Structural Analysis and Optimization of a Mono Composite Leaf Spring for Better Mechanical Properties and Weight Reduction

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1 Structural Analysis and Optimization of a Mono Composite Leaf Spring for Better Mechanical Properties and Weight Reduction Senguttuvan Nagarajan Senior Manager Spectrus Informatics Pvt Ltd Rajarajeshwari Nagar, Bangalore senguttuvan@spectrus-group.com Abstract The automotive leaf springs play a major role in the production cost and efficiency of vehicles. Manufacturers are continuously looking for improved design of automobile leaf springs to gain competitive advantage. Usage of composites materials in design and development of leaf springs can result in significant benefits to the manufacturers and end users. With the latest developments in the composites technologies and processes, it is possible to replace conventional materials used in leaf springs with composites at competitive price and for high volume production. As a result, several manufacturers are exploring different options to use composites in leaf spring production. The aim of this paper is to share our experience on design and analysis of Glass Fibre Reinforced mono composite leaf spring using Optistruct to get better structural stiffness with weight reduction. Composite structures offers an unrivalled design potential as the laminate material properties can provide exceeding mechanical properties throughout the structure. The validated leaf spring design is then optimized using Optistruct for further saving the mass of leaf spring. The optimization is concluded and it resulted with a minimum mass design, significantly reduced lapse of time prior to manufacturing. Keywords: GFRP, Optistruct, Stiffness, Leaf Spring test bench, inter laminar shear stress, Free-size Optimization Introduction The automobile industry has been looking for implementation or modification of parts to reduce the weight of the vehicle. One of the key candidates for weight reduction in a vehicle is its suspension system. Suspension system in an automobile determines the riding comfort and amount of damage to the vehicle. In this system, leaf spring accounts for 10% to 20% of the unstrung weight. The considerable improvement in mechanical properties together with the achievement of weight reduction has made composite a very good replacement material for conventional steel components. Since the composite materials have more elastic strain energy and high strength-to-weight ratio as compared to conventional materials, introduction of composite leaf spring made of glass fibre (GFRP) reduce the weight of spring without any reduction on load carrying capacity and stiffness. A spring is an elastic body that will distort when loaded and recover its original shape upon the removal of the load. Leaf spring is based upon the theory of a beam of uniform strength. Leaf springs absorb the vehicle vibrations, shocks and bump loads (induced due to road irregularities) by means of spring deflections, so that the potential energy is stored in the leaf spring and then relieved slowly. The objective of this work is to analyse a composite mono leaf spring for its stiffness and finally to optimise the composite structure for further reducing the weight maintaining the required stiffness. The predictive capability of CAE tools has progressed to the point where much of the design verification is now done using computer simulation rather than physical prototype testing. This capability is fully utilized to produce the best results that will match while practically testing in a leaf spring test bench. Finite Element Modeling The leaf spring is modelled using Finite Element Method to discretize the assembly into a finite number of elements in HyperMesh software. The analysis is limited to two dimensional analyses and the mono leaf spring assembly is meshed with QUAD elements of 3mm size. RBE2 elements are used to give connection between leaf and the associated parts and also to represent the bolt connections. 1

2 (a) (b) (c) Fig 1: a) CAD model of leaf spring, b) FEM model, and c) centre portion in the meshed model Finite Element analysis tools offer a tremendous advantage of enabling the selection of best possible designs without compromising the manufacturing cost and machine time. As the mono leaf spring is having a thick portion at the centre and tapers out to its ends, the geometry is varying along its length. HyperWorks tools has been utilised properly to model and simulate the variable geometric section with 54 composite plies along with proper setup and orientation. Fig 2: Stack up of plies Material Selection The leaf is modelled with E-Glass composites and the other parts are designed using mild steel material. The properties of the composite material type employed are listed below: Boundary conditions The leaf spring is mounted on the axle of the automobile; the frame of the vehicle is connected to the ends of the leaf spring. As leaf spring is connected to the chassis at its two eye ends, it can translate only in one plane along x axis and rotate along the y axis. All other degrees of freedom are constrained at its ends. A leaf spring is subjected to several repeated loads in vertical direction. This is simulated by gradually increasing the load from 5kN to a maximum load of 18kN.The load is uniformly distributed along the circumference of the bolt hole at the centre of the leaf spring assembly. Fig 3: loading conditions Simulation Results Static analysis was performed using Optistruct to determine the maximum safe stress and the corresponding working load. The figure 4(a) shows the deflection of composite leaf spring under the application of 18kN. The maximum deflection is at the centre of the leaf spring assembly where the value is 155.4mm. Figure 4(b) shows the equivalent maximum composite stress induced in the 2

3 assembly under the action of 18kN load. The maximum stress of 1098N/mm2 is induced at the place of contact between the leaf and the plates above and below it. (a) (b) (c) Fig 4: (a) Displacement plot, (b) Composite stress plot, and (c) Interlaminar Shear stress plot Red zone indicates the area of maximum deflection and blue zone indicates the area of minimum deflection that is shown by probe. The interlaminar shear stresses developed in the leaf laminate was given much importance in this study and it is studied carefully to predict the life of the structure. Interlaminar stresses are the out-of-plane stresses defined at the interfaces between layers in a laminated composite material. These stresses are the source of failure mechanisms uniquely characteristic of composite materials. Their existence is a major reason that laminated composites tend to delaminate near free edges, such as an edge of a plate or around a hole. A graph is plotted between the load applied and the deformation produced with load on the Y axis and deformation on the X axis. Graph 1: Load vs. Deformation It is seen from the above graph that when load increases the deformation increases linearly. So loaddeformation graph gives a straight line relationship whose slope gives the stiffness of the structure. Testing in leaf spring test rig The experimental results are validated practically in a leaf spring test bench and found the CAE results in Optistruct to be 98% accurate. Hence the analysis was found satisfactory. Fig 5: Leaf Spring Test bench 3

