Shear 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. 

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.). 

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.,... 

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