Modern Methods for Random Fatigue of Automotive Parts

Conventional approaches for the fatigue life evaluation of automotive parts like headlamps involves the evaluation of random stress conditions in either the time or frequency domain. If one is working in the frequency domain the fatigue life can be evaluated using one of the available methods like the Rayleigh (Narrow Band) approach or the more recent Dirlik method. Historically, the random stresses needed as input to these methods have been evaluated by the FEA solver (eg Abaqus, or Nastran) and these “in built” stress evaluations have limitations which relate to the fact that the stress conditions are complex and so the common “equivalents” for stress like von-Mises or Principal have not been available. There have also been limitations in the location and method of averaging for such stresses. In addition, the fatigue calculation approach for doing the evaluation has been constrained to the linear stress based (S-N) method. And finally, random methods implemented inside such solvers are inherently inefficient. Modern methods process the system properties (transfer functions) rather than the response stresses and this offers significant improvements in terms of performance. The modern methods also offer better fatigue (and fatigue material) methods like the strain based (E-N) approach and more appropriate equivalent stress options. This paper presents comparisons between the conventional methods (using Abaqus and Nastran) and the more modern methods using the CAEfatigue VIBRATION (CFV) code.