用于车辆前保险杠梁的替代轻质材料和部件制造技术【中文10100字】
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用于车辆前保险杠梁的替代轻质材料和部件制造技术【中文10100字】,中文10100字
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Contents lists available at ScienceDirectComposite Structuresjournal homepage: /locate/compstruct Composite Structures 120 (2015) 483495Alternative lightweight materials and component manufacturing technologies for vehicle frontal bumper beamG. Belingardi a, A.T. Beyene a, E.G. Koricho b, B. Martorana ca Politecnico di Torino, Department of Mechanical and Aerospace Engineering, Italyb Michigan State University, Composite Vehicle Research Center, USAc Centro Ricerche FIAT, strada Torino 50, Orbassano Torino, Italya r t i c l e i n f o Article history:Available online 25 October 2014Keywords: Bumper CrashworthinessLightweight design Composite structures Optimizationa b s t r a c t One of the vehicle subsystem where large advantage is expected in lightweight design is the bumper sub- systems. Bumper subsystems are designed to prevent or reduce physical damage to the front or rear ends of passenger motor vehicles during collusion.In this paper, detail design aspects and method of analysis with particular reference to the application of composite materials to automotive front bumper subsystem, crash box and bumper beam. Innovative design of integrated crash box and bumper beam has been considered for better crashworthiness; the proposed solution results to be of great interest also from the points of view of subassembly cost and effective production process.Three materials have been characterized under quasi static and impact tests for this bumper beam application: GMT, GMTex, and GMT-UD. Major parameters, such as impact energy, peak load, crash resis- tance, energy absorption and stiffness have been taken as evaluation criteria to compare the proposed materials solutions with pultruded and steel solutions. Finally, the results predicted by the nite element analysis have been evaluated and interpreted in comparison with other existing solutions to put in evidence the effectiveness of the proposed innovative materials and design concept solutions.。 2014 Elsevier Ltd. All rights reserved.1. IntroductionAutomobile bumper subsystem is the frontal and rear structure of the vehicle that has the purpose of energy absorption during low velocity impact. Usually, bumper subsystem consists of bumper transverse beam, stays, impact-absorbing materials (such as foam or honeycomb) connected to the structural components (generally the bumper beam) and a cover, that has both aesthetic and protec- tion purposes. Among those elements, the bumper beam is the main structural component; it is expected to be deformable enough to absorb the impact energy, in order to reduce the risks of injury for pedestrians and other vulnerable road users, but, at the same time, it should also have sufcient strength and stiffness to give place to small intrusion of the engine compartment and, therefore, to protect the nearby vehicle components.Composite materials are characterized by high specic strength, both in static and impact loading conditions, and high specic stiff- ness; they could be an interesting candidate material for this type of component, posing as targets the lightweight together with the Corresponding author.E-mail address: koricho (E.G. Koricho).maintenance of at least the same level of safety performance in comparison with the present steel solution.When designing with composite material, it is always needed not only to choice the appropriate material but to think composite (i.e. to not simply replace the metallic material with the new one, but to redesign the part) and to select the type of production tech- nology that will be used in manufacturing, as this choice will affect deeply both the structural performance and the cost and the pro- duction rate 1. Therefore material, design and manufacturing technology are strictly linked each other and should be considered all together.From the point of view of manufacturing technology we have taken into consideration two different types: pultrusion and die forming. Both of them are cost-effective and fully automated and give high quality parts in terms of geometry accuracy and degree of consistency of mechanical property (mainly due to process automation).Pultrusion has a number of advantages such as perfect ber alignment and high ber volume since polymerization takes place while the ber is under tension, capable of producing both closed and open section with a variety of end proles, etc. However, at the moment the technology is strongly limited to straight and/10.1016/pstruct.2014.10.007 0263-8223/。 