Effect of subset parameters selection on the estimation of mode-I stress intensity factor in a cracked PMMA specimen using digital image correlation

Polymethyl methacrylate (PMMA) tension specimens with double edge cracks are considered in this study. Surface displacements of the samples are obtained using digital image correlation technique using different values of subset size and subset distance. Results of displacement components from various regions in front of the crack are utilized in a least squares fitting procedure to obtain the first stress intensity factor. This is accomplished using series representation of displacement components of a cracked structure in plane loading conditions. Effects of two adjustable parameters of digital image correlation analysis, including subset size and subset spacing, on the obtained values of stress intensity factor are investigated, while the number of terms truncated from the series representation and the radius of data extraction zone are varied. Results indicate that small subsets may not properly converge to accurate results, irrespective of the value of subset spacing. Moderate subset sizes will usually yield acceptable results while large subset sizes and subset spacing may not include sufficient data to reproduce the desirable value of stress intensity factor. It is also shown that at least three or more terms in conjunction with a sufficiently large radius of data extraction region should be considered in least squares fitting to acquire a reasonable estimate of stress intensity factor in cracked specimens.