Determining SIFs Using DIC Considering Crack Closure and Blunting

Three analytical-experimental hybrid approaches for determining the range of stress intensity factors (SIF) and the pseudo-SIFs of fatigue cracks on the presence of crack closure, crack tip plasticity and blunting are presented and evaluated. These approaches use the Digital Image Correlation (DIC) technique to measure cyclic-varying displacement fields near the crack tip. The first uses displacement data of points located on the cracked component surface near and along the crack faces, finding the parallel and orthogonal displacements of symmetrical points with relation to the crack faces to determine the crack opening displacements (COD), which are placed in the crack displacement field equations to obtain the SIF values. The second determines coefficients of generalized Westergaard displacement field functions by an over-deterministic nonlinear Least Square scheme that allows the accurate localization of the crack tip coordinates. The third computes the J-integral along an arbitrary but elastic contour path placed around the crack tip to determine the SIF, using experimentally determined displacement gradients, the calculated stresses and the calculated energy densities for points along the path. The three approaches are applied to the case of cracks propagating in disk-shaped compact-tension DC (T) specimens subjected to mode I cyclic loads, considering the non-linear effects mentioned above.