Implementation of composite and recyclable ...

Int. J. Automotive Composites, Vol. 1, No. 1, 2014

Implementation of composite and recyclable thermoplastic materials for automotive bumper subsystem Giovanni Belingardi Dipartimento di Ingegneria Meccanica ed Aerospaziale, Politecnico di Torino, Corso Duca degli Abruzzi, 24 – 10129 Torino, Italy and IMAST S.c.ar.l. – Technological District on Engineering of Polymeric and Composite Materials and Structures, P.zza Bovio 22, 80133 Napoli, Italy E-mail: [email protected]

Ermias Gebrekidan Koricho* Dipartimento di Ingegneria Meccanica ed Aerospaziale, Politecnico di Torino, Corso Duca degli Abruzzi, 24 – 10129 Torino, Italy and Composite Vehicle Research Center, Michigan State University, 2727 Alliance Drive, Lansing, MI 48910, USA E-mail: [email protected] *Corresponding author

Brunetto Martorana Centro Ricerche Fiat – Strada Torino 50, 10043 Orbassano, Torino, Italy E-mail: [email protected] Abstract: In order to meet the current targets not only in terms of safety, but also in terms of lightweight that means lower polluting gas emissions and fuel consumption, for a newly developed vehicle it is necessary to perform a number of component based tests. This kind of experimental test is time consuming and very expensive. Therefore, it is recommended to develop cost effective design methodology and analysis using existing finite element methods in order to evaluate the performance of different design solutions under various loading, material and environmental conditions, from the earliest stages of the design activity. This paper intends to address such design aspects and method of analysis with particular reference to the application of composite and recyclable thermoplastic materials to automotive front bumper design. Major constraints that have been dealt with are bumper crash resistance,

Copyright © 2014 Inderscience Enterprises Ltd.

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G. Belingardi et al. absorbed energy and stiffness with particular reference to the existing bumper standards. Finally, the results predicted by the finite element analysis are evaluated and interpreted to examine the effectiveness of the proposed solution. Keywords: bumper; carbon fibre reinforced plastic; CFRP; glass mat thermoplastic; GMT; stiffness; stacking sequence; impact; low velocity. Reference to this paper should be made as follows: Belingardi, G., Belingardi, E.G. and Martorana, B. (2014) ‘Implementation of composite and recyclable thermoplastic materials for automotive bumper subsystem’, Int. J. Automotive Composites, Vol. 1, No. 1, pp.67–89. Biographical notes: Giovanni Belingardi graduated in Mechanical Engineering at Politecnico di Torino, after seven years at the FIAT Automobile Design Center. In 1983, he joined Politecnico di Torino as an Assistant Professor. Currently, he is a Full Professor of Machine and Vehicle Design and leads a research team that is focused on car body design for lightweight and crashworthiness. He is the author of more than 200 papers published in international journals and conference proceedings. He has been and is the responsible of the local research unit for a number of research projects funded by the EU, by the national and local governments, by several industrial partners, mainly in the automotive field. Ermias Gebrekidan Koricho graduated in Mechanics (PhD) at Politecnico di Torino (Italy), in Applied Mechanics (MSc) at Addis Ababa University (Ethiopia), and in Mechanical Engineering (BSc) at Bahirdar University. He was a Lecturer at Addis Ababa and Bahirdar Universities. He is an intern at FIAT Research Center where he worked on smart adhesives, composites, vehicle crashworthiness and lightweight design. At Politecnico di Torino, he was a Research Fellow in European and national projects. Currently, he is Post Doc and Research Associate at Michigan State University, Composite Vehicle Research Center. His research area includes innovative multi-material joining, vehicle crashworthiness and lightweight design. Brunetto Martorana graduated in Chemistry at Turin University. He received his PhD in Materials Engineering from University of Naples. He is a visiting research student at Institute of Material Science – Polymer Program, University of Connecticut where he worked on polymer recycling. Currently, he is with FIAT Research Center where he leads the Lightweight & Green Polymers team and he is carrying out R&D activities about smart structures based on metal/polymer composites, methodologies for evaluating the environmental impact of nanomaterials, innovative adhesive systems, new coating with improved corrosion, scratch and UV resistance, new polymer-based composites for automotive applications. He is the author of several patents and scientific papers.

