岩石の塑性についての課題

抄録

Although surfacial rocks appear brittle, the earth's crust the large aggregate of rocks is supposed to be very viscous. In the realm of rock mechanics, however, the aspect of plasticity cannot be overlooked. Rock deformations that accompany any kind of failures or fractures are declaredly plastic. Fine slips and incipient failures of rocks may take places where locally concentrated stresses satisfy the yielding condition, while the apparent general stresses are consistently below the elastic limits. The crypto-plastic flow as a result of such minor slips in the geological materials throw many problems not yet settled.
In a polycrystalline rock, boundary surfaces of grains are obstructive for moving inside dislocations and apt to concentrate local stresses matchable to the yield values. Similar concentration processes may occur in the matrices of sedimentary rocks regardless whether the cementing substance is a solid mineral or adhesive water.
Brittle tensile fractures created in homogeneous fine-grained rocks exhibit uneven surfaces with special patterns of (1) concentric undulations-conchoidal fractures, (2) radial striatures at right angles with the conchoidal waves-plumose fracture and (3) columnar fringes. Tension cracks under the high confining pressures show none of such figures.
The so-called angle of internal friction is merely expressing increase in strength with increasing lithostatic pressure. The effect of pore fluid pressure upon the fracturing of rock is revised on the basis of plasticity theory.
The lithostatic pressure or the grain to grain average rock pressure has been neglected hitherto. Reduced effective stresses are given in that the deviatoric stress minus the pore fluid pressures. The Berea Sandstone under the total given hydrostatic pressure-lithostatic pressure plus pore water pressure=2kbar-is perfectly plastic with a material constant k₀=2.2kbar, but the rock yields at k^*=k₀-pore water pressure. The value of k₀ increases with increasing total pressures. If all the principal values of reduced effective stress are tensile, the porosity increases contrary to the triaxial compression.
Strain hardening is revealed at higher confining pressures than that corresponding the fixed value of pressure, while thixotropy is evident below. Most of the subconsolidate sedimentary rocks that are indurated by means of adhesive water are perfectly plastic even at very low confining pressure.
In a simple compression test of a clay cake as a model of a large sedimentary body, there will appear a symmetrical pair of stationary surfaces dividing a central plastic region and two non-plastic prisms at the both ends. The stationary dividing surface is named ∏ surface. The intercalary plastic region between the two opposite ∏ surfaces are by no means entirely plastic during the initial stages of loading, but only certain outside parts in the test piece behave plastically. The plastic parts grow rapidly under increasing load. The moving surfaces of these parts named Σ are plastic wave surfaces, over which not all of jumps of the first derivatives of velocity and stress vanish. Plastic shock wave front Σ^* as well as Σ propagates with the speed same as that of elastic shear waves. Seismic effects of such plastic waves are themes of paramount interest. Γ surfaces are another set of singular and characteristic surfaces. If the slip discontinuity across ∏ tends to infinity, the surface develops into a fault.
The miscellaneous descriptions and statements given in the above lines automatically suggest many items to be worked out.