Effects of stress cycles on static and dynamic Young's moduli in Castlegate sandstone

It is well reported in the literature that the static moduli of rock determined from stress-strain testing are characteristically lower than dynamic moduli. We demonstrate that with a consistent definition of the static elastic modulus in terms of small load-unload cycles, the difference between static and dynamic moduli is negligible for Castlegate sandstone. This result is based on a series of stress-cycling measurements on room-dry Castlegate sandstone (UCS = 16 MPa) where we have made simultaneous measurements of both the static stress-strain behavior and the dynamic sound speeds (P and S) on the same sample of rock during unconfined, uniaxial compression. These tests include three “major” load-unload cycles up to 70% of unconfined compressive strength (UCS) and ten minor load-unload cycles of about 6% of UCS. The static minor cycle Young’s modulus is similar to the dynamic Young’s modulus at 10 MPa axial stress and has the value 10 GPa (to within 10%). We work with dry material to focus on the fundamental issues of modulus differences since fluid effects are described in the literature.