Mechanical data corrections for triaxial compression testing at high pressures and high temperatures with a Paterson...

Although restricted by a limited range of strain, the triaxial compression test is a mature and common technique for investigating the rheological properties of rock materials at high pressures and high temperatures, especially when establishing the constitutive equations for various flow laws. The Paterson gas-medium high-pressure and high-temperature mechanical testing apparatus (Paterson apparatus) is the best apparatus for triaxial compression testing due to its high stress resolution. However, to derive accurate mechanical information from the raw data recorded by the Paterson apparatus, some technical issues should be addressed, including the simultaneous distortion of the apparatus, the load force supported by the jacketing tube, and the change in the cross-sectional area of the specimen. In this paper, we introduce correction methods corresponding to these three technical issues for triaxial compression on a Paterson apparatus equipped with an internal load cell to significantly reduce experimental errors so that high-precision mechanical data for establishing the constitutive equations of flow laws, such as differential stress, strain, and strain rate, can be obtained. To facilitate corrections for the distortion of the apparatus and the load force supported by the jacketing tube, we determine the distortion of the Paterson apparatus as a function of axial load force by deforming tungsten steel specimens with a known Young's modulus and the high-temperature flow laws of two common jacketing materials, iron and copper, by triaxial compression experiments at confining pressures of 200-300 MPa. Previous flow laws of iron and copper established by Frost and Ashby (1982) using ambient mechanical data are carefully compared with the flow laws obtained in this study to evaluate their effectiveness for correcting jacket tube strength. Finally, the errors eliminated by each correction step are analyzed and discussed to better understand the necessity of mechanical data corrections.