In this paper the results of an experimental investigation on the effect of subcritical damage on the residual strength properties of notched composite laminates are presented. A procedure based on the digital image correlation method was applied to laminates subjected to static and fatigue tensile loading. The digital image correlation method (DICM) is a wholefield technique that calculates surface displacements and strains from digital images characterized by a random distribution of intensity grey levels.Graphite/PEEK (polyether ether ketone) and graphite/epoxy laminates with different stacking sequences were analysed and the damage progression near the stress riser was evaluated by means of the strain maps obtained by digital image correlation. It was found that damage developing before final fracture may significantly affect the structural performance of composite laminates. The digital image correlation technique allowed clarification of the beneficial or detrimental role played by the different failure mechanisms on the strain redistribution around the hole and, as a consequence, on the residual strength and fatigue life of notched samples.The findings of the investigation suggest that the DICM is an efficient and reliable tool for fullfield monitoring and detailed damage characterization of structural composite elements.

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Digital image correlation attempts to estimate displacement fields by digitally correlating two images acquired before and after motion. To do so, pixel intensity has to be interpolated at non‐integer locations. The ideal interpolator is the sinc, but as it requires infinite support, it is not normally used and is replaced by polynomials. Polynomial interpolation produces visually appealing results but introduces positional errors in the signal, thus causing the digital image correlation algorithms to converge to incorrect results. In this work, an experimental campaign is described, that aims to characterise the errors introduced by interpolation, focusing in particular on the systematic error and the standard deviation of displacements.

Digital image correlation (DIC) is a noninterferometric optical technique able to measure bidimensional\ud (tri-dimensional in its stereo version) displacement fields by comparing two images acquired before and after motion. In its standard formulation, the measurement is performed by comparing a small subset of pixels in the two images, looking for the best set of parameters that minimize a given error function under the assumption of an affine displacement field. This procedure is performed in several points of the image so as to allow the reconstruction of the full displacement field by interpolating the various independent samplings. Global DIC formulation avoids the a posteriori interpolation by describing the displacement field\ud with a finite-element-like mesh: the resulting minimization is global because the fitting parameters\ud (usually the nodal displacements) are shared by neighbor elements; thus the standard deviation of\ud displacements is lower. However, the new formulation raises the problem of the identification of the optimal\ud mesh. A possible solution to this problem is the adaptive mesh refinement. This work proposes a\ud simple h-refinement approach for global DIC and analyses its performance using both synthetic and\ud experimental data

Recently, a new macro-mechanical model for ductile damage was presented by Wierzbicki, Xue et al., trying to address the role assumed by Lode angle and pressure sensitivity. The new model requires several experimental tests to identify all its parameters, but should outperform the "standard" formulations at high triaxiality levels. In this work a comparison between the Lemaitre's Continuous Damage Mechanics model and the Wierzbicki's one is proposed. After calibration of both models, the displacement field measured using the Digital Image Correlation technique in a large shear test case is compared with results of Finite Element simulations obtained using the two damage models considered. Results are not conclusive but show that the new damage model is quite accurateas rupture criterion, but damage evolution history deviates to some extent from the experimentally observed behavior