Deformation Versus Modified J-Integral Resistance Curves for Ductile Materials

The J-integral resistance curve (or J-R curve) is an important fracture property of materials and has gained broad applications in assessing the fracture behavior of structural components. Because the J-integral concept was proposed based on the deformation theory of plasticity, the J-R curve is a deformation-based result. It has been known that the J-R curves of a material depend on specimen size and geometry; therefore, a modified J-integral or J_m was proposed to minimize the size dependence. Extensive experiments have shown that the J_m-R curves might remain size-dependent and could not behave better than the traditional deformation J-R curves. To date, however, it is noticed that the J_m-R curves were still used as “size-independent” results in some fracture mechanics analyses. It is necessary to revisit this topic for further clarification. This paper presents a brief review on the development of deformation and modified J-integral testing, and obtains a simple incremental J_m-integral equation. It is followed by typical experimental results with discussions on the issues of constraint or size dependence of J-R and J_m-R curves for different steels and specimens. Finally, a recommendation is made on properly selecting a resistance curve in the fracture analysis.