EFFECT OF SPECIMEN SIZE ON J_R RESISTANCE CURVE

The effect of specimen thinkness on JR resistance curve has been investigated using low and middle strength steels. As the J dominance regime is reached, the initial J-integral values, J(i), are definite even if the elastic component Ji(e) and plastic component Ji(p)vary with the thinkness of specimen. The wider the specimen width, the larger the Ji(e)/Ji and the smaller the Ji(p)/Ji. The crack propagation resistance, dJR/da and dJp/da, decreases, while dJe/da increases linearly with increasing specimen thinness.Based on the above experimental results, an empirical formula dJ/da = α+β. B is given, which can be used to evaluate the safety of practical structural components.A modified J-integral is presented, upon which a new view is Put forward that the increase in crack growth resistance is induced mainly by the increase in the plastic J-integral component with the crack propagation,

Review:J-integral resistance curve testing and evaluation

In this paper a critical review is presented of the history and current state of the art of J-integral resistance curve testing and experimental evaluation methods in conjunction with a discussion of the development of the plane strain fracture toughness test standard ASTM E1820 developed by American Society for Testing and Materials (ASTM). Early research efforts on this topic are reviewed first. These include the J-integral concept, experimental estimates of the J-integral for stationary cracks, load line displacement (LLD) and crack mouth opening displacement (CMOD) based η factor equations, different formulations of J-integral incremental equations for growing cracks, crack growth corrected J-R curve determination, and experimental test methods. Recent developments in J-R curve testing and evaluation are then described, with emphasis on accurate J-integral incremental equations, a normalization method, a modified basic method, a CMOD direct method with use of incremental equations, relationships of plastic geometry factors, constraint-dependent J-R curve testing and correction approaches. An overview of the present fracture toughness test standard ASTM E1820-08a is then presented. The review shows that after more than 40 years of investigation and development, the J-integral resistance curve test methods in ASTM E1820 have become simpler, more cost-effective and more accurate.

A Suggestion to Make Gurson Damage Model Mesh Independent for Cracked Specimens

Continuum damage mechanics is applied to compute fracture J-initiation and J-Resistance （J-R） curve for 20MnMoNi55 material by analyzing TPB and CT specimens. A strong mesh dependency of Gurson model is observed. The exponential form of q2 parameter shown earlier near crack tip is further modified to have a variable asymptotic value q2a away from crack tip. The asymptotic value q2a is suggested to be a function of crack-tip mesh size to overcome mesh dependency of J-R and load-displacement curves. The relation between q2a with the normalized crack-tip mesh size is determined numerically for CT and TPB specimens.

Ductile extension of 3-D external circumferential cracks in pipe structures

This study investigates the ductile fracture resistance of 3-D external circumferential cracks in the wall of a steel pipe under remote tension,using a damagemechanism model originally proposed by Gurson and Tvergaard.The ductile crack extension utilizes an element extinction technique implemented in the computational cell framework.The key parameter for the computational cell method,i.e.,the initial porosity ratio f0,is calibrated using both the fracture resistance and the load-deformation responses obtained from fracture tests of multiple singleedge bend[SE(B)]specimens made of high-strength steel,HY80,which has a yield strength of 630 MPa.The fracture resistance along the 3-D semi-elliptical crack front is computed from the calibrated cell model.Based on the similarity concept in the near-tip stress-strain fields,this study demonstrates that an equivalent 2-D axi-symmetric model provides conservative estimations of the fracture resistance for 3-D circumferential cracks in pipes.