# Study on Failure Mechanism of Mudstone Based on Digital Core and Digital Volume Correlation Method
## 5 Analysis of Mudstone Failure Process by Digital Volume Correlation Method
### Analysis on the Evolution Process of Strain Field  
The strain field can better reflect the evolution process of local deformation in rock, six strain components of Cauchy strain tensor were solved by local least square fitting method, and the equivalent strain and volume strain under different loads can be calculated. It can be seen from the equivalent strain nephogram (Figure 16) and the 3D volumetric strain nephogram (Figure 17) of the mudstone sample during full loading, at the initial stage of loading, the overall strain of the sample was small, and the high strain area was mainly concentrated in the XOZ plane and YOZ plane. The reduction to the real sample was in the quartz dense accumulation area of the mudstone sample, and the high strain area was scattered near the quartz particles. When loading to 18.5 MPa, there was a higher equivalent strain in the lower left corner of the sample, and the maximum equivalent effect of the sample was 0.01; according to the strain and the ultimate failure strain of mudstone, it can be inferred that there should be small cracks in this area of the sample at this time. With the progress of loading, the main strain region gradually shifted to the right side, showing the phenomenon of localized zone, which reflected the closure and change of primary pores, cracks, and other structures in the rock at the initial stage of loading. The  
Figure 15: Evolution process of displacement field u, v, and w in x, y, and z directions. equivalent strain in the mudstone went through a process of first dispersion and then concentration, which was because the microcrack near the mineral zone was closed at the initial stage of loading, resulting in a small range of strain concentration in the elastic stage, resulting in the rapid increase in the main strain of this part; with the increase in loading, the deformation of the sample tended to be gentle, and the equivalent strain entered a quiet period. When loading to 37.5 MPa, the sample began to show local deformation, and the maximum volume strain inside the specimen was 0.3, indicating that when the load is low, the volume near the mineral zone changes greatly, so it is easy to produce local deformation in the first step. When loading to 48.5 MPa, the microcracks and pores in the sample continued to develop and penetrate, forming new cracks and large-scale local deformation. The equivalent strain in the sample was rapidly redistributed with the new deformation region, forming an obvious boundary between the two mineral zones. Figure 18 shows the variation curve of volumetric strain and equivalent strain with axial stress in the process of uniaxial loading of mudstone. From the curve in the graph, it can be seen that there was a certain correlation between them. According to the change of the curve, it can be divided into three stages:  
1. The stage of compaction (a-b). During the loading process (0-18.5 MPa), the volume strain of the sample was negative, and the equivalent effect increased slowly, indicating that the sample was affected by axial load and the sample was compressed. Combined with the equivalent strain distribution nephogram, the strain region was mainly distributed in the mineral zone, indicating that the intergranular pores near the mineral particles were squeezed, the pore connectivity increased, and the overall porosity showed an increasing trend. The number of pore throats between isolated pores increased, the coordination number of pores increased, and deformation occurred. 2. The stage of microcrack development (b-c). During the loading process (18.5 MPa-48.5 MPa), the volumetric strain reached the minimum rapidly, and with the increase in load, the volumetric strain began to rebound and increased to 0. At this stage, the equivalent strain was redistributed from the local discrete strain region to form a large range of strain region, and the high strain region was gradually connected, maintaining a rapid growth trend, while the porosity continued to increase, and the connectivity of the pore network increased.