Comparison of the ASME and Bao-Wierzbicki Local Failure Models

Abstract The ASME Local Failure Criterion, discussed in Section VIII, Division 2 of the ASME Boiler & Pressure Vessel Code, is widely used in pressure vessels and piping design as a method of predicting the initiation of local fissuring under a variety of different stress states. This relation maps the expected failure strain to the degree of triaxiality in a localized region, taking the form of a monotonically decaying exponential curve that passes through the point represented by the strain to failure at a uniaxial tension triaxiality state of +1/3. In this work, we present an investigation of an alternative failure locus (Bao-Wierzbicki) in low-triaxiality states where the ASME fracture locus struggles to predict shear failures. Being a piecewise-continuous ductility model with a pronounced local minimum at the zero-triaxiality position (i.e., pure shear), the Bao-Wierzbicki (B-W) model provides a more tunable representation of mixed-mode failure than the ASME default equation, particularly in low triaxiality where the ASME model predicts unrealistically high ductility. Through a series of experimental and numerical analyses, we demonstrate the efficacy of the B-W model in predicting the thread-stripping failure mode in a study of mechanically-simulated corrosion of ASTM A563 nuts interacting with intact ASTM A325 bolts. In comparison to common pressure vessel bolting, A325 bolts have a yield-to-ultimate strength ratio (R) of 0.77, whereas more commonly used A193 Grade B7 bolts have R ratios closer to 0.84. Our findings suggest that the B-W model may be preferred over the current ASME model for some applications in Pressure Vessel design and analysis activities, such as assessment of thread stripping.