Parabolic Leaf Spring Optimization and Fatigue Strength Evaluation on the Base of Road Load Data, Endurance Rig Tests and Non Linear Finite Element Analysis

Abstract

Parabolic leaf spring plays a vital role in the suspension systems, since it has an effect on ride comfort and vehicle dynamics. Primarily, leaf spring endurance must be ensured. Presently, there are two approaches to design a leaf spring. In traditional method, fatigue tests should be repeated for each case considering different material, geometry and suspension hard points. However, it takes a long time and requires heavy budget to get the optimized solution. In the recent method, numerical approach is used to obtain the fatigue life and the leaf geometry against the environmental condition on the base of material properties. This paper presents a more precise method based on non-linear finite element solutions by evaluating the effects of the production parameters, the geometrical tolerances and the variations in the characteristics of the material. In other words, it is a hybrid method, between the traditional and the recent ones, which correlates the real life conditions and the results of computer aided engineering. The leaf springs in different characteristics have been produced and tested in the plant of OlgunCelik plant. The design methodology of this paper brings also a practical approach to the professionals in the industry. It aims to create a design tool with 2D FEA which is well correlated with 3D.The correlation of 3D and simple 2D methods with experiments are validated through a design of experiment (DOE) study. INTRODUCTION Parabolic leaf springs are the components of the suspension system. They perform isolation task in transferring vibration due to road conditions to body. There are various versions of the parabolic leaf springs such as parabolic, multi-parabolic and z-leaf spring. Development of a leaf spring is a long process which requires lots of test to validate the design and manufacturing variables. We have used CAE to shorten this development cycle by implying CAE as much as possible to reduce the tests. A systematic procedure is obtained where CAE and tests are used together. LEAF SPRING HYBRID DESIGN METHODOLOGY Every design method having CAE in the process must be based on validated virtual models. We have validated our finite element analysis models by experimental studies. These studies were carried out by the controlled manufacturing processes, measured manufacturing variables and comparison of the test results and virtual model using real variables. Primary output obtained from both test or finite element analysis of leaf springs are spring rate (force, displacement) and fatigue strength. In order to understand the parameters affecting fatigue strength of the leaf spring we have carried out a series of experiments in varying conditions. We have determined 26 parameters affecting fatigue results (Table1) depending on our experience and leaf spring boundary diagram (Table 2). Some of the parameters are major and other are minor. Design process starts with conceptual phase w general dimensions and shape of the leaf spring are determined. Conceptual phase decisions are based on experience and in-house software. Then a finite element model with nominal parameters is created. Design is optimized initially with nominal parameters. to reach required force-displacement curve, hence spring rate and stress level which is one of the important factors defining the fatigue strength. Then factors affecting fatigue are selected from the list given in Table 1 depending on project type, known tolerances, supplier data etc. The effect of these variables are studied in CAE by means of 3D and 2D models. This step is known as Design of Experiment (DOE). As a result of this DOE study, not only the CAD models but also decisions that are taken given in Table 1 are optimized. Prototypes are after having been produced. Figure 1. Hybrid Design Process