Volume 37, Issue 4 (July 2009)
Crack Size Evaluation Using Unloading Compliance in Single-Specimen Single-Edge-Notched Tension Fracture Toughness Testing
Crack-mouth-opening displacement (CMOD) unloading compliances of single-edge-notched tension (SE(T)) specimens, both pin-loaded and clamped, have been evaluated by finite element analysis (FEA), including evaluation of the effect of rotation during testing on compliance. For pin-loaded specimens, FEA results are compared with compliance correction equations based on a rigid-body rotation model recommended by Joyce and Link. The results agree reasonably well when the load line is far from the midsection of the ligament. However, the discrepancy increases when the load line approaches the midsection of the ligament where the bending load becomes small or negligible compared with the tension component and the rigid-rotation model breaks down. For clamped SE(T) specimens, factors affecting specimen rotation, such as the ratio H/W of span between load points H to width of the specimen W, normalized crack size, a/W, and material strain hardening coefficient, N, were investigated. This work was performed for specific application to surface circumferential cracks in pipes, for which the best constraint matching has been found to occur for clamped specimens with H/W = 10. For this geometry, an equation is proposed to correct compliance for rotation and to estimate crack size from rotation-corrected CMOD compliance. Crack sizes evaluated using this equation are compared with measurements on broken specimens. Excellent agreement has been achieved for initial and final crack sizes of both plain-sided and side-grooved SE(T) specimens.