Measurement of three-dimensional crack tip deformation by digital image processing

Based on a pinhole camera model, camera model equations, which account for the radial lens distortion, are used to map three-dimensional (3D) world coordinates to two-dimensional (2D) computer image coordinates by the use of camera intrinsic and extrinsic parameters. Using two cameras to form a stereo vision and applying 2D digital image correlation techniques, the 3D information can be obtained. To achieve this, a camera calibration technique was developed to obtain the optimal intrinsic and extrinsic camera parameters for each camera.; Such stereo imaging system was used to measure both in-plane and out-of-plane displacement fields around the crack tip of fracture specimens made of Aluminum 6061-T6 and 304L stainless steel. To compare with the available two-dimensional theory of fracture mechanics, the measured displacement fields expressed in the world coordinate were converted, through coordinate transformations, to the displacement fields expressed in the specimen crack tip coordinates. By using a smoothing technique, the in-plane displacement components were smoothed and the total strains were obtained. To better understand the trend of the displacement field, rigid body motion was eliminated from the smoothed in-plane displacement components and unsmoothed out-of-plane displacement.; Results were compared with theoretical elastic and elastic-plastic fields. A K{dollar}sb{lcub}rm I{rcub}{dollar} field was detected for the AL specimen, but no HRR field was found. For 304L, a K{dollar}sb{lcub}rm I{rcub}{dollar} field was observed at lower loadings and an apparent HRR field was obtained at larger loadings. The two-dimensional J integral calculated using the experimentally determined in-plane displacements is path independent for each material. The J integral values for the aluminum specimen are close to the results obtained from both Merkle-Corten's formula and the EPRI report, but are 30% to 50% lower than the results obtained from the EPRI report for the stainless steel specimen.