On the intrinsic dissipation and fracture fatigue entropy of metals

Abstract

Determination of the fracture fatigue entropy (FFE) in materials requires a proper assessment of the intrinsic dissipation terms in the entropy production equation. In this paper, physical terms that contribute to internal friction and their implications on measuring FFE in metals are discussed. We show that internal friction is one of the terms in the intrinsic dissipation equation that cannot be neglected even for low-cycle fatigue tests (LCF). Relevant to measuring internal friction, we also introduce a technique to identify the fatigue limit of metals by measuring their damping value at different stress levels. Fatigue tests are performed to examine the validity of the proposed technique for two types of steels (SS 304 and SS 1018). By comparing our results with the available data in the literature, we find that the discrepancy is less than 10%. The temperature dependence of internal friction for SS304 specimens is also investigated. The result demonstrates a linear relationship and the existence of a peak.