Doctor of Philosophy (PhD)
Mechanical & Industrial Engineering
This dissertation studies different techniques to predict the fatigue life of metals using the concept of thermodynamics by investigating the dissipated processes. To this end, a thermographic methodology is used to determine the fracture fatigue entropy for predicting low- and high cycle metal fatigue. The associated analysis includes consideration of energy dissipation via microplastic deformation and internal friction. Internal friction is shown to play an important role in high-cycle fatigue. It is shown that the proposed approach can successfully capture the material failure during low- and high-cycle fatigue experiments.
A nondestructive fatigue model is also developed that utilizes the thermographic methodology and the concept of entropy production to predict the residual life of a component subjected to variable amplitude loading. The results show that the present approach's maximum and average errors are much lower than available methods.
An in-situ technique for predicting the fatigue life of metals is developed by monitoring the specimen’s surface temperature. The method is based on the examination of the cooling characteristics of a specimen once its temperature becomes steady and actuation halted. It is shown that the cooling curve is unique for a specific material and geometry regardless of the operating conditions.
Moreover, the failure of materials subjected to torsional and axial cyclic loading is studied. It is shown that the higher magnitude of dissipated energy in torsional loading is due to the anelastic deformation in the material and its associated internal friction. Accumulation of thermodynamic entropy generation as an index parameter using the modified dissipated energy is estimated to predict the fatigue life.
Finally, an experimental and theoretical analysis of low carbon steel 1018 subjected to multiaxial loading is presented. Different loading conditions, including tension-compression, torsion, in-phase, and out-of-phase, are applied to investigate the effect of loading type on fatigue life. The FFE framework is then used to predict life using both hysteresis loops and thermography. The FFE predictions using thermography successfully captured the specimens' fatigue life subjected to the multiaxial fatigue loading independent of the loading condition.
Mehdizadeh, Mohammad, "Investigations on Metal Fatigue Degradation Using Thermodynamic approach" (2021). LSU Doctoral Dissertations. 5527.
Khonsari, Michael M.
Available for download on Friday, March 31, 2028