Micro-strain distribution around a crack tip by electron beam moiré methods

Abstract

The microscopic deformation around a crack tip in a small tensile specimen of aluminum was measured by using electron beam moire methods. The tensile test was carried out in a scanning electron microscope(SEM) by using specially designed small tensile testing apparatus for an SEM. A line grating with a pitch of 87 nm was written at the crack tip by electron beam lithography. SEM images of the grating contained(original) moire fringes at certain magnifications without loadings on the specimen. The displacement and strain in the region 10 to 50 μ m from the crack tip were evaluated by analyzing the (mismatch) moire fringes. Measured strains normal to loadings(longitudinal strains) near the crack tip were found to be comparable to those estimated using linear elastic fracture mechanics(LEFM), for the lowest load, and elastic-plastic fracture mechanics, at effective crack tip position located within the actual crack tip. The exponent of the strain singularity in the vicinity of the actual crack tip changed with applied stress, ranging between −0.64 and −1.0. The measured crack tip opening displacement(CTOD) values disagreed with CTODs based on the Hutchinson-Rice-Rosengren(HRR) field, the Dugdale model, and LEFM. The experimental plastic zones spread from the line of crack extension up to an angle of about 60°, as expected from LEFM theory.