The Measurement of Crack Tip Stresses by X-Ray Diffraction
Final rept. Jun 1976-Dec 1977
AIR FORCE FLIGHT DYNAMICS LAB WRIGHT-PATTERSON AFB OH
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To ensure the structural integrity of present day structures subjected to variable amplitude cyclic stress histories, a knowledge of the crack tip stress field is important. In this investigation, crack tip stresses were measured in specimens of 1020 and 1045 steel using a semi-automatic x-ray diffraction technique. Crack tip residual stresses were measured in the unloaded condition and crack tip applied stresses were measured under load. It was observed that, for the alloys tested and within the confines of stress analysis by x-ray diffraction, a dependence exists for the maximum crack tip residual stress on the level of the previous stress intensity factor and also for the maximum crack tip applied stress on the level of the applied stress intensity. This is in sharp contrast to theoretical models of the crack tip stress fields which predict an independence between crack tip stresses and stress intensity levels. Crack tip stresses were observed relative to the fatigue crack growth retardation process. Post overload crack tip stresses were greatly reduced from their pre-overload levels. Limited studies indicated that crack growth, after an overload, had little or no effect on the overload-induced crack tip residual stress distribution. Measurements taken at applied loads, indicated that portions of the material behind the tip of the extended crack were in compression while areas ahead of the extended crack were in tension. These results were considered in light of possible retardation mechanisms.
- Metallurgy and Metallography
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