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Introduction

With ever tougher environmental and pedestrian safety regulations and desire of higher fuel consumption efficiency, more and more lightweight materials are applied to automotive body. Generally, reduction of the weight of existing automobile models can be carried out in two ways; structural improvement by optimisation and material change (Li et al., 2004). Researches show that structural optimisation can give up to 7% weight

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reduction. On the other hand, material replacement such as full aluminium body can lead to weight reduction up to 50% (Jambor and Beyer, 1997). If further weight reduction is needed, the ideal candidate materials are fibre reinforced materials. Beside weight reduction, fibre reinforced materials have good corrosion resistance, impact cushion, noise attenuation, and allow for relevant part consolidation. One of possible application areas that allow material replacement to achieve lightweight vehicle is bumper subsystem. Optimisation of car bumper subsystem, particularly bumper beam, can improve not only weight reduction but also structural energy absorption to meet occupant and pedestrian safety standards, further easier repair or part substitution can be obtained with the use of composite instead of steel beams for the bumper subsystem. Different researchers and car manufacturers have implemented different types of composite materials such as carbon fibre reinforced plastic (CFRP), glass fibre reinforced plastic (GFRP), sheet moulding compound (SMC), and glass mat thermoplastic (GMT) for bumper beam to improve the bumper subsystem performance as it can offer lightweight as well as reduce the energy consumption (Davoodi et al., 2011; Marzbanrad et al., 2009; Cheon et al., 1995). Currently, SMC and GMT are widely used because of easy of formability, low material and manufacturing costs, even though CFRP and GFRP can offer better mechanical performance. After invention of SMC by Bayer AG, Germany, during the early 1960s, several automotive manufactures showed their interest because this technology allows for the first time a mass production process for composite parts. General Motors (GMs) implemented SMC bumper beam in Pontiac Bonneville, Cadillac Seville and Cadillac Eldorado by replacing the convectional steel material (Motors General, 1992). Also, in early 1970s Renault used SMC for bumper application instead of steel. Ford introduced SMC integrated front-end system (IFES) on Taurus and Sable (Maine, 1997). IFES demonstrates a 14% cost reduction for a platform with a production volume of about 600,000 vehicles. Besides, the consolidation of 22 steel different parts into only two SMC parts and a 22% weight reduction were obtained by joint effort of Ford’s IFES design team and Budd Plastic (Young, 1996). It is plain to understand that the weight saving was achieved through lower overhead, less floor space requirement, fewer jigs and fixtures, higher labour efficiency, and low material and component handling. All these improvements are derived from parts consolidation and modular assembly. The end-of-life vehicle directive places strong emphasis on recyclability, and it seemed that this would significantly affect the use of thermosetting composite based components in vehicle structure. In particular for automotive sector the Directive 2000/53/EC on end-of-life vehicles (ELV Directive) has as its main objective the prevention of waste from vehicles and, in addition, the reuse, recycling and other forms of recovery of end-of-life vehicles and their components so as to reduce the disposal of waste. In order to promote environmentally and economically preferable treatment of ELVs, the Directive sets up recycling and recovery targets. The Directive encourages changes in vehicle design aiming to increase vehicle reusability, recoverability and recyclability, and promotes distribution of information necessary for sustainable and safe vehicle treatment. For instant, during the late 1990s in the European Union (EU), about 75% of end-of-life vehicles, were recyclable, i.e. their metallic part. The rest (~25%) of the vehicle was considered to be waste and generally goes to landfills (Kanari et al., 2003). Recent EU legislation requires the reduction of this waste to a maximum of 5% by 2015. Since SMCs are semifinished products made essentially of fibres reinforcing a

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thermoset matrix, and thermoset resins are not easy for recycling treatments , the trend in transport sector is now towards to thermoplastic based composites. Conversely, once developed by PPG Industries (Pittsburgh, Pa., USA) in the mid1990s, GMT has been adopted by the automotive industry due to its light weight, high toughness, corrosion resistance, chemical stability during shelf life, and short shaping and curing cycles. GMT is a glass fibre based recyclable material because of its thermoplastic matrix, mostly polypropylene resin. Application includes front ends, seat frames, bumper beams, rocker panels, battery trays, valve covers, under engine covers and load floors. Since introduction of Mercedese-Benz® A Class and 2000MY Audi® A4 cars in 1997, the area behind the engine and gearbox was closed with large parts made of GMT composites. Azdel company also developed its low density (0.5–2 kg/m3) GMT for structural application (Jacob, 2001). The material was primarily designed for headliners and other interior parts such as load floors, doors trim, sunshades and parcel shelves, but with its ability to be formed into complex shapes in one step, and easy moulding with fabric and other automotive trims, low density GMT is said to be ideally suited for the modular concepts that original equipment manufacturers (OEMs) are increasingly looking for. Raghavendran and Haque (2001) also developed a lightweight GMT Composite containing long chopped fibre strands to be used in headliner and other automotive interior application. Hosseinzadeh et al. (2005) developed and proposed replacement of steel and aluminium bumpers with GMT bumper using basic parameters, i.e., shape material and impact condition. He claimed that GMT can replace SMC as recyclable material. Researchers have used different techniques for material replacement of bumper beam. Cheon et al. (1995) developed and manufactured composite bumper beams by setting up a predefined cross section and thicknesses. They performed static bending tests and results showed that the weight of the composite bumper beam was about 30% of the steel bumper beam without sacrificing the bending characteristics. Park et al. (2010) proposed a new design technique to optimise the shape of a vehicle bumper beam section satisfying both the safety equipment for a frontal rigid-wall impact and the regulations protecting pedestrian from lower leg impact. Marzbanrad et al. (2009) studied bumper beam crashworthiness improvement by analysing material replacement, predefined thickness variation, shape modifications, and impact loading parameters. Results showed that a modified SMC bumper beam is preferable to the ribbed GMT bumper beam as SMC offered minimum deflection, impact force, and stress distribution and to maximise the elastic strain energy while exhibiting the same energy absorption of the unribbed SMC bumper. Robust design of an automobile front bumper using design of experiment was also carried out by Lee and Bang (2006). Maahs and Janowiak (1987) also compared some of the materials available for manufacturing composite bumper beams. They carried out material selection task to decide the most suitable materials for bumper beam, which are categorised in three major groups, namely thermosetting, thermoplastics and structural reaction injection moulding (SRIM). Most of the abovementioned researches stresses on material replacement either with direct material substitution, without changing the thickness and the shape profile of bumper beam, or assigning predefined thicknesses and different reinforcements on replaced bumper beam. However, as per the knowledge of the authors, no articles could be found in open literature which addressed the issue of material replacement in bumper beam application in simple, but efficiently, way by using basic theory of mechanics of material, i.e., keeping the desired performance expected by bumper beam. Hence, this paper focuses on design aspects and method of analysis of

