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Alternative lightweight materials and component manufacturing technologies for vehicle frontal bumper beam G. Belingardi a, A.T. Beyenea, E.G. Korichob, B. Martoranac aPolitecnico di Torino, Department of Mechanical and Aerospace Engineering, Italy bMichigan State University, Composite Vehicle Research Center, USA cCentro Ricerche FIAT, strada Torino 50, Orbassano Torino, Italy a r t i c l ei n f o Article history: Available online 25 October 2014 Keywords: Bumper Crashworthiness Lightweight design Composite structures Optimization a 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 fi 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. Introduction Automobile 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 suffi cient 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 specifi c strength, both in static and impact loading conditions, and high specifi c stiff- ness; they could be an interesting candidate material for this type of component, posing as targets the lightweight together with the 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 fi ber alignment and high fi ber volume since polymerization takes place while the fi ber is under tension, capable of producing both closed and open section with a variety of end profi les, 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. Corresponding author. E-mail address: (E.G. Koricho). Composite Structures 120 (2015) 483495 Contents lists available at ScienceDirect Composite Structures journal homepage: /locate/compstruct constant 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 profi les that are generally less per- forming than the closed section profi les. 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: fi 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 signifi cant impact energy by material frag- mentation and large changes in the tubes cross-sectional geometry when the tube undergoes large fl 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 fi ber glass mate reinforced PP with randomly oriented glass fi bers, ? GMTex, i.e. a chopped fi ber glass mat reinforced PP laminate with randomly oriented glass fi bers and additionally reinforced with a fabric inside and ? GMT-UD, i.e. a chopped fi ber glass mat reinforced PP laminate with randomly oriented glass fi bers and additionally reinforced with unidirectional oriented glass fi ber layers. These three materials, supplied by Quadrant, were considered for front bumper application. Considering the novelty of the modifi ed 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 fi nally their capability for substituting the current steel material were numerically assessed. 2. Material characterization The 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 mechanical characteristics of the tested materials are presented in the follow- ing sections. 2.1. Experimental setup for tensile test Five specimens for each material type, in both longitudinal and transverse directions, were tested under tensile loading with a 100 kNcapacityservo-hydraulictestingmachine(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 test Similarly, fi 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 test Priortoimpacttest,quasi-staticindentationtestswere 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 fi xed to a rigid base to prevent Fig. 1. Integrated composite solution developed by Quadrant Plastic Composites International (a) and used on Mercedes for top class vehicle (b). 484G. Belingardi et al./Composite Structures 120 (2015) 483495 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 were compared by conducting a dynamic impact test at equal energy level,i.e.65%ofGMTexquasi-staticindentationenergy. Forcetime curve (data) and the actual initial impact velocity were acquired directly from the test machine through Lab VIEW Signal Fig. 2. Experimental setup for tensile and compression material characterization. Table 1 Tensile property of the material considered. PropertiesGMTGMT UDGMTex Quasi-isotropicLongitudinalTransverseLongitudinalTransverse Tensile strength (MPa)80.7180.459.2174.7170.6 Modulus (GPa)5.8111.076.489.246.00 Poissons ratio0.2840.3070.2170.3890.150 Table 2 Compressive property of the material considered. PropertiesGMTGMT UDGMTex Quasi-isotropicLongitudinalTransverseLongitudinalTransverse Compression strength (MPa)65.882.258.569.057.1 Modulus (GPa)3.265.932.644.142.66 Poissons ratio0.3380.4300.1780.2370.168 Fig. 3. Quasi-static indentation test. G. Belingardi et al./Composite Structures 120 (2015) 483495485 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 material Representative curves of Force vs. time, Energy vs. time and pic- tures of the damage mode, respectively for the fi 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 the specimen according to the specifi ed 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 fi 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 confi ned 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 fi 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 fi bers perpendicular to the crack. Conversely, in the case of GMT- UD, being the classical GMT reinforced by unidirectional fi bers, there was a chance for a crack originated at from the edge of the perforation central hole to propagate along the fi 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 design As 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 profi les 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- fi les, that are structurally weaker than the closed section profi les, and has limitations on the shape of the section profi les. Fig. 10, shows a simplifi ed model of pultruded bumper beam (a) and die formed GMT/GMTex integrated cashbox-beam bumper beam (b) considered for a nonlinear fi 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. Table 3 Energy for quasi-static perforation test. MaterialPlate thickness mmEnergy J GMT438.9 GMTex331.1 GMT-UD441.2 Fig. 4. Drop dart testing machine and specimen confi guration. Fig. 5. Drop test motion description. 486G. Belingardi et al./Composite Structures 120 (2015) 483495 3.1. E-Glass/epoxy pultruded beam solution Pultruded 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 that pultruded 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 fl exural deformation. The proposed pultruded bumper beam solution 8 intended to utilize and optimize the pseudo-ductile behavior of pultruded Fig. 6. Force vs. time (a), energy vs. time (b) and damage at the fi rst impact (ce). G. Belingardi et al./Composite Structures 120 (2015) 483495487 composite 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 profi le (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 fl 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-Glas