Stress intensity factors calculation using full-field optical techniques

Abstract

Digital photoelasticity and digital image correlation (DIC) are experimental full-field optical techniques which are used for the analysis of stresses and strains in mechanical elements. In the current paper, these techniques are compared through the determination, using flat specimens, of stress intensity factors (SIFs) and T-stress (σ0x). A polycarbonate Middle-Tension (MT) specimen under tension has been tested, capturing a series of images for different loads. By photoelasticity technique, SIFs are calculating from isochromatic fringe field near the crack tip. This isochromatic map is obtained indirectly by image processing, which involves a phase image unwrapping procedure. In addition, the application of DIC on the test images provides the displacement
field directly, from which SIFs (KI, KII) and σ0x can also be determined. In photoelasticity, the relation between the fringe order and the stress intensity factors is non-linear, so a least squares fit has been used for their determination. For DIC, simply solving a system of linear equations as the relation between the displacements and SIFs is linear. The obtained results through both techniques are compared with those calculated from theory, showing a high level of agreement that highlights the potential of the used methodology.