Entropic characterization of metal fatigue with stress concentration

The accumulation of thermodynamic entropy generation in a fatigue degradation process, beginning with a pristine specimen and ending at fracture, is referred to as the fatigue fracture entropy (FFE). In this paper, the concept of FFE is applied to study the effect of stress concentration on metal fatigue. Experiments involve uniaxial tension-compression fatigue tests carried out with solid cylindrical un-notched and V-notched specimens made of both medium-carbon steel 1045 and aluminum 6061. Finite element simulations are also performed with both types of specimens to study their thermal response under cyclic load and to predict the corresponding FFE. Experimental and theoretical results show that the FFE of these materials are nearly constant for each type of the specimens. Under the conditions tested the presence of stress concentration is shown to reduce the amount of hysteresis energy generation in the specimen gage section compared to the un-notched specimen under the identical loading conditions. Also shown is that the FFE decreases significantly due to the presence of stress concentration. An empirical correlation is proposed that can predict the fatigue life of a V-notched specimen based on the hysteresis energy per cycle and the FFE of an un-notched specimen. Predicted and experimental fatigue lives of V-notched specimens are found to be in good agreement.