ISSUES IN STRUCTURAL FATIGUE DESIGN AND EVALUATION

Current practices in the design and evaluation of structural component s for fatigue are generally semi-empirical. This is because either it is not practical or economical to realistically simulate field conditi ons in the laboratory testing or the actual conditions are not known p recisely. Fatigue life curves based on the constant amplitude cyclic l oading may result in unconservative design and evaluations where real load cycles are variable and the loads are of transient nature. There is a need to apply probability-based principles to the fatigue conside rations, particularly in the areas of developing representative fatigu e loading, and fatigue crack detection. In the area of power transmiss ion lines, there is a need for better understanding of the failure mec hanism and rationalize the fatigue design provisions. The state of str ess within a stress concentration region depends only on the local bou ndary geometry, the magnitude of the resultant force, and the material type. This equivalence of stress concentration regions can be exploit ed to develop design aids for stress concentration factors independent of the global conditions. This paper identifies the above issues in d etail and establishes some specific areas where there is a need for im provement in the current practice.