13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Table 2. Advantages and disadvantages of materials as simulants for human skulls Material Advantages Disadvantages MDF as a skull/bone simulant MDF behaves in a brittle manner similar to human bones. MDF produces material backspatter under a high velocity projectile impact. Bone is known to produce similar backspatter [3, 34]. There was no fracture of MDF other than in the area immediately around projectile impact. Fracture lines and cracks are often seen to radiate out from the impact site of a projectile as happens in a skull/bone [35]. Polycarbonate as a skull/bone simulant There are limited advantages of polycarbonate as a bone simulant. Polycarbonate did not produce any material backspatter on impact, and there was no material fracture in the form of cracking. The ductile nature of polycarbonate makes it a poor simulant for bone. (a) 0 ms (b) 0.25 ms (c) 1 ms (d) 4 ms Figure 4. Deformation behaviour during projectile impact on polycarbonate (PC1) 4.2 Computational SPH model of MDF panels Next we developed a computational model to simulate backspatter using the SPH method. To evaluate the suitability of SPH as an efficient method to capture the impact fracture, fragmentation, post-impact particulate ejection, we simulated the impact of a projectile into a 6 mm MDF panel. Fig. 5 shows the evolution of von Mises stress (left column) and plastic strain (right column) of the plate at various times after the projectile impact. Particle colouring represents a range of 0-20 MPa for von Mises stress and 0-0.5 (failure strain) for plastic strain. On impact (Fig. 5a) high impact stresses are generated around the projectile tip leading to fracture at the impact point (shown by red particle dispersion). The stressed zone expand radially but are not large enough to produce failure (only 4-12 MPa), and no plastic deformation is noticed outside the fracture site (Fig. 5b). The stress waves reach the plate boundaries, reflect and then dampen. Some fragmented particles at impact have now moved away from the impact site in the opposite direction of the projectile motion leading to backspatter (Fig. 5c-d). The majority of particles clump and fall at the impact site. Failure is only predicted at the impact site. The SPH model prediction was then compared against the equivalent MDF experiment. Fig. 6 compares the fracture pattern and particle movements against the SPH prediction. Backspatter predictions at 1 and 4 ms show particle movement over a similar distance to the experiment. This means that the velocity of the particles would also be similar. The pattern of spray is also similar representing a 3D cone pattern. The predicted entrance site is also similar to that of the experiment and no fracture is observed away from the impact site in both modelling and experiment.
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