediately recoveryand after final puncture, material within welding a andfraction solidification, of a second which [1–3, 9 occur–12]. The immediately perforation after and puncture, healing process within ais fractionvisualized of aand second summarized [1–3,9–12 in]. TheFigure perforation 1: A high-speed and healing projectile process ishits visualized the sample and summarized(a) that rapidly in Figure deforms1: A high-speed (b). The very projectile high hitsdeformation the sample and (a) strain that rapidly rate accounts deforms (b).for Thea rapid very and high consistent deformation increase and strain of temperature rate accounts in for the a rapidpunctured and consistent zone, so that increase the material of temperature melts and in breaks the punctured (c). Since zone, only a so small that thezone material melts, and melts thanks and breaksto the viscoelastic (c). Since only recovery, a small there zone is melts, an important and thanks strain to the relief viscoelastic that closes recovery, the perforation there is animmediately important strainafter the relief bullet that passage, closes the while perforation the hole immediately edges are still after molten the bullet (d), bringing passage, to while the final the hole welding. edges The are stillionomeric molten nature (d), bringing of the topolymer the final possibly welding. enhances The ionomeric the viscoelastic nature of the recovery polymer and possibly the retrieval enhances of therelevant viscoelastic mechanical recovery performances. and the retrieval While ofthe relevant general mechanical mechanism performances. is well recognized, While a the number general of mechanismissues regarding is well the recognized, energy contributions a number of issuesinvolved regarding in the theheating energy and contributions healing, in involvedaddition into thethe heatinginfluence and of healing,the material in addition conditions to the and influence bullet ch ofaracteristics the material over conditions the whole and process bullet characteristics remain to be overfully theinterpreted. whole process remain to be fully interpreted.

Figure 1. Perforation and healing process. Figure 1. Perforation and healing process. In this research, an extensive experimental campaign was carried out in which different tests wereIn performed this research, focusing an extensive on aspects experimental that were notcampaign fully approached was carried previously, out in which like different the polymer tests agingwere effects,performed the rolefocusing of friction on aspects between that bullet were and not perforated fully approached panel over previously, energy dissipation like the polymer and the responseaging effects, to multiple the role perforations.of friction between Moreover, bullet an and evaluation perforated of panel the different over energy energy dissipation contributions and the is presentedresponse to and multiple discussed perforations. in relation Moreover, to the thermal an ev effectsaluation involved of the in different the puncture energy healing contributions process. is presentedThe aging and discussed in ionomers in involvesrelation to the the formation thermal andeffects evolution involved of in ionomeric the puncture clusters healing formed process. by the associationThe aging of polar in ionomers groups; this involves is evidenced the formation by the presenceand evolution of a secondary of ionomeric transition, clusters whose formed extent by isthe expected association to affect of polar the healing groups; efficiency this is evidenced [1,2,4]. Although by the presence a number of of a non-ionomericsecondary transition, polymers whose also displayextent is self-healing expected behavior,to affect thethe presencehealing ofefficiency ionomeric [1– clusters2,4]. Although is likely toa efficientlynumber of contribute non-ionomeric to the viscoelasticpolymers also recovery display of self-healing the polymer behavior, melt and the to assist presence the restoration of ionomeric of theclusters mechanical is likely performances to efficiently aftercontribute the autonomic to the viscoelastic repair [13]. Sincerecovery the dynamics of the polymer of clusters melt reformation and to assist after meltingthe restoration and perforation of the ismechanical quite slow, performances with stabilization after the times autonomic in the orderrepair of [13]. weeks, Since an the experimental dynamics of investigationclusters reformation of the mechanicalafter melting properties and perforation evolution andis quite the puncture slow, with healing stabilization efficiency withtimes aging in the was order deemed of interesting.weeks, an Inexperimental addition, repeated investigation shots in of the the same mechanical location, i.e., properties involving evolution already-healed and material,the puncture demonstrated healing theefficiency maintenance with aging of the was self-mending deemed interesting. capacity, yet In withaddition, a somewhat repeated reduced shots in efficiency. the same location, i.e., involvingThe role already-healed of friction over material, the energy demonstrated transfer between the maintenance the impacting of bulletthe self and-mending the perforated capacity, panel yet iswith still a controversial. somewhat reduced While theefficiency presence. of friction is certainly expected to increase the fraction of bullet kineticThe energy role of that friction is dissipated over the within energy the transfer punctured between panel, the whether impacting friction bullet is fundamental and the perforated for the healingpanel is processstill controversial. is not clear While [14]. In the order presence to evidence of friction the is importance certainly expected of friction to in increase the impact the fraction energy dissipationof bullet kinetic and over energy the healing that process,is dissipated ballistic within tests withthe thepunctured presence ofpanel, lubricant whether over thefriction impact is areafundamental were conducted. for the healing The results process of tests is not with clear lubricated [14]. In specimensorder to evidence and repeated the importance shots are discussed.of friction in theA impact consistent energy sensitivity dissipation of the and EMAA over the healing healin capacityg process, on ballistic temperature tests with variation the presence has been of previouslylubricant over reported the impact for thin area films, were conducted. which is consequent The results to of temperature-dependenttests with lubricated specimens mechanical and propertiesrepeated shots and are the discussed. varying aging rate [14]. In view of an estimation of the energy contributions involved,A consistent in this research, sensitivity thick of panels the EMAA were impacted healing atcapacity different on temperatures, temperature and variation the damage has been and previously reported for thin films, which is consequent to temperature-dependent mechanical properties and the varying aging rate [14]. In view of an estimation of the energy contributions

