Determination of Light-Weighting Potential of Orthogonally Ribbed Polyamide-Steel Polymer-Metal Hybrid Material in Vehicle A- Pillar Design

Transcription

1 Determination of Light-Weighting Potential of Orthogonally Ribbed Polyamide-Steel Polymer-Metal Hybrid Material in Vehicle A- Pillar Design Rindai P. Mahoso #1, Sagar S. Parihar *2 #1 #2 Department of Mechanical Engineering, ITM University, Gwalior, India Abstract - The application of polymer-metal hybrid materials in the automotive industry is aimed at advancing vehicle light-weighting whilst reaping performance benefits at the same time. In the information available on the public domain, it indicates that the application hybrid materials has been restricted to non-safety critical components of vehicle bodies. Therefore this paper seeks to show the feasibility of application of a polyamide-steel hybrid material in a safety critical component, the A-pillar going on to show the benefits of application of the technology as far as weight saving is concerned. This paper does so through the implementation of multi-objective geometric algorithm optimisation procedure on a prototypical material setup for A-pillar design and shows that weight saving of up to 29.7% is possible with maintenance of acceptable performance in structural stiffness. Keywords A-pillar, rollover accident, vehicle light-weighting, Mass optimisation I. INTRODUCTION In modern day automotive design, operator and passenger safety are key considerations in the design process and they are major influences in structural design and material selection. As such safety devices and technologies, both active and passive are always evolving to ensure the safety of all aboard [1]. This evolution has also been greatly influenced by the industry s light-weighting drive where designers and engineers are seeking to increase motor vehicle s fuel economy and yet increasing safety and integrity of their vehicles in all aspects.[2] As is known vehicle occupant safety is at the greatest risk in the event of an accident and accidents occur in many different modes including rollover accidents, frontal impact, side impact and rear impact accidents and the key to protecting occupants in these encounters lies in ensuring the integrity of passenger compartments or safety cells so that other active safety devices such as seatbelts and airbags can perform their designed tasks effectively.[3] One of the accidents modes of gravest concern in modern day car design are rollover accidents, which are arguably the most fatal mode of accident due to their occurrence-fatality ratio.[4] It is for this reason that vehicle roof integrity is important as it is the best way to ensure passenger safety space. It has also been shown that the A-pillar is a crucial member in providing the roof integrity during rollover as top of the A-pillar is the usual point of impact thus this member has been shown to bear the greatest load in roof collapse. [5] In the light-weighting revolution, steel has stood its ground in the manufacture of automotive bodies, especially the safety critical parts like the A-pillar. This has been largely due to the material s availability and the advent of advanced steels which include dual phase steels and other multiphase steels which are easy to manipulate in terms of required physical traits for different uses on the motor vehicle. However other technologies have also gained considerable favour amongst researchers, case in point Polymer Metal Hybrid (PMH) material technology which offers the possibility of improving overall part performance, reducing part weight and also reducing part complexity. The technology has been used in several parts in motor vehicle Body-In-White (BIW) parts including front ends and roof headers by large manufacturer like Audi [6] but it has not yet been applied to safety critical components of the vehicle BIW structure. It is this researchers aim to design an A-pillar using the PMH technology and assess its performance. In the use of PMH materials different technologies are taken and used together to produce a composite material with the performance characteristics of the materials involved with the added advantage of light-weighting. [7] In this paper Direct-Adhesion Polymer-Metal-Hybrid Technology is used and the required level of adhesion strength is attained through direct adhesion where the infiltration and of the thermoplastic melt into the micron-size asperities on the metal substrate surface is facilitated through injection moulding. The general production principle is that a formed metal part is placed in the mould as the insert and an appropriate polymer is injected around it. [8] This method relies upon metal forming and plastics ISSN: Page 189

