Shallow Flaws Under Biaxial Loading Conditions—Part II: Application of a Weibull Stress Analysis of the Cruciform Bend Specimen Using a Hydrostatic Stress Criterion1

Abstract

Cruciform beam fracture mechanics specimens have been developed in the Heavy Section Steel Technology Program at Oak Ridge National Laboratory to introduce a prototypic, far-field, out-of-plane biaxial bending stress component in the test section that approximates the nonlinear biaxial stresses resulting from pressurized-thermal-shock or pressure-temperature loading of a nuclear reactor pressure vessel (RPV). Matrices of cruciform beam tests were developed to investigate and quantify the effects of temperature, biaxial loading, and specimen size on fracture initiation toughness of two-dimensional (constant-depth) shallow surface flaws. Tests were conducted under biaxial load ratios ranging from uniaxial to equibiaxial. These tests demonstrated that biaxial loading can have a pronounced effect on shallow-flaw fracture toughness in the lower transition temperature region for RPV materials. Two and three-parameter Weibull models have been calibrated using a new scheme (developed at the University of Illinois) that maps toughness data from test specimens with distinctly different levels of crack-tip constraint to a small-scale-yielding Weibull stress space. These models, with a new hydrostatic stress criterion in place of the more commonly used maximum principal stress in the kernel of the Weibull stress integral definition, have been shown to correlate the experimentally observed biaxial effect in cruciform specimens, thereby providing a scaling mechanism between uniaxial and biaxial loading states.