Fatigue damage of materials and structures assessed by Wöhler and Gassner frameworks: recent insights about load spectra for the automotive

Abstract

High cycle fatigue behaviour of materials and structures is widely assessed with respect to constant amplitude loading, because of extreme simplification of experimental procedures and numerical computations. Nevertheless, fatigue strength is usually supposed to guarantee in-service integrity face to variable amplitude loading. As the fatigue response may be very sensitive to the load time history, the gap between the former two points of view results in a long-lasting debate among the fatigue community, implying the use of a damage model and the choice of a framework. The pioneering Wöhler model is the natural reference when dealing with constant amplitude loading, whereas standard load spectra are widely applied since the seminal work of Gassner. This paper takes advantage from fruitful discussions between German & French automotive researchers. It presents an appreciation of the Gassner model, usually applied by German engineers since at least 50 years, with the point of view of French engineers, more used to the Wöhler model. Neglecting any sequence effect of the load time history, thus accepting the Palmgren-Miner’s rule as an assumption, we focus on the dual representation of high cycle fatigue behaviour expressed by Wöhler & Gassner, respectively. This necessary calls for the use of load spectra (in terms of amplitude, not frequency), which are here discussed with particular attention to the automotive field.