A RE‐EVALUATION OF THE LIFE TO RUPTURE OF DUCTILE METALS BY CYCLIC PLASTIC STRAIN

TLDR

Experiments on specimens subjected to strain cycles similar to those experienced by sub‑surface elements in rolling or sliding contact show that closed cycles lead to low‑cycle fatigue, while open cycles that accumulate unidirectional plastic strain—ratchetting—can cause a distinct ratchetting failure. The study proposes that low‑cycle fatigue and ratchetting are independent, competitive failure mechanisms, with failure occurring via the mechanism that yields the shorter life. Ratchetting failure occurs when accumulated plastic strain reaches a critical value comparable to monotonic tension failure strain, and the life can be estimated by dividing this critical strain by the ratchetting strain per cycle. Closed strain cycles lead to low‑cycle fatigue, predictable by the Coffin‑Manson relationship, and re‑analysis of uniaxial and biaxial tests—including new copper biaxial data—confirms that low‑cycle fatigue and ratchetting act independently and competitively as proposed.

Abstract

Abstract— Experiments have been performed on specimens subjected to strain cycles similar to those experienced by sub‐surface elements of material in rolling/sliding contact. It has been observed that if the strain cycle is closed then failure takes place by low cycle fatigue and the Coffin‐Manson relationship may be used to predict the number of cycles to failure. If however, the strain cycle is open, so that the material accumulates unidirectional plastic strain (the situation known as “ratchetting”) a different type of failure, which is termed ratchetting failure may occur. It occurs when the total accumulated plastic strain reaches a critical value which is comparable with the strain to failure in a monotonic tension test. The number of cycles to failure under these circumstances may be estimated by dividing this critical strain by the ratchetting strain per cycle. It is suggested that low cycle fatigue and ratchetting are independent and competitive mechanisms so that failure occurs by whichever of them corresponds to a shorter life. The results of both uniaxial and biaxial tests reported in the literature have been re‐evaluated and these, together with new data on biaxial tests on copper, found to be consistent with this hypothesis.