Non-linear ultrasonic monitoring of damage progression in disparate rocks

This paper focusses on non-destructive characterization of stress-induced damage progression in three types of rocks by experimentally capturing the elasto-dynamic non-linear response of the rocks, with the hypothesis that non-linear approaches have increased sensitivity relative to linear ultrasonic testing approaches. For this task, a non-linear ultrasonic testing procedure known as the Scaling Subtraction Method (SSM) has been implemented on uniaxially loaded rock specimens. The SSM procedure is based on exciting the damaged medium with two acoustic signals at different amplitudes consecutively. The linearly rescaled acoustic response at low excitation is subtracted from the response at large amplitude excitation for quantification of the elastic non-linearity of the medium by a non-linear indicator θ. At first, aluminum, Lyons sandstone, granodiorite and Gosford sandstone specimens were characterized using SSM and their inherent non-linearity was quantified. After this, the rock specimens were loaded under uniaxial compressive step-loading and ultrasonic measurements were performed simultaneously at each loading step. The non-linear indicator θ was calculated as the specimens were progressively damaged, and the changes in the non-linear response were linked to the different physical processes accompanying damage progression in rocks. The study concluded that the non-linear SSM technique is capable of sensitively recording signatures of the different stages of damage evolution in rocks.