No abstract

The overall objective of this study is to provide a proven methodology to allow the transfer of ductile fracture initiation properties measured in standard labora tory specimens to large, complex, flawed structures. A significant part of this work involved specifically addressing effects of constraint on transferability under large scale yielding conditions. The approach taken was to quantify constraint effects through micro-mechanical fracture models coupled with finite element generated crack tip stress-strain fields to identify the local condition corresponding to fracture initiation. Detailed finite element models predicted the influence of specimen geometry, loading mode, and material flow properties on the crack tip fields. The ability of two local, ductile fracture models (the Rice and Tracey void growth model [1] (VGM) and the stress-modified, critical strain (SMCS) criterion of Mackenzie et al. and Hancock and Cowling [2,3]) to predict fracture initiation were investigated. Predictions were made using experimentally verified, two- and three-dimensional, finite strain, large deformation, finite element analyses. Two, high toughness pressure vessel steels were investigated: A516 Gr70, a ferritic, carbon-manganese mild steel demonstrating high hardening behavior, and HY-80, a martensitic, high strength low alloy (HSLA) steel possessing medium hardening ability. Experimental verification of the ductile frac ture initiation predictions was performed in a variety of crack geometries possessing a range of a/w ratios from 0.15 to 0.70 and experiencing a range of load conditions from three point bending to nearly pure tension. The predicted constraint dependence of global ductile fracture parameters in the two materials is shown.

The effect of a realistic residual stress field on the predicted initiation of brittle and ductile fracture in a pressure and axially loaded circumferentiaUy cracked pipe is examined using finite element analysis, micromechanical models of fracture initiation, and J-Q theory. The study confirms that residual stresses contribute to the driving force and reduce fracture loads early in the loading history. In addition, results show that the residual stresses severely alter the J-value (i.e., fracture toughness) predicted for the onset of brittle fracture. The reason for this decrease is found to be the increase in constraint generated by the residual stress field. In contrast, the effect of residual stresses on the ductile fracture initiation toughness is shown to be negligible.

This study presents finite element simulations and fracture predictions for experimental conditions in a recent ASTM round-robin program on weld fracture toughness. The simulations include the effects of weld residual stress and residual stress alteration due to local compression of notched SE(B) specimens. Simulations are performed for a variety of material models and local compression details. Fracture is predicted using crack-tip stress and strain as inputs to both a local and a global prediction scheme. The local prediction is based on the RKR micromechanical model. The global prediction is based on obtaining a critical value of the J-integral, computed using a modified domain integral. Results predict that fracture toughness in the round robin was markedly reduced by local compression of the specimens. The amount of the reduction depends on the position of the compression platen relative to the notch-tip, but is largely independent of the amount of local compression applied. Available data from the round robin do not completely confirm the prediction results, but some of these data may have been influenced by warm pre-stressing during fatigue pre-cracking.