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stiffness criteria with particular reference to the application of composite and recyclable thermoplastic materials to automotive front bumper design in simple and efficient way. To take into account EU directives, in the present study a recyclable GMT material was chosen as a potential candidate for the bumper beam construction and its performance was compared with reference material, steel, and three types of laminate layups of CFRP non-recyclable composite material solutions. Particular attention is being paid to safety standard, weight reduction, energy absorption, and manufacturability of the bumper beam.

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Automotive body lightweight analysis

Vehicle body lightweight analysis is a multi-disciplinary task. Generally, the vehicle has to fulfil demands of the different attributes like safety, dynamics, statics, noise, vibration, harshness (NVH). To address these crucial issues finite element method (FEM) is used. The importance of FEM is continuously increasing due to shorting of product cycle time and competitive pressure (Hilmann et al., 2007). Open literatures and public FEM vehicles models illustrate that complete vehicle model FEM analyses have well represented the response of vehicles tested experimentally under different standard crash tests. However, still, a huge amount of time, resources, and detailed materials data are required. Furthermore, highly skilled FEM analysts are needed to address the attributes which are highly sensitive against variation of design parameters, such as material grade, assembly process, joint modelling techniques and etc. Large full-vehicle model (FVM) can contain more than 300,000 elements. Vehicle designers working on crash analysis spend months to develop a complete FEM vehicle model that represents the actual standard crash test configurations. Modifying material and geometry data, searching for analysis error, and incorporating new design features are some of challenging tasks for engineers performing a complete crash analysis. Conversely, rather than performing FVM, which is time consuming and utilises a huge storage device, nowadays, it is worth taking to adopt the modular approach. The modular based vehicle development is a unique methodology for constructing a full vehicle finite element model which allows the use of a single vehicle model, assembled using component modules, to simulate multiple test configuration (Dhar et al., 2002). In modularity approach, the ability to produce and update robust, reconfigurable, maintainable, and smaller modules that could be assembled to create any of the configuration or product variety would allow crash analyses to be performed on any of them with much reduced calculation time and less model maintenance. According to vehicle manufacture’s design trend, vehicle consists of number modules; such as, body-in-white, power train, driveline, suspension, cab, bumper system, and etc. Some modules have secondary branches or sub-modules: driveline consists of sub-modules, such as front and rear drivelines, front drive shaft and rear drive shaft; bumper system consists of front and rear bumper subsystems. In this paper, a frontal bumper subsystem of Dodge Caravan vehicle has been considered for material replacement to improve its performance under low velocity standard crash test, Figure 1. The reference solution includes main components such as bumper beam, rails, and cooling system support, as shown in Figure 2, all these components are made of steel. Other bumper components such as fascia and energy absorbers are made of polypropylene that is a recyclable thermoplastic material. A fascia is a non-structural aesthetic component that governs the aerodynamic drag force during

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driving, whereas energy absorber structure and foam layers are mainly dedicated to enhance the pedestrian safety. As a design concept, the simple substitution of the material for the bumper beam that is a major structural part of the bumper subsystem, can alter the performance of bumper subsystem during crushing condition. In this paper, two composite materials, CFRP and GMT are considered to replace the reference steel material. Those materials were chosen to explore the possibility of use of a recyclable material, GMT, with respect to the use of non recyclable material, CFRP, in bumper subsystem, bumper beam, under the same crashing load condition of reference solution. Figure 1 FEM vehicle model (a) complete FEM model (b) modular frontal bumper subsystem (see online version for colours)

