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The characterization of fatigue crack closure is an important objective because of its influence on fatigue crack propagation, particularly under conditions of variable amplitude loading. This paper describes a nontraditional technique for characterizing closure, in which ultrasonic scattering measurements are used to obtain estimates of the number density and size of asperities bridging the crack faces, with subsequent estimates of the crack-tip shielding being based on those geometrical parameters. The paper first reviews the experimental configuration and the basic elasto-dynamic theory underlying the technique. It then presents recent results obtained in studies of the influence of block overloads and load shedding on the growth of fatigue cracks in aluminum alloys. In both cases, the change in the closure state after the overload can be seen unambiguously even in the raw data. Moreover, data analysis suggests that it may be possible to predict when the crack will reinitiate based on more subtle changes in the ultrasonically inferred closure state. In the case of load shedding, a massive closure region is observed, whose characteristics appear consistent with the notion that threshold phenomena can be explained in terms of crack closure.
asperities, contact topology, crack closure, crack-tip shielding, dynamic crack-opening displacement, fatigue cracks, load shedding, overloads, through transmission, ultrasonic scattering, fracture mechanics, fatigue (materials)
Associate directorprofessor, Iowa State University, Ames, IA
Program directorprofessor, Iowa State University, Ames, IA
Associate metallurgist, Ames Laboratory, Iowa State University, Ames, IA