A study of stable crack growth using experimental methods, finite elements and fractography

Systematic analysis of the in-plane constraint influence on J-resistance curves is presented. JR curves were also recorded and analyzed beyond the limits of crack extension inside which the stress field can be assumed to be dominated by J-integral. Three steels and four types of specimen: SEN(B), SEN(T), CCT and DENT have been tested. Along with the JR curves the fracture mechanisms have been analyzed with the help of scanning microscopy. The numerical, finite element analysis has been adopted to compute the Q-stresses, as a measure of the in-plane constraint prior to the onset of crack growth. The analysis of the stress field in front of the crack has been performed to check whether the state of stress prior to the crack growth can predetermine the way the crack will grow. It turns out that characteristic features in the JR curves runs can be predicted qualitatively from the Q(a/W) and Q(J) curves. However, there is a good correlation between Q-stress and voids diameters on fractured surfaces. Several patterns in JR curves runs have been observed for tested specimens; e.g. no influence of specimen thickness on JR curves runs was observed for side-grooved specimens. Strong influence of specimen thickness on JR curve shape was observed for non-side-grooved specimens. JR curve run higher for thinner specimens unless they are dominated by plane stress. For bent specimens JR curves run higher for shorter cracks but they run lower for specimens in tension.