Load-shedding Techniques for Fatigue-threshold Determination

Current manual and automatic load-shedding methods for the testing of the fatigue threshold (delta-Kth) in materials and structures are reviewed, with a focus on the tentative ASTM standard described by Hudak et al. (1978) and Bucci (1981). The advantages, disadvantages, and some material-based alternative crack-extension requirements are discussed for conventional and experimental manual load-shedding procedures. In automatic approaches, the load shedding is continously tailored to the growth of cracks by a digital minicomputer controlling the hydraulic test apparatus. The mathematical relationships and crack parameters employed by different laboratories are examined. It is found that no universally superior method has yet been developed, so that specific material and design factors must be considered in choosing a fatigue-threshold-testing method.