Acoustic emissions during creep under triaxial compression

Granular materials exhibit time- and rate-dependent behaviour resulting from micromechanical processes at the scale of individual particles. Elastic energy is released during these processes and can be detected as acoustic emissions (AE). Using multistage creep tests under isotropic and anisotropic pressure on medium dense samples of dry silica sand, the relationship between the number of AE events N_AE and the axial creep strain ε_a was determined. In addition, the dependence on the mean pressure p and the deviator stress q was investigated. The experimental results show that the development of AE and axial strain during creep are qualitatively comparable. Within the creep phases both the change in ε_a and N_AE can be described by a logarithmic trend with time. The time-dependent development of both measured quantities exhibit a dependence on q. Moreover, the evolution of N_AE with time also shows a pronounced increase with increasing p. A time-dependent power law can be assumed to represent the rates of N_AE and the rates of ε_a with time during creep. The exponent m of the power law is similar for all experiments performed. The initial rates of N_AE and ε_a increase with increasing p as well as increasing q/p-ratio. Finally, a linear correlation between log ((Equation presented)_a) and log ((Equation presented)_AE) was found depending on two state parameters a and b, with a seems to be independent on the stress state.