| dc.description | In engineering, fatigue is defined as the decrease in strength of a material under repeated load cycles, ultimately leading to fracture. It is an issue for the design of bridges, cranes, transmission towers, ships and airplanes. The objective of this thesis was to investigate the fatigue behaviour of stiffened and unstiffened 350WT Category 5 steel plates when subjected to a number of fatigue scenarios that are commonly experienced in the life of ship plating. 350WT steel is a typical material used in Canadian bridges, offshore structures and the hulls of the City class frigates. The fatigue scenarios considered in this thesis included constant amplitude loading, multiple overloading, compressive underloading with crack surface roughness, fatigue of a brittle material and welded stiffened plates. As a result of the present research, the accuracy of two well-known fatigue models, that is, the Zheng and Hirt model and the Wheeler model, were improved. This has been shown to provide a better fatigue prediction in the threshold region and under multiple tensile overloading. A systematic study of crack surface roughness and compressive underloading, which in the past had not been given as much attention as tensile overloading, was developed. The present results showed that there appears to be some merit to the "prying" effect of the crack surface asperities, comparable to the well-known crack closure concept. By testing polymethyl methacrylate (PMMA), evidence was provided for the lack of load interaction effects in brittle materials. In other words, it was discovered that the load interaction effects are largely the result of plastic deformation. This has implications on the fatigue behaviour of steel under certain embrittling conditions. Finally, a procedure was established to predict the fatigue crack growth rates of welded stiffened steel plates using the finite element method, which involved the simulation of the residual stresses followed by a fracture mechanics analysis. The procedure was shown to offer an adequate alternative to experimental testing in predicting the fatigue behaviour of welded stiffened structures. To summarize, it is believed that fatigue life prediction of ship plating would be improved based on the results of the present research. | en_US |