2014 Elsevier Ltd. All rights reserved.484G. Belingardi et al. / Composite Structures 120 (2015) 483495constant section. Conversely, die forming composite manufactur- ing technology has also its own advantages, i.e. it allows producing structurally integrated crash box and beam, as shown in Fig. 1, that improve both manufacturing and assembling rate and eliminate connection between bumper beam and crash box. However, it is mainly limited to open section proles that are generally less per- forming than the closed section proles.As the targeted component is designed for impact loading, prior to conduct numerical impact analysis at the component level, the impact performance of composite material are assessed. In general, impact responses and damage mechanisms for the whole group of composite materials are more complex comparing with the con- ventional metallic materials and depend on a number of different parameters: ber and matrix type, section shape and dimensions, impact velocity, impact angle, shape of striker, target geometry and target material. Open literatures show that a composite tube is capable of absorbing signicant impact energy by material frag- mentation and large changes in the tubes cross-sectional geometry when the tube undergoes large exural deformation 27.In the current study six material were considered. For pultruded bumper beam solution, unidirectional pultruded E Glass/epoxy, a bidirectional fabric E Glass/epoxy and steel material were com- pared. The detailed mechanical properties documented 8. For the case of die formed integrated crash boxbeam solution, three materials were considered: A classic glass-mat-reinforced thermoplastics (GMT) i.e. an end- less ber glass mate reinforced PP with randomly oriented glass bers, GMTex, i.e. a chopped ber glass mat reinforced PP laminatewith randomly oriented glass bers and additionally reinforced with a fabric inside and GMT-UD, i.e. a chopped ber glass mat reinforced PP laminatewith randomly oriented glass bers and additionally reinforced with unidirectional oriented glass ber layers.These three materials, supplied by Quadrant, were considered for front bumper application. Considering the novelty of the modied material, extensive material characterization had been conducted to obtain the main mechanical properties of the mate- rial and to understand the failure mechanism for the intended loading case and nally their capability for substituting the current steel material were numerically assessed.2. Material characterizationThe composite materials were characterized under a tensile (both longitudinal and transverse direction), compressive (both longitudinal and transverse direction), and a drop-dart tests. A brief summary of the test set-up and of the obtained mechanicalcharacteristics of the tested materials are presented in the follow- ing sections.2.1. Experimental setup for tensile testFive specimens for each material type, in both longitudinal and transverse directions, were tested under tensile loading with a 100 kN capacity servo-hydraulic testing machine (INSTRON- 8801), as shown in Fig. 2. Each specimen was clamped by means of hydraulic wedge grips. The machine was equipped with a stan- dard load cell and a crosshead displacement measuring device.During the mount phase of the specimen, the maximum preload was controlled and set lower than 0.2 kN in order to avoid specimen damage. According to ASTM D3039, specimens were subjected to monotonic tensile loading with a stroke rate of 2 mm/min. The specimens were instrumented by strain gages to measure Youngs modulus and Poissons ratio. To acquire the strain gages data, a NI WLS-9163 data acquisition board was used and to acquire load and crosshead displacement data from the machine, a NI DAQCard-6062E was utilized. All data were acquired with a sampling rate equal to 1 kHz. The main mechanical properties are reported in Table 1.2.2. Experimental setup for compression testSimilarly, ve specimens for each material, in both longitudinal and transverse directions, were tested under compressive loading as per ASTM D6641/D6641M and the found experimental results are presented in Table 2.2.3. Experimental setup for the drop dart testPrior to impact test, quasi-static indentation tests were performed on Zweck Roell 100 universal testing machine, to inves- tigate perforation energy of the proposed composite laminates, Fig. 3. Main results are reported in Table 3.Experimental impact tests were performed according to ASTM standard 3029 using an instrumented free-fall drop dart testing machine. The impactor has a carriage mass of 5.735 kg and an hemispherical head with a radius of 10 mm and the maximum falling height of the testing machine is 2 m (see Fig. 4). The drop- weight apparatus was equipped with a motorized lifting track. The collected data were stored after each impact and the impactor was returned to its original starting height. Using this technique, the chosen impact velocity was consistently obtained in successive impacts. By means of a piezoelectric load cell, forcetime curves were acquired and, with a double integration of accelerationtime curve, forcedisplacement curves were obtained. Square specimen panels, with 100 mm edge, were clamped in the specimen holder with a 76.2 mm inner diameter, and xed to a rigid base to preventFig. 