Doctor of Philosophy (PhD)
Irreversible material degradation due to cyclic mechanical loading is investigated utilizing the concept of thermodynamic entropy, plastic strain energy, and temperature slope measurement. Uniaxial tension-compression and fully-reversed bending fatigue tests are performed over a wide range of loading conditions with metallic and composite materials subject to both constant- and variable-amplitude loading. A methodology is developed for the estimation of the fatigue fracture entropy (FFE) and fatigue toughness of metallic specimens in a rapid fashion. It is found that the FFE and the fatigue toughness of each material tested are within a small band. The value of FFE is found to be unique for a given type of a material, substantiating that FFE can be regarded as a material property. The concept of FFE is applied to study the effect of stress concentration on a metallic specimen. It is found that the FFE of a V-notched specimen with certain amount of stress concentration is fairly constant. A formula is derived for the prediction of the fatigue life of a V-notched specimen based on the fatigue test results of an un-notched specimen. The concept of FFE is utilized to study the high-cycle fatigue (HCF) of carbon steel 1018. As the stress levels in HCF are substantially smaller than the yield strength of the material, a considerable amount of anelastic energy is present in the hysteresis loop along with plastic strain energy. We propose a method to calculate anelastic energy so that entropy generation can be estimated. Finite element simulations are performed to validate the assumptions made in the development of the methodology. It is found that the FFE of this material remains within a specific band both for the low- and high-cycle fatigue. A methodology is developed for the prediction of the remaining fatigue life (RFL) of a specimen with prior history of loading in a non-destructive (NDT) fashion based on the slope of temperature rise obtained from the specimen under cyclic loading. This method which uses thermographic technique has been validated with API 5L X52, carbon steel 1018, and Glass/Epoxy composite with promising results. This approach is further extended to derive a correlation between the damage parameter and the temperature slope obtained from a fatigued specimen. This correlation and the so-called master curve of damage evolution are employed to develop a methodology for the prediction of the RFL of a metallic specimen.
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Ali, Md Liakat, "Degradation and Fatigue Involving Dissipated Processes" (2015). LSU Doctoral Dissertations. 1810.