Microstructure microcracking

The evolution of spall in a body subject to transient tensile stresses is complex. A state of homogeneous tensile stress is intrinsically unstable and small perturbations in the material microstructure (microcracks, inclusions, etc.) can lead to the opening of voids and initiation of the spall process. 

Fig. 16.7. Microstructural features of a crystalline ceramic grains, grain boundaries, pores, microcracks and second phases.

Figure 9.22. Dislocation microstructures observed at a microcrack-ladder in a single crystal of natural quartz experimentally deformed under conditions of high water fugadty (Mn)04 buffer). In all micrographs, the electron beam is parallel to [1210] and g = 10ll. The loading direction [lOTO] and [0001] are marked.

The trace of the planar central zone of the microcrack-ladder is marked by the presence of negative crystals and inclusions. This characteristic indicates that microcrack-ladders must be related to healed longitudinal fractures. The formation of unhealed microcracks normal to the loading direction (i.e., the rungs of the ladders) can be understood as follows. Undulatory extinction and dislocation microstructures observed within the microcrack-ladders indicate that plasticity is restricted to a narrow zone ( 100 / m wide) within the otherwise-undeformed host crystal. Therefore, the host crystal responded elastically while the load was applied the elastic strain A/// being about 0.5 percent. Now, in the plastic zone of the... 

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