1. Integrated composite solution developed by Quadrant Plastic Composites International (a) and used on Mercedes for top class vehicle (b).G. Belingardi et al. / Composite Structures 120 (2015) 483495485Fig. 2. Experimental setup for tensile and compression material characterization.Table 1Tensile property of the material considered.PropertiesGMTGMT UDGMTexQuasi-isotropicLongitudinalTransverseLongitudinalTransverseTensile strength (MPa)80.7180.459.2174.7170.6Modulus (GPa)5.8111.076.489.246.00Poissons ratio0.2840.3070.2170.3890.150Table 2Compressive property of the material considered.PropertiesGMTGMT UDGMTexQuasi-isotropicLongitudinalTransverseLongitudinalTransverseCompression strength (MPa)65.882.258.569.057.1Modulus (GPa)3.265.932.644.142.66Poissons ratio0.3380.4300.1780.2370.168Fig. 3. Quasi-static indentation test.slippage of the specimen. The clamping system was designed to provide an adequate uniform pressure all over the clamping area. Having got the energy required for perforation of the three materials, the response of the materials for dynamic loading werecompared by conducting a dynamic impact test at equal energy level, i.e. 65% of GMTex quasi-static indentation energy. Forcetime curve (data) and the actual initial impact velocity were acquired directly from the test machine through Lab VIEW Signal486G. Belingardi et al. / Composite Structures 120 (2015) 483495Table 3Energy for quasi-static perforation test.MaterialPlate thickness mmEnergy JGMT438.9GMTex331.1GMT-UD441.2Fig. 4. Drop dart testing machine and specimen conguration.express environment developed for this particular test scenario and the other important dynamic variables were calculated using free body motion equation 12 as described in Fig. 5.2.4. Impact response of composite materialRepresentative curves of Force vs. time, Energy vs. time and pic- tures of the damage mode, respectively for the rst and perforation impacts, are presented in Figs. 6 and 8. The number of impacts needed to perforate the plate and the damage development through the successive impacts was monitored by impacting a vir- gin specimen every time to the desired number of impacts. Figs. 7 and 9 are presenting pictures of the impacted surfaces of theFig. 5. Drop test motion description.specimen according to the specied number of impacts, respec- tively for the GMT and GMT-UD materials. A smooth Force vs. time and the relatively lower absorbed energy of GMT-UD at the rst impact can be linked to the observation that the GMT-UD plate has no visible damage, as shown in Fig. 6e. This implies that most of the energy was dissipated due to mechanisms other than mate- rial internal fracture. Whereas, Force vs. time curve of both GMT and GMTex shows an apex that can be interpreted as a sign of frac- ture and this can be linked to the visible damages that can be observed on the impacted plate shown in Fig. 6c and d.Fig. 8 shows some results at the perforation after repeated impacts. We can observe that at perforation, GMT and GMTex have similar failure behavior i.e. the impactor penetrates the plate dam- aging a conned area around the perforation hole but without extensive crack propagation into the plate (see Fig. 8c and d). This is due to the fact that classical GMT has in plane quasi-isotropic properties, i.e. it has almost uniform continues bers in all direc- tion, and this prevented the cracks at the edge of the perforation central hole from being propagated. Similarly, GMTex has a fabric ply at the midplane of classical GMT, therefore the crack propaga- tion has been impeded by the joint effects of the random and fabric bers perpendicular to the crack. Conversely, in the case of GMT- UD, being the classical GMT reinforced by unidirectional bers, there was a chance for a crack originated at from the edge of the perforation central hole to propagate along the ber direction (see Fig. 8e).It is well known that, composite material has poor plastic properties, therefore, when energy absorbing components, like bumper beam, are designed using materials of this type, the energy dissipation can mainly take place through the material fracturation. Therefore the more the component material is frag- mented the larger amount of energy is dissipated. In this respect, Energy vs. time curve and the damage mode of GMT-UD at perfo- ration impact show a better fracture behavior i.e. in addition to the impactor penetration through the plate thickness, crack prop- agate along the plate width