Application of J-Integral and Modified J-Integral to Cases of Large Crack Extension

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Abstract
The J-integral is widely accepted as a measure of elastic-plastic fracture toughness of engineering alloys. Specimen size and geometry dependence were first noted in fracture toughness measurements using the ASTM E 813 calculation of the deformation J-integral (Jd) by McCabe and Landes [1] in 1983. The modified J-integral (Jm) was introduced by Ernst [2] to attempt to minimize or eliminate size and geometry dependence. Since Jm was introduced, questions have arisen regarding the proper parameter to describe the response of a flawed body to loading. The objective of this research task is to investigate the crack growth and specimen J capacity limitations of Jd and to verify the accuracy and specimen independence of the current Jm formulation. This research is expected to affect decisions concerning the use of small laboratory specimens to predict elastic-plastic crack growth resistance in engineering structures. The J-R curve tests have been conducted on ½T, 1T, and 2T compact specimens of materials having critical fracture toughness values ranging from JIc = 140 to 455 kJ/m2. These materials include HSLA-80 steel, A106 steel, 3-Ni steel, and two A533B steels. These tests were conducted in accordance with ASTM E 1152 except that specimen loading was continued until large crack extensions were present, in many cases exceeding 50% of the initial uncracked ligament (b). All specimen data were then analyzed using the equations of ASTM E 1152 for the standard deformation J-integral (Jd) resistance curve. Additional analysis was conducted as well in terms of the modified J-integral (Jm) using equations proposed by Ernst and Landes [3]. This latter quantity was possibly useful to larger crack extensions. The test procedure was directed towards finding the useful limits of the two J quantities for the materials listed above or for materials of equivalent strength and toughness.