# 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 of the Evolution Process of Displacement Field  
On the basis of the displacement distribution histogram calculated by DVC, the displacement field of mudstone sample under initial loading was analyzed. The X-axis displacement was uniformly distributed between \(-5\) voxels and \(5\) voxels, and the frequency was concentrated at about \(20\). The displacement distribution of the Y-axis was close to that of the X-axis; there was only negative displacement in the Z-axis, which indicates that the mudstone sample was compressed and expanded in the X-axis direction. Moreover, a small abrupt increase in frequency appeared in the range of \(-5\) voxels displacement, indicating that there were a large number of quartz mineral particles, micropores, and microcracks in the rock. At the initial stage of loading, microcracks were affected by external loads, and microcracks and micropores were gradually compressed and closed, which was the reason for voxel negative value in the numerical computation. On the other hand, a large number of quartz particles in the mudstone samples were hard and widely distributed in space. Under the influence of external load, the partially agglomerated quartz particles expanded and slid towards the X-axis and Y-axis after longitudinal extrusion, resulting in both negative and positive displacement in the X-axis and Y-axis. At the same time, in the state of uniaxial compression, at the initial stage of loading, the mudstone sample was mainly affected by the external load in the loading direction; the longitudinal microcracks were closed; and the hard mineral particles slipped in the plane, resulting in the expansion of the sample in the macroscopic range (Figure 14). According to the analysis of the evolution process of the displacement fields u, v, and w in the x, y, and z directions of mudstone (in which u = u(x, y, z), v = v(x, y, z), w = w(x, y, z)) (Figure 15), we can know that the sample is mainly subjected to axial vertical compression, and because there were a large number of mineral particles in the sample, the sample showed obvious heterogeneity with the overall deformation being extremely uneven. In the first loading stage, the internal pores of mudstone samples were mostly isolated pores, and the pore connectivity was poor. The second loading stage (37.5 MPa) was the starting point of the localized deformation of the sample. The displacement value of the sample near the mineral zone was larger, and the obvious local deformation had appeared inside the sample. With the increase in the load, the overall element displacement of the specimen was gradually increasing. There were few primary cracks and micropores in the rock, and the porosity of the rock sample was low. At this time, a macroscopic crack appeared on the surface of the sample, and the pore network model was mainly developed into two clusters, with the position of the two pore connecting clusters being very close to the distribution of the mineral zone. In this stage, the rock failure increased gradually, and the number of pores and penetration was much higher than that in the previous loading stage. The large displacement area corresponded to the mineral distribution area  
Figure 14: DVC computation of x, y, and z direction displacement and ZNCC distribution histogram. in the 3D computational area shown in Figure 13 and gradually expanded, because in the first and second loading stages, the mudstone was in the elastic stage, and the lithology of the mudstone was more compact and the pores were compacted. In the third stage of loading, when the stress value reached 48.5 MPa, the internal microcracks developed rapidly, showing obvious positive and negative displacement zones, indicating that some areas had obvious local deformation, and these areas expanded further with the loading. The rapid growth of the average coordination number of pores may have been due to the fact that the pores at this stage had developed to the critical state of failure.