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C. J. Christopher et alii, Frattura ed Integrità Strutturale, 25 (2013) 161-166 ; DOI: 10.3221/IGF-ESIS.25.23 165 and the plane of the original notch in the CT specimen, e.g. K F = ( K F +K II )/√2 and K II = ( K f -K II )/√2 (or the negative of this, depending on how the force directions were chosen). However, the belief of the present authors is that it is more useful to define stress intensity factors in terms of the local crack plane, because crack growth rate is governed by these local conditions. : Geometry and dimensions (in millimetres) of the polycarbonate compact tension (CT) specimens used in this work. Note Figure 2 that the notch length (equal to the initial crack length) a i = 20 mm and the width b = W = 72 mm are defined from the centreline of the holes. DIC: y -displacement a) CJP model: y - b) DIC: x -displacement c) CJP model: x - d) : Comparison between DIC and CJP model predictions of the x and y -displacement fields at the tip of a crack under mixed Figure 3 mode fatigue loading.