DIRECT BASAL-PLANE SHEAR IN SINGLE CRYSTAL GRAPHITE.

The paper consists of an analysis of the overall stress-strain behavior of single crystals and, to a lesser extent, of pyrolytic graphite. In the order of increasing applied stress are presented the shear modulus G, the low critical resolved shear stress sigma sub c, the observations of creep and extreme glissile slip, and finally shear fracture and the effect of cyclic shearing on shear strength sigma sub s. The type and effect of bulk lattice imperfections, such as twins and delamination voids, are investigated. Correlations are made between the change in the observed G and the concentration of dislocations and bulk lattice imperfections. The average dislocation line segment length l and density N sub D are calculated from the observed sigma sub c and change in G. Attempts to circumvent the influence of dislocations by pinning or frequency damping are considered. One of these, the pinning of dislocations by boron ions, is observed here directly to be very effective in increasing G, approaching asymptotically the intrinsic lattice value G sub o. Author