Degree

Doctor of Philosophy (PhD)

Department

Mechanical & Industrial Engineering

Document Type

Dissertation

Abstract

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. The results of extensive low- and high-cycle bending fatigue experiments on stainless steel 304 and carbon steel 1018 are presented to demonstrate the utility of the proposed approach. It is shown that the proposed approach can successfully capture the material failure during cyclic loadings.

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 applicability of the model is investigated using a set of experiments on stainless steel 304 covering both low- and high-cycle fatigue regimes. Results are also presented that compare the predictions of the residual life with those obtained by applying the Miner’s rule, quantitative thermographic methodology, fatigue driving stress, and the fatigue driving energy approaches. The results show that the maximum and average errors of the present approach are much lower than the above-mentioned methods. Also presented are the results of a series of variable frequency fatigue experiments that are successfully predicted by the present methodology.

A 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 such as the loading type, amplitude, and frequency. Using this finding along with the concept of fracture fatigue entropy (FFE), a technique is introduced to readily predict fatigue life in-situ. Results of extensive sets of experiments for cyclically actuated fully-reversed bending and tension-compression (TC) types of loadings at different frequencies and for various specimens’ geometries are presented to test the validity of the technique.

Moreover, the failure of materials subjected to torsional and axial cyclic loading is studied by applying the principles of thermodynamics of dissipative processes. Extensive experimental tests are conducted using SS304 and SS316 materials to gain insight into both torsional and tension-compression fatigue life. It is shown that the higher magnitude of dissipated energy in torsional loading, as reported in many studies, is due to the anelastic deformation in the material and its associated internal friction. It is further shown that proper assessment of the damaging dissipation energy requires one to exclude the non-damaging internal friction from the total amount of energy dissipation. Accumulation of thermodynamic entropy generation as an index parameter using the modified dissipated energy is estimated to predict the fatigue life. The experimental results are found to be in good agreement with the proposed fatigue life prediction.

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 fracture fatigue entropy (FFE) framework is then used to predict life using both hysteresis loops and thermography. The results show that the out-of-phase loading leads to the minimum fatigue life, while the torsion exhibit the maximum life at a similar equivalent strain. The FFE predictions using thermography successfully captured the specimens' fatigue life subjected to the multiaxial fatigue loading independent of the loading condition. Furthermore, the results show that the strain energy obtained from the hysteresis loop is a good measure of dissipation energy in the cases of tension-compression, torsion, and in-phase loading conditions. Accordingly, the predicted life using the hysteresis loop agrees well with the thermographic and experimental results. However, in the case of out-of-phase loadings, the plastic strain energy obtained from the hysteresis loop deviates from the dissipation energy because the stored energy originated from the internal state variable variation. Accordingly, the predicted life using the thermography is more accurate than the hysteresis loop, which agrees well with the experimental results.

Committee Chair

Khonsari, Michael M.

Available for download on Monday, March 13, 2028

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