Deformation behaviour, crack initiation and crack damage of weathered composite sandstone-shale by using the ultrasonic wave and the acoustic emission under uniaxial compressive stress

Anticipating the progressive damage and strength of an anisotropic and interbedded rock mass is challenging due to the overwhelming variables which need to be considered. Therefore, the strength and progressive damage of each rock material is not a realistic model which can be used to define the deformation of the interbedded anisotropic rock mass. In this research, weathered (sandstone-shale-sandstone) composite samples under unconfined compressive stresses were concurrently assessed using acoustic emission (AE) and ultrasonic pulses. Shale interlayer significantly impacts the composite's behaviour, according to the mechanical properties and stress-strain relationships of weathered sandstone, shale, and composite. The deformation behaviour was derived following (velocity, amplitude and energy) responses of both the ultrasonic and AE waves. The intensity of AE energy reflects the best interaction with the progressive deformation of the rocks under uniaxial stress. Although the ultrasonic velocity was only sensitive toward the macro-deformation or specimen failure, the ultrasonic parameters (i.e., amplitude and energy) reflected a substantial alteration, specifically during the plastic deformation. The cumulative AE energy tangent (CSEET) method exhibited clear uncertainties in defining the CI and CD of composite specimens compared to sandstone and shale. The impact of AE energy attenuation due to the joint between shale and sandstone in the composite specimen was highlighted. For instance, the AE energy induced by CI and CD was about 6.0 E+09 mV and 2.4 E+11 mV for sandstone, 2.0 E+08 mV and 2.5 E+8 mV for shale, 1.6 E+08 mV and 2.2 E+8 mV for composite. The crack initiation (CI) (i.e., represents the plastic threshold of rock where the micro-cracks took place), and the crack damage (CD) (i.e., represents the stage where unstable macro-cracks begin to grow), which was indicated by the ultrasonic parameters, were addressed as alternative novel approaches for determining the plastic deformation of composite specimens. The CI commenced at 43%–80%, 53%–78%, and 35%–55% of the peak stress, whereas the CD occurred at 84%–97%, 78%–95%, and 75%–94% of the peak stress for the weathered sandstone, weathered shale, and composite, respectively. This research may help evaluate the deformation behaviour of the interbedded rock masses using the ultrasonic approach, presuming a sufficient material strength differential between the interbedded rocks.