(a)

(b)

Figure 2 Bumper subsystem components Bumper subsystem

Bumper beam

Fascia

Cooling support

Rails

Energy absorber

Generally, in lightweight design approach, the following criteria should be satisfied in order to be competitive in automotive market: easy maintainability, low production cost, adequate stiffness, strength, and crash resistance response, durability, availability of raw materials, and recyclability at the vehicle end of life. The design process in this paper follows the flow chart reported in Figure 3.

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FEM modelling

The reference bumper subsystem model was taken from one of the complete FEM car models available in the NHTSA website and imported into ABAQUS environment. The bumper subsystem was isolated from the complete car model and the remaining car body mass was substituted by an equivalent lumped mass at the centre of gravity of the car, as shown in Figure 4. The chosen vehicle impacting speed and appropriate boundary conditions have been imposed on centre of gravity, RP-1, Figure 4(b). At reference point RP-2 the rigid wall is constrained with all degree of freedoms.

Implementation of composite and recyclable thermoplastic materials Figure 3 Design process of bumper subsystem Achieving lightweight bumper subsystem Design problems Material selection CFRP Cross ply laminate

Angle ply laminate

GMT

Steel (reference)

Quasi-isotropic laminate Thickness determination

Layup optimisation using MIC-MAC

Layup optimisation using MIC-MAC

No

Equal stiffness with reference Yes

Evaluating design concepts Reaction force comparison

Energy absorption comparison

Mass comparison

Proposed bumper beam

Figure 4 FEM bumper model (see online version for colours)

(a)

Deformation comparison

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Figure 4 FEM bumper model (continued) (see online version for colours)

(b)

In ABAQUS, there are two approaches to analyse impact problems: standard and explicit methods. Because of the nature of crash problems which involve very large and critical modelling issues such as three dimensional contact problems, the non-linear Dynamic-Explicit solver was considered. For shell parts, such as bumper beam, fascia, rail, the S4R shell element was adopted, which is a four-node, quadrilateral, stress-displacement shell element with reduced integration and large-strain formulation (Abaqus 6.9, 2012). The element has six degrees of freedom at each node. This shell element has both bending and membrane capabilities as well as both in-plane and normal loads. Stress stiffening and large deflection capabilities are included. The three-node shell element, S3R, was also used in some specific areas to avoid a none-convergent numerical solution due to element aspect ratio. For solid model, such as the pedestrian protective foam, the eight-node linear brick element with reduced integration, C3D8R, was used. In real situation the facial is mainly fastened to the front fenders using bolt and nut. To represent this joining technique in the numerical model, abeam type multi-point-connector, MPC, was implemented at joint locations. This type of connector also used for welded joints sections such as in cooling support and rail parts. Since several parts are involved in the bumper subsystem simulation, a general contact algorithm has been chosen. When the general contact algorithm is used, Abaqus/Explicit gives a default all-inclusive, automatically defined surface that includes all element-based surface facets, all analytical rigid surfaces and all Eulerian materials in the model formulation (Abaqus 6.9, 2012). The ‘hard’ contact pressure-overclosure model was also used in the mechanical contact property option. Different material models were applied based on the material characteristics. For steel, materials data were taken directly from the original available model and imported after the conversion to yield stress-plastic strain relation since ABAQUS asks for material test data in this format.

Implementation of composite and recyclable thermoplastic materials Table 1

Material properties for PP material

Initial yield (MPa) 6

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Max. tensile stress (MPa)

Max. compressive stress (MPa)

E (GPa)

νelastic

νplastic

16.87

25.25

2.44

0.41

0.31

Figure 5 Nominal stress-strain curves of PP under different crosshead speed, (a) tensile test (b) compressive test (see online version for colours)

(a)

(b)

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Polypropylene, which was used for fascia, was experimentally characterised according to ASTM D638-3 and D695 – 02a standards for tensile and compressive tests, respectively, and results are reported in Table 1. Three additional crosshead speeds, namely, 50, 500 and 5,000 mm/min were also considered to understand the effect of strain rate on the maximum tensile and compressive strength of injection-moulded PP specimens. As it can be clearly seen in Figure 5, considerable variation of maximum tensile and compressive strengths for each strain rate was noticed. Experimental raw data were imported according to ABAQUS input data requirements for such kind of material.

