PSI - Issue 37

Theodosios Stergiou et al. / Procedia Structural Integrity 37 (2022) 250–256 T. Stergiou et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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discretised points along the geometry with the FE method. The discretisation of layers of the thin-plate target was performed by utilising a solid hexahedral mesh, with 12 elements through its thickness. In order to prevent the non physical hourglass deformation mode, viscous hourglass stabilisation was implemented. The fictitious hourglass energy was controlled to remain below 3% of the projectile’ s initial kinetic energy. Also, the projectile was discretised using a solid hexahedral mesh. Moreover, the interaction between the target and the projectile was assumed to follow a linear Coulomb friction law tangentially, with a dynamic coefficient of 0.01 and a hard-no penetration contact in the normal direction. The value of the tangential coefficient was suggested by Recht (1990) and proven suitable for applications involving metal metal impact. 3. Results and analysis Numerical simulations of impact at normal incidences were conducted at various velocities, below and above the critical velocity of penetration, to examine the effect of projectile ’s nose variations on the target response and projectile ’s deceleration profile during impact. Upon impact, sharp projectiles experienced a constant velocity drop that, for thin targets, was found to be insensitive to the impact velocity. The mechanism of target failure was identified as transverse tearing, a shear-dominated mechanism, where sections of the target were driven out by the inclined surfaces of the projectile and extracted at low velocities. The formation of such chips is shown in Fig. 2b. In Fig. 3, the absolute gradient of the projectile ’s velocity-time profile as a function of the projectile ’s half-angle is provided for a wide range of cases showing that the rate of velocity drop increases with the half-angle. The projectile with the sharpest nose considered herein ( ̂ = 15° ) experienced the lowest target resistance to penetration resulting in the lowest deceleration, while with increased half-angle, the deceleration grew linearly up to a threshold within the half-angle range of 60° and 70°. Up to this point, transverse tearing was identified as the failure mechanism, resulting to predominantly local deformation and failure of the target ’s impact zone. The data for this region is shown in Fig. 3 in blue.

half-angle,

Fig. 3: Effect of half-angle on projectile ’s deceleration.

A further increase in the projectile ’s half-angle resulted in a discontinuity of the previously observed trend, suggesting a change in the failure phenomenon. Indeed, examination of the target ’s failure mode indicated a transition of the fracture mechanism from transverse tearing to stretching (shown in Fig. 3 in purple), with the increased bluntness of the projectile ’s nose. These projectiles experienced higher rates of velocity decrease, while the target ’s reaction was associated with a global response of increased bulging and transfer of kinetic energy.