Experimental measurement of the crack tip strain field in a single crystal

Crack tip deformation fields in an aluminum single crystal are quantitatively obtained by applying a three-dimensional moiré interferometry technique. Tensile specimens made of an aluminum single crystal have an edge crack on the (2̄23) plane with their tips along the [10, 7, 2] direction. Specimens with this specific orientation undergo power law hardening with the hardening index n being 3.1. The experimental arrangement for moiré interferometry results in a displacement sensitivity of 0.47 μ/fringe. Displacement fields at the free surface near the crack tip are mapped with moiré fringe patterns. Both effective strain and maximum shear strain within a small region of less than 2 mm from the crack tip are evaluated. In partial agreement with earlier analytical and numerical solutions, the results of this experiment show that the deformation fields near a crack tip are divided into several different angular sectors, and concentrated shears are found at these sector boundaries where displacements are continuous. The experimental results also show that the effective strain and maximum shear strain remain constant (independent of angle θ) within certain angular sectors.