4 Optimization To achieve improvement on composite models certain comprehensive optimisation opportunities are available in Optistruct. One has the possibilities to make a free- size optimization, a size- optimization or a ply stacking optimization independent of each other. Composite manufacturing is a process of stacking and curing where inevitable fabrication constraints need to be considered. In this optimization sequence the objective is to achieve weight and cost saving with consideration of some manufacturing constraints. Phase I: Free Size Optimisation The Optimisation problem can be stated mathematically as given below: Minimise Subject to 0,1,, (1),1,,;1,., (2) Where fx represents the objective function, g x and g represent the j-th constraint response and its upper bound respectively. M is the total number of constraints, NE the number of elements and Np number of super plies, x!" is the thickness of i-th super ply of k-th element. At the concept design stage, free size optimisation is typically applied to a composite model consists of several super-plies. This design phase produces an overall concept of material distribution throughout the structure. The following manufacturing constraints were incorporated: 1. The minimum and maximum thickness of plies 2. A balance constraint that ensures an equal thickness distribution for +45s and -45s. Due to balance manufacturing constraint applied, the thickness distribution of +45 degree super ply and -45 degree super ply is the same. (a) (b) (c ) (d) Fig 6: (a) Composite Manufacturing Constraints, (b) Element thickness, (c) Orientation thickness, and (d) Ply thickness 4

5 Phase II: Ply bundle sizing optimisation Phase II involves identifying the optimal thicknesses of each ply bundle. Typically discrete optimisation is performed to define the number of unit plies in each ply bundle. Four ply bundles for each fibre orientation provides a good balance between true representation for the thickness field and the complexity of the ply tailoring. Then ply bundles of different fibre orientation are stacked together alternatively to form a laminate of more even orientation lay-up. Fig 7: Automatically generated ply shapes Phase III: Ply Stacking Sequence Optimisation Stacking sequence of individual plies is being shuffled during this phase to satisfy manufacturing constraints while keeping all behavioural constraints satisfied. The ply rules that control the stacking sequence are: a) The maximum successive number of plies does not exceed 4 plies b) The +45 and -45 be reversed paired The process resulted in the best stacking sequence after 4 iterations without violating the ply rules. Results & Discussions I. The static analysis completed with accurate results that are tested practically. II. Considerable mass savings are achieved by the application of this mechanics and the effect on weight of varying manufacturing constraints could be estimated.the mass of the leaf spring assembly reduced from 9kg to 8.8kg after optimization with optimum ply stack-up. III. The stiffness is maintained within the allowable limits after the process. IV. An efficient exploration of designs and delivering weight saving potential for the leaf spring is allowed successfully by applying composite optimisation process. V. An efficient testing for design sensitivity to applied loads and design constraints is made possible by this free size optimisation process. Graph 2: Optimisation History; Compliance Reduced by Modifying Stacking Sequence 5

6 Benefits Summary Benefits of using Altair products The Optistruct was employed to produce a very detailed composite design. Structural targets are reached by the optimized design of the laminate. The Optistruct also extends its application in minimizing the mass keeping all the ply-rules. The work also took advantage of other applications like HyperView and HyperGraph. Benefits to the leaf spring production Improved the stiffness Reduced the weight Better vibrational characteristics Conclusion The study shows that the ideal location of material is at the center of the assembly so as to provide required torsional as well as bending stiffness. It also gives a proper insight to minimize the mass by reducing the thickness as much as possible in other regions. The majority of plies are oriented at 0 degree as this is the direction of main load path. The final design has stiffness higher than the allowable value. The optimization process is successful in finding a minimum mass design, meeting the stiffness requirements. ACKNOWLEDGEMENTS Thanks to the Spectrus management team for giving us the opportunity to work on the advanced composites enginering projects. Thanks to the composites engineering team of Spectrus for constant guidance, support and for sharing experience to complete the project timely. Thanks for the Altair technical support team and Composites technologies customer support team for their extended support and coaching during the project. References [1]Ming Zhou, et al, Composite Design Optimisation- from Concept to Ply Book Details, 8th World Congress on Structural and Multidisciplinary Optimisation [2]David Mylett, et al, Composite Optimisation of a Formula One Front Wing, the 6 th Altair CAE Technology Conference 2009 [3]Sam Patten, Targeting Composite Wing Performance-Optimising the Composite Lay-up design, the 6 th Altair CAE Technology Conference 2009 [4]Baviskar A.C, et al, Design and analysis of a Leaf Spring for Automobile Suspension System : A Review, International Journal of Emerging Technology and Advanced Engineering (ISSN , ISO 9001:2008 Certified Journal, Volume 3, Issue 6, June 2013) [5]G Harinath Gowd, E, Venugopal Gowd, Static Analysis of Leaf Spring, International Journal of Engineering Science and Technology. 6