Sustainable Automotive Technologies, Proceedings of 4th International Conference, Springer, p.75-80,2012
Lightweight Stiffened Composite Structure with Superior Bending Strength and Stiffness for Automotive Floor Applications J. Zhang, S. He, I.H. Walton, A. Kajla, and C.H. Wang* Sir Lawrence Wackett Aerospace Research Centre, School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Bundoora, Victoria 3083, Australia
[email protected]
Abstract. The trend towards smaller and lighter, more environmentally friendly vehicles is accelerating, as the petrol price rises and the CO2 reduction target becomes more strict. As a key enabling technology, light-weight but low-cost structure plays an important role in promoting the use of fibre reinforced polymer matrix composites in automotive applications. In this work an experimental investigation is carried out to design, manufacture and analyse a stiffened composite structure, aiming at achieving required bending and torsional strength and stiffness at the minimum weight. One major application of this new lightweight structure is the load-bearing floor component. Some initial results from this work are presented in this paper.
1 Introduction Reduced CO2 emissions and increased vehicle fuel economy is a critical matter for automotive technology development. Automobile manufacturers are now using more composite materials due to their lightweighting benefits, despite of the current higher material cost of carbon fibre composites versus steel and aluminium [1]. Fibre reinforced polymer matrix (FRP) composites are materials that have high specific strength and energy absorption as well as offer other benefits such as part consolidation, styling flexibility, good noise/vibration/harshness characteristics and good corrosion resistance, which are well suited for future lighter, sustainable and more energy-efficient automotive vehicles [2,3]. For example, when advanced composites are used, instead of incremental part-by-part substitutions of metals, they can be applied in a whole-platform approach to solve system-wide issues. The 'Revolution' fuel cell vehicle developed internally by Hypercar had 77% fewer parts in primary structure than in the equivalent portion of a conventional stamped steel BIW [4]. However, there remain great challenges to expand the use of FRP composites from decorative or semi-structural parts to primary *
Corresponding author.
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load-bearing structural components, aiming at taking advantage of these remarkable benefits. Issues such as lack of experience and knowledge in design with composite materials, high material cost of composites, and difficulty with affordable processes for producing composite components in high volume to automotive production standards are main limitations to overcome [4]. Sandwich panels consisting of face sheets and cores are widely used in transportation vehicles and civil infrastructure due to their high flexural and torsional stiffness/strength-to-weight ratio. In particular, sandwich structures are increasingly applied for giving combinational bending and torsional rigidity[5, 6] to components such as body panels, floor pans and aircraft wings [7, 8]. The core keeps the face sheets apart and stabilizes them by resisting vertical deformations and enables the whole structure to act as a single thick plate. Unlike honeycomb cores, a corrugated-core resists bending and twisting in addition to vertical shear [9]. As a result, sandwich structures with corrugated cores have exceptional high flexural stiffness-to-weight ratio and are suitable for constructing structural components which require high level of stiffness characteristics while lightweight is a also an important design consideration. Electrical and hybrid vehicles are becoming increasingly popular as the petrol price rises. Comparing to other locations, the vehicle floor is the safest place for storing batteries, as it is located outside of the body’s impact and deformation zones. In this context, the corrugated sandwich structure provides spaces between the core and face sheets, which is suitable for integrating package of energy resources. When the sandwich structure is used as a vehicle floor, the housed batteries are therefore stored in a safe and secure location [10]. In the current work, composite sandwich panels were fabricated with chopped strand glass reinforcement and polyester resin matrix, due to the low cost of both the reinforcement and matrix resin. The three-point-bending tests were performed both experimentally and numerically on the composite sandwich coupons.
2 Experimental Chopped strand glass fibre mat was used as reinforcement and enydyne dicyclopentadiene modified polyester resin 1735 cured with the methylethyl ketone peroxide (MEKP) catalyst was used as matrix. An aluminium tool was fabricated for making the corrugated core; both the composite core and face sheets were prepared using wet lay-up. Both the composite core and face sheets were cured at room temperature overnight before they were joined together using epoxy adhesive Techniglue CA. Three-point-bending tests were performed using an Instron 5569 universal tester at a crosshead speed of 2 mm/min. The dimension of the sandwich coupon was 325 mm × 245 mm × 63 mm. The thickness of the face sheet was 4 mm and the core thickness was 3.5 mm. The rollers with diameter of 50 mm were exerted along different directions (Scheme 1 and 2) as shown in Fig. 1a and Fig. 1b,
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respectively. The span length was 160 mm for Scheme 1 (roller parallel to the core length direction) and was 220 mm for Scheme 2 (loading in the cross-direction). Finite element models were created with the solid element (C3D8I) in Abaqus 6.10. The sandwich specimen was supported by two rigid bodies at the lower surface; another rigid body was moved down to apply the bending. To establish the contact relationship, an initial displacement was applied to the model. The reaction force and displacement were output to compare with the experiment result.
