Experimental investigation of the effects of water content on the anisotropy of mode I fracture toughness of bedded mudstones

The influence of water content on mode I fracture toughness (K(Ic)) of mudstones has been studied using semi-circular bend (SCB) specimens subject to three-point bendings. And the mudstone SCB specimens are divided into three types, including Type-A, Type-B and Type-C, corresponding to the three configurations of the bedding planes, including divider direction, arrester direction, and transverse direction, respectively. The test results show that the values of K(Ic) for the three types of specimens are different due to the bedding structure, the Type-A specimens have the largest value of K(Ic) for the same soak period, while the Type-C specimens possess the smallest value. As the soak period increases, the K(Ic) of the three kinds of mudstone specimens decreases, and the fracture mechanisms of the specimens change gradually from the brittle failure form to the ductile failure form. Moreover, the standard deviation was used to quantify the anisotropy degree of the K(Ic) of the mudstone samples. As the water content increases, the standard deviation increases from 0.057 to 0.139, which indicates a significant increase in anisotropy of the K(Ic) of the mudstone specimens. In addition, the acoustic emission (AE) system was used to detect the AE events associated with the fracture initiation and propagation in the mudstone specimens for the different water content, with the raising water content, the cumulative AE events decrease, and the standard deviation of AE events increases, repesenting that the anisotropy of the AE events of the three types of specimens becomes more prominent. Further, the relationship between the tensile strength (σ(t)) and the K(Ic) of the three types of mudstone specimens for different water contents has been proved to be the linear relation.