## 2 Test Device, Samples, and Test Methods
### Samples and Test Methods  
The test samples were cuboid test blocks of Chifeng basalt measuring (300 \(\pm\) 5 mm) \(\times\) (70 \(\pm\) 2 mm) \(\times\) (70 \(\pm\) 2 mm) (length \(\times\) width \(\times\) height). The sample preparation met the requirement for surface evenness recommended by the International Society for Rock Mechanics (ISRM). Samples were prepared through two processes: cutting and polishing. The rock was dense basalt, and X-ray diffraction (XRD) results show that it was mainly composed of feldspar, pyroxene, olivine, and a small amount of ilmenite. The average density, average wave velocity, and average uniaxial compressive strength of the rock samples were 2.90 g/mm\({}^{3}\), 5900 m/s, and 282.50 MPa, respectively [27].  
The microwave heating tests were performed in the modified commercial microwave oven. In the microwave oven, a multite cushion (a microwave-insensitive material used to elevate the sample) was placed, on which the basalt sample was put to be heated by microwaves. The used microwave heating power was 2100 W, and the preset microwave heating durations were 0 (blank control), 3, 6, 9, 12, and 15 min (the microwave heating duration was relatively long due to the large volume of the rock samples in the tests, while in practical production, an industrial microwave oven is generally used). After microwave heating, the samples were removed and naturally cooled to room temperature.  
The cutter wear and cutting tests were conducted on the combined abrasion test device for the SP3-I TBM cutter. The material of the reduced-scale TBM cutter used is H13, the diameter of the cutter ring is 48.26 mm, the surface hardness of the cutter ring is 58 HRC, the blade width is 1.5 mm, and the ratio of the prototype TBM cutter is 1:10. The bearing specification is 6800 ZZ deep groove ball bearing with an inner diameter of 10 mm and outer diameter of 19 mm. Before testing, the reduced-scale cutter was first wiped with acetone and then weighed using a balance with a resolution of 0.001 g after being dried. Thereafter, the reduced-scale cutter was fixed on the fixture (Figure 4), which was then mounted on the test device. After adjusting the reduced-scale TBM cutter to the lower part of the sample, a basalt sample was fixed on the fixture (Figure 5). In the process of cutting rocks, the three-dimensional cutting forces (driving force and rolling force) applied on the reduced-scale TBM cutter were measured by the three-way force sensor on the tool apron.  
Figure 4: Installation of the reduced-scale TBM cutter and the fixture: (**a**) Reduced-scale TBM cutter; (**b**) Cutter weighing; (**c**) Cutter shaft; (**d**) Cutter fixture.  
In the cutter wear and cutting tests, the penetration speed, penetration depth, single rolling stroke, rolling speed, cumulative number of rolling events, and cumulative rolling distance of the reduced-scale cutter were set to be 0.08 mm/min, 3 mm, 250 mm, 17.8 mm/min, 150, and 37.5 m, respectively. The cutting marks of the reduced-scale TBM cutter with different spacings could be set by adjusting the horizontal installation location of the rock sample. In the tests, each sample was cut twice, on which two cutting marks were left, and the cutter spacing was set to 20 mm. After cutting the basalt samples with the reduced-scale cutter, the cutter was removed from the fixture and then weighed. The rock debris falling therefrom was also collected, weighed, and screened.