Failure loci of some igneous and Metamorphic]Rocks

Ps. Theocaris, Failure loci of some igneous and Metamorphic]Rocks, ROCK MECH R, 32(4), 1999, pp. 267-290
Citations number
Categorie Soggetti
Geological Petroleum & Minig Engineering
Journal title
ISSN journal
0723-2632 → ACNP
Year of publication
267 - 290
SICI code
Experimental evidence from true triaxial tests on dense rocks are analysed with emphasis on the failure modes of these materials under multiaxial load ing, ambient temperature and external pressure. The strong dependence of th e modes of fracture on the secondary components of applied stresses, and es pecially on the intermediate principal stress, indicated that the failure s urface of these brittle materials may be appropriately described by a failu re tensor polynomial criterion. As such, the elliptic paraboloid failure cr iterion was found to conveniently describe their mode of failure, by consid ering also the severe influence of anisotropy of the material. For this purpose, a method developed recently (Theocaris and Panagiotopoulo s, 1995a, 1995b) was applied, defining anisotropic hardening plasticity thr ough an appropriate sequence of anisotropic elasticity problems. Assuming a particular path of loading or unloading, we measured the instantaneous ten sion and compression yield stresses along the transient principal-stress di rections. These parameters completely define the instantaneous state of ani sotropy of the body for the corresponding loading step, by applying the the ory of the elliptic paraboloid failure locus (EPFS) (Theocaris, 1989a). A p arameter identification problem was formulated on the constitutive expressi ons for this most general failure criterion. Then, by applying convenient c onstraints derived from the EPFS theory, which serve as filters throughout the whole procedure, the characteristic values of terms defining the variab le components of the failure tensor polynomial were calculated, as the mate rial was continuously loaded from the elastic into the plastic region and u p to the ultimate failure load. Accurate simple tests in uniaxial tension a nd compression provided sufficient data for the definition of the yield loc i of the material, at the considered loading step. These tests may be compl emented with biaxial and triaxial modes of loading of the specimens. The re sults improve the accuracy and sensitivity of the method. All such data wer e used as input values, for establishing the mode of plastic deformation of the body during particular loading paths. Moreover, the method employed allows the complete definition of the compone nts of the failure, H, and the strength differential effect, h, tensors at each loading step. These quantities define completely the failure tensor po lynomial for each material. Therefore, it presents the important advantage over other experimental methods by clearly indicating the parts contributed to the failure mode (either by plasticity, or by the strength differential effect) and their evolution during plastic deformation. As convenient prototype materials for testing the method, specimens of meta morphic rocks such as Westerly granite (G), or quartzite (Q) were selected. Interesting results concerning the mechanical and especially the failure m odes of such materials were obtained. Furthermore, the mechanical tests ind icated clearly some basic properties of these materials as concerns the mod e of their structure.