The stress-strain state and stress-intensity coefficients in a plate at the tip of a crack-like defect are investigated on the basis of photoelasticity and holographic interferometry. The components of the stress tensor, the primary stress, the stress intensity, and the elastic strain at the tip of a cut are determined. Various methods of determining the stress-intensity coefficient from maps of isolines of the total primary stress, maximum tangential stress, and absolute path differences at the tip of an elliptical cut are analyzed. New analytical formulas are obtained for determining the stress-intensity coefficient on the basis of linear failure mechanics.