3.1 Modelling of composite material The progressive damage model used in this paper to model the behaviour of the composite material is based on the Hashin theory (Abaqus 6.9, 2012). In this model four different modes of failure are considered, namely, fibre rupture in tension, fibre buckling and kinking in compression, matrix cracking under transverse tension and shear, and matrix crushing under transverse compression and shear. The response of the material during the post-damage initiation phase is computed as, σ = Sε

(1)

where σ and ε are, as usual, stress and strain vectors and S is damaged elastic matrix ⎡ (1 − d f ) E1 1⎢ S = ⎢(1 d f ) (1 − d m ) v12 E2 D⎢ 0 ⎣ D = 1 − (1 − d f

(1 − d f ) v21 E1 (1 − d m ) E1 0

⎤ ⎥ 0 ⎥ 1 d GD − ( s ) ⎥⎦ 0

(2)

)(1 − d f ) v12 v21

The three scalar parameters, df, dm, and ds represent the current state of fibre damage, matrix damage, and shear damage, respectively. The damage parameters have 0 ≤ di ≤ 1 values, with monotonically increasing values up to di = 1, when complete fracture takes place. GMT material was considered as isotropic in the in-plane directions, while different mechanical properties have been used in the thickness direction. Detailed material characteristics of GMT were taken from Dear and Brown (2003). Steel and CFRP test data were taken from previous works of Belingardi and Koricho (2010), Koricho (2012) and Koricho et al. (2013), respectively, i.e., detailed material data for modelling of damage initiation and evolution for CFRP were set according to Abaqus code. Furthermore, to incorporate energy damage evolution in the code, fracture energy values were referred from Kepple et al. (2008).

3.2 Standards for low-speed frontal impact To setup the appropriate boundary conditions and the needed general variables of the bumper subsystem, it is worth briefly surveying the existing standards related to design of bumper under impact load condition. Currently, there are three low-speed impact regulations to check the performance of bumper during crashing condition: the National Highway Traffic Safety Administration (NHTSA) Code 49 part 58 (NHTSA, 1990), the

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United Nations Economic Commission for Europe (ECE) regulation No. 42 (ECE, 1980), and the Canadian Motor Vehicle Safety Regulation (CMVSR) (CRC, 2009). The NHTSA safety regulation has the same limitation and safety damage requirement as the CMVSR, however, the speed is reduced by half. In this paper, NHTSA standard was chosen to perform car-into-barrier impact tests. The impact test against the barrier was conducted at 4 km/h on the full-width of the frontal bumper, as shown in Figure 4. This standard requires that the light system, bonnet, and doors can be operated after the impact as in the normal operation conditions, beside all essential features should be still appropriately functional or serviceable. Similarly, ECE requires that a car's safety systems continue to operate normally after the car has been impacted by a pendulum or moving barrier at the front or rear, longitudinally at 4 km/h of velocity and on the front and rear corner at 2.5 km/h of velocity, at an height of 455 mm above the ground under loaded and unloaded conditions.

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Bumper beam wall thickness determination

To understand the effect of material replacement on crashing behaviour of bumper subsystem, two approaches were adopted; material replacement but maintaining the wall thickness equal to the reference solution (steel) and material replacement with wall thickness modification to get the deflection (bending stiffness) equal to reference solution. The first approach is the simplest one which allows substituting the material without altering the geometry profile of bumper beam. The second one needs some technique and analysis to determine a profile which gives the same stiffness performance as the reference solution. So, in the following section a simplified procedure is outlined to find appropriate thickness for each material type and laminate layup. Composite materials have high specific stiffness and strength comparing with conventional materials such as steel and aluminium. A flat plate is considered for determination of the thicknesses for the material types of interest under the same loading condition, in our case load type is assumed to be bending load. In Figure 6, the plate with dimensions b × l × h, made of steel, is subjected to maximum bending load Mmax. Figure 6 Plate under bending load

To choose the appropriate thickness for the proposed material, a simple equation has been used on the bases of maximum deflection formula,

δ max =

Ml 2 8 EI

(3)

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and in particular for the steel solution Ml 2 bh3 , where I s = s 8 Es I s 12

δ max, s =

(4)

where Es is the Young modulus of the steel. If the steel is replaced with another material such as composite material, with the same applied bending load, M, the maximum deflection can be expressed as Ml 2 , 8 Ec I c

δ max,c =

(5)

where Ic =

bhc3 12

(6)

and Ec is the flexural modulus of the composite laminate. The value of Ec for each laminate layup is calculated on the bases of laminate theory using MIC-MAC tool (Tsai, 2008). The results are shown in Table 2, where r depends on the available ply thickness to achieve the required laminate thickness. Table 2

Effective flexural modulus for each material type CFRP [0/90]rs

CFRP [45/–45] rs

CFRP [0/90/45/–45] rs

GMT

54.3

10.4

45.2

4.7

Ec[GPa]

