Regularities and features of the failure process with hard loading

Abstract

Dynamic phenomena in rock masses occur with a combination of a number of conditions [i, 2], one of which is the fact that the stiffness of an external system governing the load on a failing element should be not greater than its own stiffness. These phenomena may be studied under laboratory conditions since failure of specimens in the latter stages of a compressive test are an analog of the dynamic phenomenon if this failure is brittle and related to the release of excess energy accumulated by the test machine.In view of the fact that no~lly failure is only observed on reaching limiting loads and external manifestations of failure in a normal soft test machine are quite similar for different rocks, there is interest in the features of rock behavior at the strength limit [3][4][5].In the present work an attempt is made to compare the results of phenomenological studies of the deformation diagram for rock specimens of different genesis differing in tendency toward brittle failure with data for the failure process obtained using the phenomenon of acoustic emission (AE). EXPERIMEN~rAL PROCEDUREThe rocks were granite and sandstone from the Kaindy and Dzhergalan deposits, respectively. Granite is cataclastic hornblende mixed mainly with potassium feldspars.The structure is hypidiomorphic granular, and granitic. Quartz grains are cataclastic.Hornblende is strongly chloritized.Open rectilinear cracks are noted crossing the rock in different directions having a width of 12-18 ~m. The density of open cracks is To = 40 i/m, porosity over a microsection is 1.5-2%.Secondary pores (leaching) are located along the path of open cracks and within feldspar grains.Pores have a size of 0.2-0.35~nand they are of angular elongated shape.Sandstone is medium-to fine-grained and polymictic.The composition of the cement is hydromicaceous-kaolin, and carbonate in areas. The type of cement is basal-interstitial. The cement comprises 25-30%, fragmented material 75-70%, and it contains quartz, feldspars, and mica. Open cracks are encountered 18-20 ~m wide. Density To = 10-12 I/m. Porosity over a microsection is 3-5%, pores are of primary and secondary leaching, they have a size of 0.05-0.15 mm, and they are right-angled and rarely circular in shape.Cylindrical specimens were prepared from these rocks with a height-to-diameter ratio equal to two, and with a diemeter of 30 mm according to the specifications of the GOST [6].Experiments were carried out in a PR-500 press from the Armavirskii test machine factory having a stiffness of i-I0 s kg/cm (soft loading). Stiffness was increased to 2.10 ~ kg/cm by loading in parallel with the specimen three columns of steel U-10 whose size was selected from the conditions for equality of their elastic deformations (not exceeding the yield strength) to the value of longitudinal strains with which specimens of the test rocks are beyond the strength limit, i.e., the post-limit region (hard loading).In both cases a specimen was placed in the dynamometer with which the directly operating force was measured.Pos...