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Fig. 1. Three-point-bending tests on the corrugated sandwich composite coupon. a) and b) show the set-up for coupons loaded in the machine-direction and the cross-direction, respectively.
3 Results and Discussion Fig. 2 presents the experimental load-displacement curves under loading in the machine- direction and the cross-direction. In the parallel-direction, the downfall load reached 50 kN, which is the limit of the testing machine, after the adhesion between facesheet and core failed. In the case of the cross-direction, the sudden load drop became less significant and the loss of linearity was more progressive. The first load-drop occurred at 41 kN and the second at 33 kN, which correspond to the failure between face sheet and core and the failure in the up facesheet, respectively. The bonding moduli of the corrugated sandwich coupon were calculated by using the following function:
E = ∆ ⋅ l3 / 4 ⋅ b ⋅ H 3 Where Δ denotes the slope of the load-displacement curve, l is the span length, b is the specimen width and H the specimen thickness. The Bending Moduli was calculated to be 232 MPa for the coupon in the parallel-direction and was 333 MPa for the coupon in the cross-direction.
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Fig. 2. Experimental load-displacement curves of corrugated composite sandwich coupons under bending conditions of: a) Scheme 1 and b) Scheme 2.
Fig. 3. Stress distributions of composite sandwich coupons loaded in both a) the machinedirection and b) the cross-direction
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Fig. 4. Comparison between experimental and numerical results from load-displacement curves. a). loading in the machine-direction; b) loading in the cross-direction
In order to simulate the mechanical behaviour of corrugated sandwich structure, a FE model was created using Abaqus (Figure 3). The FE simulation will reduce the development costs and accelerate the development of the optimised structure in the early stage of design. At this stage, the adhesion between the core and the face sheets was ignored. As it can be seen from Figure 3 that the stress concentration occured on the corrugation core part and the region where the core and the face sheet meet. The model was able to simulate the load-displacement curves at the initial loading stage. The comparisons between the experimental and the numerical results are shown in Figure 4. It can be seen that at the initial loading stage, the FE models could simulate the experimental results reasonably well.
4 Conclusion A lightweight glass fibre composite structure has been fabricated for applications where high bending strength and stiffness are needed. The sandwich structure with corrugations as core has demonstrated good bending properties. Three-pointbending tests have been performed in both the parallel-direction and the crossdirection of the corrugated sandwich coupons. The FE simulation results simulated the load-displacement curves reasonably well at the initial loading stage. The spaces between core and face sheets provide opportunities for integrating energy resources into the vehicle floor that is a safe and secure location for this purpose. In the next stage, optimisation of the sandwich structure will be conducted. Factors such as corrugation angle, thickness of core and face sheets, fibre alignment and hybridisation will be considered for maximizing the bending performance with minimum weight. Acknowledgments. This work was funded by the “Lightweight Modular Vehicle Platform” project of Australian Cooperative Research Centre for Advanced Automotive Technology (Auto CRC). The authors would like to acknowledge the lab assistance provided by Mr Robert Ryan and Mr Peter Thatchyk from RMIT University.
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References [1] Stewart, R.: Automotive composites offer lighter solutions. Reinforced Plastics (2010) [2] Brylawski, M., Lovins, A.: Ultralight hybrid vehicle design: overcoming the barriers to using advanced composites in the automotive industry. Rocky Mountain Institute, RMI (1995) [3] Beardmore, P., Johnson, C.F.: The potential for composites in structural automotive applications. Composites Science and Technology 26(4), 251–281 (1986) [4] Cramer, D.R., Taggart, D.F.: Design and manufacture of an affordable advancedcomposite automotive body structure. In: The 19th International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium & Exhibition (2002) [5] Li, X., Li, G., Wang, C.H., You, M.: Minimum-weight sandwich structure optimum design subjected to torsional loading. Applied Composite Materials, 1–10 [6] Li, X., Li, G., Wang, C.H., You, M.: Optimum design of composite sandwich structures subjected to combined torsion and bending loads. Applied Composite Materials, 1–17 [7] Belingardi, G., Cavatorta, M.P., Duella, R.: Material characterization of a composite– foam sandwich for the front structure of a high speed train. Composite Structures 61(1-2), 13–25 (2003) [8] Gustin, J., Joneson, A., Mahinfalah, M., Stone, J.: Low velocity impact of combination Kevlar/carbon fiber sandwich composites. Composite Structures 69(4), 396–406 (2005) [9] Chang, W.S., Ventsel, E., Krauthammer, T., John, J.: Bending behavior of corrugated-core sandwich plates. Composite Structures 70(1), 81–89 (2005) [10] http://www.emercedesbenz.com/autos/mercedes-benz/conceptvehicles/mercedes-benz-at-the-detroit-auto-show-mercedesbenz-concept-bluezero-vehicles/ (accessed on October 1, 2011)
