Assessment of the material properties effect on the response of Morrow’s Model for estimating the dissipation of energy in fatigue of metals

Abstract

Morrow’s model has been widely used in recent years as part of new methodologies for assessing the fatigue life of metallic materials through the laws of thermodynamics. It is used to quantify the rate of dissipated energy in the fatigue process, represented by the hysteresis loop of the stress–strain diagram of the material. Then, the dissipated energy is used to quantify the entropy generation of the fatigue process. Morrow’s model is a function of the amplitude of the cyclic loading, and a few constants that represent material properties. However, despite its apparent simplicity, the material properties, when are experimentally obtained, usually exhibit dispersions that can lead to inaccurate calculations. In this work, the sensitivity of Morrow’s model to the variability of these parameters is evaluated. In order to assess the sensitivity, a constant amplitude value was set, and the quotient \({{\sigma }_f^{'}}/{{\sigma }_U}\), the fatigue strength coefficient \(\sigma_f^{'}\), the fatigue ductility coefficient \({ \epsilon_f^{'}}\) and the cyclic strain hardening exponent \(n^{'}\) were varied. A factorial design was conducted to determine the effects of the interactions of the material parameters on the response of Morrow’s model. The results showed that Morrow’s model is strongly sensitive to the above mentioned parameters, affecting its accuracy. In an extended case study, the fatigue fracture entropy and the fatigue life of Al2024-T3 were determined using material parameters from different sources in order to determine the influence of the material parameters in the assessment of the estimation of fatigue damage. The study allowed concluding that a meticulous statistical treatment is required when characterizing the material properties and the design engineer must be aware of possible inaccuracies if Morrow’s model is used for estimating fatigue life, specially in the new thermodynamic approaches.