Measurement of crack tip strains using neutron diffraction

Residual elastic strains and plastic zones near fatigue cracks and monotonically loaded fatigue cracks in compact tension specimens of AISI 4340 steel were measured using neutron diffraction. Compressive residual strain fields were observed close to the crack tip. Further from the crack tip the residual strains were tensile, reaching a maximum within 5 mm from the crack. Mode I stress intensity factors required to compensate for the residual compressive crack tip strains were approximated using crack tip profiles estimated from stretch zone measurements. Diffraction peak broadening indicating plastic deformation was observed within regions up to 2 mm from the crack tips. Estimation of the plastic zone sizes from residual strain distributions and peak broadening measurements agree well with theoretical predictions.