New Design of Composite Structures Used in Automotive Engineering

S S symmetry

Article New Design of Composite Structures Used in Automotive Engineering

Vasile Gheorghe, Maria Luminita Scutaru *, Virgil Barbu Ungureanu, Eliza Chircan and Mihai Ulea

Department of Mechanical Engineering, Transilvania University of Brasov, B-dul Eroilor 29, 500036 Brasov, Romania; [email protected] (V.G.); [email protected] (V.B.U.); [email protected] (E.C.); [email protected] (M.U.) * Correspondence: [email protected]

Abstract: The paper proposes composite materials for the manufacturing of parts of the car body structure, namely a door. This work aims to analyze the possibility of replacing the metal door of a vehicle with a door made of composite materials. Specific issues related to this replacement are analyzed in the paper. Test specimens were made of composite materials of different sizes, using several types of constituents to determine which material might be most suitable to replace metal in the manufacturing of the door. The choice of materials for the car door was made starting from the characteristics of the analyzed composite materials, but also taking into account the manufacturing possibilities and other engineering limitations. The behavior of the automotive structure as analyzed, using the finite element method for determining the stresses in the structure. Experimental verifications were performed on an experimental stand which has been specially designed for this purpose, to validate the proposed model. Keywords: vehicles; composite materials; metal materials; plastics; impact Citation: Gheorghe, V.; Scutaru, M.L.; Ungureanu, V.B.; Chircan, E.; Ulea, M. New Design of Composite Structures Used in Automotive 1. Introduction Engineering. Symmetry 2021, 13, 383. The replacement of metal materials used so far in the automotive industry with plastics https://doi.org/10.3390/sym13030383 and composites has led to increased service life, increased noise and vibration absorption, and intercompartmental insulation of vehicles to absorb the kinetic energy of shocks in the Academic Editor: Dumitru Baleanu case of accidents in an environmentally friendly way. and Marin Marin The use of composite materials in the automotive industry has increased significantly in recent decades so that, at present, automotive models incorporate in their construction Received: 31 January 2021 up to 50% plastics and composite materials. Many parts of today’s vehicles are made of Accepted: 24 February 2021 Published: 27 February 2021 composite materials. The composite materials from which these parts are made are chosen depending on the role that these components have in the whole vehicle. The technology

Publisher’s Note: MDPI stays neutral of obtaining these components also has an important role in choosing the material used with regard to jurisdictional claims in to make the car parts. The parts can be made of one or more subassemblies, of the same published maps and institutional afﬁl- type of composite material, or of different materials, removable or non-removable. Many iations. parts of today’s vehicles are made of composite materials. The technology of obtaining these components also has an important role in choosing the material used to make the car parts. The parts can be made of one or more subassemblies, of the same type of composite material or of different materials, removable or non-removable. In most cases, the design of devices or composite parts is a challenge due to the Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. demands that the structure must meet, the need for a low weight, and ensuring low This article is an open access article production costs. In order to achieve the optimal structure, strategies are developed distributed under the terms and regarding the limits of the compromise in the design process, the selection of materials, the conditions of the Creative Commons geometric design, and the possibilities of realization. Such cases are studied in [1] where Attribution (CC BY) license (https:// monolithic, U-beam, sandwich-insert, and sandwich-stiffened plates are analyzed. creativecommons.org/licenses/by/ SMART textiles, through their multifunctionality, low energy required in manufac- 4.0/). turing, small size and weight, ease of processing, and low costs offer diversiﬁed uses in

Symmetry 2021, 13, 383. https://doi.org/10.3390/sym13030383 https://www.mdpi.com/journal/symmetry Symmetry 2021, 13, 383 2 of 21

very different fields such as the automotive industry, aerospace engineering, military, med- ical, sports equipment industry, civil engineering, energy industry, etc. The development of textile-based composite materials is done quickly and with important consequences, which redefine the design and engineering of material science and improve the quality of life and the environment [2]. Composite materials manufactured for applications in the automotive industry are presented in [3]. The advantage of the proposed materials is the low weight and low costs. The materials are manufactured using scraps of freshly recycled rubber, epoxy resin, and graphene nanoparticles (GnPs). The Halpin–Tsai method is used to estimate the engineering constants of the manufactured materials. The finite element method is used to verify the in-service behavior of manufactured components. The rigidity of these materials is high enough and are very suitable for applications in the automotive industry. Natural fibers are a realistic and environmentally friendly solution as a reinforcing element for polymeric composites thanks to their mechanical properties, low manufacturing cost, environmental friendliness, easy procurement, biodegradable nature, and good weight resistance ratio [4]. According to the mentioned study, the potential of natural fibers is enormous for a series of applications in automotive engineering, sports, electrical and electronic articles, civil engineering, etc. Modern computational methods can also be applied for the calculation of natural fiber composites (NFC) which offer a good alternative to synthetic fiber composites. In [5], the properties of NFC for their use in automotive construction, aviation, marine equipment, sports equipment, and other engineering fields are analyzed. Thus, the finite element method proves to be a powerful tool in the analysis of such types of composites. In the context of the diversification of reinforcement components from composite materials, wood can become a promising choice and the automotive industry is one of the main beneficiaries of these new types of materials. Wood composites offer a number of very useful properties in practice such as high rigidity, good strength, very good damping properties, high fatigue resistance, low density, and low manufacturing costs. They thus become competitive compared to metals or other materials if they are properly designed and used. In order to apply these composites in automotive engineering, data on materials are needed, necessary to establish the composition of the material following numerical calculations. A detailed study of this problem is done in [6]. Following the studies, a database is compiled for three species of hardwood. The results demonstrate a very good behavior of these composites in the manufacturing of automotive components. Many papers study various aspects of the use of composites in the automotive industry. The rubber compound that has been used for the engine housing has thermal and sound insulation properties within acceptable limits, as well as superior properties when it comes to impact and tensile strength compared to plain polypropylene [7]. The compound based on rubber powder as a reinforcing element has a positive impact on the environment, reducing material waste by using old rubbers from car tires. At the same time, the mass of the product is significantly reduced, having at the same time properties within the allowed limits, such as products made of conventional materials. Different types of composite materials made from plastic and natural fibers show a new way to improve the automotive industry, by reducing weight and increasing performance [8]. To improve the composite structure of automotive bumpers, a study that proposed a new structure, with lighter weight and improved impact resistance was presented in [9]. Due to the high costs of obtaining carbon fibers used in various industries such as aerospace and automotive, the transition from metallic materials to carbon fiber-based composites is difficult. For this reason, polyacrylonitrile is used as a precursor for the manufacture of carbon fibers [10]. New solutions for the composite materials used in automotive engineering are presented in [11,12]. Different numerical procedures and modeling methods were used for the study of composites. To determine the mechanical characteristics of a two-component composite material with homogeneous and transversely isotropic reinforcing fibers, the Mori–Tanaka field method in the viscoelastic domain was applied. The method gave significant errors in the 45-degree test. To increase the calculation accuracy, fluorescence tests were performed Symmetry 2021, 13, x FOR PEER REVIEW 3 of 20

Tanaka field method in the viscoelastic domain was applied. The method gave significant errors in the 45-degree test. To increase the calculation accuracy, fluorescence tests were performed for carbon fiber reinforced polyether-ether-ketone epoxy resin laminates, thus determining the nonlinear behavior of the material [13,14]. Theoretical problems of numerical calculus in such cases are presented in [15]. Interesting applications of the composite in the field of automotive engineering and experimental results and procedures are studied in [16–18]. This paper proposes composite materials for the manufacturing of parts of the car body structure, namely a door. Test specimens were made of composite materials of different sizes, using several types of constituents to determine which material might be most suitable for making the door. The materials used to reinforce the plates are fiberglass and Symmetry 2021, 13, 383 3 of 21 carbon fiber. From these plates the specimens necessary for the tests on the stand were cut to establish the characteristics of the composite materials studied. The data recorded dur-

