Book Description

Fracture in steel structures represents a critical limit state in evaluating the safety and resiliency of civil infrastructure during earthquakes. This importance was demonstrated by the widespread fractures observed in older steel connections during the 1994 Northridge Earthquake, and in modern connections during the 2011 Christchurch Earthquake. The application of traditional crack-tip fracture mechanics to structural design provisions has successfully delayed the onset of Northridge-type brittle fracture. However, the extreme strain capacity in modern ductile connections increases the relevance of ductile fracture. Recent developments in 'local' fracture models have proven successful at predicting ductile fracture under many conditions. However, the application of these models has been limited due to their limited scope and difficulty in evaluation of the necessary continuum parameters. The current objective in the structural engineering community of replacing full-scale experiments with advanced finite element simulations require accurate models and calibration techniques to evaluate cyclic plasticity and fracture predictions. Motivated by the above requirements, the objectives of the present study are to (1) further the understanding of the ductile fracture mechanism for all stress, (2) develop robust methods for the calibration of constitutive parameters and local fracture models in highly plastic materials, and (3) to develop a new damage-based model to predict ductile fracture under all relevant structural conditions states (especially those with low stress triaxiality). These objectives are accomplished through an extensive experimental program, including 48 monotonic and cyclic specimens in geometries designed to effectively interrogate the fracture criteria. A total of six specimen designs are tested, including three original designs developed for the current study. Complementary finite element analyses are used to evaluate the local fracture criteria, and micrographic examination and void cell simulations provide insight into the fracture mechanism at varying stress states. The data from these experiments and the derived fracture model demonstrate the importance of the deviatoric stress state, in addition to the hydrostatic pressure, in the fracture ductility of steel. Specifically, material in a plane strain condition is found to exhibit about 50\% more fracture ductility than material in an axisymmetric stress condition. Through meta-analysis of test data from this and previous studies, ductile fracture is found to be prohibited under negative (compressive) hydrostatic pressure.