2 injection moulding which are two very economical methods of series production, thus boosting the possibility of offering solutions for structural automotive parts that are light, cheap, reproducible with functional manufacturing integration and functionally competitive in a single process step. This gives this technology its economic advantage over other methods being researched for BIW design. [8] [9] Direct adhesion under optimal process conditions an adhesion strength as high as 40MPa which has been shown by several researchers as adequate for use in BIW uses. [6] [10] [11]. This paper therefore seeks to show the potential weight saving that can be achieved by using polyamide-steel polymer metal hybrid material for a safety critical component like the A-pillar. It achieves this by explaining the optimisation problem itself with its major parameters and constraints and goes on to describe the simulation setup to test this which was followed by an explanation and discussion of the results. II. OPTIMISATION PROBLEM One of the major draws of applying the PMH technology to the motor vehicle parts is that of lightweighting and in this paper the application of a polyamide-steel PMH material to BIW members is to be assessed for its merits in the weight saving criterion. The BIW member of note in this paper is the A-pillar. Its main functions on a vehicle are to provide structural support to the roof at the foremost end, to form a support structure for the windshield and to form part of the closure for the doorframe of the front doors. However in this paper we seek the optimal material combination design and the extent to which the material can offer weight saving. For the purposes of this assessment we employ the use of an Erlanger-Trager beam which has bounding box dimensions approximate to those of a mid-size sedan A-pillar as a prototype member. The control member will be a prototypical all steel pillar sharing the same bounding box dimensions of 100mm depth and width and 800mm length. The main objectives in this paper s optimisation problem are the minimisation of mass and the maximisation of stiffness. The parameters that we are to focus on to achieve our best stiffness and mass for the A-pillar member are the geometric parameters of steel insert thickness and polyamide rib thickness. The average sheet thickness used in mid-range sedan A-pillars is 1mm and a material availability of high strength steel sheets is there to a gauge of 0.55mm and the polymer rib thickness was initially set at the maximum advised for reinforced injection moulded wall and rib thickness at 5mm but reduced to 3mm due to the maximum weight restriction. The minimised output parameter will be the maximum deflection under a nominal axial compressive load of 30kN as this will give us a direct indication of the stiffness performance of the prototype member which we seek to maximise. The Erlanger-Trager beam polymer reinforcement design parameters are in line with the guidelines given by a few polymer part design handbooks and the metal insert design parameters are all within the limits of material supply and general current manufacturer general practise. [12] [13] [14] The above stated optimisation problem, the maximisation of stiffness: k =AE/L It is clear that by maximising the elastic modulus of our hybrid material, we will also maximise our part stiffness. Therefore, using the rule of mixtures, we can estimate our material elastic modulus using volume fractions of the constituent materials. After expressing the volume fractions in terms of metal insert thickness (m), t s, and polyamide rib thickness (m), t p, we can seek the values of these variables that: Max f(e c ) = [E s (0.24t s -1.6t s 2 )+E p (0.3663t p t p 2 )] / (0.24t s -1.6t s t p t p 2 ) Subject to t s t p Where E s and E p are elastic moduli for Dual Phase 980 HSS and 43% Glass Fibre reinforced Polyamide respectively. For the second objective to minimise mass, we can use density volume relationships to t s and t p to give us the following optimisation problem: Min f(m) = ρ s (0.24t s -1.6t s 2 )+ρ p (0.3663t p t p 2 ) Subject to t s t p As can be noticed that these objectives do not have an obvious optimal point, however trade-offs need to be done to produce an optimal point. Hence for this complicated optimisation we will employ the use of ANSYS tool DesignXplorer. An overall objective function for the optimisation problem for the system can be written as: f(x) = α 1 f 1 (E c ) + α 2 f 2 (M) Where α 1 and α 2 are weighting constants. III. SIMULATION SETUP The aim of the work in this paper is to minimize the part s mass yet maximising its structural performance. Therefore the prototypical A-pillar members are modelled in Autodesk Inventor and thereafter the optimisation building blocks are set up in ANSYS Workbench. The first step is to select the most favourable reinforcement design to be used in the prototypical PMH A-pillar comparing the performance to the control all-steel A-pillar. Then the chosen design member is set up as the geometry ISSN: Page 190