Aerospace 2019, 6, 14 3 of 17 healed areas were analyzed. The analysis of energy contributions can follow two distinct, but related approaches; from the mechanical point of view, the bullet energy transferred to the panel during impact induces extensive plastic deformation. A comparison between the energy lost by the bullet during puncture and the deformation energy provides a basis for the analysis of energy contributions leading to the melting of the material. From the thermal point of view, the evaluation of the energy required to raise the temperature of the material at the impact area above the melting temperature can provide confirmation of the melting/welding process and an estimation of the polymer volume involved in healing. The influence of temperature conditions over these aspects was also experimentally investigated and discussed. With a more applicative view, in order to improve the safety in the design and manufacture of containers for fuels, a potential use of these new materials can be their integration in the tank walls to suppress or reduce liquid spilling in case of punctures or bullet perforations. The focus of this investigation is mainly related to aircraft and helicopter fuel tanks; however, the same concepts and solutions may be applied for the containment of dangerous fluids in general. Fire and deflagration involving the full tank often start with the leakage of fuel as a consequence of the rupturing of the tank walls; this is then followed by vapor ignition due to electric or friction sparks. On this basis, possible solutions should be directed towards the limitation or prevention of leakages and the suppression of the explosion. Various concepts have been explored over the years, for the inhibition of tanks explosion, including pressurization of the ullage space with inert gas [15]. Among these, foam and metallic fillers are the most used in military aircraft and helicopters. The coupling of such self-healing polymers as tank walls with cellular fillers can lead to relevant improvements, allowing high impact resistance, repeatable and environmentally stable self-healing capability, lightness, mechanical stiffness and strength. The integration of self-mending walls and filler made from expanded aluminum foil is expected to enhance the safety of tanks in case of puncture by high-speed objects and limit the fuel loss, thus extending the functioning capability even after damage. In this study, the self-healing capacity of such configurations has been experimentally explored, also considering the effects of bullet diameter/wall thickness and of the presence of internal tank pressure over the healing efficiency.

2. Materials and Methods The thermoplastic self-healing ionomer used as a tank wall was a copolymer poly(ethylene co-methacrylic acid) neutralized with sodium; it is available as a commercial material with the name of Surlyn®8940 (from DuPont de Nemours—USA, Wilmington, DE, USA). It contains 5.4% mol. of methacrylic acid groups distributed along the main polyethylene chains; about 30% of the acid groups are neutralized with sodium. The material, received in granular form, was compression molded in a hot-platens press to obtain polymer plates of 130 mm × 130 mm sides, with thicknesses ranging from 1.5 mm to 3 mm, which were employed in subsequent thermal and mechanical tests. The ﬂat plates were aged at room temperature for the prescribed time period before tests. Material aged for more than 30 days was assumed to have reached stable conditions. The thermo-mechanical behavior of the material was assessed by Differential Scanning Calorimetry (DSC 2920–TA Instruments—New Castle, DE, USA) tests performed at a 10 ◦C/min heating/cooling rate. Tests at a higher ramp rate (20 ◦C/min and 30 ◦C/min) were also performed in the attempt to evidence possible thermal events. Dynamic mechanical tests and complex viscosity measurements were performed with a Rheometrics RDAII torsional rheometer at 1 Hz, in a temperature range between 25 ◦C and 220 ◦C. Tensile tests of dog-bone ionomeric specimens were carried out at room temperature and a 5 mm/min crosshead rate (gage length 50 mm, width 10 mm) with an Instron mod. 4302 Dynamometer. Specimens for DMA (dynamic-mechanical analysis) and tensile tests were directly cut from molded plates. The ﬂat plates used in the ballistic proofs were shot with different bullets at different velocities. The bullets used at low speeds (160 m/s–180 m/s) were steel spheres with a diameter of 9.55 mm (mass Aerospace 2019, 6, 14 4 of 17 Aerospace 2019, 6, x FOR PEER REVIEW 4 of 17

=facility 3.52 g). is Thecomposed speed was of an measured air chamber by means pressurized of a high-speed at 6.5 bar camera. by an The external available air shooting compressor facility and is composedseparated offrom an airthe chamber outside pressurizedambient through at 6.5 bara 0.075 by an mm external thick air metal compressor foil. An and electronic separated switch from theactivates outside a solenoid ambient which through triggers a 0.075 a mmsharp thick point metal toward foil. the An metal electronic foil. After switch the activates foil puncture, a solenoid the whichdifferential triggers pressure a sharp causes point the toward sudden the expansion metal foil. Afterand flow the foilof the puncture, compressed the differential air through pressure a long causesaluminum the suddentube (length expansion 7 m, andinternal ﬂow ofdiameter the compressed 40 mm, airexternal through diameter a long aluminum70 mm). The tube air (length flow 7launches m, internal a high-density diameter 40 polyethylene mm, external (HDPE) diameter sabot, 70 mm). which The carries air ﬂow the launchessteel sphere a high-density inside the polyethylenealuminum tube (HDPE) until it sabot, reaches which the carriesend of thethe steelbarre spherel; a dissipative inside the aluminum aluminum tube tube absorber until it reachesand an thealuminum end of thestopper barrel; block a dissipative the sabot aluminum at the end tube of the absorber tube, while and an the aluminum steel sphere stopper is projected block the against sabot atthe the target. end of the tube, while the steel sphere is projected against the target. A high-speedhigh-speed PhantomPhantom v5.1 v5.1 camera camera was was employed employed to to measure measure the the projectile projectile velocity velocity prior prior to and to afterand after the impact the impact by recording by recording the time the requiredtime required to cover to cover a short a distance.short distance. Projectile Projectile speeds speeds in the in range the 150range to 200150 m/sto 200 are m/s reached are reached dependent dependent on the air on reservoir the air pressurereservoir [16 pressure]. In Figure [16].2, theIn Figure schematic 2, the of theschematic ballistic of test the set-up ballistic is shown.test set-up is shown.

