Framework to Correlate a/W Ratio Effects on Elastic-Plastic Fracture Toughness (Jsub c)

摘要

Single edge-notched bend (SENB) specimens containing shallow cracks (al W < 0.2)are commonly employed for fracture testing of ferritic material in the lower-transition region where extensive plasticity (but no significant ductile crack growth) precedes unstable fracture. Critical J-values (Jc) for shallow crack specimens are significantly larger (factor of 2-3) than the Jc-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 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 (J) characterization of the crack-tip stress field. Moreover, these analyses provide the first quantitative framework to correlate Jc-values with a/W ratio once the deformation limits are exceeded. Keywords: Elastic-plastic fracture toughness, Constraint, Shallow crack. (JES)