The influence of the fracture toughness

The influence of the fracture toughness of the ceramic material on the transition pressure is illustrated in Fig. 8. Experimentally determined as well as estimated transition velocities and pressures versus prestress are shown in Fig. 9. Unless otherwise stated, the material data for SiC-B in Table 1 have been used in Figs. 5–8.
Discussion Fig. 4 shows the development of interface defeat at an impact velocity just below the transition velocity in an unconfined SiC-X1 target. The copper cover in front of the ceramic reduces the initial effect of impact by establishing erosion of the projectile before the latter reaches the ceramic surface. Other studies [10,26] have shown that after radial flow had been established, it phospholipase c inhibitor continued steadily for a long time, i.e., the projectile load can be seen as quasi-static. Although the transition velocity increases with prestress, the velocity interval from interface defeat to penetration remains narrow and is not affected by the prestress and the flow onto of the ceramic surface look similar for the different targets, see Fig. 5. The effect of prestress is clearly seen in the experiments; the transition from interface defeat to penetration is moved to significantly higher levels as the confining pressure increases. The velocity at transition in unconfined SiC-X1 target was found to be 982 m/s whereas a confining pressure of   increased this transition velocity to 1367 m/s. This relatively low level of prestress almost doubles the projectile pressure at transition. The two leftmost open circles in Fig. 9 correspond to these values. Further increase of the confining pressure did not show the same strong influence and a transition velocity of   was found unaffected in spite of increased confining pressure from 100 to 168 MPa. The SiC-B seams to behave in a similar way as the SiC-X1, though no data is available in-between unconfined and 168 MPa of prestress. Although the number of experiments is not sufficient for a statistical analysis, the SiC-B seems to perform slightly better than SiC-X1. This could be a result of the slightly higher fracture toughness for SiC-B or due to natural variations in the properties of the projectile which determines the transition pressure. Use of Eqs. (1)–(3) and typical uncertainties for the material data in Table 3 give an indication of an maximum error in transition pressure within ±1.7 GPa, i.e., the observed differences between SiC-B and SiC-X1 may simply be within statistical fluctuations. The SiC-X1 experiments indicate a shift in behaviour for a confining prestress around 100 MPa. This is illustrated with two grey sectors in Fig. 9(a) and (b). The shift may indicate zygospore another fracture mode, e.g., modus-II cracking, has been activated. A higher transition velocity for a similar combination of ceramic and projectile materials has been reported [8] but then with strong axial and radial confinement. Such a high overall confining pressure could affect both modus-I and II fracture. The model for the projectile pressure in Eqs. (1)–(3) gives similar results as numerical simulations in [17] where more detailed material models for both the tungsten and the SiC material were used. The projectile pressure distribution used here is not identical to the one in [17] but gives only minor changes in the overall stress distribution and does not appreciably change the results. The influence of the confining prestress in the model consists of two parts: (i) the change in tensile stress field due to the prestress and (ii) the influence of prestress on the opening process of the crack itself. The model for the tensile stress over the crack assumes that the stress field will be unaffected by the crack. This is a simplification but does not significantly change the general behaviour of the model. Fig. 6 shows that the principal tensile stress over the crack decreases and changes direction towards the impact surface as a result of the confining prestress. This will gradually reduce the effect of a radial prestress on the crack propagation.