which increases the amount of energy absorption.3. Bumper beam designAs indicated in the introductive paragraph, two different types of manufacturing technology, namely pultrusion and die forming, have been considered to manufacture the beam with the desired shape. Both of them are cost-effective and fully automated manu- facturing technologies and give high quality parts in terms of geometry accuracy and degree of consistency of mechanical prop- erty. Pultrusion manufacturing solution is very effective to make a great variety of end proles but is currently limited to straight beams, moreover is not suitable to develop integrated crash box beam solutions. Die forming manufacturing technology is suitable to develop integrated crash boxbeam solution and with no limita- tion on the beam curvature, but it is limited to open section pro- les, that are structurally weaker than the closed section proles, and has limitations on the shape of the section proles.Fig. 10, shows a simplied model of pultruded bumper beam (a) and die formed GMT/GMTex integrated cashbox-beam bumper beam (b) considered for a nonlinear nite element simulation using commercial code ABAQUS/Explicit version 6.12-1. In both cases, the rigid bodies were modeled as discrete rigid surfaces in order to create higher mesh density at critical contact areas. A mass of 1000 kg was rigidly coupled with the two rear extremities of the crash boxes, in order to simulate the vehicle mass. Boundary condition was also applied on vehicle mass point whose initial velocities towards the rigid wall were set in turn equal to 4, 8 and 15 km/h, in order to simulate three different impact situations relevant for the bumper system.G. Belingardi et al. / Composite Structures 120 (2015) 483495487Fig. 6. Force vs. time (a), energy vs. time (b) and damage at the rst impact (ce).3.1. E-Glass/epoxy pultruded beam solutionPultruded beam solution have been studied by a number of scholars 10,11, for roadside barrier structures which is a similar to lateral loading case as in vehicle bumper. Roadside barrier are usually designed to shield motorists from man-made or natural hazards, to redirect errant vehicles back on to roadway and for energy dissipation in case crashing. These studies indicated thatpultruded composite materials are viable for use in guardrail sys- tem due to their pseudo-ductile characteristics that arise primarily from material fragmentation (crushing, separation and tearing of composite materials) and large changes in the tubes cross- sectional geometry when the tube undergoes large exural deformation.The proposed pultruded bumper beam solution 8 intended to utilize and optimize the pseudo-ductile behavior of pultruded488G. Belingardi et al. / Composite Structures 120 (2015) 483495Fig. 7. Damage development through the successive impact for GMT and GMTposite beam for effective energy dissipation at low velocity vehicle frontal crash. The pseudo-ductile behavior was optimized through a structural optimization procedure of the beam section prole (that can be easily obtained by means of a properly shaped die section) and of the curvature (that at present is not offered by main manufacturers with this technology) aimed to obtain a pro- gressive energy absorption and a stable exural failure of the com- posite bumper beam.A numerical study has been conducted according to the meth- odology developed in 8,9 in order to explore the possibility of substituting the current metallic bumper beam with E-Glass/epoxy pultruded composite beam. The resulting structures are compared in terms of shape and in terms of energy absorbing capability, com- parison is also established with steel normal production solution. The pseudo-ductile behavior of pultruded beams arise from material fragmentation (crushing, separation and tearing of com- posite materials) and large changes in the tubes cross-sectional geometry when the tube undergoes large exural deformation. Therefore, the analysis has been conducted based on the hypothe- sis that a properly optimized and predened stress concentration zone i.e. beam longitudinal groves (through an optimization process of the end prole shape) can serve as crash triggering mechanism, i.e. to initiate cracks formation and to develop pro- gressive tear along beam longitudinal axis. The optimization has been conducted using as design variables the number of groves on the height (h) of the beam end prole and the distribution of the wall thickness, taking advantage from the capability of the pultrusion technology to produce such a particular prole. The per- formance comparison among the proposed end proles was done through the investigation of impact event characteristic data, such as forcetime, forcedisplacement, energydisplacement and displacementtime curves. The optimized beam section prole