2009 Volume 58 Issue 7 Pages 588-595
Fracture mechanics have been successfully applied for damage tolerant design of engineering structures against fatigue and stress corrosion cracking. The stress intensity factor is a key parameter to predict the progress of cracking behavior, and is computed from the shape of cracks and the loading stress distribution. In the present study, using high energy monochromatic X-rays of energy 66.4keV from the synchrotron radiation source, SPring-8, we have developed a system to perform the hybrid measurement of imaging of cracks and strain distribution around cracks. This system was applied to a fatigue crack made in a round bar made of carbon steel with the diameter of 4mm. Computed tomography (CT) of the specimen gave the three-dimensional shape of a thumb-nail crack. Scanning of lattice strain along the loading axis around cracks was conducted under the zero and maximum applied stresses. High tensile strain ahead of the crack was measured at the maximum stress, while the strain on the crack face was low because of crack opening. The full width at half maximum (FWHM) increased near the crack tip under loading, and then decreased after unloading. This recoverable part of FWHM by unloading was caused by the steep distribution of the applied stress in the vicinity of the crack tip. The increase of FWHM due to plastic deformation did not change by unloading.
