Volume 21, Issue 6 (November 1993)
Pressure and Flexible Membrane Effects on Direct-Contact Extensometer Measurements in Axisymmetric Compression Tests
Axisymmetric compression tests have become routine tests in laboratory experiments on pressure-seasitive materials like geologic material. The usual compression test unes right-circular-cylindrical specimens that are encased in a protective membrane, placed in a pressure vessel, and then subjected to some combination of axial stresses and confining pressures. The best measurement of the resulting specimen deformation can be made by installing pressure-rated instrumentation directly on the specimen. However, responsible experimental practice requires that the effects of pressure and the protective membrane on the deformation measurements be understood. This paper attempts to quantify such effects.
The investigation began with an evaluation of the pressure effects on the instrumentation. Significant pressure effects required that correction factors be introduced. After these pressure effects had been quantified, four types of protective-membrane conditions were tested in three common types of compression tests. The four conditions were: (1) no jacket, (2) thin jacket, (3) thick jacket, and (4) ported jacket. The three compression tests included hydrostatic compression (HC), triaxial compression (TC), and uniaxial strain (US). A single result at each combination of jacket condition and test mode required that twelve tests be performed. The results show that protective membranes can have a very significant effect on the elastic moduli calculated from the deformation measurements, but in some cases the errors can be reduced or eliminated through the use of correction factors or specially designed membranes.