## 4 Conclusions  
Direct tensile, tensile-shear, and compressive-shear tests were performed on rock-like specimens utilizing a self-developed multi-functional test equipment, focused on AE amplitude and peak frequency evolution during failure. Based on AE amplitude and peak frequency, the percentage of tensile and shear cracks at peak strength moments was estimated. The results were compared with RA-AF-based classification results. A summary of specific conclusions is as follows:  
(1) AE signals from compressive-shear failure have a \"quiet period phenomenon\" compared to tensile and tensile-shear failure. The different tests reflect the failure evolution pattern of accumulation of small-scale microcracks at the beginning of loading until the large-scale rupture near the peak strength moment. In intact specimens, tensile-shear failure amplitude distribution is similar to compressive-shear failure, but in jointed specimens, it is more similar to tensile failure; this is due to the presence of tensile stresses. The specimens of both test loading methods are initially almost in the elastic deformation stage, with little damage and few cracks produced;  
(2) Low-frequency signals dominate at the beginning of loading in tensile tests, but medium- and high-frequency signals grow towards failure. In compressive-shear tests, high-frequency signals are generated at the start of stress and increase towards failure. During tensile-shear tests, all three bands continuously provide signals. All three test types show an increase in mid-frequency signals near failure, which can be utilized to predict failure;  
(3) The moving average of peak frequencies data demonstrate that the overall trend of the peak frequencies of the three test types fluctuates and decreases as the stresses are loaded, with the tensile and tensile-shear tests following similar patterns. The compressive-shear test varies from the other two in that the peak frequency fluctuates considerably at the beginning of loading but stabilizes towards failure;  
(4) Medium- and high-frequency bands generally correlate to tensile cracks, whereas low-frequency bands comprise both tensile and shear cracks. Tensile cracks produce signals with low frequency and low amplitude, whereas shear cracks produce signals with low frequency and high amplitude. The amplitude-PF and RA-AF crack classification results were in accord. Even though there was no horizontal shear stress in the direct tensile test, shear cracks were more common. Because the material is non-homogeneous, crack-sprouting paths may not be perpendicular to tensile stress, generating shear cracks. Figure 9: Determination of the secant line equation. (**a**) A diagram of secant lines that do not pass through the origin [28]; (**b**) crack classification results; (**b**) crack classification results based on different secant line equations (take the T1 as an example). **Author Contributions:** Conceptualization, Y.X. and J.H. ; methodology, Y.X., G.W., P.H. and H.P. ; software, Y.X., G.W., P.H. and H.P. ; validation, H.P. and J.H. ; formal analysis, Y.X., H.P., P.H. and J.H. ; investigation, Y.X., H.P. and J.H. ; resources, [PERSON] and J.H. ; data curation, [PERSON] and G.W. ; writing-original draft preparation, [PERSON], [PERSON], P.H. and Y.X. ; writing-review and editing, [PERSON] and J.H. ; visualization, [PERSON], H.P. and J.H. ; supervision, [PERSON] and J.H. ; project administration, Y.X. ; funding acquisition, J.H. All authors have read and agreed to the published version of the manuscript. **Funding:** Much of the work presented in this article was supported by the National Natural Science Foundation of China (Grant No. 52209133); China Postdoctoral Science Foundation (2022M711643); China Postdoctoral Science Foundation (2022T150324). **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Data available on request from the authors.