Figure 2. Schematic of the ballistic test system. Figure 2. Schematic of the ballistic test system. The tests at medium velocities (280 m/s–450 m/s) were carried out at the TSN-National Shooting The tests at medium velocities (280 m/s–450 m/s) were carried out at the TSN-National Shooting Range in Milan and at the Ballistic Laboratory of the Italian bullet manufacturing company, Fiocchi Range in Milan and at the Ballistic Laboratory of the Italian bullet manufacturing company, Fiocchi Munizioni SpA, Lecco, Italy. In this case, the bullets were 9X21 and .45ACP calibers, shot by a Kimber Munizioni SpA, Lecco, Italy. In this case, the bullets were 9X21 and .45ACP calibers, shot by a Custom II handgun and a Beretta CX4-Storm Carabine. 9X21 and .45ACP bullets have a rounded Kimber Custom II handgun and a Beretta CX4-Storm Carabine. 9X21 and .45ACP bullets have a impact nose with a diameter of 9.5 mm and 11.5 mm, respectively. The speed was measured by a laser rounded impact nose with a diameter of 9.5 mm and 11.5 mm, respectively. The speed was chronograph. For the temperature-controlled tests, the ionomeric panels were kept in thermostatic measured by a laser chronograph. For the temperature-controlled tests, the ionomeric panels were chambers at temperatures between −40 ◦C and +70 ◦C before shooting; the actual temperature at the kept in thermostatic chambers at temperatures between −40 °C and +70 °C before shooting; the moment of shooting was checked by a non-contact infrared thermometer. actual temperature at the moment of shooting was checked by a non-contact infrared thermometer. All shots with flat panels at low and medium velocities, at low and high temperatures, with All shots with flat panels at low and medium velocities, at low and high temperatures, with different bullets, were repeated at least two times. different bullets, were repeated at least two times. Leakage tests, imposing a differential pressure on the two sides of the perforated item, were Leakage tests, imposing a differential pressure on the two sides of the perforated item, were used to detect the presence of non-healed holes and check for the self-healing efficiency. Scanning used to detect the presence of non-healed holes and check for the self-healing efficiency. Scanning electron microscopy (SEM TM3000-Hitachi, Tokyo, Japan) analyses, at different magnifications, were electron microscopy (SEM TM3000-Hitachi, Tokyo, Japan) analyses, at different magnifications, used to observe the morphology of the punctured/healed holes and get evidence of the melting and were used to observe the morphology of the punctured/healed holes and get evidence of the melting re-solidification features of the material after each ballistic test. Tensile tests after ballistic impacts were and re-solidification features of the material after each ballistic test. Tensile tests after ballistic

Aerospace 2019,, 6,, 14x FOR PEER REVIEW 55 of of 17 impacts were carried out with specimens having an impacted zone in the middle of the gage length carriedand results out withwere specimens compared havingto analogous an impacted test data zone with in thenot middle impacted of the specimens. gage length and results were comparedFinally, to ballistic analogous tests test on data a mock-up with not of impacted the new specimens. conceptual solution for fuel tanks were carried out. TheFinally, tests ballistic were conducted tests on a mock-upusing the of same the newair gu conceptualn as that used solution in the for low-speed fuel tanks wereballistic carried impacts out. Theon flat tests plates. were conductedThe tank usingmock-up the sameconsisted air gun of asa that6064 used T6 aluminum in the low-speed alloy ballistictube (92 impacts mm internal on flat plates.diameter, The 366 tank mm mock-up length), consisted with flanged of a 6064 ends T6 to aluminum fit the ionomer alloy tube Surlyn (92 mm® 8940 internal plates. diameter, The tank 366 could mm ® length),be pressurized with flanged with water ends toinside fit the and ionomer with the Surlyn presence8940 of plates. the anti-sloshing The tank could filler. be pressurizedPressure sensors with waterwere placed inside andin different with the positions presence along of the the anti-sloshing tube. A similar filler. tank Pressure with sensorsa transparent were placed PMMA in tube different was positionsused for tests along requiring the tube. internal A similar observation; tank with no a transparent pressure sensors PMMA and tube no was pressurization used for tests option requiring were internaladopted observation;in this case. no pressure sensors and no pressurization option were adopted in this case. ® Explosafe® waswas used used as as an an anti-sloshing anti-sloshing material material in in th thee tank; tank; it it is made of an aluminum sheet manufactured by slitting and expanding a foil toto createcreate hexagonalhexagonal holes.holes. The average density of ® 3 Explosafe® isis 30 30 kg/m kg/m3. .It It was was used used both both as as a a single single rolled rolled block block or or as as small small cylindrical cylindrical pellets pellets fitting fitting the tank space. Its Its presence presence in in aircraft fuel tanks is expected to reduce liquidliquid sloshing, to dampen pressure peakspeaks in in case case of impactsof impacts and and to slow to slow down do fuelwn spilling fuel spilling in case ofin perforation,case of perforation, thus providing thus explosionproviding suppressionexplosion suppression effects [17– effects19]. [17–19]. Ballistic teststests ofof aa simplified simplified model model tank tank were were performed performed using using steel steel balls balls as projectiles as projectiles with sizeswith rangingsizes ranging between between 2.35 mm 2.35 and mm 16.6 and mm 16.6 (massmm (mass between between 0.053 0.053 g and g 18.74 and 18.74 g). Figure g). Figure3 shows 3 shows the tank the mock-uptank mock-up configurations configurations employed. employed.

Figure 3. Fuel tank sketch indicating the location of the pressure gauges (top) and an image of the Figurealuminium 3. Fuel and tank transparent sketch indicating tanks (with the Explosafe location® offiller the inside)pressure (below gauges). (top) and an image of the aluminium and transparent tanks (with Explosafe® filler inside) (below). 3. Results and Discussion 3. Results and Discussion 3.1. Testing of Single Flat Panels 3.1. Testing of Single Flat Panels 3.1.1. Aging Effects 3.1.1.In Aging order Effects to investigate how the effects of aging influenced the thermal and mechanical properties of theIn polymer, order to five investigate different ionomerhow the molded effects plates of aging with differentinfluenced time the spans thermal were and selected: mechanical 1, 7, 14, 28,properties and 42 days.of the Thermal polymer, and five mechanical different testsionome werer molded carried outplates after with the selecteddifferent aging time times,spans eitherwere selected: 1, 7, 14, 28, and 42 days. Thermal and mechanical tests were carried out after the selected

