## 3 Results and Analysis
### Three-Point Bending Test  
The critical stress intensity factor (KIC) of tonalite specimens can be calculated according to the three-point bending test. The KIC of tonalite specimens treated with nitric acid at pH = 1 (G1&G2) are shown in Figure 8. Generally speaking, the KIC of G2 decreases with the increase of soaking time. The KIC of granite specimens soaked in nitric acid at pH = 1 of soaking time 30, 60 and 90 days are 5.9%, 12.6% and 12.8% of the original value, respectively. The trend of reduction is very fast from 30 days to 60 days, but the decline is almost the same from 60 days to 90 days. The KIC of tonalite specimens under freeze-thaw (G3) and the combined effect of chemical corrosion and freeze-thaw cycles (G4) are shown in Figure 9. KIC of untreated tonalite specimens is 2.34 MPa. The KIC of G3 and G4 both decreases as the freezing temperature reduces. The KIC of tonalite specimens with freezing temperatures of \(-\)30 \({}^{\circ}\)C, \(-\)40 \({}^{\circ}\)C and \(-\)50 \({}^{\circ}\)C are 82.2%, 82.0% and 86.1% of G1. The KIC of tonalite specimens combined effect of chemical corrosion and after \(-\)30, \(-\)40, and \(-\)50 \({}^{\circ}\)C freeze-thaw cycles decreased by 1.3%, 5.2% and 9.3% of G2. It can be seen that the KIC of G4 declined less than the KIC of G3 compared with the control group. In the G3, the KIC is slightly increased at a freezing temperature of \(-\)50 \({}^{\circ}\)C, which can be attributed to the error caused by the discreteness of the tonalite itself. Figure 8: The KIC of tonalite specimens after nitric acid treatment at pH = 1. Figure 9: The KIC of tonalite specimens under freeze-thaw (G3) and the combined effect of chemical corrosion and freeze-thaw cycles (G4). Factors Influencing the Mechanical Properties of Tonalite under the Combined Effect of Chemical Corrosion and Freeze-Thaw Cycles  
Several factors can affect the mechanical properties of tonalite subjected to the coupled effect of freeze-thaw weathering and chemical corrosion. The influence factors have been separated into internal and external causes. Internal causes include (but are not limited to) the type of rock, the development of cracks, density, porosity and permeability. External causes include freezing methods (i.e., freezing times, freezing cycles and freezing temperature), initial moisture content, water supply condition, and the stress state of the rock. Based on analysis of the results of the experiments carried out in this work, the factors that affect the mechanical properties of tonalite specimens subjected to the combined effect of freeze-thaw weathering and chemical corrosion are summarized as follows:  
(1) Freezing temperature  
At different freezing and thawing temperatures, the UCS of tonalite specimens does not differ much. However, the peak strain decreases with decreasing freezing temperature. The KIC of specimens decreases as the freezing temperature reduces. (2) Chemical reaction of minerals  
Based on the comparison of the peak strains for G3 and G4 specimens, it can be inferred that cementation takes place inside the specimens. This enhances the peak strain and the peak stress of G4. Otherwise, the UCS and KIC of samples subjected to combined effect should be much lower compared with those subjected to freeze-thaw or chemical corrosion. Additionally, a cream suspension liquid was found in the experimental container when tonalite was soaked in nitric acid solution.