Using the local approach to evaluate scaling effects in ductile fracture

摘要

This paper uses a local model to predict ductilefracture in geometrically similar structures of different sizescontaining either sharp cracks or blunt stress concentrators.Simple theoretical considerations suggest that when fractureoccurs by quasi-isotropic void growth, fracture initiation at bluntnotches follows replica scaling, whereas fracture initiation atsharp cracks does not. Simulations with a local fracture model offracture events in (1) fatigue precracked compact specimens and(2) three-point-bend bars containing blunt notches confirm theseconclusions. However, a comparison of simulations with actualexperimental results with HY-130 steel specimens leads to mixedconclusions. Predicted and observed behaviors for fracture atsharp cracks agree well, but the discrepancy is considerable forfracture initiating at blunt notches loaded in bending. Significantscaling effects are observed in the experiments for the conditionsof fracture initiation at blunt notches.Fractographic analysis reveals that the reason for thisdiscrepancy is a difference in he micromechanisms controllingfracture at sharp cracks as opposed to blunt notches. At sharpcracks, quasi-isotropic void growth dominates, whereas fractureinitiates at blunt notches by a shear localization process and thenucleation, growth, and coalescence of voids in a mixed shearand tensile deformation field. The transition from one mode tothe other may be governed by the hardening rate and, if so, ismaterial dependent. Therefore, when using local fracture modelsfor predicting fracture under generalized geometric and loadingconditions, care must be taken, that the micromechanisms ofductile fracture invoked in the actual material match thoseassumed by the local fracture model. If this correspondence isverified, local fracture models can be used to predict fractureconditions and associated scaling effects for situations notamenable to treatment by classical elasto-plastic fracture mechanics.However, new or expanded models that can treat ductilefracture in localized shear zones should be developed to realizethe full potential of these local fracture methodologies.