CRACK TIP BEHAVIOUR IN RESIDUAL STRESS FIELD: FINITE ELEMENT MODELLING AND NEUTRON DIFFRACTION MEASUREMENTS

Simple analyses of fracture and fatigue often make use of the stress intensity at a crack tip or the J-integral surrounding it. However, there is no universally accepted method of including the effect of residual stress in these values, even though the qualitative effect of residual stress on crack growth is well known. In this work, we create a cracked compact tension C(T) specimen with a residual stress field that affects the crack tip behaviour, in particular by altering the level of expected crack closure. Neutron diffraction measurements under in situ applied loading reveal strain distributions consistent with an increased level of closure when the crack tip is in a state of compressive residual stress. Through finite element modelling of the samples studied, we show that the residual stress in these samples redistributes as the crack grows, which changes the level of crack closure for any given crack length and applied load. As crack closure is often considered in fatigue analysis by deriving an 'effective' stress intensity based on the applied load needed to overcome the closure and open the crack, the model is used to compare this approach with numerical calculations of the J-integral for different crack lengths.