isbeam and crash box structure thus eliminating the need of joints in between.A nonlinear nite element simulation, with a simplied bumper beam model, as shown in Fig. 10b, has been carried out using the commercial code ABAQUS/Explicit version 6.12-1. The model com- prises two parts, one rigid part, i.e. the impact rigid wall, and one deformable part that integrates crash-boxes and transverse beam and its back cover. The integrated beam solution has been developed in three alternatives according to the three considered composite materials, GMT, GMtx and GMT-UD. A mass of 1000 kg is rigidly attached at the two rear extremities of the crash boxes, in order to simulate the vehicle mass, it moves with an ini- tial velocity of 4 or 8 km/h towards the rigid wall. Considering the load path, different sections have been used at different portions of the proposed structure as shown in Fig. 10b. Hollow tapered trun- cated square based pyramids were proposed for crash boxes, in order to obtain a progressive failure.The design solutions that are using the proposed materials were developed starting from the normal production solution (that is the reference solution) made by steel by means of two approaches:- by direct substitution of the current steel beam, through inte- gration of the composite beam with crash boxes with minormodications to the base plate only for joining purpose, using the wall thickness recommended by the company i.e. 臼8 mm,and- through equal bending stiffness approach 9, i.e. for a given thickness and stiffness of the reference material, the thickness to be adopted with the targeted material can be approximately calculated by Eq. (1).s3 Espresented on Fig. 11.Even if the current pultruded manufacturing technology ishx hsEx1mainly limited to straight beam (curved pultrusion technology is still in infant stage) an optimization has also been conducted on beam curvature radius (R) Fig. 12. A large number of beam curva- ture radius, from straight axis to smaller radius, were considered. The failure phenomenon and the bumper beam performance were closely monitored using the already mentioned impact event characteristic data.3.2. GMT/GMTex die forming integrated crash boxbeam solutionDie forming manufacturing technology is capable of producing structurally integrated crash box and beam as a single component. This is an extremely interesting feature of this technology because it leads to remarkable improvements both from the point of view of the manufacturing/assembly rate and from the point of view of a relevant reduction of the number of different components that should be produced and assembled to construct the front end structure. Besides, since joining is one of the critical issue in using composite part in automotive structures (as structures often have their weak points where their parts are joined together), The die forming technology is suitable for producing an integrated bumperwhere hs and hx are respectively the wall thickness of steel and of the targeted material solutions and Es and Ex are the elastic modulus of steel and the targeted material respectively.During low velocity impact, such as small parking load, the bumper beam is expected only to bump i.e. it has to operate within elastic limit without any form of permanent damage. Therefore, for the current study, the allowable minimum thickness of the bumper for such small load was determined through monitoring impact energy curve. Having got the threshold value the thickness, it was gradually increased up to a value where the beam gives a sim- ilar impact performance as with the reference material. Finally the mass reduction has been evaluated.3.2.1. Design consideration for the integrated bumper beamWhen metallic components are substituted by composite com- ponents, taking into account the very different failure modes of the two materials, new design hypothesis has to be followed, only in this way the advantage that comes from the important features of the new material can be maximized. Therefore, in the current integrated bumper system the following three design consider- ations were made:G. Belingardi et al. / Composite Structures 120 (2015) 483495489Fig. 8. Force vs. time (a), energy vs. time (b) and damage during perforation (ce). In traditional bumper system, the structural integrity betweenthe bumper beam and crash box is obtained by mechanical fas- tening (welding or bolting) of the crash box to the beam. The failure mode of axially loaded composite pyramidal tube isinitiated by front end triggering mechanism and progressively is propagated along the length of the tube, the adoption of the traditional connection scheme for composite bumper system assembly will result in an early beam and crash box detachmentG. Belingardi et al. / Composite Structures 120 (2015) 483495490Fig. 9. Damage