Calibration of Weibull stress parameters using fracture toughness data

The Weibull stress model for cleavage fracture of ferritic steels requires calibration of two micromechanics parameters (m, sigma(u)). Notched tensile bars, often used for such calibrations at lower-shelf temperatures, do not fracture in the transition region without extensive plasticity and prior d uctile tearing. However, deep-notch bend and compact tension specimens test ed in the transition region can provide toughness values under essentially small-scale yielding (SSY) conditions to support Weibull stress calibration s. We show analytically, and demonstrate numerically, that a nonuniqueness arises in the calibrated values, i.e., many pairs of (m, sigma(u)) provide equally good correlation of critical Weibull stress values with the distrib ution of measured (SSY) fracture toughness values. This work proposes a new calibration scheme to find (m, sigma(u)) which uses toughness values measu red under both low and high constraint conditions at the crack front. The n ew procedure reveals a strong sensitivity to m and provides the necessary m icromechanical values to conduct defect assessments of flawed structural co mponents operating at or near the calibration temperature in the transition region. Results of a parameter study illustrate the expected values of m f or a typical range of material flow properties and toughness levels. A spec ific calibration is carried out for a mild structural steel (ASTM A36).