## 3 Results and Discussion
### Microscopic Observations  
#### 3.2.1 Characterization of Contact Networks and Force Transmission  
Considering that intergranular forces are strongly dependent on the orientation of contacts, a complete description of load transfer in granular assemblies requires information on the distribution of contact orientations. The morphological characteristics of force chain networks (strong and weak) of FRS with different fiber orientations before and after shearing provide insights into the paths transmitting contact force in FRS. Figure 9 presents polar histograms in the XZ orthogonal view, showing the distribution of contact normals before and after shearing in specimens with different fiber orientations under 200 kPa of normal stress. As shown in Figure 9, a similar fabric of strong normal contacts is observed in the four samples, which indicates that fiber orientations have little effect on strong contact distribution. In the initial state, strong force chains are found with their major orientation aligned in the horizontal direction, and the histogram of normal contacts has a spindle shape. In the final state, the principal direction of anisotropy of the contact normals for strong contacts is about 150\({}^{\circ}\), which signifies that the strong contacts are apt to be orientated in the diagonal direction going from upper-left to lower-right of the box. The contact normals in the direction of 60\({}^{\circ}\) mainly decrease, so the distribution diagram transforms to a peanut-shaped distribution. Interestingly, the principal direction of anisotropy of the contact normals for strong contacts was observed to be perpendicular to the optimum orientation for maximum fiber contribution to shear strength. When the orientation of fibers is perpendicular to the main direction of strong contact force, more fibers are being subjected to radial compression, which is conducive to the formation of axial tensile forces of fibers and can contribute considerably to the strength of the composite. These results may help us to understand the strength anisotropy of FRS induced by fiber orientation. Figure 8: Stress–horizontal displacement curves of FRS with different fiber orientations (0\({}^{\circ}\), 30\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\)). The results indicated that the fiber orientation plays a key role in the anisotropy of the contact normals for weak contacts (see Figure 9). At the initial state, the rose diagrams of the contact normals for weak contacts are approximately spindle-shaped. The principal directions of anisotropy of the contact normals for weak contacts varied with the initial fiber angle. At the final state, the contact normals for weak contacts oriented in the direction of maximum extension decreased to a certain extent. In addition, the contact normals for weak contacts in horizontal orientations increased greatly under horizontal load. Furthermore, the stress state also influenced the evolution of anisotropy and the force chain network [38]. A representative stress for any sub-region of the granular material can be determined by summing together the stresses in the material cells within the chosen measurement region. We defined the measurement region in the middle section along the y direction of the generated FRS sample. Figure 10 shows the measured major principal stress field in FRS with various fiber orientations under a vertical pressure of 200 kPa. It is noticed that the distribution of principal stress magnitude is very close. In the initial state, a uniform distribution of major principal stress was observed, as shown in Figure 10. In the final state, the major principal stress mainly concentrated in the direction from upper left to lower right. [PERSON] et al. [39] and [PERSON] et al. [40] have shown that the major principal stresses have an angle of 60\({}^{\circ}\) relative to the vertical in direct shear tests with the DEM.