CRACK MODELING - A NOVEL TECHNIQUE FOR THE PREDICTION OF FATIGUE FAILURE IN THE PRESENCE OF STRESS-CONCENTRATIONS

Finite element (FE) analysis and other computational methods have deve loped rapidly in recent years, allowing accurate predictions of elasti c stresses in components of complex geometry. However, the prediction of fatigue failure in these components is still a non-trivial problem; one reason for this is the difficulty of assessing stress concentrati ons and regions of high stress-gradient.This paper describes a new tec hnique, called ''crack modelling'', which addresses the problem throug h a modification of linear-elastic fracture mechanics (LEFM). LEFM is designed to deal with cracks in nominally elastic stress fields, using elastic analysis to derive a characteristic stress intensity, K or, f or cyclic loading, a range Delta K. This methodology is modified in tw o ways. Firstly it is shown that LEFM can be extended to predict the f atigue behaviour of bodies containing notches of standard geometry, in stead of cracks. Secondly, FE analysis is used in conjunction with a m odelling exercise in order to extend the method to include bodies of a rbitrary shape subjected to any set of loads. The method was first tes ted using standard notch geometries (blunt and sharp notches in beams) , where accurate predictions of fatigue limit could be achieved. It wa s then applied to an industrial problem, giving a prediction of high-c ycle fatigue behaviour for an automotive crankshaft. The method requir es only simple mechanical-property data (the material fatigue limit an d stress-intensity threshold) and uses only linear-elastic FE modellin g. It allows fracture mechanics theory to be used without the need to specifically model the presence of a crack and uses far-field elastic stresses to infer behaviour in the region of a stress concentration.