ing forthe carbon tests fiber allowed reinforced the polyether-ether-ketone choice of appropriate epoxy resin materials laminates, for thus the determining manufacturing of a car structure.the nonlinear The behaviorcar structure of the material studied [13,14 in]. Theoreticalthe paper problems is a car of numericaldoor. The calculus door is made of two parts,in such an outer cases are face presented and an in [inner15]. Interesting frame. applicationsBoth parts of were the composite made inof theco fieldmposite of materials. The automotive engineering and experimental results and procedures are studied in [16–18]. outer faceThis was paper made proposes of composite carbon materialsfiber, and for thethe manufacturing inner frame of partswas ofmade the car of fiberglass. The choicebody of structure, materials namely from a door.which Test the specimens two parts were of madethe car of compositedoor were materials made of was made starting fromdifferent the characteristics sizes, using several oftypes the analyzed of constituents composite to determine materials, which material but mightalso taking be into account most suitable for making the door. The materials used to reinforce the plates are fiberglass the andmanufacturing carbon fiber. From possibilities. these plates the specimensThe behavior necessary of for the the automotive tests on the stand structure were made of the chosencut to composite establish the characteristicsmaterials was of the analyzed, composite usin materialsg the studied. finite The element data recorded method for determining duringthe stresses the tests allowedin the thestructure. choice of Experiment appropriate materialsal verifications for the manufacturing were performed of a in order to car structure. The car structure studied in the paper is a car door. The door is made of validatetwo parts, the an proposed outer face andmodel. an inner frame. Both parts were made of composite materials. The outer face was made of carbon fiber, and the inner frame was made of fiberglass. The 2. Materialschoice of materials and Methods from which the two parts of the car door were made was made starting from the characteristics of the analyzed composite materials, but also taking into account theIn manufacturing order to achieve possibilities. the design The behavior of the of the door automotive it is necessary structure made to use of the some appropriate materialschosen composite and to materialsknow the was analyzed,characteristics using the finiteof these element materials, method for determiningwhich will be determined experimentally.the stresses in the For structure. the Experimentalmaterials used, verifications the werebending performed tests in orderare the to validate most suitable for the the proposed model. analysis, using standard tests specimen. The specimen is not clamped at either end, allow- ing 2.pure Materials bending. and Methods The three-point bending stress scheme of the specimen is shown in Fig- ure 1. AsIn a order result to achieve that only the design bending of the stress door ittests is necessary were performed, to use some appropriate the specimens were made materials and to know the characteristics of these materials, which will be determined in theexperimentally. form of a Forrectangular the materials parallelepiped used, the bending with tests the are thedimensions: most suitable the for thelength of the speci- menanalysis, (A) is using100 mm; standard the testswidth specimen. of the test The specimenpiece (B) is is not between clamped 10 at eitherand 15 end, mm; the thickness of theallowing specimen pure bending. (C) depends The three-point on the bending thickness stress of scheme the plate of the specimenfrom which is shown the specimens were in Figure1. As a result that only bending stress tests were performed, the specimens obtained.were made The in thetest form piece of arests rectangular on two parallelepiped supports with and the is dimensions:subjected theto lengthbending at a constant speedof the until specimen it breaks. (A) is 100 During mm; the the width test, of the the test force piece (B)applied is between to 10the and specimen 15 mm; and its deformationthe thickness (displacement of the specimen of a point (C) depends in the on middle the thickness of the of distance the plate from between which thethe support points) specimens were obtained. The test piece rests on two supports and is subjected to bending shallat abe constant measured. speed until The it test breaks. piece During is theconsider test, theed force broken applied at to thethe specimen first drop and of the force–dis- placementits deformation chart. (displacement of a point in the middle of the distance between the support points) shall be measured. The test piece is considered broken at the first drop of the force–displacement chart.

Figure 1. Three-point bending stress scheme of the test piece. Figure 1. Three-point bending stress scheme of the test piece. The following five types of materials were studied: 1. The first class of composite test specimens are made of polyester resin, reinforced Thewith following 5 layers of five fiberglass types fabric, of materials which we were call RT-800, studied: with a specific mass of 845g/m2. After deposition of the five layers, the material was allowed to polymerize

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1. The first class of composite test specimens are made of polyester resin, reinforced with 5 layers of fiberglass fabric, which we call RT-800, with a specific mass of 845g/m2. After deposition of the five layers, the material was allowed to polymerize at ambient temperature. From the first material, specimens with a thickness of 4.5 mm were made. 2. The second material tested was also made of polyester resin, reinforced with 7 layers of fiberglass fabric with a specific mass of 845g/m2. These two types of materials were made of the same constituents, differing only in the number of layers of fiberglass fabric used. The specimens made of the second material had a thickness of 6 mm. Symmetry 2021, 13, 383 3. The third material subjected to bending tests was a material4 ofmade 21 of polyester resin, of the “sandwich” type, made of layers of fiberglass fabric, but also composed of coremat. The specimens obtained from this material have a thickness of 8 mm. at ambient temperature. From the first material, specimens with a thickness of 4.5 mm 4. wereThe made. fourth material is fiberglass in the form of mat was used to reinforce a plate of 2. Thepolyester second material resin. tested Fifteen was alsolayers made of of mat polyester were resin, used reinforced to make with this 7 layers composite. 2 5. ofThe fiberglass fifth fabric material with a specifictested mass was of made 845g/m of. Theseepoxy two resin, types ofreinforced materials were with five layers of car- made of the same constituents, differing only in the number of layers of fiberglass fabricbon used. fiber The fabrics specimens and made one oflayer the second of polyester. material had These a thickness specimens of 6 mm. suffered total ruptures, 3. Theonly third one material showing subjected detachments to bending tests between was a material the laye maders. of polyesterIt could resin, rather be said that these ofspecimens the “sandwich” were type, broken made of during layers of the fiberglass bending fabric, tests. but also composed of coremat. The specimens obtained from this material have a thickness of 8 mm. 4. TheThere fourth are material two methods is fiberglass to in test the form the ofcomposite mat was used material to reinforce to determine a plate of the bending properties.polyester The resin.three-point Fifteen layers bending of mat wereflexural used to test make help this composite.us to obtain the bending modulus of 5. The fifth material tested was made of epoxy resin, reinforced with five layers of carbon elasticity,fiber fabrics flexural and one stress, layer of polyester.and strain These and specimens the material suffered total flexura ruptures,l stress–strain only response. To realizeone showingthe test detachments is a three-point between bend the layers. fixture. It could The rathermain be advantage said that these of this method is deter- minedspecimens by the were ease broken of duringpreparation the bending of tests.the experiment and testing. The disadvantages are thatThere the results are two methodsare sensitive to test to the loading, composite strain material rate, to determine and specimen. the bending The other method is the properties. The three-point bending flexural test help us to obtain the bending modulus offour-point elasticity, flexural flexural stress, test, and very strain similar and the to material the three-point flexural stress–strain bending response. flexural test. In the paper, Tothe realize three-point the test is abending three-point flexural bend fixture. test has The mainbeen advantage used, due of this to methodthe ease is of testing. The tests determinedhave been by themade ease ofconsidering preparation of the the experimentrecommend andations testing. Thepresented disadvantages in the standards ASTM are that the results are sensitive to loading, strain rate, and specimen. The other method isD790, the four-point ASTM flexuralD7264, test, ASTM very C1161, similar to ASTM the three-point D6272, bendingASTM flexuralC393, ASTM test. In D7249, ASTM D7250 the[19–26]. paper, the three-point bending flexural test has been used, due to the ease of testing. The testsThe have first been material made considering for which the recommendations results were presentedobtained in theis the standards composite material rein- ASTM D790, ASTM D7264, ASTM C1161, ASTM D6272, ASTM C393, ASTM D7249, ASTM D7250forced [19 with–26]. 5 layers of fiberglass fabric. From this plate were obtained, by cutting, 20 specimens,The firsthaving material the fordimensions: which results length—100 were obtained mm; is the width—10 composite material mm; thickness—4.5 re- mm. The inforcedspecimens with 5were layers numbered of fiberglass and fabric. marked From this for plate identification were obtained, with by cutting, numbers from 1 to 20 (Fig- 20 specimens, having the dimensions: length—100 mm; width—10 mm; thickness— 4.5ure mm. 2). TheThe specimens test pushing were numbered speed on and the marked specimen for identification is 0.1 mm/s, with at numbers an ambient temperature of from20 °C. 1 to The 20 (Figure specimens2). The have test pushing the same speed temperatur on the specimene as isthe 0.1 environment. mm/s, at an Figure 3 shows the ◦ ambientdeformation temperature of the of 20specimenC. The specimens during havethe thebend sameing temperature test on the as thestand. envi- A Force-Arrow chart ronment. Figure3 shows the deformation of the specimen during the bending test on thewas stand. created A Force-Arrow for each chart test was piece created tested. for each The test piecedeformation tested. The of deformation the specimen is recorded in ofmillimeters, the specimen and is recorded the pressing in millimeters, force and is recorded the pressing in force N. is recorded in N.

FigureFigure 2. Test2. Test specimens. specimens.

