## 6 Conclusions  
In this study, controlled laboratory experiments on salt rock samples under uniaxial stress were performed to study ultrasonic velocity and AE activity during deformation and damage. The conclusions are summarized as follows:  
(1) An integrated apparatus of ultrasonic wave measurement and AE monitor was developed; it was composed of an ultrasonic wave testing system, an AE testing system, and a data recording system. It can be used to measure ultrasonic velocity and record AE information and location of  
Figure 10: (**a**) Relationship between AE energy and loading time for sample SA-2 under uniaxial compression; (**b**) relationship between AE number and loading time for sample SA-2 under uniaxial compression; (**c**) relationship between axial stress and \(r\)-values for sample SA-2 under uniaxial compression.  
a rock sample under uniaxial or triaxial loading. However, further improvements are needed to better separate the two types of data. In this paper, the results of ultrasonic wave and AE have been obtained on salt rock under uniaxial loading. This was an innovative experiment, which laid an important foundation for future research to reveal the coupling laws among rock cracks, ultrasonic waves and acoustic emission.  
(2) The damage process of salt rock under uniaxial loading included five stages. In the compression fissures stage, the volumetric strain of salt rock increased while the crack volume decreased, but the crack volume strain was constant during the linear deformation stage. During the stabilized-growth crack stage, salt rock suffered unrecoverable damage and the compressed volumetric strain increased gradually and reached a peak. The beginning stresses of rock damage and dilatancy were about 42-50% and 62-67% of the uniaxial compressive strength, respectively. After dilatancy strength, internal cracks in the rocks expanded rapidly and connected, and the samples soon lost their carrying capacity.  
(3) The ultrasonic wave velocity of salt rock first increased, stabilized for a time, and then finally decreased. S-waves were more sensitive to change than P-waves because micro-cracks were mostly parallel to the loading direction.  
(4) In the compression fissures and linear deformation stage, AE activities in salt rock samples were very weak; however, they increased to account for 13.8% of the total AE activity in the stabilized-growth crack stage. When the loading stress was higher than the dilatation stress, AE activities were the most intense, accounting for 75.27% of the total AE events. After the stress peak, AE activities fell. We found that the AE location was a good predictor of crack development and rock damage. The ultrasonic wave velocity ratio, _r_-value, and _lb_-value can predict damage to rock samples; however, _r_-values are superior owing to their sensitivity and ease of measurement.  
The authors acknowledge the financial support of the National Natural Science Foundation for Young Scientists of China (Grant nos. 41602328, 51509261) and the Natural Science Foundation of Hebei Province (Grant no. D2017210182).  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON] and [PERSON] analyzed the data; [PERSON], [PERSON] and [PERSON] wrote the paper.  
The authors declare no conflict of interest.