Three-Dimensional Crack-Tip Deformation in a Plastically Deformed Three-Point Bending Specimen

A new technique of holospeckle interferometry using twin-plate recording has been developed in which three-dimensional crack-tip deformation within an area of 1 mm² can be mapped in detail. The resulting fringe has a sensitivity of 0 to 3 μm for out-of-plane displacement and 1.3 μm for in-plane displacement. The difficulty due to a difference in sensitivity between the hologram and specklegram techniques is eliminated by judiciously pairing the front (or back) plate of one holospecklegram with the back (or front) plate of another at different stages of loading. The technique was applied to a notched three-point-bending aluminum specimen under large-scale yielding. A three-dimensional crack-tip deformation field was obtained, and strain contours were plotted. This shows that the region directly above the crack experiences a relatively small amount of strain.