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FigureFigure 3. 3.Test 3. Test Test specimen specimen specimen on stand. on onstand. stand. In Figure4 is the graph of the diagram force–displacement recorded during the bendingInIn testFigure Figure for the 4 is4 test isthe specimenthe graph graph of 12. the Theof thediagram breaking diagram point force–displacement offorce–displacement the specimen is considered recorded recorded the during during the bend- the bend- ingﬁrsting test point test for wherefor the the atest drop test specimen of specimen the loading 12. 12. forceThe The breaking occurs. breaking InFigure point point5 ,of in the the of force–displacementspecimen the specimen is considered is considered the first the first pointdiagrampoint where where the records a adrop drop are putof ofthe together, the loading loading cumulatively, force force occurs. for occurs. the bendingIn Figure In stress.Figure 5, Onlyin the5, nine inforce–displacement arethe force–displacement presented, in order not to load the ﬁgure. diagramdiagramIn the the Figure the records 6records is presented are are put the put together, Young’s together, Modulus cumulati cumulati andvely, breakvely, for stress the for and bending the in Figure bending st7 ress.the stOnlyress. nine Only are nine are presented,displacementpresented, in and in order theorder break not not force,to loadto allload the for the thefigure. material figure. RT-800.

Figure 4. 4.Diagram Diagram of force–displacement of force–displacement recorded recorded for one test for specimen one test (test specimen specimen 12(test made specimen by 12 made

byRT-800). RT-800). Figure 4. Diagram of force–displacement recorded for one test specimen (test specimen 12 made by RT-800).

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Figure 3. Test specimen on stand.

In Figure 4 is the graph of the diagram force–displacement recorded during the bending test for the test specimen 12. The breaking point of the specimen is considered the first point where a drop of the loading force occurs. In Figure 5, in the force–displacement diagram the records are put together, cumulatively, for the bending stress. Only nine are presented, in order not to load the figure.

Symmetry 2021, 13, 383 Figure 4. Diagram of force–displacement recorded for one test specimen (test 6specimen of 21 12 made by RT-800).

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FigureIn 5. the Cumulative Figure 6 force–displacementis presented the Young’s diagram recordsModulus for andRT-800 break specimens. stress and in Figure 7 Figure 5. the displacementCumulative and the break force–displacement force, all for diagramthe material records RT-800. for RT-800 specimens. In the Figure 6 is presented the Young’s Modulus and break stress and in Figure 7 the displacement and the break force, all for the material RT-800

Young's Modulus Maximum stress 70 50 70 50 60 60 40 50 40 50 40 30 40 30 30 30 20 20 20 20 10 10 10 10 0 0 0 0

Young's Modulus [MPa] Young's 12345678910 12345678910 Maximum stress Maximum [MPa]

Young's Modulus [MPa] Young's 12345678910 12345678910 No. of Specimen stress Maximum [MPa] No. of Specimen No. of Specimen No. of Specimen

Figure Figure6. Young’s 6. Young’s modulus modulus and break and stress break for stress test for specimens test specimens RT-800. RT-800. Figure 6. Young’s modulus and break stress for test specimens RT-800. Displacement 80 Displacement 80 60 60 40

v [mm] 40 v [mm] 20 20 0 0 12345678910 12345678910 No. of Specimen No. of Specimen

Figure 7. Displacement and break force for the test specimens RT-800. Figure Figure7. Displacement 7. Displacement and break and force break for force the fortest the specimens test specimens RT-800. RT-800. Based on these tests, several conclusions can be drawn. A few layers, of those re- quiredBased for stretching, on these tests, were severalcompletely conclusions torn. Detachment can be drawn. did not A appearfew layers, between of those the lay- re- ersquired of any for sample, stretching, except were for completely the median torn. area, Detachment where the rupture did not occurred. appear between Bending the stress lay- aters break, of any σ sample,, ranged except from 22.23 for the to median38.90 [MPa]. area, where the rupture occurred. Bending stress at break,In material σ, ranged type from no. 22.23 3, the to breaking 38.90 [MPa]. bending stress for these specimens had lower valuesIn than material that oftype the no.samples 3, the that breaking did not bending contain stresscoremat, for and these ranged specimens from 5.85had to lower 8.81 [MPa].values Thisthan wasthat aof material the samples made that of polyester did not contain resin, of coremat, the “sandwich” and ranged type, from made 5.85 of tolayers 8.81 of[MPa]. fiberglass This was fabric, a material but also made having of polyester of coremat resin, in the of themiddle “sandwich” of the sandwich. type, made The of speci-layers mensof fiberglass obtained fabric, from bu thist also material having have of coremat a thickness in the of middle 8 mm. of These the sandwich. specimens The suffered speci- damagemens obtained to the coremat from this layer material before have the rupt a thicknessure occurred. of 8 mm. Absolutely These specimensall specimens suffered have suffereddamage detachmentsto the coremat between layer before the constituent the rupture la yersoccurred. due to Absolutely the built-in all coremat specimens layer, have the corematsuffered coredetachments does not betweenprovide goodthe constituent properties la toyers the due composite. to the built-in coremat layer, the corematSample core with does material not provide no. 4. good The fourthproperties materi to althe is composite.composed by 15 layers of fiberglass in theSample form of with mat, material used to no.reinforce 4. The afourth plate materiof polyesteral is composed resin, from by which15 layers were of fiberglass cut spec- imensin the formwith aof thickness mat, used of to 12 reinforce mm. Those a plate tested of atpolyester bending, resin, at ambient from which temperature were cut (20 spec- °C), showedimens with ruptures a thickness of the of layers 12 mm. subjected Those tested to stretching. at bending, Bending at ambient stress temperature at rupture (20varied °C), betweenshowed ruptures22.15 and of 23.69 the [MPa],layers subjectedvalues comparable to stretching. to those Bending obtained stress for at samples rupture without varied coremat.between 22.15The samples and 23.69 of [MPa],this material, values tested comparable at a temperature to those obtained of 50 °C, for also samples suffered without rup- turescoremat. of the The layers samples subjected of this material,to stretching tested. The at a breakingtemperature bending of 50 load°C, also for sufferedthe samples rup- testedtures ofat the50 °C layers had valuessubjected lower to stretchingthan the va. Thelues breakingrecorded bendingfor breaking load the for specimens the samples at

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Based on these tests, several conclusions can be drawn. A few layers, of those required for stretching, were completely torn. Detachment did not appear between the layers of any sample, except for the median area, where the rupture occurred. Bending stress at break, σ, ranged from 22.23 to 38.90 [MPa]. In material type no. 3, the breaking bending stress for these specimens had lower values than that of the samples that did not contain coremat, and ranged from 5.85 to 8.81 [MPa]. This was a material made of polyester resin, of the “sandwich” type, made of layers of fiberglass fabric, but also having of coremat in the middle of the sandwich. The specimens obtained from this material have a thickness of 8 mm. These specimens suffered damage to the coremat layer before the rupture occurred. Absolutely all specimens have suffered detachments between the constituent layers due to the built-in coremat layer, the coremat core does not provide good properties to the composite. Sample with material no. 4. The fourth material is composed by 15 layers of fiberglass in the form of mat, used to reinforce a plate of polyester resin, from which were cut specimens with a thickness of 12 mm. Those tested at bending, at ambient temperature (20 ◦C), showed ruptures of the layers subjected to stretching. Bending stress at rupture varied between 22.15 and 23.69 [MPa], values comparable to those obtained for samples without coremat. The samples of this material, tested at a temperature of 50 ◦C, also suffered ruptures of the layers subjected to stretching. The breaking bending load for the samples tested at 50 ◦C had values lower than the values recorded for breaking the specimens at ambient temperature, and ranged from 16.41 to 17.15 [MPa]. The samples subjected to the bending rupture test at a temperature of 65 ◦C, suffered ruptures of the layers subjected to stretching, but also multiple detachments between layers. During these tests, the detachments between the layers appeared long before the samples broke. Basically, the resin allowed the layers of felt to slide against each other. Bending stress at recorded rupture, σ, had very low values, between 1.49 and 2.31 [MPa]. From this material, a set of samples was heated to a temperature of 100 ◦C, after which it was allowed to cool to ambient temperature, when the bending test was performed. Although they were heated, the specimens behaved similarly to those that were not heated at all. The bending load at break for the samples heated to 100 ◦C and tested at ambient temperature, had values between 19.51 and 22.25 [MPa]. The rupture also occurred in the layers subjected to stretching and there were also detachments between the layers, in the middle area. In Figure8 the recordings during the bending rupture tests are graphically represented, for a test piece made of mat, required in the four test conditions. From this graph, it is observed that the samples tested at ambient temperature (recording in blue) have the same behavior as the ones heated to 100 ◦C and allowed to cool to ambient temperature, when they were tested (recording in green). Additionally, from this graph it appears that the resistance of the samples made of polyester resin, reinforced with fiberglass felt, decreases with increasing temperature of the resin. Among the constituents of the composite material, the matrix was affected by the increase in temperature. Carbon fiber samples from material no. 5. The bending load recorded at break for these samples tested increased to 52.95 to 77.87 [MPa]. Table1 shows that the composite material made of fiberglass, both fabric and mat layers, has values close to σ, the bending load, at ambient temperature (20 ◦C). Even if the use of coremat in the structure of the composite material has many advantages, from an economic point of view, its presence diminishes the performance of the composite material. Symmetry 2021, 13, x FOR PEER REVIEW 7 of 20

