Visualizing Stress

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Plotting the Mohr Failure Envelope

In Visualizing Stress select File from the main menu and then Open. In the Open File dialog box select the file BereaSandstone.txt and then click Enter. Plot the Mohr diagram for the sandstone from Berea, Ohio. With these data plotted, a failure envelope can be drawn. Select the menu item Graphics and then select Plot Controls. Place a check mark in the box labeled Mohr Envelope. The failure envelope is the best-fit line representing the locus of shear and normal stresses at failure for the sandstone from Berea, Ohio under the given experimental conditions (Figure 22). From Figure 22 we would predict that for stress states below the failure envelope, the sandstone from Berea would be stable. However, if the stress state yielded shear and normal stresses plotting above the envelope, then such a condition would be unstable and failure would be likely to occur.

Figure 22. Mohr failure envelope plotted with Visualizing Stress.

The failure envelope reveals other useful information besides the stress state that causes failure in a material. Where the failure envelope intersects the shear stress axis the normal stress is zero. The magnitude of the shear stress that causes failure in a material subjected to no normal stress is related to the materials cohesive strength. The cohesive strength of a material is the strength of bonding between the particles or surfaces that make up that material (Keary 1996). In rock mechanics the cohesive strength is more specifically the inherent shear strength of a plane across which there is no normal stress (Keary 1996). The slope of the Mohr failure envelope is the ratio of the shear stress to the normal stress at failure. This ratio is termed the coefficient of internal friction.,


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