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Aerospace 2019, 6, x FOR PEER REVIEW 6 of 17 with not impacted specimens or with specimens that were impacted with low-speed steel spheres and whichaging showed times, healing.either with For thenot latterimpacted case, specimensspecimens foror with mechanical specimens tests that were were cut soimpacted that the healedwith arealow-speed was in the steel middle spheres of and the specimens.which showed It washealing. previously For the reportedlatter case, [2 specimens,3,8,11,13] for that mechanical the healing efﬁciencytests were increases cut so that in correspondence the healed area was with in ionic the mi clusterddle of evolution, the specimens. reaching It was stabilization previously afterreported about 30[2,3,8,11,13] days. In our that tests, the the healing provided efficiency projectile increases impact in speedcorrespondenc was abovee with 160 ionic m/s, cluster and all evolution, specimens reaching stabilization after about 30 days. In our tests, the provided projectile impact speed was showed full healing. Only when the projectile speed was lower, partial healing or non-perforated above 160 m/s, and all specimens showed full healing. Only when the projectile speed was lower, panels were observed. partial healing or non-perforated panels were observed. ◦ DSC tests show a clear change of the secondary transition temperature, Ti, between 40 C and DSC tests show a clear change of the secondary transition temperature, Ti, between 40 °C and 60 60 ◦C and the corresponding heat transfer; the peak, which may be related to the ionic presence, °C and the corresponding heat transfer; the peak, which may be related to the ionic presence, becomesbecomes more more deﬁned defined and and shifted shifted to to higher highertemperatures temperatures with the increasing increasing aging aging time. time. According According to theto the tests tests performed, performed, the the secondary secondary transition transition cannotcannot be detected for for a a sample sample aged aged 1 1day day which which mightmight be be due due to to the the low low size size of of the the ordered ordered microstructure microstructure formations formations forfor thisthis agingaging time, see Figure 4. The4. lowerThe lower peak peak appears appears for for samples samples aged aged for for more more than than 1 1 dayday andand gets bigger bigger with with the the aging aging time, time, as reportedas reported in in Table Table1. These1. These results results show show that that aggregates aggregates growgrow soso thatthat molecular mobility mobility and and a a ◦ re-organizationre-organization of theof the polymer polymer chains chains is possibleis possible during during the the aging aging of of the the material. material. At At about about 90 90C, °C, full meltingfull melting occurs asoccurs indicated as indicated by the largeby the endothermic large endothermic peaks; nopeaks; relevant no relevant effect of effect aging of is evidencedaging is overevidenced the melting over process. the melting process.

Figure 4. DSC curves of the EMAA ionomer at different aging times (curves are vertically shifted for Figure 4. DSC curves of the EMAA ionomer at different aging times (curves are vertically shifted for better readability). better readability). Table 1. Thermal properties estimated by DSC after different aging times. Indices od and m refer to the order-disorderTable 1. Thermal transition properties and to estimated the melting by DSC respectively. after different aging times. Indices od and m refer to the order-disorder transition and to the melting respectively. ◦ ◦ Aging Days Tod ( C) Hod (J/g) Tm ( C) Hm (J/g) Hod Hm Aging1 Days Tod - (°C) -Tm (°C) 92.4 23.9 7 47.7(J/g) 0.9 93.4(J/g) 19.4 141 48.8- 1.0- 92.4 95.1 23.9 21.5 287 49.947.7 0.9 1.2 93.4 93.8 19.4 18.9 4214 52.948.8 1.0 1.31 95.1 93.5 21.5 22.6 28 49.9 1.2 93.8 18.9 DMA tests conﬁrm these42 results: A52.9 drop of storage1.31 modulus,93.5 due22.6 to the secondary transition, shows that some molecular motion is activated between 40 ◦C and 70 ◦C; in this case, evidence of a transition is also shown after the shortest aging time, see Figure5.

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DMA tests confirm these results: A drop of storage modulus, due to the secondary transition, shows that some molecular motion is activated between 40 °C and 70 °C; in this case, evidence of a transition is also shown after the shortest aging time, see Figure 5. The main results of the tensile tests performed are shown in Figure 6. An increase of the stiffness and the yield stress with aging time can be noticed. Stabilization occurs after about 30 days. FailureAerospace of 2019the, 6specimens, x FOR PEER REVIEWcannot be observed at the low strain rate applied due to the 7limited of 17 Aerospace 2019, 6, 14 7 of 17 displacement capability of the testing system. At the crosshead speed of 5 mm/min, failure always DMA tests confirm these results: A drop of storage modulus, due to the secondary transition, occurs for strains exceeding 400%. shows that some molecular motion is activated between 40 °C and 70 °C; in this case, evidence of a transition is also shown after the shortest aging time, see Figure 5. The main results of the tensile tests performed are shown in Figure 6. An increase of the stiffness and the yield stress with aging time can be noticed. Stabilization occurs after about 30 days. Failure of the specimens cannot be observed at the low strain rate applied due to the limited displacement capability of the testing system. At the crosshead speed of 5 mm/min, failure always occurs for strains exceeding 400%.

FigureFigure 5. 5. StorageStorage shear shear modulus, modulus, G’ G’ (– (–)) and and dissipation dissipation factor, factor, tan tanδ δ(--)(–) measured measured by by DMA DMA after after one one dayday of of aging. aging. The main results of the tensile tests performed are shown in Figure6. An increase of the stiffness and the yield stress with aging time can be noticed. Stabilization occurs after about 30 days. Failure of the specimens cannot be observed at the low strain rate applied due to the limited displacement capabilityFigure of the5. Storage testing shear system. modulus, At the G’ crosshead(–) and dissipation speed offactor, 5 mm/min, tanδ (--) measured failure always by DMA occurs after one for strains exceedingday of 400%. aging.

Figure 6. Stress–strain curves at different aging times (from 1 to 42 days).

