MICRO-STRAIN DISTRIBUTION AROUND A CRACK-TIP BY ELECTRON-BEAM MOIRE METHODS

The microscopic deformation around a crack tip in a small tensile spec imen of aluminum was measured by using electron beam moire and methods . The tensile test was carried out in a scanning electron microscope(S EM) 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 ti p by electron beam lithography. SEM images of the grating contained(or iginal) moire fringes at certain magnifications without loadings on th e specimen. The displacement and strain in the region 10 to 50 mu m fr om the crack tip were evaluated by analyzing the (mismatch) moire and fringes. Measured strains normal to loadings(longitudinal strains) nea r the crack tip were found to be comparable to those estimated using l inear elastic fracture mechanics(LEFM), for the lowest load, and elast ic-plastic fracture mechanics, at effective crack tip position located within the actual crack tip. The exponent of the strain singularity i n the vicinity of the actual crack tip changed with applied stress, ra nging between -0.64 and - 1.0. The measured crack tip opening displace ment(CTOD) values disagreed with CTODs based on the Hutchinson-Rice-Ro sengren(HRR) field, the Dugdale model, and LEFM. The experimental plas tic zones spread from the line of crack extension up to an angle of ab out 60 degrees, as expected from LEFM theory.