Whole grain stress cracks

Concerning the deformability, especially because of its sensitive dependence on the state of stress, there is, typically, insufficient information available. This indicates the importance of the stress state while only the deviator-tensor is responsible for plastic deformation, fracture is determined by the whole stress state. The clearly ductile fracture is a consequence of the joining of concentrated dislocation masses to a continuos deficiency at the grain boundaries. This process is retarded by the component of hydrostatic compression of the stress state. Ductile fracture is determined by the crack propagation, which is a consequence of an... 

Plastic flow in crystals occurs by dislocations moving on slip planes that slide over one another. Most experiments with single crystals to determine the CRSS are performed with tensile specimens. The shear stress on the active slip plane is computed by resolving the tensile stress onto the active slip plane. With polycrystalline tensile specimens, plastic flow still occurs on slip planes in the individual grains and these are grouped into wide shear bands that traverse the specimen at 45° from the tensile axis. The shear stress is usually calculated approximately for the whole specimen by dividing the tensile stress in half. Failure of a ductile tensile specimen usually occurs by initiation of a shear crack that is followed by a mode I crack that is normal to the tensile axis. Sudden flnal failure occurs when the true tensile stress exceeds the mode I fracture stress. 

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