In order to understand how the healing process influences the mechanical behavior of the ionomer, completely healed impacted specimens were tensile tested after the same aging times as the not impacted specimens. It should be noted that the material volume interested by melting and FigureFigure 6. 6.Stress–strain Stress–strain curves at at different different aging aging times times (from (from 1 1to to 42 42 days). days). re-welding is a small portion of the total volume of impacted specimens; most of the impacted region,In Inalthough order order to understandtodeformed understand during how how the impact, the healing healing is process not processsubj inﬂuencesected influences to an the actual mechanical the mechanicalphase behaviorchange. behavior As of thea matter of ionomer, the of completelyionomer, completely healed impacted healed impacted specimens specimens were tensile weretested tensileafter tested the after same the agingsame aging times times as the as not impactedthe not impacted specimens. specimens. It should It be should noted be that noted the material that the volumematerial interestedvolume interested by melting by andmelting re-welding and isre-welding a small portion is a small of the portion total volume of the of total impacted volume specimens; of impacted most specimens; of the impacted most of region,the impacted although deformedregion, although during impact,deformed is during not subjected impact, tois not an actualsubjected phase to an change. actual phase As a matter change. of As fact, a matter the results of did indicate an appreciable variation in terms of the yield strength and stiffness. On the other hand, impacted and healed specimens showed a lower elongation and stress at break, which reduced with AerospaceAerospace2019 2019, 6,, 14 6, x FOR PEER REVIEW 8 of8 17 of 17

fact, the results did indicate an appreciable variation in terms of the yield strength and stiffness. On increasingthe other aging hand, time, impacted but remained and healed quite specimens large and sh alwaysowed a well lower above elongation yield values. and stress Figure at7 break,reports, as examples,which reduced the stress–strain with increasing curves aging recorded time, but in impactedremained andquite not large impacted and always specimens well above after 7 yield and 28 agingvalues. days. Figure 7 reports, as examples, the stress–strain curves recorded in impacted and not impacted specimens after 7 and 28 aging days.

(a)

(b)

FigureFigure 7. 7.Stress–strain Stress–strain curvescurves collected from from specimens specimens aged aged 7 days 7 days (a) and (a) and28 days 28 days(b) before (b) before the theimpact. impact.

3.1.2.3.1.2. Friction Friction Reduced Reduced Tests Tests InIn order order to to investigate investigate the the inﬂuence influence ofof frictionfriction inin the healing process, process, shooting shooting tests tests were were conducted,conducted, applying applying a a 1 1 mm mm thick thick layerlayer ofof lubricantlubricant grease on on the the specimens specimens in in the the impact impact area. area. Tests with medium-speed bullets shot by ﬁrearms and with low-speed steel spheres launched by an air cannon were performed. Aerospace 2019, 6, x FOR PEER REVIEW 9 of 17

Tests with medium-speed bullets shot by firearms and with low-speed steel spheres launched by an Aerospace 2019, 6, 14 9 of 17 air cannon were performed.

MediumMedium SpeedSpeed BallisticBallistic teststests werewere carriedcarried outout usingusing 99× × 2121 bulletsbullets (diameter(diameter 9.039.03 mm)mm) shotshot withwith thethe rifle.rifle. FullFull healinghealing waswas obtainedobtained inin allall cases,cases, asas confirmedconfirmed byby leakageleakage tests.tests. InIn FigureFigure8 a,8a, an an example example of of a a healed healed panelpanel afterafter bulletbullet puncturepuncture isis shown;shown; SEMSEM images,images, seesee FigureFigure8 b,c,8b,c, show show the the difference difference of of bullet bullet passagepassage through through ionomer ionomer panels panels in thein the two two situations, situatio i.e.,ns, with i.e., andwith without and without lubricant. lubricant. In both images,In both twoimages, different two circulardifferent areas circular can be areas distinguished; can be distin an internalguished; area, an whereinternal the area, material where melted, the whichmaterial is muchmelted, smaller which than is much the bullet smaller diameter than andthe anbullet outer diameter area where and most an ofouter the viscoelasticarea where deformationmost of the occurredviscoelastic with deformation no phase change. occurred It with is in theno outerphase areachange. where It is the in main the outer differences area where can be the noticed; main thedifferences absence can of the be striationsnoticed; the suggests absence that of the lubricant,striations suggests with its slipperythat the effect,lubricant, allows with the its bullet slippery to passeffect, through allows the the ionomer bullet to with pass lower through material the stretchingionomer with (low lower plastic material deformation). stretching This observation(low plastic maydeformation). be linked toThis a rapidobservation viscoelastic may returnbe linked and to a a lower rapid drop viscoelastic of the temperature return and a in lower the melted drop of area; the thetemperature consequence in the of melted which seemsarea; the to beconsequence improved of healing. which Thisseems was to be confirmed improved by healing. the tensile This tests was carriedconfirmed out afterby the the tensile shot tests; tests thecarried samples out onafter which the shot the lubricanttests; the wassamples applied on showedwhich the no lubricant failure in was the perforatedapplied showed area, while no failure in all thein the other perforated cases the area, failure wh startedile in all from the thisother area. cases On the this failure basis, started considering from thatthis meltingarea. On occurs this basis, in any considering case, the friction that melting energy occurs seems in to any represent case, the a significant friction energy but not seems critical to contributionrepresent a significant to the total but melting not critical energy, contribution whose main to origin the total is thus melting attributed energy, to thewhose strain main energy. origin is thus attributed to the strain energy.

(a)

(b) (c)

FigureFigure 8.8.Self-healed Self-healed panel panel after after bullet bullet puncture puncture (a ).(a SEM). SEM images images of puncturedof punctured EMAA EMAA panels panels with with (b) and(b) and without without (c) the (c) presencethe presence of lubricant. of lubricant.