tested at 50 °C had values lower than the values recorded for breaking the specimens at ambient temperature, and ranged from 16.41 to 17.15 [MPa]. The samples subjected to the bending rupture test at a temperature of 65 °C, suffered ruptures of the layers subjected to stretching, but also multiple detachments between layers. During these tests, the detachments between the layers appeared long before the samples broke. Basically, the resin allowed the layers of felt to slide against each other. Bending stress at recorded rupture, σ, had very low values, between 1.49 and 2.31 [MPa]. From this material, a set of samples was heated to a temperature of 100 °C, after which it was allowed to cool to ambient temperature, when the bending test was performed. Although they were heated, the specimens behaved similarly to those that were not heated at all. The bending load at break for the samples heated to 100 °C and tested at ambient temperature, had values between 19.51 and 22.25 [MPa]. The rupture also occurred in the layers subjected to stretching and there were also detachments between the layers, in the middle area. In Figure 8 the recordings during the bending rupture tests are graphically represented, for a test piece made of mat, required in the four test conditions. From this graph, it is observed that the samples tested at ambient temperature (recording in blue) have the same behavior as the ones heated to 100 °C and allowed to cool to ambient temperature, when they were tested (recording in green). Additionally, from this graph it appears that the resistance of the samples made of polyester resin, reinforced with fiberglass felt, decreases with increasing temperature Symmetry 2021, 13, 383 of the resin. Among the constituents of the composite material, the matrix8 of 21 was affected by the increase in temperature.

Figure 8. 8.Behavior Behavior under under different differ testent conditions. test conditions.

Table 1. The values for the bending loads recorded for all types of specimens tested. Carbon fiber samples from material no. 5. The bending load recorded at break for Type of Bending Load at theseMaterial samples tested increased to 52.95Test Temperature to 77.87 [MPa]. Table 1 showsStructure that the composite material made of fiberglass,Break both fabric and mat σ [MPa] layers, has values close to σ, the bending◦ Cload, at ambient temperature (20 °C). Even if the use of coremat in the structure of the composite material hasMin manyMax advantages, from an ◦ economic pointFiber of view, fabric its presence dimini at 20 Cshes the performance 22.23 of 38.90 the composite mate- Fabric + coremat at 20 ◦C 5.85 8.81 rial. at 20 ◦C 22.15 23.69 Fiberglass at 50 ◦C 16.41 17.15 Mat layer at 65 ◦C 1.49 2.31 at 20 ◦C after heating to 100 ◦C 19.51 22.25 ◦ Carbon ﬁber at 20 C 52.95 77.87

The values recorded in Table1 are represented graphically in Figure9. Analyzing Figure9, it is found that the composite material made of carbon ﬁber fabric bears the highest load. The composite material made of ﬁberglass supports a considerable load if it is in normal temperature conditions and if it does not contain materials that can damage its homogeneous structure. Symmetry 2021, 13, x FOR PEER REVIEW 8 of 20

Table 1. The values for the bending loads recorded for all types of specimens tested.

Material Type of Structure Test Temperature Bending Load at Break σ[MPa] °C Min Max Fiber fabric at 20 °C 22.23 38.90 Fabric + coremat at 20 °C 5.85 8.81 at 20 °C 22.15 23.69 Fiberglass at 50 °C 16.41 17.15 Mat layer at 65 °C 1.49 2.31 at 20 °C after heating to 19.51 22.25 100 °C Carbon fiber at 20 °C 52.95 77.87

The values recorded in Table 1 are represented graphically in Figure 9. Analyzing Figure 9, it is found that the composite material made of carbon fiber fabric bears the highest load. The composite material made of fiberglass supports a considerable load if it is in Symmetry 2021, 13, 383 9 of 21 normal temperature conditions and if it does not contain materials that can damage its homogeneous structure.

FigureFigure 9. Graphical 9. Graphical representation representation of σ obtainedobtained in in the the tests tests performed. performed.

DesignDesign of ofthe the Door Door DuringDuring the the work, work, it it was was decided to to manufacturing manufacturing a car a car door door using using composite composite materi- mate- rials.als. The The variant variant chosenchosen was was a righta right front front door door fora for five-door a five-door car. In car. order In to order build to the build car the cardoor, door, it it was was necessary necessary to maketo make two two pieces: pieces: an outer an outer face and face an and inner an frame. inner frame. AnAn existing existing (metal) (metal) door door model waswas chosen chosen in in order order to maketo make comparative comparative tests. tests. The The constructive solution established by the designer, based on previous experience, it was that constructive solution established by the designer, based on previous experience, it was the outer face of the door should be made of carbon fiber and the inner frame of fiberglass. that theA outer drawing face of of this the exterior door doorshould face be with made the 3Dof carbon model wasfiber used. and Basedthe inner on it, frame the of fiberglass.mold in which the molding will be made was performed. The mold was made by polyester resin,A drawing reinforced of with this fiberglass. exterior door Classical face technologies with the 3D were model applied was to used. make theBased piece. on it, the mold inFigure which 10 thea shows molding the 3D will model be made of the was outer performed. face of the door The andmold Figure was 10 madeb the by piece polyester made.resin, Basedreinforced on the with drawing fiberglass. (Figure Classical 11a), a polyester technologies resin mold were was applied built and to make reinforced the piece. with fiberglass (Figure 11b). The mold of the inner door frame has pronounced cavities. Fiberglass is more malleable than carbon fiber, which is why it was decided that the inner door frame should be made of fiberglass. The interior door frame was made of polyester resin, reinforced with two layers of fiberglass felt. The door assembly is made by mounting together the outer face of the door and the inner frame of the door. The overall drawing is shown in Figure 12b. The two parts were mounted together, not by gluing with adhesive, but with screws. The door will not be mounted on a car, but will be mounted on a stand, where it will be subjected to deformation resistance tests, compared to a door of the same type, but made of sheet steel. Additional elements identical to those of the steel sheet version were attached to the composite door. The inner frame and the outer face were fixed to each other to form the car door with M4 screws, on the entire contour, to allow mounting and dismounting as many times as needed during the tests on the stand. Symmetry 2021, 13, x FOR PEER REVIEW 9 of 20

Symmetry 2021, 13, x FOR PEER REVIEW 9 of 20 Figure 10a shows the 3D model of the outer face of the door and Figure 10b the piece made. Based on the drawing (Figure 11a), a polyester resin mold was built and reinforced with fiberglass (Figure 11b). The mold of the inner door frame has pronounced cavities. FiberglassFigure 10a is showsmore themalleable 3D model than of the carbon outer face fibe ofr, the which door andis why Figure it was10b the decided piece that the inner Symmetry 2021, 13, 383 made. Based on the drawing (Figure 11a), a polyester resin mold was built and reinforced10 of 21 withdoor fiberglass frame should (Figure 11b).be made The mold of fiberglass. of the inner The door interior frame has door pronounced frame was cavities. made of polyester Fiberglassresin, reinforced is more malleable with two than layers carbon of fibe fiberglassr, which is felt.why it was decided that the inner door frame should be made of fiberglass. The interior door frame was made of polyester resin, reinforced with two layers of fiberglass felt.

(a) (a) (b) (b) Figure 10. (a) 3D model of the outer face of the car door. (b) Door face made of carbon fiber. FigureFigure 10. 10. (a)) 3D3D model model of theof the outer outer face offace the of car the door. car (b )door. Door face(b) Door made offace carbon made ﬁber. of carbon fiber.

Symmetry 2021, 13, x FOR PEER REVIEW 10 of 20 (a) (b)

FigureFigure 11. 11.(a) (3Da) 3D model model of ofthe the outer outer face face of ofthe the car car door. door. (b ()b The) The inner inner door door frame frame made made of of fiber- ﬁberglass. glass. (a) (b) The door assembly is made by mounting together the outer face of the door and the innerFigure frame 11. (ofa) the3D door.model The of overallthe outer drawing face of is theshown car indoor. Figure (b) 12b. The The inner two door parts frame were made of fiber- mountedglass. together, not by gluing with adhesive, but with screws. The door will not be mounted on a car, but will be mounted on a stand, where it will be subjected to deformation resistance tests, compared to a door of the same type, but made of sheet steel. The door assembly is made by mounting together the outer face of the door and the inner frame of the door. The overall drawing is shown in Figure 12b. The two parts were mounted together, not by gluing with adhesive, but with screws. The door will not be mounted on a car, but will be mounted on a stand, where it will be subjected to deformation resistance tests, compared to a door of the same type, but made of sheet steel.

