# Fracture Prediction in Weldox 700E Steel Subjected to High Velocity Impact Using LS-DYNA
## 3 Results and Discussion
### Investigation of Fracture Behaviour
#### 3.3.1 Fracture Behaviour with Varying Critical Fracture Parameter (Dc)  
It is learned that the value of the critical fracture parameter is a crucial factor in the analysis of fracture in a material. The model is applied to evaluate the effect of this parameter. The analysis is performed with a target plate thickness of 4 mm, a projectile velocity of 400 m/s having a mass of 6.4 g, while the Dc values are varied at 0.6, 0.8, and 0.99. (a) Plug formation Figure 8 shows the plug formation in the target plate with varying critical fracture parameters. It is observed that at a higher Dc value, a smoother plug is formed with larger deformation, while at a lower Dc value, a less smooth plug is formed with less deformation. Figure 7: Experimental results showing cavity in target plate due to plug formation under high-velocity impact obtained by [PERSON] et al. (2016). 2. Contour plots for von Mises stress stress  
The von Mises stress contour is presented in Figure 9 at different time steps to demonstrate the fracture progression along the thickness in the target plate. The results are presented for 4 mm target plate thickness under high-velocity impact using 400 m/s velocity and 6.4 g mass of projectile. The blunt-shaped projectile strikes the flat target plate. The elements along the projectile edge at the impacting face experience a rapid increase in von Mises stress (around 1600 MPa) at around 1 \(\upmu\)s. The stresses also propagate across the thickness under impact. The high values of von Mises stress affect the values of other material and fracture parameters which are discussed in the subsequent sections. The fracture starts from the top face of the target plate along the periphery of the blunt projectile at around 2 \(\upmu\)s. The fracture progresses to mid elements and the bottom elements experience stretching at around 6 \(\upmu\)s. It  
Figure 8: Plug formation with varying critical fracture parameters (Dc). Figure 9: Contour plot for von Mises stress at different time steps demonstrating fracture propagation across target thickness. is observed that at around 8 us, the complete plug is formed in the flat target plate of 4 mm thickness. The von Mises stresses are relieved in the plug as elements are separated from the target (around 1350 MPa). The projectile has completely penetrated the target plate across the thickness at around 16 us, ejecting the circular plug from the flat target plate. 3. Effect on fracture parameters  
Figure 10 illustrates that fracture in the \"top\" elements occurs earliest with a Dc value of 0.6, followed by 0.8 and 0.99. This is due to the rapid increase in triaxiality, which leads to an increase in equivalent plastic strain (Figure 11) and a decrease in load-carrying capacity, as shown in Figure 12. The trend for triaxiality is presented in Figure 13. Therefore, the elements experience the maximum value of fracture parameter and are deleted without appreciable elongation. It is also observed that the temperature does not affect the top element (Figure 14); thus, failure occurs primarily due to an increase in equivalent plastic strain (Equation (2)). Figure 11: Trend for equivalent plastic strain with varying critical fracture parameter. Figure 10: Trend for fracture parameter with varying critical fracture parameter. The \"mid\" elements experience significant elongation before failure due to the considerable effect of plastic strain rate (Figure 11) and temperature (Figure 14). Figure 12 shows that von Mises stress increases as a result of strain hardening in the element. It is observed that the triaxiality value remains either zero or negative with a Dc value of 0.6, whereas for Dc values of 0.8 and 0.99, its value is positive. As a result, the mid element fails due to  
Figure 14: Trend for temperature with varying critical fracture parameter. Figure 12: Trend for von Mises stress with varying critical fracture parameter. Figure 13: Trend for triaxiality with varying critical fracture parameter.