## 5 Influence of Axial Joint Transfixion Degree on Specim Damage  
### Analysis of Pore Size Distribution  
MRI can detect the signal intensity of water in each pore and the signal intensity of water in the total pores, and the ratio between the two can characterize the proportion of pores with different pore sizes in the rock to the total pores as the pore size distribution. To study the mesoscopic damage of different transfixion joint specimens under the combined action of water-saturated freezing and impact load, the pore size distribution structure of the specimens was analyzed. The pore size in the cement mortar specimen is divided into 3 grades [33; 36; 38]; 0-0.1 \(\upmu\)m is microscopic pores, 0.1\(\sim\)1 \(\upmu\)m is medium-sized pores, and larger than 1 \(\upmu\)m is large-sized pores. It can be seen that the unjointed specimens before impact are mainly composed of tiny pores, accompanied by a small number of medium-sized pores, and almost no large-sized pores. Figure 8 is a comparison chart of the pore-size distribution of the specimens with different joint transfixion degrees frozen in saturated water under the action of cyclic impact. It can be seen from Figure 8 that before the SHPB impact, a large number of tiny pores and a small amount of medium and large pores were developed inside all types of specimens. The continuity between the three types of pores is poor, and the proportion of pores in the original state of the specimen is similar to the trend of pore size. There are subtle individual differences between the specimens, but these do not affect the overall test effect. Analysis of the pore-size distribution trend after impact shows that in the first two impact tests, the micro-pores of the specimen were compacted, and the medium-sized pores and the large-sized pores gradually increased, and the increasing phenomenon was not significant. It shows that the inside of the specimen belongs to the crack initiation state in which the tiny pores are compacted during the test. When the number of cyclic impact tests reaches 4-10 times, the medium-sized pores and the large-sized pores increase rapidly, the micro-sized pores do not change much, and the connection of the three pore intervals has a vertical upward trend, gradually softening and smoothing. It shows that the connectivity between pores of different sizes becomes better. In general, under the combined action of water-saturated freezing and impact load, the newly sprouted pores in the specimen and the compaction, redevelopment, and expansion of the primary pores are the reasons for the change in the pore-size distribution of the specimen. The ice crystal filling in the primary pores increases the strength of the specimen but reduces the expansion capacity of the original pores due to incident energy. Therefore, the increase of medium-sized pores and large-sized pores caused by the absorption of energy by the specimen dominates the damage degree of the specimen. According to the pore-size distribution comparison chart of the specimens with different joint transfixion degrees in saturated water under cyclic impact conditions, the peak pore sizes of different pore types and their proportions are shown in Table 3.  
Figure 8: Test pore-size distribution curves for different transfixion joint specimens: (**a**) 0%, (**b**) 20%, (**c**) 40%, (**d**) 60%, (**e**) 80%, (**f**) 100%. By comparing the data in Table 3, it can be seen that the peak pore sizes of small pores, medium pores, and large pores of all specimens did not change much before and after impact, but the proportion of peak pore sizes of all specimens changed significantly before and after the cyclic impact test.