Entropy as a damage metric for fatigue life predictions

Damage mechanics describes the degradation process that takes place in materials and structures. Traditionally, Coffin-Manson type empirical curves are used to determine the fatigue life. Damage mechanics allows us to determine the fatigue life without the need for empirical curves. The main problem in damage mechanics has always been a lack of universally agreed upon definition of a damage metric. In this paper a damage metric based on the second law of thermodynamics and statistical mechanics is presented. The proposed thermodynamic framework treats a solid body as a thermodynamic system and requires that the entropy production be nonnegative. Verification of the damage model has been performed by extensive comparisons with laboratory test data of low cycle fatigue of Pb40/Sn60 solder alloy.