ACCURATE MEASUREMENT OF 3-DIMENSIONAL DEFORMATIONS IN DEFORMABLE AND RIGID BODIES USING COMPUTER VISION

Recently, digital-image-correlation techniques have been used to accur ately determine two-dimensional in-plane displacements and strains. An extension of the two-dimensional method to the acquisition of accurat e, three-dimensional surface-displacement data from a stereo pair of C CD cameras is presented in this paper. A pin-hole camera model is used to express the transformation relating three-dimensional world coordi nates to two-dimensional computer-image coordinates by the use of came ra extrinsic and intrinsic parameters. Accurate camera model parameter s are obtained for each camera independently by (a) using several poin ts which have three-dimensional world coordinates that are accurate wi thin 0.001 mm and (b) using two-dimensional image-correlation methods that are accurate to within 0.05 pixels to obtain the computer-image c oordinates of various object positions. A nonlinear, least-squares met hod is used to select the optimal camera parameters such that the devi ations between the measured and estimated image positions are minimize d. Using multiple orientations of the cameras, the accuracy of the met hodology is tested by performing translation tests. Using theoretical error estimates, error analyses are presented. To verity the methodolo gy for actual tests both the displacement field for a cantilever beam and also the surface, three-dimensional displacement and strain fields for a 304L stainless-steel compact-tension specimen were experimental ly obtained using stereo vision. Results indicate that the three-dimen sional measurement methodology, when combined with two-dimensional dig ital correlation for subpixel accuracy, is a viable tool for the accur ate measurement of surface displacements and strains.