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Low Speed Low Speed Additionally, after low-speed tests, the healing was confirmed to be complete both in lubricated and not-lubricatedAdditionally, aftersamples. low-speed By means tests, of the a high-s healingpeed was camera, confirmed the tospeed be complete of the 9.55 both mm in lubricatedspherical bulletsand not-lubricated was measured samples. before and By meansafter perforation. of a high-speed In Table camera, 2, the the speed speed and of kinetic the 9.55 energy mm sphericalchanges arebullets reported; was measured the last column before andreports after an perforation. indication of In the Table area2, thesubjected speed andto permanent kinetic energy deformation, changes measuredare reported; using the SEM last columnobservations. reports It ancan indication be observed of thethat area the subjectedkinetic energy to permanent lost by the deformation, projectiles, whichmeasured is absorbed using SEM by the observations. sample, is lower It can when be observed lubricant that is theused. kinetic Since energypart of lostthe byenergy the projectiles,is used in thewhich permanent is absorbed deformation by the sample, and part is lower in friction, when alth lubricantough difficult is used. Sinceto be quantified, part of the energya lower is value used of in thethe permanent permanent deformation deformation is and congruent part in friction,with a lower although energy difficult change. to be quantified, a lower value of the permanent deformation is congruent with a lower energy change. Table 2. Comparison of puncture test results with and without lubricant in the impact area. Table 2. Comparison of puncture test results with and without lubricant in the impact area. Test Vin (m/s) Vout (m/s) % ΔEkin (J) Deformed Zone Diam. (mm) Test Vin183.8(m/s) Vout76.4(m/s) 41 % 49.2∆Ekin (J) Deformed 8.2 Zone Diam. (mm) No lubricant 183.8 76.4 41 49.2 8.2 No lubricant 190.9 88.4 46.3 50.4 8.2 190.9 88.4 46.3 50.4 8.2 190.9 118.1 62.8 39.6 7.8 Lubricant 190.9 118.1 62.8 39.6 7.8 Lubricant 157.5 112 71.1 21.6 7.4 157.5 112 71.1 21.6 7.4 3.1.3. Multiple-shot tests 3.1.3. Multiple-Shot Tests Ionomeric panels tested at different temperatures in the range −40 °C/70 °C with firearm shots ◦ ◦ (.45ACPIonomeric bullets) panels showed tested full athealing different after temperatures single impacts. in the Multiple-shot range −40 C/70 tests Cwere with carried firearm out shots to investigate(.45ACP bullets) the ability showed to heal full healingin the case after of single consecutive impacts. shots Multiple-shot in the same tests location. were carried The results out to obtainedinvestigate by the these ability tests to showed heal in thethat case at up of consecutiveto two perforations shots in theat the same same location. point, The leaving results about obtained 30 s betweenby these testseach showedother, the that material at up to healed two perforations properly. All at the the same samples point, passed leaving the about leakage 30 s betweentests. Oneach the otherother, hand, the material SEM observations, healed properly. see Fi Allgure the 9, samples showed passed a somewhat the leakage worse tests. re-welding On the with other respect hand, SEMto a singleobservations, healing. see Figure9, showed a somewhat worse re-welding with respect to a single healing.

(a) (b)

FigureFigure 9. 9. SEMSEM images images projectile projectile entrance entrance side side ( (aa)) exit exit side side ( (bb)) after after two two shots shots in in the the same same location. location.

It was observed that, after the ﬁrst shot, plastic deformation occurs that changes the thickness of the sample in the impacted area, as can be seen in Figure 10. In particular, material stretching in the outer part of impacted area leads to a somewhat reduction in the thickness.

Aerospace 2019, 6, x FOR PEER REVIEW 11 of 17

It was observed that, after the first shot, plastic deformation occurs that changes the thickness of Aerospacethe sample2019 ,in6, 14the impacted area, as can be seen in Figure 10. In particular, material stretching 11in ofthe 17 outer part of impacted area leads to a somewhat reduction in the thickness.

Figure 10. Enlarged photograph of a specimenspecimen sectioned at thethe puncturepuncture location.location. Thickness proﬁleprofile after the ﬁrstfirst shot.

As long asas thethe panelpanel thickness/projectilethickness/projectile diameterdiameter ratioratio isis greatergreater thanthan thethe criticalcritical one one (( ((t/t/dd))critcrit ~ 0.2 [[20])20]) thethe healing healing still still takes takes place, place, but but the the local lo mechanicalcal mechanical characteristics characteristics are lowered are lowered by the by phase the changesphase changes and the and permanent the permanent plastic plastic deformation deformation of the previousof the previous shot. shot. A third shot was carried out at the same point of perforation and, as expected, the material did not healheal completely.completely. More More speciﬁcally, specifically, an an elastic elastic return return was was observed, observed, but but the edgesthe edges of the of moltenthe molten area didarea not did weld not properly,weld properly, creating creating a small a hole small through hole through which a which slight leakagea slight ofleakage water couldof water be observed.could be However,observed. itHowever, should be it pointedshould be out pointed that in out all cases,that in the all residualcases, the hole residual is far hole smaller is far than smaller the projectile, than the soprojectile, that healing, so that although healing, partial, although is stillpartial, effective. is still effective. Tests werewere repeated repeated at at a differenta different temperature temperature (Vin 282(Vin m/s, 282 Em/s,in 592 Ein J), 592 and J), the and results theare results reported are inreported Table3 .in It isTable interesting 3. It is to interesting observe that to even observe at the that lowest even temperatures at the lowest tested, temperatures a melting/welding tested, a effectmelting/welding and self-healing effect wereand self-healing evident. In were a few eviden instances,t. In a at few very instances, low temperatures, at very low panels temperatures, showed extensivepanels showed damage extensive and cracks damage when and impacted. cracks when However, impacted. locally, However, in the locally, shot area, in the hole shot healing area, hole was alwayshealing observed.was always observed.

TableTable 3.3.Variation Variation of of kinetic kinetic energy energy after after multiple multiple perforations perforations ( V(Vinin282 282m/s, m/s, Ein 592592 J). J).

