| This research uses micromechanical analyses to define the relationship between global, ductile fracture initiation parameters and constraint in large scale yielding conditions. Two local fracture criteria corresponding to ductile fracture micromechanisms were evaluated: a stress-modified, critical strain (SMCS) criterion for void coalescence and a critical void ratio criterion for void growth. Predictions were made in thin fracture specimens for a variety a/w ratios and load conditions ranging from three point bending to pure tension, and in circumferentially cracked pipes of various sizes. A516-70, a high-hardening, ferritic, C-Mn pressure vessel steel, and HY-80, a medium-hardening, martensitic HSLA steel were investigated.; In the criteria studied, the initiation of ductile crack extension was assumed to occur when the critical value of the local fracture parameter was reached over a critical length characteristic of the material microstructure. Critical values of the fracture parameters were obtained by numerical analysis and experimental testing of circumferentially notched tensile specimens of varying triaxiality. The characteristic length was similarly obtained using a crack geometry and verified through microstructural measurements. These values were then used to predict initiation in other geometries. Experimentally verified, large strain, large deformation, two- and three-dimensional finite element analyses were used to predict crack tip stress-strain distributions and the global fracture parameters associated with ductile fracture initiation. Initiation predictions were verified experimentally; microscopy was used to measure the crack tip opening displacement (CTOD) and the amount of stable crack growth in each specimen following testing to (and beyond) the predicted initiation load.; Results for A516-70 and HY-80 indicate that the SMCS criterion with a critical length approximately equal to twice the grain size (ferritic and prior austenitic, respectively) provides accurate predictions of ductile fracture initiation. The critical void ratio criterion was found to considerably underpredict crack initiation loads with the same critical length. The relationships defined by the SMCS criterion between global fracture parameters (J-integral, CTOD, flow stress) at initiation and constraint indicate that these parameters vary as much as 50%. Strain hardening did not significantly influence the observed constraint dependence. |
