Global vs local strain measurements in triaxial tests – Implications

Accurate stress-strain measurements in triaxial compression tests are critical to derive correct values of stiffness, Poisson’s ratio, and the Biot α-parameter. Yet, inherent biases can produce unrepresentative rock properties. This study investigates the impact of different measurements using strain gauges and LVDTs. A detailed analysis reveals the impact of surface compliance at the interfaces between the specimen and end caps. Tested materials include: standard aluminum, Eagle Ford shale, Berea sandstone, and Jubaila carbonate. Results reveal: 1) Contact deformation adds non-linear behavior to the stress-strain response. 2) Seating effects lower the stiffness computed from cap-to-cap deformation measurements. 3) Strain gauges do not show hysteresis evident in cap-to-cap LVDT systems. 4) Bending due to uneven surfaces and misalignment affect cap-to-cap deformation measurements. 5) Confining pressures improve the contact at the interface and reduce partial slippage. 6) Mounting strain gauges on sleeves is ill-advised. 7) The dynamic modulus is higher than the static modulus. 8) The static and dynamic moduli are sensitive to the imposed axial deviatoric stresses. 9) The estimation of the Biot α-parameter is affected by seating effects. We conclude that specimen–bonded strain gauges are preferred to minimize and possibly avoid any of the above effects for pre-peak strain measurements.