On the anomalous mean stress sensitivity of Ti-6Al-4V and its consideration in high cycle fatigue lifetime analysis

This paper gives a critical overview of the literature available on the topic of the anomalously high mean stress sensitivity of Ti-6Al-4V. It summarizes key facts and offers insights gained by further experimental investigation. The pronounced mean stress dependence has to be considered adequately in fatigue lifetime analysis. Common models such as, for example, the Goodman relation are not sufficient and can lead to a significantly non-conservative or over-conservative estimation, depending on the considered cycles to failure. In this paper, the mean stress behavior of Ti-6Al-4V is analyzed in the high cycle fatigue region from 104 up to 108 load cycles. Explanations for the mean stress dependence for a wide range of stress ratios are given by introducing four fatigue limit determining mechanisms. One of them, the maximum stress controlled basal crack initiation, is identified as the underlying mechanism for anomalous mean stress sensitivity. New models are proposed to approximate the mean stress behavior in the high cycle fatigue region. Experimental validation confirms the suitability of the chosen approach. In addition, an anomalously low supporting effect is observed.