3 for a non-linear axial loading simulation which will be our benchmark for the stiffness of different parameter settings. At this point a Central Composite Design type Design of Experiments (DOE) is run to assess parameter correlation and sensitivities. Thereafter the sensitivities guide us as to which parameters are more effective and important to our desired goal of maximising stiffness. The desired parameters can then be plotted on the Kriging Response surface. This then gives us an appreciation of how our parameters vary with each other and finally we use the Multi-Objective Genetic Algorithm (MOGA) optimisation to find candidate design points that give us the best trade-offs which are closest to our desired goals. The polyamide used for the simulation is a 43 percent Nylon 6/6 and the steel used is 600MPa dual phase steel for the control beam and 980MPa dual phase steel for the PMH beam. These steels are common automotive steels and the 600MPa steel is the steel usually used in A-pillar construction by automotive constructors. The ANSYS material models are shown below: TABLE I: 43% GLASS FIBRE REINFORCED NYLON 6/6 MATERIAL MODEL Density 1480 kg/m 3 2.6e8Pa Poisson s Ratio e8Pa 1.4e10Pa e10Pa e9Pa Table II: DUAL PHASE 600 VERY HIGH STRENGTH STEEL Density 7850 kg/m 3 Tensile Yield Strength Poisson s Ratio e8Pa 4.29e8Pa 5.8e8Pa 2e11Pa 1.75e11Pa e10Pa Table III: DUAL PHASE 980 Y700 VERY HIGH STRENGTH STEEL Density 7850 kg/m 3 Tensile Yield Strength 7e8Pa Poisson s Ratio e8Pa 9.8e8Pa 2.1e11Pa 1.75e11Pa e10Pa For modelling the contact between the steel and polyamide, the contact debonding tool in ANSYS will be used which uses Cohesive Zone Modelling (CZM) material. This cohesive zone material is used in modelling the contact elements between contact surfaces applying the bilinear behaviour model. The metal sheets in the control member are modelled with spot weld elements. Fig. 1 Prototypical 3 component all-steel member Fig. 2 Prototypical Erlanger-Trager type PMH member ISSN: Page 191

4 Table IV: STIFFNESS TEST RESULTS USED FOR REINFORCEMENT CONCEPT DESIGN Reinforcement type Mass (kg) Axial loading Reinforcement side Wall side buckling buckling buckling All steel pillar Longitudinal Orthogonal Diagonal The Erlanger-Trager composite members are designed initially using the upper design limits permitted for injection moulded polyamide parts and this results in the all steel part being almost equal to the PMH member in mass. The all-steel prototype member was roughly designed with the 3 sheet configuration in most A-pillars, where there is an outer member, inner member and a middle reinforcement member spot welded together. IV. RESULTS Initially different designs of reinforcement were tested to find the most appropriate ribbing design for the Erlanger-Trager beam. When the different ribbing methods were designed and tested through linear buckling simulations, the concepts compared as shown in Table IV. The first step in the design optimisation process was to run a central composite design type design of experiments (DOE). This method came up with a set of points to determine the parameter relevance. The sensitivity graph and linear correlation matrix derived from the DOE are shown below: Figure 4 Parameter sensitivity chart As can be determined from the linear correlation matrix from the ANSYS DesignXplorer tool shows very good linear correlation between the chosen parameters. Showing a very strong inverse relationship between the geometry mass and the total deformation of the member. With the reinforcement thickness having a stronger correlation with the total deformation. The sensitivity graph also shows that the rib thickness has a greater effect on the output parameters, which shows the importance of the polymer reinforcement to the performance of the design. This also shows that there is greater freedom to play with the insert material gauge than it is to tweak the reinforcement thickness as far as maximising rigidity is concerned. After the DOE, a response surface is then plotted to show a more complete set of results through extrapolation of the DOE points. Fig. 3 Parameter Linear correlation matrix Fig. 5 Optimisation response surface ISSN: Page 192

