Multiaxial Fatigue Analysis—Approach Toward Real-World Life Prediction

Abstract

Conventional fatigue analysis procedures followed in daily practices are at least 10 or more years old. Typically, equivalent stress (on a plane) approach is used for fatigue analysis. However, variation of stresses/strains in a particular plane may not yield accurate results for fatigue life calculation. With new developments in analysis practices and better solver capabilities, it is imperative to evaluate these techniques for fatigue analysis followed today. One such need is to evaluate the fatigue life when loads are highly complex and nonproportional. Study is done here to perform multiaxial fatigue analysis of the structure. It is compared against the existing methods for applicability in terms of cost and solution time. n-code fatigue analysis module is used for the study based on multiaxiality. Multiaxial fatigue analysis requires use of yield criteria to fatigue under combined loading. With nonproportional loading, there is increased strain hardening which needs to be considered appropriately. Hence, based on nonproportionality and biaxiality, a decision matrix is prepared to decide the appropriate notch correction and equivalent stress or strain approach. Notch correction methods studied are Neuber, Hoffmann–Seeger, Glinka and Jiang Sehitoglu. Available n-code multiaxiality options are explored and comparative study among these approaches is summarized here. The n-code multiaxial fatigue analysis proposes two approaches to predict the fatigue life. Two-step analysis and multistep analysis give closer solution to fatigue life, but time required for solution increases drastically. Two-step analysis can be selected as a trade-off for life prediction with multiaxiality in terms of time taken for solution. Multiaxial fatigue analysis is performed on one of our vehicle structure to correlate the results with actual life of the components predicted from the rig test.