To avoid excessive deformation and protect the remaining nearby components, the maximum deflection of composite bumper should not be greater than the reference steel bumper. Therefore, equation (4) and equation (5) should be equated as follows to determine the desired thickness of composite bumper. Ml 2 Ml 2 , = 8 Es I s 8Ec I c

hc = hs 3

(7)

Es Ec

(8)

Most of automobile body parts are mainly sheet metal parts that are generally subjected to both tensile and bending load under normal working condition (Li et al., 2004). Particularly, during frontal crash, the bumper beam is the main frontal part of automotive structure, which is generally subjected to bending load. Therefore, equation (8) has to be used to predict the thickness of composite bumper beam under the same deflection as for the reference steel bumper beam. Obtained beam wall thicknesses for each material type are reported in Table 3. Table 3

hc [mm]

bumper beam thickness for each material type [0/90]rs, r = 3

[(45/–45)r /45]s, r = 5

[0/90/45/–45] rs, r = 2

GMT

3.18

5.5

3.38

7.18

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Results and discussion

In the following sections the main obtained results are reported and discussed, particular attention is paid to the effect of the adoption of different materials that were implemented to make the bumper beam when submitted to the same standard loading conditions, and designed according to the two described approaches of the same thickness and the same deflection (stiffness) with respect to the normal production solution. The other components of bumper subsystem such as the energy absorbing layers (structural foam), the rail, the cooling system supports, etc, remained unchanged for all cases.

5.1 Bumper composite beams with the same wall thickness To study the effect of stacking sequence and elastic module on bumper crash behaviour, five types of material were chosen, the shell thickness have been maintained equal in all the considered cases and equal to the normal production solution. Figure 7 Reaction force versus time of the bumper subsystem equipped with composite beam with the same thickness as the normal production solution (see online version for colours)

The bumper (and the constrained lumped mass that models the rest of the vehicle) collided perpendicularly against the rigid flat barrier with a speed of 4 km/h (1.11 m/s). The impact reaction force time history is analysed as it describes how the car decelerates due to the interaction between the rigid wall barrier and bumper subsystem. A portion of reaction force (deceleration), a in Figure 7 for the steel solution, is originated from the load action on fascia and polypropylene foam, while the portion b from the load acting mainly on bumper beam itself. As it is clearly shown in the figure, the values of these reaction forces are varied proportionally with stiffness of bumper beam materials. According to Figure 7, the reaction force for the GMT bumper is lower, whereas the steel bumper exhibits the highest reaction force value; the CFRP solutions give intermediate behaviours. This is due to the lowest and the highest elastic module of GMT and steel, respectively. Another result obtained from the observation of this diagram concerns the effect of the CFRP stacking sequence on reaction forces. Among cross-ply, angle-ply,

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and quasi-isotropic CFRP laminates, slight differences are visible, however the angle-ply laminate solution exhibits the lowest reaction force. This can be explained, once again, by the lowest modules of elasticity of angle-ply CFRP. Irrespective of low values of longitudinal and transverse stiffness, quasi-isotropic laminates exhibit slightly higher reaction force than cross-ply laminate. This result can be attributed to higher shear modules value enhanced by the presence of ± 45 plies in quasi-isotropic laminate, [0/90/45/–45]rs. It is worth noting that the quasi-isotropic laminate is more able to resist the impacting load which create a random stress propagation on bumper beam during frontal crashing, i.e., fibre directions in quasi-isotropic are more dispersed than angle-ply and cross-ply laminates that allow the laminate to carry a load in respective fibre orientations. Figure 8 Displacement-time diagram of the bumper subsystem equipped with composite beam with the same thickness as the normal production solution (see online version for colours)

Figure 9 Strain energy- time diagram of the bumper subsystem equipped with composite beam with the same thickness as the normal production solution (see online version for colours)

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Figure 8 shows a comparison of the displacement history among the five considered bumpers. This displacement gives a measure of the intrusion motion of the bumper subsystem into the engine compartment. The reported displacement is measured with respect to the centre of gravity of the car to take in to account the global response of the vehicle due to materials and layups change on bumper beam. The maximum and minimum displacements occurred in the case of GMT and steel bumper beam, respectively. Regarding the effect of stacking sequence, among CFRP quasi-isotropic layup, cross-ply layup and angle-ply layup, the quasi-isotropic layup solution shows minimum deflection. Finally Figure 9 shows the time diagram of the strain energy absorbed by the bumper in the five considered material solutions. It can be noted that the highest strain energy value for each material type occurred at the highest deflection. The bumper will stretch and deform until maximum deflection is reached, if the collision still occurs. If the energy turns to zero, this means that an elastic collision has occurred, however according to Figure 9, minor strain energy has remained at the conclusion of the test. This can be explained mainly by plastic deformation of energy absorber, polypropylene foam, and, somehow, bumper beam. Figure 10

Kinetic energy versus time with the same thickness as the normal production solution (see online version for colours)

