Wide Range CTOD Estimation Formulae for SE(B) Specimens

Abstract

Fracture testing of single edge notch bend, SE(B), specimens containing shallow cracks is often undertaken to match constraint conditions between specimen and structure, thereby obtaining a more accurate assessment of structural integrity. However, standardized methods for measuring fracture toughness in terms of the crack-tip opening displacement (CTOD) do not currently provide formulae which relate experimentally measurable quantities to CTOD for shallow cracks. A CTOD estimation equation, based on results from plane-strain finite element analyses, is proposed to fill this gap. This equation applies to specimens having a wide range of crack depths (0.05 ≤ a/W ≤ 0.70) and Ramberg-Osgood work hardening exponents (4 ≤ n ≤ 50). The proposed equation provides CTOD estimates which are up to five times more accurate than those obtained using the procedures recommended by ASTM E1290. This improved accuracy is achieved by estimating the plastic part of CTOD from a work-like quantity rather than as a scalar multiple of CMODpl (as is done in E1290). Accuracy improvements of the new equation over the E1290 equation are especially notable for materials having a moderate to high work hardening capacity because the linear proportionality that E1290 assumes between CTODpl and CMODpl breaks down in the presence of work hardening. A procedure for estimating the value of the work hardening exponent (n) from the results of a simple tension test is presented and validated. Use of this procedure permits routine application of the proposed CTOD estimation equation in the characterization of fracture toughness. The only material property data that is needed to apply this equation in addition to that currently required by E1290 is the ultimate tensile strength.