Shape optimization of notched parts for maximum fatigue life

The objective of the thesis study is to develop a shape optimization procedure to maximize fatigue life of notched components under in-phase or out-of-phase multiaxial loading conditions. Two critical-plane fatigue assessment models, Smith-Watson-Topper (SWT) and Fatemi-Socie (FS), are used to estimate the fatigue life of the parts. Two different notch geometry, fillet and groove, is considered. Considering that typical machine components under fluctuating loads are designed to endure long fatigue lives, no significant plastic deformation is expected to occur at the notch. Accordingly, the part is assumed to be linear elastic. Using ANSYS Parametric Design Language, codes are developed to carry out structural analysis of the parts, evaluate the fatigue life according to SWT and FS models, and optimize the notch shapes. First, the validity of the models for the particular notch geometries is investigated by comparing their predictions with the experimental results reported in the literature for circular notches. The results are generally accurate within acceptable limits. The boundary line of the notch is defined by spline curves passing through key points and the positions of the key points are selected as optimization variables. The objective function to be minimized is chosen as the SWT or FS damage parameter, which is inversely proportional to fatigue life. Using a modified simulated annealing algorithm, optimum notch shapes are obtained for notched shafts subjected to different combinations of torsional, bending, and axial loadings. Significant improvements are observed in fatigue life with the optimum notch shapes in comparison to circular notches.