This paper presents an outline of the development, verification and application of a new model of
crack tip stress fields in the presence of a plastic enclave around a growing fatigue crack. The approach taken
rests on capturing the effects of the crack tip and crack wake plastic zones through elastic stress distributions
applied at the elastic-plastic boundary. A Muskhelishvili complex potential extension to the Williams crack tip
stress field is found for four stress parameters representing a K-stress, a T-stress, a crack retardation stress and a
compatibility-induced shear stress at the elastic-plastic boundary. This model is validated via full field fitting to
photoelastic stress fringe patterns, obtained from epoxy resin and polycarbonate specimens. It has also been
extended to the strain fields measured in digital image correlation techniques, which allows its application to
metallic alloys.