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A quantitative analysis of the interaction fatigue crack propagation and ofthe shielding effects associated with the plasticity along the flanks and at the tip of acrack has been performed. In two CT polycarbonate specimens, fatigue cracks weregrown under low frequency loading: the first followed a nominal cycling, while anoverload was applied to the second one and the shielding level monitored. Images of thetwo tests were collected by means of digital phase-stepping photoelasticity, andisochromatic fringe patterns of the cracks were obtained by a recently developedunwrapping technique. Stress intensity factors were evaluated fitting a mathematicalmodel to isochromatic fringe data. The model separates the effects of applied load andof shielding due to the plasticity at the crack tip and flanks on the propagation and thusallows some qualitative observations to be made on the underlying fatigue mechanisms.