Continuum mechanics modeling and experimental confirmation for the prediction of nanoscale transitional fracture behavior

A continuum mechanics-based model is used to explore the fracture properties of nanoscale multilayered materials. Length scale appears via inclusion of potential emission of discrete dislocations from a crack tip following the Rice-Thomson approach. For nanoscale multilayers, the high density of interfaces has a strong effect on the propensity for a crack to emit blunting dislocations, which relieve high local stresses, or not emit them, which promotes cleavage. The model is used to predict how material combination, layer thickness, crystal structure, and flaw size influence the effective fracture toughness of the material system for the “mesoscopic” size regime that bridges atomistics and continuum mechanics.; To confirm validity of the continuum model at the nanoscale and test certain critical predictions, viz. the directions of dislocation emission from crack tips, Bragg bubble raft experiments are performed. Perfect (dislocation-free) two-dimensional bubble rafts with sharp internal and interfacial cracks are subjected to the bounding cases of uniform displacement or traction to examine the nucleation and emission of dislocations. The experiments verify, qualitatively and quantitatively, the trends predicted by the model. Theory and experiment demonstrate transitions in emission behavior that take place over a large range of crack dimensions, crack locations, and loading conditions. Remarkably, the continuum model is confirmed to predict the crack dislocation emission behavior down to length scales approaching several atomic (bubble) spacings.; The continuum model is applied to materials and material systems to make predictions of nominal crack response as a function of specimen size and geometry. The model predicts substantial modifications to crack behavior on the nanoscale. Most notable are predictions of brittle-to-ductile transition in macroscopically brittle materials such as silicon and germanium. Finally, it is proposed that the theoretical model may have utility in making predictions of brittle-to-ductile transitions during precision machining operations involving nanoscale depths of cut. A preliminary investigation into an elastic cutting model, featuring a simple chip formation geometry, predicts a physical transition from purely elastic response to elastoplastic response in silicon and germanium on the nanoscale. These predictions exhibit intriguing consistency with experimental machining data.