Firstly Kármán (1910, 1911), after Griggs (1936) later many other researchers (e.g. Handin and Hager 1957, Paterson 1958, Heard 1960, Byerlee 1968, Edmond and Paterson 1972, Rutter 1972, Gowd and Rummel 1980, Scott and Nielsen 1991, etc.) observed the influence of confining pressure on the behaviour of rocks which were brittle at zero and low confining pressure. At high confining pressure, however, the same rocks may be ductile. That means they may fault or otherwise deform without loss of compressive strength. The amount of ductile deformation and the strength increase progressively with increasing confining pressure until fully ductile deformation occurred with apparent work-hardening.
The aim of this paper is to show the influence of confining pressure during a three- point bending test. An anisotropic gneiss was investigated in 5 different directions of the foliation under 4 different confining pressures. The function of the tensile stress vs. hydrostatic pressure was determined, as well.