# Evaluation of Rock Brittleness Index under Dynamic Load
## 4 Discussion
### Changes in Brittleness Characteristics of Rock under Dynamic Load  
The total stress-strain curve obtained from the rock compression test is a direct reflection of the mechanical behavior of rock, and can reveal the internal mechanism of rock failure. Therefore, the total stress-strain curve is often used as a qualitative and quantitative method to evaluate the brittleness of rock. Based on the summary of the existing brittleness indices and the previous experimental study, this section summarizes the rock brittleness in terms of the stress-strain curve in four categories.  
The first brittleness characteristic is the strain variation in the stress-strain curve, which is mainly represented by the relationship between the plastic strain and the peak strain. The more brittle the rock, the smaller the ratio of the plastic strain to the peak strain. The second brittleness characteristic is the stress variation in the stress-strain curve, which is mainly manifested by the change in peak stress amplitude. The more brittle the rock, the greater the peak strength and the greater the post\(-\)peak stress drop. The third brittleness characteristic is the stress-strain relationship in the stress-strain curve, which is mainly represented by the pre\(-\)peak elastic modulus and the post\(-\)peak elastic modulus. The more brittle the rock, the larger the elastic modulus and the stronger the resistance to deformation. At the same time, the stress drop rate is faster and the post\(-\)peak elastic modulus is larger. The fourth brittleness characteristic is the energy relationship in the stress-strain curve, which is mainly manifested as the energy absorption or release of the rock after failure. The more brittle the rock is, the less energy it can absorb after reaching the peak, and even self\(-\)sustaining failure can occur without absorbing energy.  
According to the test results, when a static load changes to a dynamic load, the plastic strain ratio of the rock decreases, the peak strength of the rock increases significantly, the pre\(-\)peak elastic modulus of the rock increases to varying degrees, the energy released after the rock's failure is greater, and the four types of brittleness characteristics are enhanced, so that the overall brittleness of the rock is stronger.  
In Section 3, it was described that the brittleness of rock was enhanced to different degrees under dynamic load, and the enhancement amplitude was positively correlated with the brittleness of rock under static load. At different strain rates, the brittleness of sandstone increases with the increase in strain rate, and the higher the strain rate, the greater the increase in brittleness. This is mainly due to the loading rate, which amplifies the changes in brittleness characteristics. The energy absorbed by the rock is used to compress itself and spread the fracture before failure. Under static load, the loading rate is very low, all the energy can be fully absorbed, and fracture propagation in the rock is only related to the mineral composition and structural composition. However, under dynamic load, the loading rate is relatively high, and not all of the energy can be fully absorbed by the rock to be compressed in a short period of time. Part of this energy is used to accelerate the expansion of cracks, resulting in more obvious brittleness characteristics.