In the paper, experimental studies on Strain Gradient (SG) of a marble were performed in macro and messo scope. The conventional triaxial compression test were carried out to study the initiation and nucleation of local SG along macro fracture plane of the marble. In the messo scope, investigations were carried out based on direct shear tests by means of Scanning Electric Microscope (SEM).
The failure patterns of rock changed with the confining pressures in conventional triaxial compression. In general, the confining pressures for rocks under engineering conditions were 0–40 MPa. Brace (1964), Wawersik & Faihurst (1970), Wawersik & Brace (1971) had found that the failure of rock under these confining pressures range was due to the localization engendered in the mode of narrow shear band. Based on the section analyses on macro fracture of sandstones in triaxial compression, Yang C.H. et al (1998) had found that the fracture surface, which was smooth in macro scope, was composed of the sliding zone and the rupture zone. By SEM, Wang YQ. (1997) found that there was intensive ductile deformation around the macro shear fracture surface of a mylonite. Those observations indicated that shear fracture engendered in triaxial compression result from local ductile deformation accumulated to full extent (Sec Fig.I). Shear band phenomena of localization were common in metals. This topic for geomaterial research on shear bands could be traced back to similar formation of in-situ geological structure and specimen failure in laboratory tests. Ramsay (1980) had classified shear bands into three classes, such as the brittle shear band, the ductile shear band and the brittle-ductile shear band. All of them could be found in in-situ geological structure. Various researchers had also conducted theoretical work on this topic, for example. Rice (1973), Hill & Hutchinson (1975), Young (1976), Chau (1990.1992, 1993) and Chau & Rudinicki (1990) had analyzed the bifurcation behaviors of shear bands based on the finite/large deformation theory, Sun J, (1996), Zheng Y.R. et al (1996), Xie H.P.et al (1996) emphasized the importance of bifurcation analyses and their widespread in applications in their papers. Yin G.Z. et al(1999), Liu X.C. (1997) had studied the phenomenon on the non-related flowing theory of plane elastic-plastic and the large deformation theory. Because the in-situ shear band was one of the characteristic deformation patterns, which showed minerals were contained in the geological structure, studies on them were important and Practical. In order to study the mechanism that the Initiated and then lesd to shear fracture, the macro process localization was studied on the conventional triaxial compression.
Rock specimens were Daye marble, which belong to T1D4 of Pliocene Epoch in the Triassic System. It was grayish white in color, densely lumpy in shape and nicely uniform in structure. The axial maximum load was 2000kN. Deformation was measured by BE(BA.BQ)120–2BA strain meters.
