A local approach to cleavage fracture incorporating the statistical distribution of microcracks

Abstract

This research addresses and further advances a local approach to cleavage fracture in terms of a modified form of the Weibull stress and associated probabilistic formulation. Within the present methodology, cleavage fracture is viewed as a multiscale problem in which the coupling between macroscopic (remote) loading, here characterized by the J-integral, and material failure at the microlevel is given by the measured statistical distribution of Griffith-like microcracks. Development of such probabilistic formulation thus requires: i) the local crack-tip stress and strain fields determined by detailed non-linear finite element analysis; ii) fracture toughness values in the ductile-to-brittle transition (DBT) region obtained by an extensive experimental program; iii) microstructure and fractographic characterization to determine the statistical distribution of Griffith-like microcracks (which are associated with the cleavage-nucleation sites) in the ferritic matrix and iv) operation history of the structural component or fracture specimen, including specifically temperature effects on fracture behavior. Development and exploration of the present research addresses the five key objectives described as follows: 1) Development and establishment of a structural integrity methodology based on a micromechanics (local) model based on the modified Weibull stress concept as and effective crack driving force; 2) Development of a probabilistic formulation applicable to brittle fracture, which derives explicitly from the experimental measurement of the statistical distribution of Griffith-like microcracks; 3) Parameter calibration of the local model to cleavage fracture by means of an experimental program to measure cleavage fracture toughness and to experimentally measure the statistical distribution of microcracks for typical structural steels, including pressure vessel steels; 4) Investigation of the microfeatures associated with transgranular cleavage in ferritic materials and their connection with local approaches to cleavage fracture, specifically related to the Griffith fracture energy; 5) Application and validation of the local methodology with particular emphasis on the structural integrity assessments of large structural components using fracture toughness values derived from subsize specimens. (AU)