A framework to correlate a/W ratio effects on elastic-plastic fracture toughness (J c )

Abstract

Single edge-notched bend (SENB) specimens containing shallow cracks (a/W < 0.2) are commonly employed for fracture testing of ferritic materials in the lower-transition region where extensive plasticity (but no significant ductile crack growth) precedes unstable fracture. Critical J-values (J,.) for shallow crack specimens are significantly larger (factor of 2-3) than the J,-values for corresponding deep crack specimens at identical temperatures. The increase of fracture toughness arises from the loss of constraint that occurs when the gross plastic zones of bending impinge on the otherwise autonomous crack-tip plastic zones. Consequently, SENB specimens with small and large a/W ratios loaded to the same J-value have markedly different crack-tip stresses under large-scale plasticity. Detailed, plane-strain finite-element analyses and a local stress-based criterion for cleavage fracture are combined to establish specimen size requirements (deformation limits) for testing in the transition region which assure a single parameter characterization of the crack-tip stress field. Moreover, these analyses provide a framework to correlate J~-values with a/W ratio once the deformation limits are exceeded. The correlation procedure is shown to remove the geometry dependence of fracture toughness values for an A36 steel in the transition region across a/W ratios and to reduce the scatter of toughness values for nominally identical specimens.

Review of fracture toughness (G, K, J, CTOD, CTOA) testing and standardization

“…Thus, the biaxiality ratio or the Tstress corresponds to the observed experimental trends, and is a qualitative index of the relative crack-tip constraint levels for various geometries with a positive value representing a high level of constraint and a negative value denoting a low level of constraint. The elastic-plastic FEA results of Dodds et al [253] have shown that for any two cracks having the same K (or J) at their crack tips the crack-tip stress and strain fields can be different if the constraint levels are different. Bilby et al [254] showed that the T-stress can dramatically affect the magnitude of hydrostatic triaxiality in the immediate vicinity of the crack tip.…”

Approximate techniques of J estimation applicable to part-through surface cracks

“…fail by transgranular cleavage [10]. This more detailed study would require meshes with 2-3 times the number of nodes and elements.…”

Study on Thickness Effect of Three-Point-Bend Specimen

“…This is called the constraint effect. Many studies have been conducted on the effect of the constraint (4), (5) . Practically, the local approach method (6), (7) gave one solution to the constraint condition problem.…”