A REVIEW OF THE J-INTEGRAL AND I-INTEGRAL AND THEIR IMPLICATIONS FOR CRACK-GROWTH RESISTANCE AND TOUGHNESS IN DUCTILE FRACTURE

The application of the J and the I-integrals to ductile fracture are d iscussed. It is shown that, because of the finite size of the fracture process zone (FPZ), the initiation value of the J-integral is specime n dependent even if the plastic constraint conditions are constant. Th e paradox that the I-integral for steady state elasto-plastic crack gr owth is apparently zero is examined. It is shown that, if the FPZ at t he crack tip is modelled, the I-integral is equal to the work performe d in its fracture. Thus it is essential to model the fracture process zone in ductile fracture. The I-integral is then used to demonstrate t hat the breakdown in applicability of the J-integral to crack growth i n ductile fracture is as much due to the inclusion in the J-integral o f progressively more work performed in the plastic zone as it is to no n-proportional deformation during unloading behind the crack tip. Thus J(R)-curves combine the essential work of fracture performed in the F PZ with the plastic work performed outside of the FPZ. These two work terms scale differently and produce size and geometry dependence. It i s suggested that the future direction of modelling in ductile fracture should be to include the FPZ. Strides have already been made in this direction.