Finite element analysis of the modified ring test for determining mode I fracture toughness
Review articleOpen access

AbstractPlane strain fracture toughness (KIc) values are determined for the modified ring (MR) test through numerical simulation of crack growth to highlight the sensitivity of MR KIc values on applied displacement or force boundary conditions, slip conditions at the specimen-platen interface, and the Poisson ratio (v) of the test material. Numerical calculation of fracture toughness in the MR test is traditionally conducted assuming a uniform force along the specimen loading surfaces and no slip between the specimen and the loading platens. Under these conditions KIc increases by 30–40% as v decreases from 0.4 to 0.1. When slip is allowed at the specimen-platen interface under a uniform force, KIc values are independent of v, and for any given v, are 5–25% less than those determined under a no-slip boundary condition. Under a uniform displacement of the specimen loading surfaces, KIc is essentially independent of v, regardless of specimen-platen interaction. Moreover, although KIc values determined under unifor displacement and no-slip boundary conditions are always higher than those determined under uniform displacement and slip-allowed boundary conditions, the average difference in KIc for these two methods is less than 5% for the two specimen geometries examined. This suggests that under uniform displacement conditions, KIc is essentially independent of specimen-platen interaction. Because KIc values determined from MR testing are strongly dependent on the modeling procedure, future reports of KIc determined from this test should be accompanied by detailed reports of the modeling procedure. Until further testing reveals the most accurate simulation technique, we advocate use of a uniform displacement formulation for KIc determination from MR testing because results from this method are insensitive to most modeling parameters. Numerical results from models conducted under uniform force, no-slip boundary conditions should be viewed as an upper bound to KIc.

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