development through the successive impact for GMT-UD.Fig. 10. Simplied FEM bumper models:(a) pultruded beam solution, (b) die forming integrated beamcrash box solution.Fig. 11. Optimized beam end section prole for pultruded solution 8.and, therefore, will not meet the intended energy absorbing goal. A new design approach has to be followed for this group of materials. As previously pointed out, with a proper design of integrated bumper system the problem can be soundly addressed. The proposed solution has a free frontal crash box end with the required crash trigger. While the structural integ- rity can be obtained through optimization of the trickiness of the connecting rim. In particular the rim thickness has to be optimized to withstand the shearing load resulting from frontal impact. In case of small low velocity impact only the bumper beamshould be involved and should behave fully elastic, without the direct involvement of the crash boxes. Therefore, the clear- ance C between the front of the beam and the front of the crash box need to be optimized. Energy absorption for the crashed object is the area under forcedisplacement curve, that is proportional to the product of the force by the crush length. Hence, the crush length L is anFig. 12. Bumper beam Curvature considered for optimization.Fig. 13. Point considered for integrated beamcrash box design.G. Belingardi et al. / Composite Structures 120 (2015) 483495491important parameter for crash component design. During inte- grated bumper system design, improper placing of connecting rim will affect the crash length and affect the energy absorption of the system by blocking and controlling the progressive failure of the crash box. Thus, the relative position of the crash box and bumper beam rim has to be optimized.Having in mind the above stated design considerations, a sim- plied integrated bumper beam model, as shown in Fig. 13, was modeled in CATIA 5 and mesh renement was conducted in ABAQUS. The model comprises only two parts: a deformable inte- grated bumper system and the rigid wall. The rigid bodies were modeled as analytical rigid surfaces.4. Results and discussion4.1. Pultruded bumper beam solutionThe beam end prole has been optimized through the number of groves on the height (h) of the beam and the distribution the wall thickness. The detailed optimization process has been reported in 8. When the bumper beam is subjected to frontal impact, concentrated stresses develop at the grove vertexes; points on the fold sides at equal distance from the impacted surface have the same stress levels. This is substantially uniform in case of straight beam while a change in the beam curvature has an effect both on the stress distribution along the beam and on the stress values.Fig. 14 is showing the nal deformed shape of the bumper beam for three different solutions characterized by different values of the curvature radius from 2400 mm (case a) to straight beam (case c). Fig. 14d shows the reaction force histories for those three solu- tions. It is well visible that the case of the small curvature radius is generating a concentrated failure hinge close to the beam mid- span and a very large load peak comes out; the other two cases are giving more diffuse energy absorption and smoother curves; the solution with the intermediate values of the curvature radius is giving the minimum load peak. As a rst general observation on low velocity impact analysis, when the beam curvature radius is increased, the formation of local stress concentration is reduced. This is due to the fact that larger zones of the bumper beam are in contact with the at rigid wall at the same time. This leads to higher load peak that promote the formation of diffuse fractures on the portions of the folds which have the same stress level.The worst case is when the bumper beam is straight Fig. 14c, which corresponds to a solution currently used by some vehicles. In this situation the portion of the beam extremities just in front of the crash box, with length equal to the crash box width will frac- ture at the same time, since that portion of the beam is under equal stress level and there is not possibility for crack propagation and proper energy absorption.On the other hand, when the beam curvature radius is reduced below some critical curvature radius, 2862 mm in this particularcase, crack propagation is not taking place, but instead a high local stress line is developed at the apical portion of the beam, which results in unstable localized failure, as shown Fig. 14a.Finally, the performance of the proposed pultruded composite bumper beam solution can be compared with the steel and the glass fabric/epoxy composite solutions in terms of impact energy absorption and weight reduction. Three parameters, namely the amount of absorbed energy, the