Stress triaxial compression tests

The occurrence condition of shear fracture is examined on the basis of the Coulomb criterion. The averaged shear stress across the fracture plane in the simulated hydraulic stimulation tests is plotted in Fig. 5, as a function of the effective normal stress across the fracture plane. The steady-stale pore pressure distribution given from Equation (1) is averaged over the fracture plane and is used to compute the effective normal stress. Triaxial compression tests have been performed on the granite using the same apparatus shown in Fig. 

A flexible boundary cubical triaxial test is another commonly used test for compression studies (Kamath et al., 1993 Li and Puri, 1996). A picture of a triaxial compression tester is shown in Figure 8. It allows not only the application of the three principal stresses independently, but also constant monitoring of the volumetric deformation and deformations in three principal directions. In a triaxial compression test, the specimen is at an initial isotropic state of stress then the three pressure lines apply the same pressure at the same rate to all six faces thus pressure is the same in all three directions (i.e., cti = 02 = 03). 

I (Takahashi, 2000). The peak shear stresses obtained from the triaxial compression tests are also plotted in Fig. A2. In the plot of Fig. 5, there is general agreement between the two types of the experimental results. Thus, it is thought that the occurrence of the shear fracture in the simulated hydraulic stimulation tests can be approximately predicted by the Coulomb criterion. Based on the comparison, the critical condition for the shear fracture due to hydraulic stimulation was estimated using the experimental results of the triaxial compression tests, as given in Fig. A2, and the averaged value of pore pressure. The detailed discussion of the triaxial compression tests can be found elsewhere (Takahashi, 2000.). 

Figure 2. Stress-Strain curves obtained by triaxial compression test. Confining pressure is lOOMPa and pore pressure is 25MPa.

Stress-strain relationships for triaxial compression test of carpet-fiber-soil confined at 34.5 kPa [87],... 

The transition point where the number of AE events drastically increases can be graphically determined under the assumption that the number of events linearly increases as a function of the axial stress. Fig. 11.41 exem-plarily shows the number of events versus the axial stress which was measured during a triaxial compression test of a rock salt specimen (diameter 60 mm, length 300 mm) at a confining pressure of 6 MPa (Alkan and Pusch, 2002). 

Figure 1.46 illustrates the stress-strain curves of an unconhned, uniaxial, compressive deformation of a SiC-N specimen. For uniaxial compression, the axial load was applied without conhning pressure (P = 0). The conhning pressure is indicated in Fig. 1.47, where stress-strain plots obtained from the uniaxial/ triaxial compression tests of SiC-N are illustrated. In general, the application of... 

Fig. 1.47 Stress-strain plots obtained from the uniaxial/triaxial compression tests of SiC-N specimens (tr —axial stress, —axial strain, Si—lateral strain, —volumetric strain), a without confining pressure b confining pressure of 350 MPa [33], With kind permission of Professor Brannon...

In the case of loose soil ( i/> 0), the intrinsic behaviour is readily observed in an undrained triaxial compression test and Figure 8.48 illustrates stress path followed by the sample to the left and the stress-strain behaviour to the right. The important behaviour is the rapid loss of soil strength following the peak static liquefaction—if such behaviour develops where the loads cannot reduce, which can be the situation in a slope, then there will be large scale movement. This is generally referred to as a flow slide as it develops rapidly and usually with little warning by way of tension cracks. 

Deformation Under Loa.d. The mechanical behavior of coal is strongly affected by the presence of cracks, as shown by the lack of proportionahty between stress and strain in compression tests or between strength and rank. However, tests in triaxial compression indicate that as the confirming pressure is increased different coals tend to exhibit similar values of compressive strength perpendicular to the directions of these confining pressures. Except for anthracites, different coals exhibit small amounts of recoverable and irrecoverable strain underload. 

Figure 10.7 (a) Failure lines for grouted and ungrouted granular soils, (b) Drained triaxial test results for silicate grouted coarse and medium sands. (From Ref. 11.15.) (c) Typical stress-strain curve from unconfined compression test on chemically grouted sand, (d) Compression versus time data for creep test on chemically grouted sand, under constant load, (e) Failure time versus percent of unconfined compression failure load. (+) indicates unconfined compression tests, and ( ) indicates triaxial tests with S3 = 25% of Si. 

For short-term or undrained loading analysis (( ) = 0) the factor N is zero and the factor N c is selected from Figure 10.43 for the anticipated relative embedment depth D/B and soil cohesion (a distance B above the anchor) in the soft rupture zone. The soil cohesion can be measured in place with a field vane or cone penetrometer or determined from laboratory tests on core samples such as the unconfined compression or triaxial CU test. If soil strength data are not available, the strength profile can be estimated using d CTot = 0.30, where the vertical effective stress (o J is estimated using a buoyant unit weight of 4 kN/m. ... 

AnhDan, L., Tatsuoka, R, and Koseki, J. 2006. Viscous effects on the stress-strain behavior of gravelly soil in drained triaxial compression. Geotechnical Testing Journal, 29(4) 330-340. 

Test method B determines the creep characteristics of bituminous mixtures by means of the triaxial cyclic compression test. In this test, the confining stress is induced by vacuum and the cylindrical specimen is subjected to a cyclic axial stress. This test is most often used for the purpose of evaluation and development of new types of mixtures. 

The concept of shear failure in thick sections of brittle material such as concrete is obscure and in many instances it could be misleading. One clear concept is that concrete failure can easily be put to the test if it is assumed that it is governed by the principal tensile and compressive stress caused by the so-called shear . The problem with this simple concept is the limitation on these stresses. In the prestressed concrete reactor vessels due to variations in loading conditions these principal stresses at any time at any point may vary from biaxial and triaxial compression to compression—tension—tension in any combination. These instant changes can bring about any kind of failure. For example, it may be pure flexural-cumnominal shear failure or principal tensile or compressive failure or by the so-called shear compression failure or in any combination of these. It must be borne in mind that the type of failure is directly related to the vessel overall layout. Before discussing the individual sample examples, it is necessary to know what the above-mentioned terms are and what effect they have on the prestressed concrete vessels. 

The tensile strength of compacts [30] also provides useful information. Excellent specimens of square compacts are necessary to conduct the tensile testing. For this reason, a split die [31 ] (Fig. 2) is used to make compacts that are not flawed. The split die permits triaxial decompression, which relieves the stresses in the compact more uniformly in three dimensions and minimizes cracking. These specimens are then compressed with platens 0.4 times the width of the square compacts in the tensile testing apparatus. (Fig. 3). Occasionally nylon platens and side supports are used to reduce the tendency to fail in shear rather than tension. The force necessary to cause tensile failure (tensile forces are a maximum... 

In a recent attempt to bring an engineering approach to multiaxial failure in solid propellants, Siron and Duerr (92) tested two composite double-base formulations under nine distinct states of stress. The tests included triaxial poker chip, biaxial strip, uniaxial extension, shear, diametral compression, uniaxial compression, and pressurized uniaxial extension at several temperatures and strain rates. The data were reduced in terms of an empirically defined constraint parameter which ranged from —1.0 (hydrostatic compression) to +1.0 (hydrostatic tension). The parameter () is defined in terms of principal stresses and indicates the tensile or compressive nature of the stress field at any point in a structure —i.e.,... 

The elastic-plastic constitutive model for triaxial simulations is introduced to illustrate the Mohr-Coulomb strength criterion and dilatancy. Figure 11 shows the soil model for the simulation of a displaeement-controlled drained triaxial test, where total stresses from the previous isotropic compression are taken as initial conditions. A vertical displacement... 

---