A possible way to suppress

A possible way to suppress the formation and growth of macroscopic cracks is to prestress the ceramic material. The influence of prestress and the related failure modes of impacted ceramics have been studied by several authors. The papers [18–20] report experimental data on small calibre projectiles impacting thin prestressed ceramics (i.e., the thickness is of the same order as the diameter of the projectile). These studies show that prestress reduces damage in the form of fewer macroscopic cracks and that the trajectory of possible cone cracks becomes shallower. An increase in protective performance was also observed. The papers [8,9] report experimental data on model scale long rod projectiles impacting thick ceramic targets (i.e., the thickness is much larger than the diameter of the projectile). The experiments in [8] with large and heavily confined and prestressed targets showed similar interface defeat velocities as small, unconfined targets in [9]. This indicates the need of prestress in larger targets. Holmquist and Johnson [21] and later Runqiang et al. [22] conducted a computational study on the responses of a small scale thick prestressed ceramic target tested by Lundberg et al. [7]. Various levels of prestress and stress states were simulated. Their studies showed that prestress enhanced the performance and that the velocity at which ceramic penetration occurred, i.e., the transition velocity, could be increased by prestress. This paper explores the influence of a radial confining prestress on the transition from interface defeat to penetration for a thick ceramic target. Although the physical background of the influence of prestress on the transition velocity in ceramic targets is not fully explained, impact experiments as well as modelling indicate that it is intimately linked to ceramic fracture. A phospholipase a2 inhibitor proposed in [11] is that the centre part of the ceramic suddenly loses radial support as a result of the cone cracking. A confining prestress will suppress the growth of the cone crack by lowering the stress intensity over the crack tip. In order to overcome this virtual toughening of the ceramic, the projectile pressure on the surface of the target must be increased relative to osteoporosis for an unconfined target in order to initiate critical fracture. A set of impact experiments have been performed in which the transition velocities for four different levels of prestress were determined. Two grades of silicon carbide ceramics with slightly different mechanical properties were used. The experimental technique used is presented in the paper together with the determined transition velocities versus radial confining prestress. The experimental data are compared to an extended version of the model presented in [11].
Model of cone crack under confining prestress
Experiments The impact experiments were performed using a reverse impact technique. The stationary projectiles were suspended in blocks of Divinycell material (density 45 kg/m3) and mounted in front of the muzzle of the gun, see Fig. 2. Two different qualities of silicon carbide materials have been used: SiC-B and SiC-X1, both materials are from CoorsTek (former BAE Systems Advanced Ceramics Division and Cercom Inc, Vista, CA). The SIC-B material was initially delivered as large cylinders with diameter 50 mm and length 50 mm. From these, smaller cylinders with diameter 20 mm and length 20 mm were produced. The SiC-X1 material was delivered as cylinders with diameter 20 mm and length 20 mm. Data on microstructure, Young\'s modulus and fracture toughness of the SiC-B used has been published by Wereszczak et al. [23] and properties for SiC-X1 has been provided by CoorsTek. The properties are given in Table 1. The Vickers hardness and the fracture toughness were estimated by means of a Wholpert macro hardness indenter equipped with video-system and imaging software. SiC-B and SIC-X1 samples were carefully polished using a semi-automatic polishing machine, and each material was indented 9 times. The fracture toughness was determined according to Anstis et al. [24]. The estimated Vickers hardness and fracture toughness of SiC-B and SiC-X1, normalised to these quantities for SiC-B, are given in Table 2.