(a) (b)

FigureFigure 12. 12. (a).( a3D) 3D model model of ofthe the car car door. door. (b ().b Mounted) Mounted door. door.

AdditionalFigure 12 elementsb shows theidentical mounted to those model of of th thee steel door, sheet which version was madewere attached starting fromto the the compositeexecution door. drawing The inner in Figure frame 12 a.and the outer face were fixed to each other to form the car door with M4 screws, on the entire contour, to allow mounting and dismounting as many times as needed during the tests on the stand. Figure 12b shows the mounted model of the door, which was made starting from the execution drawing in Figure 12a.

3. Modeling and Experimental Research of the Car Door In order to be able to perform a finite element analysis the model of the structure must be elaborated. The assumptions considered are the following:

- each sheet is shaped in the form of a continuous, linear elastic medium. The theory does not include cracks, air gaps, etc; - the laminates are orthotropic, parallel, and perfectly glued to each other; - the fibers are not examined in isolation from the matrix or the adhesive layer (interface effects are neglected); - the individual layers are ideally glued to each other. In case of application of loads, no relative slips appear; - the materials behave linearly, ideally elastic, i.e., for each individual layer the laws of linear elasticity are valid; - the connections between the components of a mechanical assembly are relatively difficult to make; - the damping of the mechanical system is generally ignored. The 3D models made of the door elements were used to perform the analysis with a finite element. The Hyperworks/Hypermesh software suite was used to discretize the CAD model.

3.1. Test Stand for Car Structure The experimental research of the car door was carried out on a stand specially designed for this type of test. The stand must reproduce, as accurately as possible, the way the door is attached to the car. For this reason, a five-door body was used to make the stand, on which the door made of composite materials could be mounted. The doors, hood, rear tailgate, windshield are not mounted on the body. The body does not have the suspension system, steering or power group. The body was fixed, with the help of clamps, on the laboratory platform. In order for the door to be mounted exactly like on a car, the door sealing gasket (cheddar), the door hinges, its locking system were kept on the body.

Symmetry 2021, 13, 383 11 of 21

3. Modeling and Experimental Research of the Car Door In order to be able to perform a finite element analysis the model of the structure must be elaborated. The assumptions considered are the following: - each sheet is shaped in the form of a continuous, linear elastic medium. The theory does not include cracks, air gaps, etc; - the laminates are orthotropic, parallel, and perfectly glued to each other; - the fibers are not examined in isolation from the matrix or the adhesive layer (interface effects are neglected); - the individual layers are ideally glued to each other. In case of application of loads, no relative slips appear; - the materials behave linearly, ideally elastic, i.e., for each individual layer the laws of linear elasticity are valid; - the connections between the components of a mechanical assembly are relatively difficult to make; - the damping of the mechanical system is generally ignored. The 3D models made of the door elements were used to perform the analysis with a finite element. The Hyperworks/Hypermesh software suite was used to discretize the CAD model.

3.1. Test Stand for Car Structure The experimental research of the car door was carried out on a stand specially designed for this type of test. The stand must reproduce, as accurately as possible, the way the door is attached to the car. For this reason, a ﬁve-door body was used to make the stand, on which the door made of composite materials could be mounted. The doors, hood, rear Symmetry 2021, 13, x FOR PEER REVIEW 11 of 20 tailgate, windshield are not mounted on the body. The body does not have the suspension system, steering or power group. The body was ﬁxed, with the help of clamps, on the laboratory platform. In order for the door to be mounted exactly like on a car, the door sealing gasket (cheddar), the door hinges,On its locking the stand, system were the kept doors, on the body.both metallic and composite, were stressed statically and On the stand, the doors, both metallic and composite, were stressed statically and dynamically.dynamically. The diagram The diagram of the stand of is presentedthe stand in Figure is presented 13. in Figure 13.

FigureFigure 13. The13. carThe body car used body on the used stand. on the stand. To monitor the surface of the door structure subject to stress, the Digital Image CorrelationTo Methodmonitor (VIC) the was surface used, using of the the system door produced structure by ISI-Sys subject GmbH, to Kassel, stress, the Digital Image Cor- relation Method (VIC) was used, using the system produced by ISI-Sys GmbH, Kassel, Germany. The system is made with the help of two digital cameras, mounted on a tripod, whose high-fidelity images are processed using software. The door test was made with a static charging device. The device is made of an upright, also fixed on the metal platform of the laboratory, on which is mounted a mechanism by means of which the door is operated and its request is recorded. The device is made of an upright, also fixed on the metal platform of the laboratory, on which is mounted a mechanism by means of which the door is operated and the force is recorded. On the upright (3) of Figure 14, a support (4) is fixed in which the loading mechanism is located. The loading mechanism of the door (8), which is mounted on the vehicle body (2), by means of the rod (6), on which the force transducer (7) follows. The support can be moved vertically with respect to the upright, by means of a screw–nut mechanism. The door loading mechanism is also made on the nut–nut principle. Thus, turning the nut (5) produces a controlled, horizontal movement of the mechanism rod (6), which in turn acts on the door, by means of the force transducer (8).

For this test, the car doors, both metallic and composite, were covered on the outside with a layer of matte white paint. Over this layer of white paint, in the monitored area, will be applied, by spraying, stains with random dimensions, shape, and distribution, which will ensure a good contrast and an easy subsequent identification for the cameras of the VIC system.

Symmetry 2021, 13, x FOR PEER REVIEW 11 of 20

On the stand, the doors, both metallic and composite, were stressed statically and dynamically. The diagram of the stand is presented in Figure 13.

Figure 13. The car body used on the stand.

To monitor the surface of the door structure subject to stress, the Digital Image Cor- relation Method (VIC) was used, using the system produced by ISI-Sys GmbH, Kassel, Symmetry 2021, 13, 383 12 of 21 Germany. The system is made with the help of two digital cameras, mounted on a tripod, whose high-fidelity images are processed using software. The door test was made with a static charging device. The device is made of an up- Germany. The system is made with the help of two digital cameras, mounted on a tripod, right,whose also high-fidelity fixed on the images metal are processedplatform using of the software. laboratory, on which is mounted a mechanism byThe means door testof waswhich made the with door a static is chargingoperated device. and Theits devicerequest is made is recorded. of an upright, The device is madealso of fixed an onupright, the metal also platform fixed of theon laboratory,the metal on platform which is mounted of the alaboratory, mechanism byon which is mountedmeans a of mechanism which the door by ismeans operated of which and its requestthe door is recorded.is operated The and device the is force made is recorded. of an upright, also fixed on the metal platform of the laboratory, on which is mounted On athe mechanism upright by(3) meansof Figure of which 14, a the support door is (4) operated is fixed and in the which force the is recorded. loading Onmechanism is located.the upright The loading (3) of Figure mechanism 14, a support of the (4) door is fixed (8), in which the is loadingmounted mechanism on the vehicle is body (2), located.by means The of loading the rod mechanism (6), on which of the doorthe force (8), which transducer is mounted (7) onfollows. the vehicle The body support can be moved(2), by vertically means of thewith rod respect (6), on which to the the uprigh force transducert, by means (7) follows. of a scre Thew–nut support canmechanism. be The doormoved loading vertically mechanism with respect is also to the made upright, on the by means nut–nut of a principle. screw–nut mechanism.Thus, turning The the nut (5) door loading mechanism is also made on the nut–nut principle. Thus, turning the nut (5) producesproduces a controlled, a controlled, horizontalhorizontal movement movement of the of mechanism the mechanism rod (6), whichrod (6), in turnwhich acts in turn acts on theon thedoor, door, by by means means ofof the the force force transducer transducer (8). (8).

