The Crack Detection Using Dynamic X-ray Stress Measurement during Fatigue Test

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The purpose of this investigation is to detect both a surface and an internal fatigue cracks from actual stress distribution in the surface of specimen by using X-ray diffraction technique during fatigue test.In 1999, an apparatus to measure actual stress distribution along circumference of specimen was fabricated by use of a cantilever type rotary bending fatigue machine and a stress analyzer based on single exposure technique with two position sensitive proportional counters. The method to collect separately the diffraction profile at each position along circumference of specimen was developed. Actual stress distributions at the maximum tensile applied stress were measured using an annealed carbon steel S45C specimen during fatigue test.As a result in 1999, the shape of actual stress distribution was keeping stable with increasing number of stress cycles until surface crack initiation. After surface crack initiation, the actual stress at the position of surface crack initiation was d … More ecreased suddenly near 0 MPa, and the distribution was shaped "V" type. The "V" type distribution was appeared when crack length reached greater in size than a width of irradiation area.In 2000, the actual stress distributions on shot-peened S45C specimen were measured during fatigue tests to clarify the behavior of compressive surface residual stress through fatigue process. As a result in 2000, the actual stress at every position on specimen circumference was roughly the same value as the algebraic sum of the compressive residual stress and the maximum tensile applied stress until surface crack initiation. However, after a macro surface crack was appeared, the actual stress distribution was showed V-shape at crack position. The actual stress at the bottom of V-shaped distribution was the same value as the compressive residual stress.In 2001, the actual stress distributions on shot-peened ductile cast iron FCD600 specimens were measured during fatigue tests to clarify a possibility of the internal fatigue crack detection using change in surface actual stresses distribution. As a result in 2001, it was confirmed that the peak position in the actual stress distribution agreed with the internal crack position.The method proposed in this investigation is useful for detecting a position of both a surface and an internal cracks. On the other hand, the residual stress distributions showed no significant change even where the internal crack and the surface cracks existed, whereby the crack could not be detected by change in the residual stress distribution. Less