A non-linear ultrasonic method for damage characterization in a shaly sandstone

It is a well-established fact that the ultimate failure of rocks is preceded by the initiation, growth, and coalescence of micro-cracks. A plethora of techniques have been developed over the years for the characterization of the different stages of micro-cracking in rocks, but an objective approach for monitoring damage evolution in rocks has still not been fully established. In this paper, a new non-linear ultrasonic testing approach, the Scaling Subtraction Method (SSM), has been used to evaluate its potential to detect the signatures of the mechanical changes that accompanies a rock at different levels of damage. This approach is implemented based on the hypothesis that non-linear components of ultrasonic waves have increased sensitivity to damage. Firstly, aluminium, Gosford sandstone and Lyon’s sandstone were characterized and their inherent non-linearity was established using a SSM non-linear indicator θ. Then, a Gosford sandstone specimen was damaged under uniaxial compressive step-loading and ultrasonic measurements were performed at each loading step. The non-linear indicator θ was calculated as the specimens were progressively damaged, and in such a way, the efficiency of SSM technique in capturing damage signatures was evaluated. The study concluded that the elastic non-linearity in a specimen, either due to its inherent micro-structure or due to damage, can be successfully quantified using the non-linear SSM indicator (θ).