◦ T ( C) Shot Vout (m/s) Eout (J) ∆EEkinkin (J) T (°C) Shot Vout (m/s) Eout (J) 1 250 466(J) 126 −40 21 247250 466 455 126 137 −40 12 255247 455 485 137 107 0 21 268255 485 536 107 56 0 12 256268 536 489 56 103 0 21 268256 489 536 103 56 3 279 580 12 0 2 268 536 56 1 257 492 100 50 3 279 580 12 2 279 580 12 1 257 492 100 50 1 268 536 56 70 22 270279 580 544 12 48 1 268 536 56 70 2 270 544 48 Additional multi-shot tests were conducted at different points within a range of 20 mm. Complete healing was observed in all tests, see Figure 11.

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AerospaceAdditional2019, 6, 14 multi-shot tests were conducted at different points within a range of 2012 mm. of 17 Complete healing was observed in all tests, see Figure 11.

Figure 11. Multiple-shots at different locations.

3.2. Tests with Integrated Fluid Container Previous teststests conducted conducted on on self-healing self-healing panels pane withls awith range a ofrange thickness of thickness and spherical and projectilesspherical withprojectiles different with sizes different indicated sizes the indicat paneled thickness/sphere the panel thickness/sphere diameter (t/ ddiameter) ratio as ( at/d characteristic) ratio as a parameter.characteristic It wasparameter. observed It was that observed a t/d ratio that of abouta t/d ratio 0.2 results, of about which 0.2 results, distinguishes which distinguishes full healing from full partialhealing healing from partial conditions healing [19 conditions]. [19]. ConfidentConfident withwith the the results results obtained obtained by theby ballisticthe ballistic tests on tests the ionomer,on the ionomer, authors have authors integrated have theintegrated self-healing the self-healing polymer in polymer a simplified in a simplified model of model a fuel tankof a fuel filled tank with filled water, with see water, Figures see2 Figures and3, investigating2 and 3, investigatingt/d ratios t/ comparabled ratios comparable to previous to previous tests. The tests. addition The addition of water of inside water the inside tank seemsthe tank to haveseems a to quite have limited a quite effect limited over effect the self-healing over the self-healing capacity of thecapacity ionomeric of the wall; ionomeric a slightly wall; easier a slightly healing occurseasier healing with any occurs panel thickness,with any possiblypanel thickness, due to the possibly lower panel due deflectionto the lower and panel material deflection deformation and inmaterial the surroundings deformation of in the the impact. surroundings of the impact. A somewhatsomewhat lowerlower healing healing efficiency efficiency was was observed observ whened when both both water water and Explosafeand Explosafe®®filler (single filler block(single or block pellets) or pellets) were inserted were inserted into the into tank. the Figuretank. Figure 12 compares 12 compares the results the results of ballistic of ballistic tests withtests ® andwith withoutand without water water and Explosafe and Explosafein the® in tank; the “airtank; leakage” “air leakage” specifies specifies conditions conditions when onlywhen partial only healingpartial healing was indicated was indicated by leakage by leakage tests. Atests. higher A highert/d threshold t/d threshold than than in previous in previous configurations configurations was noted,was noted, indicating indicating a lower a lower healing healing efficiency. efficiency. A possible A possible explanation explanation of this can of be this linked can tobe alinked temporary to a pressuretemporary increase pressure during increase impact during due toimpact a reflection due to wave, a reflection interfering wave, with interfering the healing with process. the healing process.Leakage tests and SEM analysis gave additional information regarding the healing efficiency. Most of the tested panels did not show any evidence of water spilling after bullet perforation; however, air leakage tests resulted more critical and about half of the panels showed air passage. This confirms that self-mending is a general tendency in this material, although in some cases, it is not able to provide full hole sealing. A comparison of SEM images of the impact area observed after tests indicates that the presence of water leads to lower petalling, possible indication of minor local deformation in the hole area, compared to the configuration in air, see Figure 13.

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Figure 12. Comparison of self-healing results of ionomeric panels with and without water and Figure 12. Comparison of self-healing results of ionomeric panels with and without water and AerospaceExplosafe 2019, 6,® x. FOR PEER REVIEW 14 of 17 Explosafe®.

Leakage tests and SEM analysis gave additional information regarding the healing efficiency. Most of the tested panels did not show any evidence of water spilling after bullet perforation; however, air leakage tests resulted more critical and about half of the panels showed air passage. This confirms that self-mending is a general tendency in this material, although in some cases, it is not able to provide full hole sealing. A comparison of SEM images of the impact area observed after tests indicates that the presence of water leads to lower petalling, possible indication of minor local deformation in the hole area, compared to the configuration in air, see Figure 13. Tests with an imposed overpressure inside the tank showed that in all cases, the hole generated by the projectile passage was fully closed or at least extensively reduced, see Table 4. When a small residual hole remained, spilling of water was observed, although at a very limited rate. Again, as in previous cases with a water-filled tank, inhibited petalling was noticed and the presence of anti-sloshing filler, either as a single block or as pellets, seems to be somewhat detrimental for the self-healing efficiency.

Table 4. Results of ballistic tests with a pressurized tank.