peak load value and failure mode, are considered for material comparison.A shown in Fig. 15, the three design solutions absorbed the same amount of energy, however the peak load values and the mode of failure are completely different. During vehicle frontal crash, peak load is relevant for the vehicle occupant risk, as a mat- ter of fact lower peak load yields to lower decelerations and vice versa, so this parameter should be carefully controlled. In addi- tions, by comparison of the failure modes of the two composite material solutions, i.e. pultruded and fabric, it comes out that the energydisplacement curve of pultruded beam is almost linear and the load deection curve of pultruded beam resembles the uni- axial stressstrain diagram of an elasto-plastic ductile material, that is technically termed as pseudo-ductile. Therefore, as far as it is possible to control the displacement or to keep the displace- ment within the design limits, the pseudo-ductile behavior of the pultruded solution is an important feature in the passive safety behavior of the bumper component.4.2. Die forming integrated bumper beamcrash box solutionFrom our previous related activity, it has been learned that a closed section beam has better structural integrity and energy absorbing capacity than an open section beam. Hence, even if an open section beam was considered and recommended by the material supplier company, for sake of production feasibility and simplicity, a closed section beam has also been numerically investigated.The rst attempt was conducted by direct substitution of the current steel beam with integration of the crash boxes and with minor modications on the base plate only for joining purpose. As recommended by the material supplier, the composite beamwall thickness for each material conguration was set 臼8 mm. Asit can be seen on force vs. time and force vs. displacement curves resulting from the simulation of impact events a 4 km/h (see Fig. 16), all the three material solutions are structurally weak. GMT-UD and GMtx solutions show an early sharp break at the cen- ter of the beam while GMT solution shows relatively higher elastic deformation.The design changes were made by increasing the section dimensions, particularly the base plate. The wall thickness of the integrated bumper beamcrash-box was determined on the basis of the data of the reference material solution. For a given wall thickness and stiffness of the reference material (steel), the thick- ness of the targeted material can be calculated approximately with Eq. (1). The obtained approximated wall thickness and the mass ofFig. 14. Failure mode and peak load for curvature radius (a) 2400 mm, (b) 3200 mm and (c) straight 8.G. Belingardi et al. / Composite Structures 120 (2015) 483495492Fig. 15. Reaction force vs. displacement and energy vs. displacement for the three materials considered 8.Fig. 16. Force vs. time and force vs. displacement curves for the design solutions in steel and in equal thickness of the three considered materials.the integrated bumper beamcrash box solutions are reported in Table 4.As presented in Table 1, GMT-UD has tensile longitudinal mod- ulus approximately 50% and 25% higher than GMT and GMtx, respectively. therefore it has better mechanical performance. Furthermore, as it was explained in the previous Section 2.4, the introduction of unidirectional bers in the classical GMT, makes the material to crack along the width of the plate, which is improv- ing the energy absorption capability. This failure behavior is also observed on energy vs. displacement curves of dynamic drop dart test.Through the comparison of force vs. time and displacement vs. time curves (see Fig. 17) related to the targeted four materials, it comes out that GMT-UD solution has the minimum peak load,i.e. 25 kN, this is one of the important parameters that the designer has to control, and has a failure mode similar to the reference material solution but has the maximum intrusion, i.e. 37 mm, the beam results to be fractured at the selected loading. This results is not acceptable since it was stated that for the 4 km/h impact cases the bumper beam should remain in the elastic region behav- ior without any structural damage.The failure behavior can also be tracked using load displace- ment curves and energy time history curves as shown respectively in Figs. 1820. Fig. 20a and b show that the GMTex and GMT solu- tions after having transformed the whole kinetic energy into inter- nal deformation energy, are able to restitute most of this energy and an elastic rebound takes place. A completely different situation is visible in Fig. 19b, for the GMT-D solution the restitution phase is missing and this is due to the beam crack.Table 4MaterialSteelGMTGMtxGMT-UDThickness mm5.8Thickness and mass of the combined