Micas macroscopic-grained

Previous work indicates that the indentation response of mica glass-ceramic materials is a function of the macroscopic uniaxial compressive yield stress, Y and a strain-hardening characteristic. A range of behavior, from quasi-ductile to nominally brittle, could be attained by simply varying the size and shape of the grains through... 

It is thus claimed that any resistance to internal sliding depends upon the inherent shear strength of the mica platelets. This in turn depends upon the grain size, which then impacts on the macroscopic yield strength of the material. 

The relevance to the present work is that the scale of contacts in the mica platelets studied here is in the order of 1 to 10 pm. This is the intermediate zone referred to by Johnson in which the frictional stress depends upon the inverse square root of the grain size in the manner of a Mode II crack. It has been previously shown that estimations of yield stress for a range of platelet sizes based on an inverse square root law yielded predictions of macroscopic behavior that agreed well with experimentally observed behavior and that the macroscopic yield stress was determined to be in the order of 1 GPa. A yield stress in the order of 1 GPa gives a value of So in the order of 500 MPa, precisely within the intermediate zone described above. The nature of the observed Mode II behaviour on the microstructural scale is thus explained. 

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