Modelling of Crack-tip Blunting using Finite Element Method (FEM) (GAP-0088)

Abstract

For determination of fracture toughness in engineering structural materials, the standard procedure specifies the generation of a crack growth resistance curve (R curve) experimentally and identification of the critical toughness parameter by the intersection of an offset to a blunting line on the R curve. The use of a theoretical blunting line, as proposed in the standards, often gives conservative fracture toughness values particularly in high toughness materials. In this project, through finite element simulation of standard fracture mechanics test specimen, the crack blunting process in ductile materials is proposed to be investigated. From the load-displacement data generated by FEM, the crack tip opening displacement (CTOD) as well as J, an energy parameter of elastic plastic fracture mechanics, and Δa, the crack extension, accompanying the blunting process have been calculated. The effect of variation of material properties like Young’s modulus, E, yield stress, σy and strain hardening parameter, n, on the shape and slope of the blunting line has been established. The numerically obtained blunting line has been validated using experimental results. Important conclusions drawn from this investigation are the following ♦ the blunting line is mildly sensitive to the level of flow stress, with the inflection occurring at a lower flow stress and with reduced sharpness as the flow stress is increased. The blunting line slope is higher in low strength materials. ♦ the nature of the blunting line is greatly dependent with the work hardening capacity of the material, with the overall slope of the blunting line increases with increased hardening behaviour. ♦ the crack tip profile is elliptical not semicircular. For the low strain hardening materials the major axis is more than the minor axis i.e. CTOD<2*da. In case of high strain hardening materials reverse co-relation is observed i.e. CTOD>2*da.