# Drill Hole Orientation: Its Role and Importance on the Compression Response of Pure Magnesium
## Materials and Methods  
### Simulations  
#### 2.1.1 Simulation Parameters  
For a specimen with a vertical drill hole orientation, [PERSON] et al. [36] reported that a 1.5 mm diameter drill hole offered optimum material properties. Hence, to maintain a comparison, three different drill orientations (vertical, angular, and horizontal) were analysed in this study while maintaining a drill diameter of 1.5 mm. In the case of vertical and horizontal drills, the drill position was centralized in the sample. However, for the angular drill condition, due to practical difficulty in obtaining a 45\({}^{\circ}\) orientation (i.e., drilling hole on the edge of a sample while maintaining stability), a drill position, slightly lower than the edge of the sample, was chosen, as shown in Figure 1b.  
Similar to previous experiments conducted by [PERSON] et al. [36], 8 mm diameter cylinders with 8 mm height were considered in these simulations as well. This resulted in a hole length of 8 mm each on the vertical drill and horizontal drill cases (Figure 1a,c) while it resulted in 9.94 mm in the angular drill scenario (Figure 1b). These lengths were measured along the axis of the drill hole. The above models were built in ANSYS DesignModeler, following which, they were meshed using structural hexahedral and tetrahedral elements in ANSYS Mechanical v19. The vertical, angular, and horizontal drill hole models had 50,200, 245,292, and 77,922 elements, respectively, with a minimum element size of 0.1 mm around the hole. A visual representation of the mesh around the hole is shown in Figure 2.  
Figure 1: Drill hole positions and orientations for (**a**) Vertical (**b**) Angular and (**c**) Horizontal drills.  
The material behaviour was set to be deformable in elastic and plastic regions with a Young's Modulus of 45 GPa, a Poisson's ratio of 0.28 (as reported by [PERSON] et al. [37]), and a true stress-true strain curve for plastic behaviour obtained from monolithic magnesium experiments (i.e., with no drill hole). The engineering stress-strain curve for monolithic magnesium, from which true stress-true strain curve was derived, is represented in Figure 3.  
To simulate compression testing, a rigid top support with a fixed constraint was used to restrain the movement of the cylinder in the upward direction while the compressive load was applied using a rigid plate at the bottom of the sample. Additionally, deformable-deformable frictional contact pairs with a coefficient of friction of 0.3 were used in the presence of an augmented Lagrange contact algorithm with contact detection at every Gaussian integration point to ensure that simulations mimic  
Figure 3: Experimentally obtained Engineering Stress–Strain curve for undrilled monolithic magnesium that was converted to true stress–true strain curve for use in simulations.  
Figure 2: Finite Element Mesh around the drill hole in the three cases (**a**) Vertical drill hole, (**b**) Angular drill hole and (**c**) Horizontal drill hole.  
experimental conditions. Incremental compressive loads up to 15,000 N were applied on the bottom support in each case to study their deformation behaviour. This is illustrated in Figure 4. With this setup, a Newton-Raphson based elastic-plastic analysis was carried out using a sparse matrix direct solver with large deformation mode active.