5 The Kriging response surface obtained from the ANSYS DesignXplorer minimization of the total deformation which we desire correlates with maximisation of both the insert thickness and the rib thickness. This works the in the direct opposite way with the objective of mass reduction. Therefore trade-offs would have to be made. This can be shown in by the distribution of the feasible points which are marked by the blue shade on the surface. In light of this the multi-objective optimisation was run and to yield the candidate point distribution shown below: Fig. 6 Candidate spread from optimisation In the candidate point spread, the green lines are the feasible ones. The results of the MOGA optimisation therefore suggest that the advised rib thickness range from mm to mm and the metal insert thickness range from mm to These ranges yield a deflection of our polyamide-steel prototype member that ranges from mm to 7.213mm. The best candidate yielded by the MOGA optimisation gave us the metal insert thickness of mm and polyamide rib thickness of mm and this yielded a maximum deformation under the 30kN load of mm at a part mass of mm which represents a 29.7% weight saving. A linear buckling simulation was then performed on the optimised design and it yielded at a buckling load of 35387N which indicates a reduced performance from the non-optimal ribbing but very much competitive compared to the steel prototype pillar. V. CONCLUSIONS The ANSYS DesignXplorer optimisation clearly shows how well the PMH technology can achieve reasonable levels of weight-saving whilst at the same time maintaining performance of the member. This therefore shows that there is a future for application of this PMH technology in the safety critical members of vehicle BIW. The maintenance of performance within acceptable levels whilst offering a 29.7 % weight saving is a huge promise towards the potential benefits that further application of PMH technology in the automotive industry can hold. REFERENCES [1] K. V. Mutya and S. Rudra, Road Safety Mechanism to Prevent Overtaking Accidents, International Journal of Engineering Trends and Technology, vol. 38, p , 2015 [2] E. Ghassemieh, "Materials in Automotive Application, State of the Art and Prospects," in New Trends and Developments in Automotive Industry, M. Chiaberge, Ed., Rijeka, INTECH, 2011, pp [3] S.. L. Oesch, "Statement before the US Senate Committee on Commerce, Science, and Transportation: IIHS research on vehicle roof crush," Insurance Institute for Highway Safety, Arlington, [4] K. M. Dobbertin, M. D. Freeman, W. E. Lambert, M. R. Lasarev and S. S. Kohles, "The relationship between vehicle roof crush and head, neck and spine injury in rollover crashes," Accident Analysis and Prevention (Elsevier), vol. 58, p , [5] R. Pathare, M. Mirdamadi and O. Bijjargi, "Roof crush enhancement incorporating structural body enhancement systems," Dow Automotive, [6] M. Grujicic, V. Sellappan, M. A. Omar, N. Seyr, M. Erdmann and J. Holzleitner, "An overview of the polymerto-metal direct-adhesion hybrid technologies for loadbearing automotive components," Journal of Materials Processing Technology, [7] S. Müller, M. Brand, K. Dröder and D. Meiners, "Increasing the Structural Integrity of Hybrid Plastics-Metal Parts by an Innovative Mechanical Interlocking Effect," Materials Science Forum, Vols , pp , [8] Lanxess Corporation, "Application Information: Hybrid Components in Series Production," [Online]. [Accessed 15 November 2015]. [9] A. Jäschke and U. Dajek, "Roof-frame design using hybrid technology," LANXESS Deutschland GmbH, Leverkusen, [10] M. Grujicic, V. Sellappan, G. Arakere, N. Seyr and M. Erdmann, "Computational feasibility analysis of directadhesion polymer-to-metal hybrid technology for loadbearing body-in-white structural components," Journal of Materials Processing Technology, vol. 195, pp , [11] K. Ramani and B. Moriarty, "Thermoplastic Bonding to Metals Via Injection Molding for Macro-Composite Manufacture," POLYMER ENGINEERING AND SCIENCE, vol. Vol. 38, no. No. 5, pp , [12] Modern Plastics, Modern plastics handbook, C. A. Harper, Ed., Lutherville: McGraw-Hill, [13] GE Plastics, GE Engineering Thermoplastics Design Guide, GE Plastics. [14] C. Maier, Design Guides for Plastics, Tangram Technology Ltd., ISSN: Page 193