Abstract: Using strengthened 40Cr steel specimens, the authors examine the suitability of the method of surface reflected caustics in determining K_I of metallic specimens. The experiments show. that in order to get the correct values K_I for specimens of different thickness, the condition of r₀/t≥0.5must be satisfied. Furthermore, the off-plane displacement near the crack tip is measured by means of reflected holo-graphic interferometry and its partial derivatives are calculated by the method of cubic-spline interpolation. Thus the conclusion mentioned above is further verified.