Life Assessment Based on Numerical Thermodynamic Entropy Estimation: Case Study of Metal Fatigue

Thermodynamically, all damage mechanisms share common feature of energy dissipation. Dissipation is fundamental measure of irreversibility. The irreversibility is quantified in thermodynamic approach by entropy generation. Crack initiation and growth is irreversible phenomena in fatigue process, resulting entropy generation in the system. In this paper, concept of thermodynamic entropy generation is studied in crack initiation for reliability assessment. Analytical and numerical analysis are carried out to evaluate the temperature of specimen during the test. Thereafter, entropy generation is calculated by using evaluated temperature and the empirical relations (such as Morrow) as plastic work per cycle. The finite element software (ANSYS) is used for numerical simulation. It is shown that both entropy generations obtained from analytical analysis and simulation are in good agreement with experimental data. In continuation and for probabilistic analysis, a Mont Carlo simulation method is utilized to determine uncertainties and finally reliability analysis performed with entropy generation approach. As a case study, Aluminum 7075-T651 is used which has common use in airframe constructions. The result shows that the entropy generation is not constant at crack initiation. Although it has specific mean value with low dispersion at same loading condition.