Figure 14. Stand diagram. 1. Metal platform; 2. car body; 3.mount; 4. support loading mechanism; 5. nut; 6. rod loading mechanism; 7. force transducer; 8. door; 9. speaker without membrane; Figure10. sound14. Stand level diagram. meter; 11. 1. camerasMetal platform; of the Video 2. car Image body; Correlation 3.mount; (VIC) 4. suppor system;t 12.loading tripod; mechanism; 5. nut;13. 6. computer. rod loading mechanism; 7. force transducer; 8. door; 9. speaker without membrane; 10. sound level meter; 11. cameras of the Video Image Correlation (VIC) system; 12. tripod; 13. com- For this test, the car doors, both metallic and composite, were covered on the outside Symmetry 2021, 13, x FOR PEER REVIEWputer. 12 of 20 with a layer of matte white paint. Over this layer of white paint, in the monitored area, will be applied, by spraying, stains with random dimensions, shape, and distribution, which willFor ensure this test, a good the contrast car doors, and an both easy metallic subsequent and identiﬁcation composite, for were the cameras covered of on the the outside withVIC a layer system. of matte white paint. Over this layer of white paint, in the monitored area, Figure 15 shows the two doors, metallic and composite, after being covered with will be Figureapplied, 15 shows by spraying, the two doors, stains metallic with and random composite, dimensions, after being covered shape, with and matte distribution, whitematte paint. white The paint. metal doorThe ismetal blue, anddoor the is composite blue, and door the is black. composite door is black. which will ensure a good contrast and an easy subsequent identification for the cameras of the VIC system.

Figure 15. 15.Metal Metal door door (left) and(left composite) and composite materials door materials (right). door (right).

On the car body, fixed on the laboratory platform, will be mounted, in turn, the metal door and the composite door to be loaded both statically and dynamically. To monitor the surface of the door subjected to static load, the Digital Image Corre- lation Method (VIC) (Video Image Correlation) is used, which allows high-precision experimental investigations (of the order of microns, μm). This is an optical method of investigation, without direct contact with the surface of the analyzed part and is not de- pendent on its material either. It does not intervene in the intimate process of changing the field of displacements and deformations of the structure under the action of external influencing factors, to which the piece would be subjected. The main parts of the VIC-3D system are: two high-resolution video cameras; a rigid tripod; a computer. The two chambers (1) are arranged on the crosspiece (2) so as to see the analyzed object at the same angles, arranged symmetrically (Figure 16). The analyzed object is painted with spots of random size, shape, and distribution, which on the white background of the door surface, will ensure a good contrast.

Figure 16. 1. High resolution video camera; 2. support; 3. tripod; 4. LED lamp. Basic elements of the VIC-3D system.

The three-dimensional Video Image Correlation (VIC-3D) represents a displacement/strain measurement method developed by Correlated Solutions, Inc. Basically, the technique uses a proprietary mathematical correlation method to for the digital image data taken while a test specimen is subjected to load. This method can offer very accurately the full-field. It uses a simple specimen preparation, the non-contact nature of the measurement, the low sensitivity to vibrations, the ability to measure large strains (>500%), and the ability to measure initial specimen shape and surface displacements in

Symmetry 2021, 13, x FOR PEER REVIEW 12 of 20

Figure 15 shows the two doors, metallic and composite, after being covered with matte white paint. The metal door is blue, and the composite door is black.

Figure 15. Metal door (left) and composite materials door (right).

On the car body, fixed on the laboratory platform, will be mounted, in turn, the metal door and the composite door to be loaded both statically and dynamically.

Symmetry 2021, 13, 383 To monitor the surface of the door subjected to static13 of 21 load, the Digital Image Corre- lation Method (VIC) (Video Image Correlation) is used, which allows high-precision experimental investigations (of the order of microns, μm). This is an optical method of in- On the car body, fixed on the laboratory platform, will be mounted, in turn, the metal doorvestigation, and the composite without door to bedirect loaded bothcontact statically with and dynamically.the surface of the analyzed part and is not de- pendentTo monitor on the its surface material of the door either. subjected It todoes static not load, intervene the Digital Image in Cor-the intimate process of changing relation Method (VIC) (Video Image Correlation) is used, which allows high-precision experimentalthe field investigationsof displacements (of the order and of microns, deformationsµm). This is anof optical the structure method of under the action of external investigation,influencing without factors, direct contact to which with the the surface piece of the would analyzed be part subjected. and is not de- pendent on its material either. It does not intervene in the intimate process of changing the fieldThe of displacements main parts and deformationsof the VIC-3D of the structure system under are: the two action high-resolution of external video cameras; a rigid influencingtripod; factors,a computer. to which the The piece two would chambers be subjected. (1) are arranged on the crosspiece (2) so as to see The main parts of the VIC-3D system are: two high-resolution video cameras; a rigid tripod;the analyzed a computer. Theobject two chambers at the (1)same are arranged angles, on thearranged crosspiece symmetrically (2) so as to see (Figure 16). The analyzed theobject analyzed is objectpainted at the samewith angles, spots arranged of random symmetrically size (Figure, shape, 16). The and analyzed distribution, which on the white object is painted with spots of random size, shape, and distribution, which on the white backgroundbackground of the door of the surface, door will ensuresurface, a good will contrast. ensure a good contrast.

FigureFigure 16. 1. 16. High 1. resolution High resolution video camera; 2. video support; camera; 3. tripod; 4. LED2. support; lamp. Basic elements3. tripod; of the 4. LED lamp. Basic elements of VIC-3D system. the VIC-3D system. The three-dimensional Video Image Correlation (VIC-3D) represents a displacement/ strain measurement method developed by Correlated Solutions, Inc. Basically, the technique usesThe a proprietary three-dimensional mathematical correlation Video method Image to for Correla the digital imagetion data(VIC-3D) represents a displace- takenment/strain while a test specimen measurement is subjected tomethod load. This methoddeveloped can offer veryby accuratelyCorrelated the Solutions, Inc. Basically, the full-ﬁeld. It uses a simple specimen preparation, the non-contact nature of the measurement,technique the low sensitivity uses toa vibrations,proprietary the ability mathematical to measure large strains correlation (>500%), and method the to for the digital image ability to measure initial specimen shape and surface displacements in three dimensions. Thedata method taken is based while on the usea test of two specimen video cameras, is located subject at a distanceed to from load. each other,This method can offer very accu- whoserately images the recordedfull-field. simultaneously It uses providea simple a spatial specimen image of the preparation, analyzed object, the non-contact nature of the similar to the human eye. measurement,The static door loading the mechanismlow sensitivity works on the to principle vibrations, of the nut–screw. the Thus,ability to measure large strains by(>500%), rotating the and nut (5)the in Figureability 14, to there measure is a horizontal initial movement specimen of the rod shape of the and surface displacements in mechanism (6), which, in turn, loads the door, through the force transducer (8). While the force transducer records the pressing or compression force applied to the door face, the VIC system records the deformations that occur on its surface.

Symmetry 2021, 13, x FOR PEER REVIEW 13 of 20

three dimensions. The method is based on the use of two video cameras, located at a distance from each other, whose images recorded simultaneously provide a spatial image of the analyzed object, similar to the human eye. The static door loading mechanism works on the principle of the nut–screw. Thus, by rotating the nut (5) in Figure 14, there is a horizontal movement of the rod of the mechanism (6), which, in turn, loads the door, through the force transducer (8). While the force transducer records the pressing or compression force applied to the door face, the VIC system records the deformations that occur on its surface.

Symmetry 2021, 13, x FOR PEER REVIEW 13 of 20 3.2. Experimental Research of a Classical Metal Structure Symmetry 2021, 13, 383 14 of 21 The metal door was mounted on the body (Figure 17). It is fastened in hinges and three dimensions. The method is based on the use of two video cameras, located at a dis- tancerests from on each the other, sealing whose images gasket recor ded(cheder). simultaneously The provide static a spatial door image loading of mechanism works on the prin- 3.2. Experimental Research of a Classical Metal Structure the analyzed object, similar to the human eye. cipleTheThe static metalof the door door nut–screw.loading was mounted mechanism on the When bodyworks (Figure on the the 17 principlemechanism). It is fastened of the innut–screw. hinges presses and Thus, rests on the power transducer, the door byonpanel rotating the sealing isthe applied gasketnut (5) (cheder).in Figure from The14, there staticthe is doorinside a horizontal loading of mechanismmovement the vehicle of works the rod onto of the thethe principle mech- outside (we call this experiment trac- anismof the (6), nut–screw. which, in When turn, the loads mechanism the door, presses through on the the force power transducer transducer, (8). the While door the panel force is transducerappliedtion), from and records the when inside the pressing of thethe vehicle ormechanism compression to the outside force (wepulls applied call this the to experiment the force door face,transducer, traction), the VIC and the door panel is applied from systemwhen the records mechanism the deformations pulls the force that transducer, occur on its the surface. door panel is applied from the outside tothe the insideoutside of the vehicleto the (we inside call this experimentof the vehicle compression). (we call this experiment compression). 3.2. Experimental Research of a Classical Metal Structure The metal door was mounted on the body (Figure 17). It is fastened in hinges and rests on the sealing gasket (cheder). The static door loading mechanism works on the principle of the nut–screw. When the mechanism presses on the power transducer, the door panel is applied from the inside of the vehicle to the outside (we call this experiment traction), and when the mechanism pulls the force transducer, the door panel is applied from the outside to the inside of the vehicle (we call this experiment compression).

