A generalized approach for determining the stress intensity factors (SIFs) K I and K II for any mode mixity directly from displacement fields obtained by digital image correlation is presented using a centre fatigue cracked aluminium plate as an example problem. It was found that the crack-tip position could be determined on average to within 50 per cent of the displacement vector spacing (60 μm). The approach has been shown to be fairly robust, both in terms of the stability of the SIFs thus obtained and their sensitivity (less than 0.07 MPa

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A B S T R A C T This study presents a methodology for evaluating crack closure and the effect of cracktip plasticity on stress intensity. Full-field displacement maps obtained by digital image correlation are used to obtain the mixed-mode, crack-driving force. The methodology allows the quantification of the effect of a range of contact phenomena: effects arising from interlocking, plastic deformation of crack face asperities and wedging generated as a consequence of sliding displacements of fatigue cracks have been identified. By evaluating the effective crack-tip stress intensity factor, crack opening levels can be quantified for both mode I and mode II. Moreover, the approach can take into account plasticity effects local to the crack in determining the stress intensity factor. All the information can be extracted in a non-contacting fashion with equipment that can be easily incorporated into industrial environments.Keywords digital image correlation; effective stress intensity factor; fatigue crack closure; plastic zone. N O M E N C L A T U R Ea = crack length a k , b k = complex coefficient for Fourier series representation A = coefficient matrix of the system Aχ = b b = right-hand side term of the system Aχ = b c = cross-correlation function C j k , D j k , E j k , F j k = coefficients for point j E = Young's modulus G = error function i = square root of −1 I(x, y) = image intensity function k = summation index for Fourier series K = complex stress intensity factor, K = K I -iK II K I = opening mode stress intensity factor K II = shear mode stress intensity factor L = contour in real space m = shape parameter of mapping function N = limit in Fourier series p = maximum number of collected displacement data points r p = Dugdale approximation of the extent of the plastic zone R = scale parameter of mapping function Correspondence: P. Lopez-Crespo.