Observation of Internal Crack of Steel Bar by Computed Tomography and Strain Mapping near Its Tip Using Synchrotron White X-Ray

Observation of the internal crack of steel by a computed tomography (CT) and strain mapping near its tip were investigated using a white X-ray obtained from BL28B2 beam line at SPring-8 in Japan. A low-alloy and high-tensile steel was used as a specimen prepared in the tensile bar with a parallel part of 5mm diameter. A fatigue crack was introduced into the specimen under the pulsating tension load (stress ratio R = 0). CT observation of the crack in the specimen was carried out by using the CCD camera that can detect indirectly the X-ray transmitted through the specimen. To measure the strain using the energy dispersive X-ray diffraction technique, the synchrotron white X-ray beam, which had a height of 100μm and a width of 100μm, was incident on the specimen with the Bragg angle θ of 5 degrees. As a result, the crack in the specimen under the tensile loading could be observed by the CT using the white X-ray. The resolution of CT, which was about 18μm3, was considerably lower than it using a monochromatic X-ray reported previously. The large tensile strain was measured near the crack tip. It was, however, relatively small under the influence of the long slender gauge volume as compared with the crack tip distributed circularly. It was confirmed that the FWHM of diffracted X-ray profile measured near the crack tip was increased due to the steep change in the strain distribution