As stated above, next to the reference steel bumper, Quasi-isotropic CFRP exhibits smaller deflection, as shown in Figure 6. For cross-ply CFRP, angle-ply CFRP, and GMT, all need a longer period in order to absorb the energy and to attain the maximum deflection. Exceptionally, under the same thickness for all materials, GMT exhibited high deformation and strain energy, as shown in Figures 8 and 9, respectively, which needs more attention to avoid deep intrusion and protect the remaining parts nearby the bumper subsystem. While observing in Figure 9 it is evident that the initial part of the strain energy curves are nearly superimposed, without particular differences between the different material solutions. Then the steel bumper beam gives the lower values with respect to the other solutions due to more stability of energy absorption with minimum plastic deformation. Inelastic impact response is more pronounced when bumper material

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is more deformable when submitted to impact loading. According the FEM results, next to steel solution, the three CFRP solutions are quite close each other, however the quasi-isotropic CFRP laminate solution exhibited moderate plastic deformation (response curve next to steel), followed by cross-ply and angle-ply laminates, respectively. The GMT solution curve is characterised by the highest values. Besides, as it is well known for low velocity impact, the bumper system is responsible for absorbing the kinetic energy and the amount of energy that is expected to be restituted should be as small as possible. From this perspective the GMT solution showed the worst performance comparing with other proposed solutions as it exhibited high rebound velocity, see Figure 10. Actually, the higher value of the rebound velocity is due to catastrophic failure of GMT bumper beam that induces a larger involvement of the crash box, which is stiffer than the GMT bumper beam. For what concerns the achieved weight reduction, we can say that the weight of bumper beam is reduced from 8.06 kg of the steel solution to 1.53 kg and 2.39 kg of CFRP and GMT ones, respectively, even though it is not reasonable to compare under this circumstance; i.e., material replacement with the same wall thickness is not able to offer the same performance as reference steel solution did.

5.2 Bumper composite beams with the same bending stiffness According to the second approach, when the bumper beam material was replaced with composite materials, calculation of the adjusted wall thickness was made on the basis of equation (8) to retain the original bending stiffness of the steel beam. Thus, it becomes easy to understand the influence of material replacement on crashworthiness. Figure 11

Reaction force-time diagram of the bumper subsystem equipped with composite beam with the same bending stiffness as the normal production solution (see online version for colours)

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The diagram of Figure 11 shows the time history of the impact reaction force for the five considered solutions. The obtained curves are rather grouped together, as expected, however it may be noticed that the material replacement lead to a small reduction of the reaction force in comparison with reference steel bumper beam. A portion of reaction forces (deceleration), a in Figure 11, which originate from the load action on fascia and polypropylene foam, are also grouped together, unlike equal thickness solution. Maximum reduction is found when GMT material is used. From point of view of occupant safety, the found lower reaction force using GMT material is appreciated. On the other hand, CFRP angle-ply exhibited nearly equivalent reaction force with the reference steel material. This phenomenon can be explained by the effect of homogenised repeated sub-laminates. Use of highly repeated sub-laminate in composite structure helps to reduce damage occurrence and improve load carrying capacity. Figure 12

Displacement – time diagram of the bumper subsystem equipped with composite beam with the same bending stiffness as the normal production solution (see online version for colours)

Figure 12 shows a comparison of the displacement history among the five considered bumpers. Observing the curves of Figure 12, it can be easily seen that the differences between the maximum displacements for all the five materials are small in comparison with the results obtained with the previous approach. The devised approach is leading to design a beam solution that performs competitively with steel solution during crash. Figure 13 shows the time diagram of the strain energy absorbed by the bumper in the five considered material solutions. From the diagram of Figure 13 it is immediate to observe that the GMT solution exhibits higher strain energy than the other materials. The CFRP solutions have intermediate behaviour between the GMT solution (that absorbs the greatest strain energy) and the steel solution (that absorbs the lowest strain energy). This implies that larger plastic deformations have taken place when GMT material is implemented on bumper beam. Regarding kinetic energy transfer, unlike in the equal thickness material substitution, GMT showed a comparable performance of absorbing the kinetic energy with the reference steel solution, Figure 14.

