## 3 Results
### Mechanical and Fracturing Behaviour of Intact Samples
#### 3.1.2 Effects of Printing Direction and Sintering Process  
The printing direction and the sintering process of the PP 3 DP method will affect the mechanical properties of the specimens. This feature makes the PP printing capable of simulating the sedimentary rocks regarding the sedimentary direction and the cementation condition between grains. This section selected five different printing inclination angles and four sintering temperatures to systematically study the influence of the printing parameters of PP 3 DP on mechanical properties and failure modes. 1. Inclination angle  
Table 1 lists the physical and mechanical properties of the 3 DP specimens, including the dry density, UCS, and failure strain, with five printing inclination angles (0\({}^{\circ}\), 30\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\)) and the sintering temperature of 100 \({}^{\circ}\)C. The UCS is significantly affected by the printing inclination angle. The specimens with the 45\({}^{\circ}\) inclination angle have the lowest UCS value consistent with the experimental data on the sedimentary rocks in previous studies [54; 55; 56; 57], while those with the 0\({}^{\circ}\) and 90\({}^{\circ}\) inclination angles are the most consistent with the theoretical result based on single-joint mode [58] and the experimental result given by [PERSON] and [PERSON] (2016) [20]. Fortunately, the failure strains of most specimens are less than 3%, supporting a brittle failure characteristic. Figure 3 shows the typical stress-strain curve of these 3 DP specimens. The inclination angle also has a conspicuous influence on elastic modulus, post-failure modulus, and brittleness, which should be considered in the research using 3 DP methods. Coplanar failure patterns may dominate the failure mode for complicated structures manufactured by these layered printing methods. The specimens with printing inclination angles of 30\({}^{\circ}\), 45\({}^{\circ}\), and 60\({}^{\circ}\) have a shear plane coplanar with the printing direction, as shown in Figure 3. The specimens with a printing inclination angle of 90\({}^{\circ}\) have a diagonal or X-shaped failure plane, similar to the failure mode of natural homogenous and isotropic brittle rocks. Therefore, the printing inclination angle is chosen to be 90\({}^{\circ}\) to minimize the influence of the printing inclination angle in this study. 2. Sintering temperature  
A certain period of high-temperature curing can improve the strength of the PP 3 DP sample. It is noted that all the samples are printed with a printing inclination angle of 90\({}^{\circ}\). Since it is found that the change in sample mass becomes very little after eight hours of sintering, the samples are heated and cured in an oven for eight hours at different temperatures (20 \({}^{\circ}\)C, 60 \({}^{\circ}\)C, 100 \({}^{\circ}\)C, and 140 \({}^{\circ}\)C) in this study. The stress-strain curves and the corresponding UCS and failure strain are shown in Figure 4 and Table 2. It is found that the mechanical properties are very sensitive to the sintering temperature, with nonlinear and nonmonotonic variation trends. The UCS increases with the sintering temperature within 100 \({}^{\circ}\)C, while it decreases sharply over 100 \({}^{\circ}\)C. In contrast, the failure strain decreases significantly with the sintering temperature within 100 \({}^{\circ}\)C, while it increases at 140 \({}^{\circ}\)C. The surface colour changes from pure white to light yellow at 140 \({}^{\circ}\)C (Figure 4), implying the chemical change of the gypsum or the binder. However, the sintering temperature has little influence on the failure pattern, mainly diagonal or X-shaped failure.