Fatigue Life of Elastomeric Engineering Components Under Biaxial Loading Using Finite Element Analysis

A key challenge in engineering design with elastomers for automotive applications, such as chassis suspension mounts and engine mounts, is to integrate fatigue life calculations into the design process. This will be required for life cycle engineering but is a difficult and complex task and this paper will outline some recent progress that has been made using a fracture mechanics approach together with a new finite element code, FLEXPAC, developed especially for this purpose. Finite element analysis enables the fracture mechanics approach to be generalised in principle to any geometry. In practice, however there have been serious difficulties in obtaining numerical solutions when rubber components containing internal cracks, whose surfaces are in contact, are modelled with large deformations and non linear elasticity properties. The overall problem has been approached in three parts. First, materials models are input for elasticity, stress softening and fatigue crack growth behaviour. This has involved a considerable amount of experimental work to develop and verify appropriate materials models. Second, solutions are obtained for cracks growing within rubber layers under the chosen mode of deformation. These solutions output tearing energy as a function of crack depth as well as other parameters that provide a more comprehensive basis for fatigue life calculation than that attainable from analytical approaches. The third stage is catered for by a fatigue calculator which uses the materials fatigue model, an input fatigue spectrum and the solutions for tearing energy to compute the depth of a selected crack as a function of cycle number.

This paper describes the methodology of this approach to fatigue life calculations of elastomeric engineering components and illustrates it with a case study of a model rubber bearing deformed under a bi-axial combination of shear and compression. Questions of calculating crack location and crack growth direction are also discussed in this context.