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Figure 13

Strain energy-time diagram of the bumper subsystem equipped with composite beam with the same bending stiffness as the normal production solution (see online version for colours)

Figure 14

Kinetic energy versus time with the same bending stiffness as the normal production solution (see online version for colours)

Figure 15 shows the dependence between displacement and absorbed energy. The maximum absorbed energies in all four materials are nearly same; however, in the case of steel based bumper beam, the energy absorption is faster than the remaining candidate materials because of high strength and isotropic nature. Conversely, GMT absorbed the impact energy slowly with larger deformation, which is due the fact that GMT has low strength and more lenient mechanical properties. This result is highly appreciated from

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point of view of occupant safety standard, i.e. during crushing, the major portion the crash is absorbed by the frontal parts of vehicle intentionally designed to absorb a great amount of energy to reduce the transmitted impact load to the occupant compartment. To have the same absorbed energy and equivalent vehicle mass but with different intrusion values implies that the material, that exhibited low intrusion with respect to other candidate materials, is supposed to exhibit high impulse force which pass through the frontal stack-up of non-crushable power-train components to occupant compartment, and vice versa. This behaviour can be clearly seen in Figure 16a. A snap window of Figure 16(a) is shown in Figure 16(b) to observe the threshold reaction force value of each material at the final stage of the impact. Figure 15

Energy-displacement diagram for deferent material and layup types with the same bending stiffness as the normal production solution (see online version for colours)

Figure 16

Reaction force-displacement diagram for deferent material and layup types with the same bending stiffness as the normal production solution (see online version for colours)

(a)

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Figure 16

Reaction force-displacement diagram for deferent material and layup types with the same bending stiffness as the normal production solution (continued) (see online version for colours)

(b)

For what concerns the achieved weight reduction, it can be said that the weight of the bumper beam is reduced with respect to reference steel solution both with the CFRP and the GMT ones. In particular, when GMT material is used, a weight reduction up to 46.4% can be achieved. A further point has to be taken in consideration: the manufacturing easiness for the different competing solutions. It is worth noting that the manufacturing process of composite bumper beams asks for longer manufacturing process time with respect to the steel solution and thus it should be verified if this manufacturing process time is still compatible for the automotive industry production pace, especially in the case of large mass production. GMT material is made of short glass fibres mixed with polypropylene thermoplastic resin and is produced in the form of sheet. In this study, the considered GMT sheets have to be placed in the die and formed to the desired bumper beam shape. In this process, easy of melting, short curing cycle, and recyclability are three major interesting characteristics that give reason for their increasing usage in automotive industries, particularly in application areas where medium and low tolerance accuracy are required. On the other hand, the considered CFRP twill fabric sheets have to be placed in the die, subjected to vacuum bagging to extract the trapped air and excessive resin, and then placed in autoclave for curing at elevated temperature and pressure. This process needs longer manufacturing process time while few parts can be placed into the autoclave at the same time. Therefore GMT material is more economical not only from the point of view of the material costs but also from point of view of easy of manufacturability (Li et al., 2004; Marzbanrad et al., 2009).

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Conclusions

In this study three different materials – namely steel (that is the normal production one), GMT and CFRP (with three different laminate types) – have been evaluated for the bumper transverse beam, to understand the effect of material replacement on crash behaviour of automobile front end during low-speed frontal crash test. The study has been performed by means of numerical simulation with the ABAQUS code, according to the standard NHTSA impact tests. Modular based vehicle crash analysis allows to investigate the performance of bumper subsystem for change in design variables and material type. This approach permits to reduce very largely the dimensions of the model to be used for the simulation, for example in the present study the full-vehicle-model, which consists of 329,300 elements, has been lamped into one vehicle mass and the pure frontal bumper subsystem, which consists of only 13,488 elements. To design the material replacement for the bumper beam, two different approaches have been explored: without changing the wall thickness with respect to the base (steel) solution and with the appropriate adjustment of the wall thickness in order to obtain the same bending stiffness as in the steel solution. When the material replacement of bumper beam is done under equal sheet thickness, CFRP reduced the weight by 6.53 kg (to 18% of the original weight), with a large decrement of the maximum reaction force and a large increment in the displacement (that is the intrusion into the engine compartment) as far as crashworthiness performance are considered. According to the obtained result stacking sequence plies can alter, although in a slight way, the crash behaviour of bumper. GMT was unable to resist the considered impact load and showed very week characteristics which might expose the frontal parts of automobile to damage. On the other hand, when the material replacement of bumper beam is done under the equal bending stiffness criteria, GMT solution exhibits better crashworthiness by reducing the reaction force compared with steel and CFRP solutions. Also, the maximum intrusion values exhibited by all proposed materials is found to be less than 80 mm, which is a reasonable value that left free of damage or with small damage the lighting system, bonnet and doors without affecting their normal operation conditions after impact. For what concerns the weight saving, up to 46.4% weight reduction of the bumper beam could be achieved with GMT with respect to the steel normal production solution. Because of its weight convenience, taking into account the easier manufacturing process and recyclability points of view, in comparison with CFRP, GMT results to be the more interesting choice for composite material application.

Acknowledgements Authors would like to acknowledge that part of the research activity, whose main results are presented in this paper, has been conducted within the frame of the research project ‘MACADI’ (DM 60703) granted to IMAST S.c.a.r.l. and funded by the M.I.U.R.

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