Application of tensile stresses (with negative sign according to our geomechanics convention) on a metal bar results in tensile strains (negative too). In this example the state of stress is relatively simple with tensile stress in the axial direction and zero-stress in any direction perpendicular to the axis of the bar (Figure 4.4). The maximum tensile stress taken by the bar is called tensile strength. Metals are usually “ductile” and deform after reaching a peak stress. When unstressed, the bar in the example figure does not recover its original length but remains with “plastic deformation”.
The one-dimensional tensile strength test for metals (Fig. 4.4) is not easy to implement in rocks. You would have to grab the rock on its sides or glue it on the ends to perform such tests. Even in that case, your rock may break at the those “grabbing” points. One alternative is to “machine” the rock to a convenient shape, so that, you can pull it without using glues or grabbing jaws (shown in Figure 4.5). However, rocks are not easy to “machine” in general, and thus this test becomes impractical in many situations. Rock failure in simple tension usually displays “brittle” failure, no plastic strains follow after reaching tensile strength. It just breaks quickly.
The Brazilian test is a convenient method to measure tensile strength. It uses short cylindrical samples and takes advantage of the shape of the rock specimen to create tensile stresses with application of a compressive force along the sample diameter (Figure 4.6). A solution of the state of stress within the rock (assuming a linear elastic homogeneous material) yields the tensile strength value equal to
PROBLEM 4.1: Determine the tensile strength of the shale sample shown in Fig. 4.6. The sample diameter is 1.00 in and the length is 1.00 in.
The sample dimensions are
Typical values of tensile strength for cemented sedimentary rocks range from 0.5 MPa to 10 MPa. Uncemented sediments -very common in sedimentary basins- have zero tensile strength. Figure 4.7 summarizes typical values of tensile strength for rocks.