Doctor of Philosophy (PhD)
The present dissertation concentrates on the fatigue life of metals and fatigue crack growth (FCG). The basic concept of fatigue thermodynamics is used for this purpose. With the application of thermoelectricity, a method is developed on the basis of fracture fatigue entropy (FFE) for the rapid estimation of the fatigue life of components. To this end, thermoelectric signals captured from a specimen during cyclic loading are analyzed, and a procedure is proposed for the calculation of fatigue life based on the thermoelectric signals in the beginning and the steady-state zone of fatigue. Temperature gradient and electric potential gradients in metals are correlated. This concept, which is well-known as Seebeck and Peltier effect, can be used as a bridge between the thermodynamics of fatigue and thermoelectricity. It is shown that the electric potential signal captured from metal during the cyclic loading can be used to determine the heat dissipation per unit volume of material, which is ultimately indicative of the fatigue life of the material.
In addition, an approach is presented to predict fatigue life and energy dissipation based on the measurement of the electrical power consumption of an external heat source. In this method, the steady-state temperature profile of a specimen experiencing cyclic fatigue test is reproduced simply by externally heating the specimen with an electric coil with a DC source. The results show a direct relationship between the electric power consumption of heating coils and the fatigue plastic energy dissipation. This relationship is used as a tool to predict fatigue life.
To analyze FCG, a model is proposed based on plastic dissipation from the tip of a moving fatigue crack to find the heat generation and temperature rise around the tip of the crack. The model is based on the Hutchinson-Rice-Rosengren (HRR) singularity model. A relation is derived that relates the stress intensity factor (SIF) to the maximum temperature rise around the moving crack. The presented model is used to find the entropy generation from a moving crack tip. It is shown that the Degradation-Entropy generation theorem (DEG) can provide an estimation of the coefficient of the empirically derived Paris law. Finally, an approach is proposed to rapidly estimate the propagation rate based on the temperature evolution around a moving crack at the onset of loading. The fatigue crack growth rate is measured at the beginning of loading for different stress intensity facture for stainless steel 304. This crack growth rate is found to have a linear relation with the slope of temperature rise with respect to time at the beginning of loading.
A simple model based on realistic assumptions for determining the fatigue crack propagation rate is presented. It relies on the measurement of the temperature evolution around the crack tip thermographically. The approach is capable of predicting crack propagation speed in both low- and high-growth rates. Results can be used to determine the stress intensity factor.
Hajshirmohammadi, Behnam, "Metal Fatigue and Crack Propagation Using Thermodynamic/Thermoelectric Approach" (2021). LSU Doctoral Dissertations. 5710.
Khonsari, Michael M.
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