Panel Sphere Residual ∆P Water Air Configuration Thickness Diameter Hole [bar] Spillage Leakage [mm] [mm] (diameter) water 0.2 2 7.95 No Yes (<1 mm) Figurewater 13. SEM image0.2 of the projectile2 exit side of7.95 a healed ionomericNo tank wallNo (water inside).no Figurewater 13. SEM image0.4 of the projectile2 exit side of7.95 a healed ionomericNo tank wallNo (water inside).no water,Tests Expl. with anblock imposed 0.4 overpressure 2 inside the 7.95 tank showed thatYes in all cases, Yes the hole (<1 generated mm) In all tests with a water-filled tank, pressure variations were recorded during the entire impact bywater, the projectile Expl. pellets passage was 0.4 fully closed 2 or at least 7.95 extensively reduced, Yes see TableYes 4. When (<1 amm) small residualevent by holesensors remained, placed near spilling the impacted of water wall was (P1) observed, and near although the opposite at a very wall limited (P2); sensors rate. placed Again, asin other in previous locations cases did with not provide a water-filled clear results. tank, inhibited As a general petalling trend, was it was noticed observed and thethat presence the bigger of anti-sloshingthe impacting filler, sphere, either the ashigher a single the blockpressure or as peak pellets,s. This seems is expected to be somewhat both considering detrimental the forlarger the self-healingvolume of the efficiency. projectile entering the tank and the higher deflection of the panel due to the impact energy. Figure 14 shows, for example, the pressure peaks recorded near the impacted wall of the non-pressurized tank in tests with spheres of different size. A comparison between different configurations shows that a remarkable reduction of pressure peaks is observed when Explosafe® anti-sloshing filler is present, see Figure 15, with pellets leading to a more efficient pressure peak reduction. In the case of a pressurized tank, a similar trend was observed, again with a more efficient peak reduction effect given by filler in pellets form. It was noticed that during impacts, the internal overpressure was maintained and, only when a small hole remained unsealed after impact, a slow reduction of pressure was recorded afterword.

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Table 4. Results of ballistic tests with a pressurized tank.

Panel Sphere Water Air Residual Hole Conﬁguration ∆P [bar] Thickness Diameter Spillage Leakage (diameter) [mm] [mm] water 0.2 2 7.95 No Yes (<1 mm) water 0.2 2 7.95 No No no water 0.4 2 7.95 No No no water, Expl. block 0.4 2 7.95 Yes Yes (<1 mm) water, Expl. pellets 0.4 2 7.95 Yes Yes (<1 mm)

In all tests with a water-filled tank, pressure variations were recorded during the entire impact event by sensors placed near the impacted wall (P1) and near the opposite wall (P2); sensors placed in other locations did not provide clear results. As a general trend, it was observed that the bigger the impacting sphere, the higher the pressure peaks. This is expected both considering the larger volume of the projectile entering the tank and the higher deflection of the panel due to the impact energy. Figure 14 shows, for example, the pressure peaks recorded near the impacted wall of the non-pressurized tank in tests with spheres of different size. A comparison between different configurations shows that a remarkable reduction of pressure peaks is observed when Explosafe®anti-sloshing filler is present, see Figure 15, with pellets leading to a more efficient pressure peak reduction. In the case of a pressurized tank, a similar trend was observed, again with a more efficient peak reduction effect given by filler in pellets form. It was noticed that during impacts, the internal overpressure was maintained and, only when a small hole remained unsealed after impact, a slow reduction of pressure Aerospace 2019, 6, x FOR PEER REVIEW 15 of 17 was recorded afterword.

Figure 14. Pressure peaks recorded next to the tank impacted wall (pressure sensor P1) during Figureperforation 14. Pressure with steel peaks spheres recorded of different next size to the (between tank impacted 9.55 and 14 wall mm (pressure diameter). sensor P1) during perforation with steel spheres of different size (between 9.55 and 14 mm diameter).

Figure 15. Pressure peaks recorded in the tank far from the impacted wall (pressure sensor P2) with different configurations (t = 3 mm, steel sphere 14 mm).

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Figure 14. Pressure peaks recorded next to the tank impacted wall (pressure sensor P1) during Aerospaceperforation2019, 6, 14 with steel spheres of different size (between 9.55 and 14 mm diameter). 15 of 17

Figure 15. Pressure peaks recorded in the tank far from the impacted wall (pressure sensor P2) with different conﬁgurations (t = 3 mm, steel sphere 14 mm). Figure 15. Pressure peaks recorded in the tank far from the impacted wall (pressure sensor P2) with 4. Conclusionsdifferent configurations (t = 3 mm, steel sphere 14 mm).

4. ConclusionsIn this work, the self-healing behavior of an ionomeric system was explored in view of safety- related aeronautical applications. Ballistic tests were conducted in a variety of aging and environmental conditions;In this the mechanicalwork, the characterizationself-healing behavior of healed of materialsan ionomeric evidenced system the capacitywas explored of the ionomericin view of systemsafety-related to react to aeronautical damages and applications. provided information Ballistic tests regarding were theconducted damage andin a healing variety mechanisms. of aging and Shooting tests showed that self-mending behavior and hole closure can also be observed after multiple shots and at different temperatures. These results support the interest in the development of ionomeric systems and their employment for the design of new applicative solutions where an efficient autonomic repairability is required. In the case of punctures by high-speed objects, the safety level of fuel tanks may be improved by the integration of self-healing walls and explosion suppression systems based on aluminum cellular fillers. The response of such configurations to bullet impacts has been experimentally explored and showed encouraging results even though a reduction of the healing efficiency at the lowest temperature suggests that improvements in the behavior in these conditions require further research. The presence of the inner liquid improves the mending efficiency, supporting the deformation recovery during the healing process; although, its pressurization can affect the healing efficacy. However, even when full healing was not attained, a remarkable and consistent reduction of the punctured area and fluid spilling was recorded. The presence of an anti-sloshing filler placed inside the tank at a sufficient distance from the self-healing wall effectively helps in reducing the over-pressure peaks in the case of a perforating impact.

Author Contributions: Conceptualization: G.J., A.M.G. and L.D.L.; Formal analysis: G.J., G.C., A.M.G. and L.D.L.; Investigation: G.J., G.C. and A.M.G.; Methodology: G.J., G.C., A.M.G. and L.D.L.; Writing–original draft: G.J. and G.C.; Writing–review & editing: A.M.G. and L.D.L. Acknowledgments: The authors wish to thank Fiocchi Munizioni Ballistic Laboratory, Lecco, Italy, for their support in carrying out ballistic tests and Explosafe Ltd., Thun, Switzerland. for providing aluminum foil filler. Part of the characterization tests were performed at AMALA lab of Politecnico di Milano. Conflicts of Interest: The authors declare no conflict of interest. Aerospace 2019, 6, 14 16 of 17

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© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).