FigureFigureFigure 17. 17. StainedStained 17. Stainedmetal metal door. door. metal door. ToTo begin begin with, we test the loadload actingacting onon thethe panel, panel, from from the the inside inside of of the the door door (vehicle) (vehicle)to the to the outside. outside. The The pressing pressing force force on theon doorthe door panel panel increased increased over over time, time, from from a minimum a min- imumto a maximum to Toa maximum begin of 549.249 of with, 549.249 N, then N,we decreased then test decreas the to 0ed N, loadto with 0 N, the withacting entire the entire cycle on lasting cyclethe lastingpanel, 82 s. The 82 from the inside of the door (vehi- s.variationcle) The variation to of the the of force outside. the overforce time over The is time shown ispressing shown in the in graph the force graph in Figure in on Figure 18. the 18. door panel increased over time, from a minimum to a maximum of 549.249 N, then decreased to 0 N, with the entire cycle lasting 82 s. The variation of the force over time is shown in the graph in Figure 18.

FigureFigure 18. 18. VariationVariation of of the the pressing pressing force, force, over over time, time, for for the the metal metal door. door.

TheThe deformation deformation of of the the metal metal door door surface surface was was monitored, monitored, by the by Digital the Digital Image Image Cor- relationCorrelation method, method, using using the VIC-3D the VIC-3D system. system. From From the points the points made, made, by painting by painting on the on door the panel,door panel,12 points 12 pointswere identified, were identiﬁed, arranged arranged as in Figure as in Figure19, which 19, the which Digital the DigitalImage Corre- Image lationCorrelation system, system, VIC-3D, VIC-3D, monitored. monitored. With the With help the of these help ofpoints, these the points, deformation the deformation mode of themode door of panel the door was paneldetermined, was determined, when pressed, when from pressed, the inside from of the the inside door to of the the outside. door to the outside.

Figure 18. Variation of the pressing force, over time, for the metal door.

The deformation of the metal door surface was monitored, by the Digital Image Cor- relation method, using the VIC-3D system. From the points made, by painting on the door panel, 12 points were identified, arranged as in Figure 19, which the Digital Image Corre- lation system, VIC-3D, monitored. With the help of these points, the deformation mode of the door panel was determined, when pressed, from the inside of the door to the outside.

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Figure 19. Chosen points for the study of compression displacement. Figure 19. Chosen points for the study of compression displacement. FigureUsing 19. Chosen the records points forfor thethe study considered of compression points in displacement. Figures 20–22 it was possible to graphically drawUsing the the vertical records section for the considered of the door points panel in Figures for the 20– 22initial it was force, possible for to an graphi- intermediate cally draw the vertical section of the door panel for the initial force, for an intermediate load loadUsing and forfor the the the records maximum maximum for load. the load. Theconsidered maximum The maximu points deformationm in deformation Figures of the door20–22 of panel itthe was was door possible 3.13 panel mm, to was graph- 3.13 icallymm, fordraw aa maximum maximum the vertical force force ofsection 549.24 of 549.24 N.of the N. door panel for the initial force, for an intermediate load and for the maximum load. The maximum deformation of the door panel was 3.13 mm, for a maximum force of 549.24 N.

Figure 20.Figure 3D deformation 20. 3D deformation at traction at traction and and compression compression of of the the metal metal door. door. Simulation Simulation using FEM. using FEM.

Figure 20. 3D deformation at traction and compression of the metal door. Simulation using FEM.

SymmetrySymmetrySymmetry 202120212021,, 1313,,13, xx , FORFOR 383 PEERPEER REVIEWREVIEW 16 of 21 1515 ofof 2020

FigureFigureFigure 21. 21. DeformationDeformation at atat traction tractiontraction obtained obtainedobtained by measurement. byby measurement.measurement.

FigureFigureFigure 22. 22. DeformationDeformation at atat compression compressioncompression obtained obtainedobtained by measurement. byby measurement.measurement.

The test was resumed, but this time the force of action acted from the outside of the TheThe testtest waswas resumed,resumed, butbut thisthis timetime thethe forcforcee ofof actionaction actedacted fromfrom thethe outsideoutside ofof thethe panel to the inside of the vehicle. The pulling force ranged from 0 N to 240,894 N and again panelpanelto 0 N, toto within thethe insideinside 88 s. The ofof recordingthethe vehicle.vehicle. of the TheThe variation pullpullinging of forceforce the force rangedranged in time fromfrom is illustrated 00 NN toto 240,894240,894 in the NN andand againagaingraph toto from 00 N,N, Figure withinwithin 18. 8888 The s.s. maximum TheThe recordingrecording deformation ofof ththee variation madevariation by the ofof doorthethe forceforce panel inin was timetime 5.93 isis mm, illustratedillustrated ininfor thethe a maximum graphgraph fromfrom force FigureFigure of 240.89 18.18. TheThe N. maximummaximum deformationdeformation mademade byby thethe doordoor panelpanel waswas 5.935.93 mm,mm, forfor aa maximummaximum forceforce ofof 240.89240.89 N.N. 3.3. Experimental Research of a Structure Made of Composite Materials 3.3.3.3. ExperimentalExperimentalFor the door Research madeResearch of composite ofof aa StruStructurecture materials, MadeMade theofof CompositeComposite same tests MaterialsMaterials were performed. The door was mounted on the body with the help of hinges and locks, as in the metal door, and was ForFor thethe doordoor mademade ofof composcompositeite materials,materials, thethe samesame teststests werewere performed.performed. TheThe doordoor supported on the sealing gasket (cheder). waswas mountedmountedIn the Ø8mm onon thethe hole, bodybody the loadingwithwith thethe rod helphelp of theofof hinghing staticeses door andand loader locks,locks, was asas inin mounted. thethe metalmetal The door,door, door andand waswas supportedsupportedsurface was onon deformed thethe sealingsealing to the gasketgasket outside (cheder).(cheder). and inside of the vehicle, and the deformation of the doorInIn surface thethe Ø8mmØ8mm was monitored, hole,hole, thethe by loadingloading the Digital rodrod Imageofof thethe Correlationstaticstatic doordoor method, loaderloader waswas using mounted.mounted. the VIC-3D TheThe doordoor surfacesurfacesystem (Figurewaswas deformeddeformed 23). toto thethe outsideoutside andand insideinside ofof thethe vehicle,vehicle, andand thethe deformationdeformation ofof thethe doordoor Thesurfacesurface door waswas panel monitored,monitored, was loaded byby from thethe inside DigitalDigital the ImageImage vehicle CorrelationCorrelation to the outside. method,method, The pressing usingusing force thethe VIC-3DVIC-3D systemsystemon the panel (Figure(Figure increased, 23).23). over time, from the minimum to the maximum value of 70.709 N, after which it decreased again to a minimum within 74 s. The variation of the force over time is given in the graph in Figure 24.

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Figure 23. Door surface monitoring with VIC-3D system.

The door panel was loaded from inside the vehicle to the outside. The pressing force on theFigure panel 23. Doorincreased, surface monitoring over time, with from VIC-3D th system.e minimum to the maximum value of 70.709 N, after which it decreased again to a minimum within 74 s. The variation of the force over The door panel was loaded from inside the vehicle to the outside. The pressing force timeon FigureisFigure thegiven panel 23. 23. Doorin increased, theDoor surface graph surface monitoring over in time, Figuremonitoring with from VIC-3D24. th e minimumwith system. VIC-3D to the system. maximum value of 70.709 N, after which it decreased again to a minimum within 74 s. The variation of the force over time is givenThe indoor the graphpanel in wasFigure loaded 24. from inside the vehicle to the outside. The pressing force on the panel increased, over time, from the minimum to the maximum value of 70.709 N, after which it decreased again to a minimum within 74 s. The variation of the force over time is given in the graph in Figure 24.

Figure Figure24.Figure Variation 24. 24. VariationVariation of the of of the thepressing pressing pressing and and and traction traction fo force, force,rce, over over over time, time, time, for for the the for composite composite the composite door. door. door.

TheThe deformation deformation of of the the door door surface, surface, made made of of composite composite materi materials,als, was was monitored, monitored, alsoThealso with withdeformation the the VIC-3D VIC-3D of system. system. the door Figure Figure surface, 25 25 shshowsows made the the deformations deformationsof composite for for materia a pressure pressureals, force forcewas of ofmonitored, also 3.45with3.45 N N theand and aVIC-3D pressure system. forceforce ofof 24.9524.95 Figure N, N, and and25 Figure Figureshows 26 26 showsthe shows deformations the the deformations deformations for for afor a pressure a pres- force of 3.45 sureNforce and force of a 52.74 ofpressure 52.74 N and N and forforce the for maximumofthe 24.95 maximum N, force and force of 70.71Figure of 70.71N. N. 26 shows the deformations for a pressure force of 52.74 N and for the maximum force of 70.71N. Figure 24. Variation of the pressing and traction force, over time, for the composite door.

