Fatigue Life Estimation and Validation Using Full-Field Strain Measurements

Abstract

Fatigue is defined as the weakening effect of materials when subjected to cyclic loads that are less than the materials’ yielding stress. The effect of frequency of loading on Stress as a function of Number of Cycles or S-N curve in the presence of temperature variation is not well understood. Effect of elastic-plastic stress-strain field near the crack tip due to stress concentration on fatigue life estimation is still a topic of continued research. The objectives of this thesis are to study these two challenges. Specific objectives are to study (1) effect of frequency of loading on fatigue experimentally in the presence of temperature variation and then evaluate it numerically using existing finite element simulation, (2) effect of elastic-plastic stress-strain field near the crack tip on fatigue experimentally using the smart skin technique developed at Rice University and the evaluate it numerically using finite element simulation with suitable plasticity models, and attempt fatigue life estimation. To achieve these objectives a set of rotating fatigue experiments were performed to study the influence of frequency and temperature on fatigue life. Due to the limitation of the experimental setup tests could not be continued. However, the test results were used to validate the FEM model developed LS-Dyna software. Fatigue life estimation under random loading was studied. The FEM model was used to evaluate the effect of frequency; it was found to have important effect on fatigue life. The research was continued to study the effect of elastic-plastic stress-strain field near the crack tip. The smart skin, SWCNTs coating, was used to measure plastic strain field on an aluminum specimen with a crack under tensile loading. When the specimen and smart skin were subjected to plastic load, the peaks in spectrum shifted and it could be used to determine plastic strain map at the crack tip. The stress –strain concentration on the crack tip was evaluated after processing data. After the experiment, LS-Dyna finite element model was validated using the test results. Attempts to estimate fatigue life were unsuccessful due to the presence of plasticity.