IS INTERNAL-FRICTION FRICTION

Mogi [1974] proposed a simple model of the incipient rupture surface t o explain the Coulomb failure criterion. We show here that this model can plausibly be extended to explain the Mohr failure criterion. In Mo gi's model the incipient rupture surface immediately before fracture c onsists of areas across which material integrity is maintained (intact areas) and areas across which it is not (cracks). The strength of the incipient rupture surface is made up of the inherent strength of the intact areas plus the frictional resistance to sliding offered by the cracked areas. Although the coefficient of internal friction (slope of the strength versus normal stress curve) depends upon both the fricti onal and inherent strengths, the phenomenon of internal friction call be identified with the frictional part. The curvature of the Mohr fail ure envelope is interpreted as a consequence of differences in damage (cracking) accumulated in prefailure loading at different confining pr essures.