The deformation of the door surface, made of composite materials, was monitored, also with the VIC-3D system. Figure 25 shows the deformations for a pressure force of 3.45 N and a pressure force of 24.95 N, and Figure 26 shows the deformations for a pressure force of 52.74 N and for the maximum force of 70.71N.

FigureFigure 25. 25. DeformationDeformation of of the the panel, panel, at at pressure, pressure, for for 3.45 3.45 N, N, respectively respectively the the force force of 24.95 N.

Figure 25. Deformation of the panel, at pressure, for 3.45 N, respectively the force of 24.95 N.

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Figure 26. Deformation, at pressure, for the force 52.74 N, respectively the force of 70.71 N. FigureFigure 26. 26. Deformation,Deformation, at at pressure, pressure, for for the the force force 52.74 52.74 N, N, respectively respectively the the force force of 70.71 N.

TheTheThe colored colored colored areas areas areas appearappear appear on on the thethe door doordoor panel panel panel depending depending depending on on theon the sizethe size sizeof of the the of deformation deformationthe deformation of oftheof the the panel, panel, panel, in in in the the the direction direction direction ofof of the the pressing pressingpressing force. force. force. The The The colors colors colors are are areaccording according according to to the the to legend: legend: the legend: purplepurplepurple for for for deformation deformation deformation 0 0 mm, mm, red red for forfor deformation deformation deformation of of of5 5 mm. 5 mm. InIn InFigure Figure Figure 27 2727 is isis a a a three-dimensionalthree-dimensional three-dimensional imag image image ofe of ofthe the the deformation deformation deformation area, area, area, made made made using using using FEM FEM FEM simulation. The maximum deformation made by the door panel was 3.24 mm, for a simulation.simulation. The The maximum maximum deformation made made by by the the door door panel panel was was 3.24 3.24mm, mm,for a max-for a maximummaximum force forceof 70.71 of 70.71N. For N. the For composite the composite door the door test the was test resumed, was resumed, the stress the stressacting acting from imum force of 70.71 N. For the composite door the test was resumed, the stress acting from thefrom outside the outside of the ofpanel the to panel the inside to the insideof the vehicle. of the vehicle. The pulling The pulling force ranged force rangedfrom 0 fromN to the45.830 outside N to N 45.83and of againthe N and panel to again 0 N,to towithinthe 0 N,inside 68 within seconds. of the 68 s. vehicle. The The colored colored The areaspulling areas that that force appear appear ranged on on the the from door door 0 N to 45.83panelpanel N dependand depend again on on the theto 0size size N, of ofwithin the the deformation deformation 68 seconds. of of theThe the panel, panel,colored in in the theareas direction direction that appear of of the the traction on the door panelforceforce depend (Figure (Figure 28).on28). theThe The sizecolors colors of are arethe according according deformation to to the the oflegend: legend: the panel, red red for for in deformation deformation the direction 0 0 mm, mm, of purplethe purple traction forceforfor (Figuredeformation deformation 28). Theof of 5 5 mm.colors mm. are according to the legend: red for deformation 0 mm, purple for deformationThe maximum of 5 mm. deformation of the door panel was 4.34 mm.

Figure 27. 3D deformation at traction and compression of the metal door. Simulation using FEM.

FigureFigure 27. 3D 27. deformation3D deformation at attraction traction and and compression compression ofof the the metal metal door. door. Simulation Simulation using using FEM. FEM.

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(a) (b)

Figure 28. TractionFigure panel 28. deformation Traction panel for deformation forces (a) 20.14 for forces N; (b )(a 102.44) 20.14 N;N; ((cb)) 157.76 102.44 N;N; ((dc)) 157.76 240.46 N; N. (d) 240.46 N. 4. Discussions Comparative Evaluation of Solution The simulationThe made maximum on the deformation 3D model shows of the thatdoor the panel use was of the 4.34 caisson mm. stiffens the surface of the door panel, ensuring a higher resistance of the panel. Table2 shows the load 4. Discussions Comparative Evaluation of Solution forces and the maximum deformations recorded during the tests, both for the metal door and for the compositeThe door. simulation made on the 3D model shows that the use of the caisson stiffens the surface of the door panel, ensuring a higher resistance of the panel. Table 2 shows the load Table 2. Load forcesforces and and maximum the maximum deformations. deformations recorded during the tests, both for the metal door and for the composite door. Compression Traction Table 2. Load forces and maximum deformations. Force [N] Deformation [mm] Force [N] Deformation [mm] Metal door 549.24 3.13Compression 240.89 5.93 Traction Composite door 70.71 3.24 3.24 4.34 Deformation Deformation Force [N] Force [N] [mm] [mm] Although the pressure forces from the inside of the door to the outside have higher values, the deformationsMetal in thisdoor direction are 549.24smaller than if the3.13 doors are required 240.89 from 5.93 the outside to the inside,Composite for both door types of door. 70.71 This is due to the 3.24 constructive 3.24 shape of the 4.34 door panel. Analyzing theAlthough data in this the table,pressure it is forces found from that the the insi deformationsde of the door of theto the door outside made have higher of composite materialsvalues, the have deformations values close in this to those direction recorded are smaller for the than metal if the door. doors The are same required from the outside to the inside, for both types of door. This is due to the constructive shape of cannot be said about the stresses of the two doors. For the deformation of the metal door, the door panel. in both cases, compression and traction, much greater forces were needed, of the order of Analyzing the data in this table, it is found that the deformations of the door made hundreds of Newtons. For the door made of composite materials, the loads were only in of composite materials have values close to those recorded for the metal door. The same the order of tens of Newtons, for the similar values of deformation. cannot be said about the stresses of the two doors. For the deformation of the metal door, 5. Conclusionsin both cases, compression and traction, much greater forces were needed, of the order of Composite materials have advantages that make them very suitable for applications in the automotive industry. The paper made a study of several types of materials that could be used in the automotive industry, presenting an application for the construction of a door.

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The behavior of this subassembly, made of composite materials, was studied, compared to a door made of metal. It was found that composite materials will have different properties depending on the constituents that make them up and their performance is influenced by the environmental conditions in which they are forced to work. The studied materials were composite made of polyester resin, reinforced with fiberglass s fabric, with a different number of layers, composites of polyester resin reinforced with fiberglass mat, and composites reinforced with carbon fibers. As the increase in temperature leads to a decrease in the rigidity of this material, this type of material cannot be used to make structural elements in the vicinity of heat engines, flue gas systems, or other areas of vehicles exposed to high temperatures. The presence of coremat in the composition of the composite material reinforced with glass fibers reduces its performance. Such a material can be used successfully for ornamental landmarks, or without an important role in the strength structure of vehicles. The presence of coremat in the composition of the resistance structures would only decrease their quality. The weight of the metal door, not equipped (without hinges, closing mechanism, anti-fog bar, window and window actuation mechanism) is 17.5 kg, the door made of composite materials, under the same conditions has a mass of 5 kg. The structures made of composite materials must be rigid enough to prevent the phenomenon of cracking of the material matrix. The composite material made of polyester resin, reinforced with glass fibers withstands a considerable load if it is in normal temperature conditions. The increase in temperature leads to a decrease in the rigidity of this material. For this reason, structural elements in the vicinity of heat engines, flue gas systems or other areas of vehicles exposed to high temperatures cannot be made from this type of material. For the manufacture of doors, however, the use of this material proves to be very convenient. The composite material made of epoxy resin reinforced with carbon fiber fabric supports the highest load, but is very rigid. The high price still keeps it away from many mass-produced car parts. The idea of making small closed structures, such as caisson, must be further developed and exploited. Composite materials allow the realization of such structures, compared to metals, where such an approach would be expensive.

Author Contributions: Conceptualization, V.G. and M.L.S.; methodology, V.G. and V.B.U.; software, V.G., M.L.S., and E.C.; validation, V.G., M.L.S., and M.U.; formal analysis, V.G., V.B.U., and M.L.S.; investigation, V.G.; resources, V.G., M.L.S., E.C., and M.U.; data curation, V.G., V.B.U., and M.L.S.; writing—original draft preparation, V.G. and M.L.S.; writing—review and editing, M.L.S., E.C., and M.U.; visualization, M.L.S. and E.C.; supervision, M.L.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: We want to thank the reviewers who have read the manuscript carefully and have proposed pertinent corrections that have led to an improvement in our manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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