## 2 Rock Composition and Wear Characteristics of Composite Strata
### Wear Types and Wear Phenomena of Disc Cutter  
There is contact friction during the cutter-rock cutting process, which produces different wear phenomena. In order to analyse the mechanism of cutter wear and its influencing factors, scholars have sorted out and classified different types of wear. [PERSON], [PERSON] et al. studied a shield construction project for a water tunnel in Norway and collected data on  
Figure 1: Horizontal distribution of the composite strata and a cross-section of the shield tunnel path. the wear of cutters over 7 km of shield excavation [2]. The excavation surface stratigraphy is distributed with hard rocks, mainly granite, quartzite and marble, with strengths of up to 264 MPa and quartz contents of up to 61%. The statistical data of the cutter wear type are shown in Figure 2; the proportion of abrasive wear is 71%, and the proportion of cutter ring fracture is 18%. Cutting hard rock is more wear intensive, with the total number of cutter replacements amounting to 1068, of which 575 were of the even-wear type, accounting for 54% of the total wear. In a 500 m long tunnel project in Austria, the cross-section was unevenly distributed between soft and hard rocks [5], and statistical data on the type of cutter wear are shown in Figure 3. The percentage of abrasive wear in this tunnel section decreases to only about 49% for uneven strata and blocky rock conditions and 41% for cutter ring fractures. The decrease in the percentage of abrasive wear for cutting composite strata indicates that wear in soft rock requires a different wear calculation method. Cutting soft rock tends to lead to cutter ring sharpening, and cutting inhomogeneous formations leads to vibration damage of the cutter and fracture. [PERSON]'s study showed that even wear was the most probable type of cutter wear, with homogeneous or abrasive wear dominating. This was followed by bias wear and cutter ring chipping. Low wear has relatively little effect on the rock-breaking capacity. This study focuses on the type of uniform wear of the cutter ring. Wear areas on the outer edge and sides are analysed, and a wear prediction model is constructed. The degree of cutter wear often varies during the life cycle. Clarifying the wear failure and replacement cycle of the cutter can optimally improve the rock-breaking performance and cutting efficiency of the TBM [23]. The distribution of cutter life and wear is shown in Figure 4; the wear increases gradually with the wear distance, and the cutting distance affects wear degree and cutter life [3]. The probability of even wear is high, and there is a correlation between different factors. Figure 3: Proportion of wear types for cutter cutting soft and hard composite strata. Figure 2: Proportion of wear types for cutter cutting hard rock. Wear occurs on the surface of the material, and the hardness of the alloy material of H13 steel reflects the characteristics of the cutter ring, which is an important factor affecting the wear of the cutter. Hardness reflects the abrasiveness of harder materials on softer materials. Analysis and determination with a hardness index is helpful to describe the degree of wear and calculate the amount of wear. [PERSON], [PERSON] et al. [4] and [PERSON] et al. [13] investigated the relationship between the hardness of the cutter ring material, the average wear depth and the mass loss. Figure 5 shows the variation in wear with hardness. It indicates that there is a negative correlation between the hardness of the cutter ring material and the average wear amount [24]. The greater the hardness of the cutter ring, the lower the wear of cutting soft and hard rocks. The wear state of the cutter ring is affected by the abrasiveness of the rock and the degree of penetration, reflecting the rock characteristics of the shield excavation face. [PERSON], et al. investigated the relationship between penetration, external thrust and ultimate torque [12].