The true triaxial stress are typical stress state in the deep underground. 3D surface flaws, one of the most common type of flaws, extensively existed in the rocks. Therefore, the study on evolution of the 3D surface flaw under true triaxial stress is crucial to determine the fracture behaviors of the rock in some deep underground spaces. Gypsum, as a rock-like material, has been extensively used in studies of crack initiation and propagation. In this study, we prefabricate a pair of 3D surface flaws at 45° in the cubic gypsum specimen and investigates the effect of the intermediate principal stress on initiation and peak stresses (characteristic stress thresholds) of flaws and crack propagation patterns of 3D surface flaws parallel to the intermediate principal stress. The true triaxial apparatus and acoustic emission (AE) technique were used to test and monitor the mechanical behaviors of the samples. The internal crack propagation pattern was observed by X-ray CT scan. The results demonstrate that the intermediate principal stress strongly affects crack patterns but has a limited influence on characteristic stress thresholds. Both the intermediate and minimum principal stresses affect the difference in the crack peak and initiation stress, which elucidates how the true triaxial stress affects the fracture behavior of the specimen. Additionally, the intermediate principal stress effect on characteristic stresses is closely related to the magnitude of minimum principal stress. When the magnitude of minimum principal stress is small, with the rising intermediate principal stress, the characteristic stresses increase slowly. When the magnitude of minimum principal stress is large, the intermediate principal stress almost has no effect on characteristic stresses. The surface wing cracks and anti-wing cracks initiate from the flaw when the magnitude of intermediate principal stress is relatively small. With the intermediate principal stress increasing, the surface crack propagation pattern is shift from tensile crack to shear crack. Through the CT image reconstruction technique, the propagation patterns of the inner tips of single 3D surface flaw were illustrated in this paper. It is observed that the large intermediate principal stress can restrict the crack wrapping and even make the internal flaw propagation patterns same with that on the specimen surface, providing insights into the validity of simplifying 3D flaws as 2D flaws for analyzing and computing crack propagation.

Externally bonded (EB) carbon fiber reinforced polymer (CFRP) is widely used in structural strengthening and retrofitting. Premature debonding of the FRP severely limits the efficiency of CFRP utilization. The application of CRRP anchorage system offers a solution to the debonding problem. However, the understanding of damage mode identification of this combined system still remains elusive. Acoustic emission (AE) technique is employed to identify the damage mode of this CFRP anchorage system, due to its high sensitivity and the ability to detect damage in real-time. The objective of the current study is to identify the failure mechanisms of CFRP-strengthened beam by applying advanced pattern recognition techniques to the collected AE data. Firstly, four-point test of CFRP-strengthened beam was carried out until failure with simultaneous recording of AE signals. Then, correlation analysis was adopted to select the AE characteristic parameters, and principal component analysis (PCA) was used for dimensionality reduction. Lastly, the AE signals of the CFRP-strengthened beam was clustered to track the evolutionary behavior of the different damage modes by Gaussian mixture model (GMM) algorithm. Three main damage modes of CFRP-strengthened beam were identified by GMM clustering: concrete cracking, debonding of CFRP sheet and fracture of CFRP sheet. This study explores the damage evolution mechanism of combined system and provides a basis for achieving health monitoring of CFRP-strengthened structures.