A review of the proposed K_Isi offset-secant method for size-insensitive linear-elastic fracture toughness evaluation

Recently proposed modifications to ASTM E399, Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness K_Ic of Metallic Materials, would provide a new size-insensitive approach to analyzing the force-displacement test record. The proposed size-insensitive linear-elastic fracture toughness, K_Isi, targets a consistent 0.5-mm crack extension for all specimen sizes by using an offset secant that is a function of the specimen ligament length. With intent to increase flexibility in the test method, the K_Isi evaluation method removes the P_max/P_Q criterion, which can penalize materials with rising tearing resistance curves, and increases the limits on specimen deformation, which allows more plasticity at the crack tip at the onset of crack extension. The primary motivation for this investigation is to confirm the validity of this new interpretation of the force-displacement test record in regard to the increase in acceptable specimen deformation. This article summarizes a finite element study of the effects of increased crack tip plasticity on the K_Isi evaluation method, with two primary points of investigation: the continued validity of linear-elastic fracture mechanics (LEFM) to describe the crack front conditions and the effect of crack tip plasticity on compliance change in the force-displacement record. The analytical study illustrates that LEFM assumptions remain valid at the increased deformation limit; however, the influence of plasticity on the compliance change in the test record is problematic. A proposed revision to the validity criteria for the K_Isi test method is described.