Evaluation of a Punch Shear Test Device

摘要

The report presents results of a U.S. Bureau of Mines study of a punch shear test device for the direct evaluation of the shear strength of rock core. Sixteen sets of laboratory tests were conducted, under varied test conditions. Results were analyzed using factorial analysis and graphical methods. Finite-element modeling was also conducted to evaluate the stress distribution created within the rock specimen by the punching action of the test device. The test results show that a specimen thickness-punch diameter ratio of less than 0.14 is a key factor in producing reasonable data. The finite-element modeling shows that the sheared portion of the disk produces nonuniform stress distribution with significant confining stresses, especially in thicker specimens. This may explain why the punch shear strength results were considerably higher than the shear strengths obtained with a triaxial test. The results indicate that the punch shear test apparatus is not appropriate for determining accurate shear strength values unless a determined correction factor is applied to the punch shear strength equation. The punch shear test may prove useful for comparative evaluation of core pieces too small to be evaluated with conventional tests.