beamcrash-box.Mass kg7.673.723.323.2Both load and energydisplacement curves of GMT-UD solution (which shows the amount of energy absorption and the behavior of the impacted system during energy dissipation) conrm that the material is already fractured at the selected velocity. Similar phe- nomenon is also observed on the reference material (steel), which might be due to the strength of the selected steel. As metallic materials have a higher plastic range, the energy curves of steel show the energy dissipation through plastic deformation. Whereas, composite materials have very limited plastic range, therefore, energy curve shows that GMT-UD has already passed its elastic limit and, as a consequence, the energy dissipation resulted from the material fragmentation.On the contrary, both GMT and GMtx solutions remain within elastic range, this can be observed from load vs. displacement and energy timehistory curves in Figs. 18 and 20. Failure mode are shown in Fig. 21. For 4 km/h impact velocity, which is close to parking load, the bumper has to operate within elastic range, therefore besides bumping and, eventually, a minor cosmetic dam- age, a complete fracture, as we observed on GMT-UD solution, is not acceptable. Therefore, with the proposed beam conguration and for the considered loading conditions (i.e. at low velocity impact) the wall thickness obtained from Eq. (6) i.e. using equal stiffness approach, leads to acceptable results only for GMT and GMtx and can be considered for material replacement with signif- icant weight saving but for GMT-UD the beam is fractured and therefore the wall thickness asks for further changes.The above proposed solutions have been also cheeked at higher impact velocity, i.e. at 8 km/h. From force vs. time curve of Fig. 22, it can be observed that there are two modes of deformation at this higher velocity, after the initial phase of elastic deformation up to臼2540 kN, which is a behavior similar that observed at lowervelocity impact (at 4 km/h), a complete fracture of GMT and GMtx beams at their central part comes out. Finally the other portions of the fractured beam impact against the rigid wall but without fur- ther material fracture. This phenomenon yields relatively higher peak loads i.e. 臼265 kN, and 300 kN for GMT and GMtx solutionsG. Belingardi et al. / Composite Structures 120 (2015) 483495493Fig. 17. Force vs. time and displacement vs. time curve for the modied bumper system.Fig. 18. Energy vs. displacement and force vs. displacement curves for the modied bumper system in case of impact at 4 km/h.Fig. 19. Energy curve for (a) steel and (b) GMT-UD solutions in case of impact at 4 km/h.Fig. 20. Energy curve for (a) GMtx and (b) GMT solutions in case of impact at 4 km/h.respectively. Whereas, GMT-UD solution shows symptom of further fractures during the second phase of the impact which are denitely due to fractures along the width of the plate. Thisresults in relatively smaller peak load i.e. 臼170 kN. The failurebehaviors of the two solutions based on GMT-UD and GMTex can be observed in Fig. 23.In general, the modication made on classical GMT is indeed improving both the tensile and impact performance of the original material and can be used for structural purposes in someapplication in place of steel and aluminum. However, coming to energy absorbing components, having the composite materials a completely different failure behavior than the conventional metal- lic materials, the energy absorbing performance is strongly affected by the geometry of the component. The direct adoption of the traditional metallic energy absorbing geometry may lead to a catastrophic failure and yield higher peak loads. As it has been pointed out in the previous study 8 in the case of transversally loaded energy absorbing composite components, like automotive494G. Belingardi et al. / Composite Structures 120 (2015) 483495Fig. 22. Force vs. time and displacement vs. time curves for the modied bumper system in case of impact at 8 km/h.Fig. 23. Energy vs. time curve and failure mode of GMT-UD (a) and GMtx (b) solutions in case of impact at 8 km/h.Fig. 21. Failure mode of the modied bumper system in case of impact at 4 km/h.bumper beam, a progressive mode of failure can be obtained through properly optimized beam end prole in such a way that beam corners can serve as stress concentration zone or crack trig- gering point. In this way cracks can initiate and progressively prop- agate along beam longitudinal axis. Whereas in the cases of the three considered materials for Die forming integrated bumper beamcrash box solution, after proper thickness and cross section optimization, GMT-UD, that has the better performance in terms mechanical property and fracture behavior, leads to interesting solution for
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