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C. J. Christopher et alii, Frattura ed Integrità Strutturale, 25 (2013) 161-166 ; DOI: 10.3221/IGF-ESIS.25.23 161 Special Issue: Characterization of Crack Tip Stress Field Extension of the CJP model to mixed mode I and mode II C. J. Christopher, G. Laboviciute, M. N. James University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK mjames@plymouth.ac.uk E. A. Patterson University of Liverpool, Brownlow Hill, Liverpool, L69 3GH, UK A BSTRACT . The present authors have previously proposed a novel ‘plastic inclusion’ approach for dealing with the local plasticity which occurs at the tip of a growing fatigue crack. This meso-scale model provides a modified set of crack tip stress intensity factors that include the magnitude of plastic wake-induced crack tip shielding and which have the potential to help resolve some long-standing controversies associated with plasticity-induced closure. The present work extends the CJP model to deal with the case of mixed Mode I and Mode II loading and thus opens up enhanced possibilities for testing it on inclined cracks in metallic specimens. This extension requires the addition of only one new force parameter to the model, i.e. an anti-symmetric shear force on either side of the crack. K EYWORDS . Mixed mode fatigue; CJP crack tip stress model; plastic inclusion; crack tip shielding. I NTRODUCTION he present authors have previously proposed a novel ‘plastic inclusion’ approach for dealing with the local plasticity which occurs at the tip of a growing fatigue crack [1]. Localised plasticity arises from crack growth mechanisms and essentially blunts the crack, creates a reversed cyclic plastic zone, and induces shear along the crack flanks, along with the possible generation of wake contact stresses which act on the applied elastic stress field at the boundary of the elastic-plastic enclave surrounding the crack. The outcome of this meso-scale model is a modified set of crack tip stress intensity factors that include the magnitude of plastic wake-induced crack tip shielding and which have the potential to help resolve some long-standing controversies associated with plasticity-induced closure. A full-field approach has been developed for stress using photoelasticity and also for displacement using digital image correlation. This model has been termed the CJP model by the authors, and is independent of the mechanisms of plastic deformation and is therefore potentially applicable to a variety of materials. The definition of the forces on the crack allows roughness-induced closure to also be accounted for in the calculated stress intensity factors. The model can also be used to mathematically explore the effect on shape of the crack tip stress field of changes in magnitude of the various parameters; for, example, the effect of variation in magnitude of increasing positive or negative T-stress on crack tip fringe patterns. Under Mode I loading, the new model uses four parameters to characterize the stress fields generated by the forces in Fig. 1; an opening mode stress intensity factor K F , the shear stress intensity factor K S , the retardation stress intensity factor K R , and the T-stress. In applications involving the DIC technique, stress intensity factors in the new four-parameter model can be solved directly from measured displacement fields using Muskhelishvili’s potential functions. T