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    • The Hybrid cased charge split along the

    2018-10-22

    The Hybrid cased charge split along the axial grooves producing large heavy fragments (Fig. 15). The Buxton liner in the high mode generated regular shallow cuts into the case, but the cuts did not appear deep enough to promote regular fracture across the strips defined by the grooves. Occasionally the strips had fractured at a location of a Buxton liner formed cut, but the timing of the fracture was unknown. When the Hybrid cased charge operated in the low mode, the case split in a similar manner. The Buxton liner again generated regular small cuts into the case (Fig. 16).
    Case expansion The Phantom camera images of the charges allowed the observation of the case expansion at early times. Measurements were taken to calculate the case radius at the grid position marked on the case of the charges, this gave up to four radius values at each position along the case length. At later times, obscuration prevented some measurements and limited how many times were measurable. The case radius from the trial has been compared with a GRIM 2D simulation for the high modes of both charge designs. Fig. 17 shows three times from the Hybrid high mode from round 7. The GRIM modelling results are plotted with a time offset from the experiment to take account of the detonator and pellet delay. This is assumed to be approximately 7 µs. The points from the experiment show some variation which is due to errors in measurement; however they show good agreement with the modelling. These data will also help validate the early expansion of the case for 3D modelling of the design. Low brisance explosives typically continue to accelerate fragments during the expansion of the explosive products; due to the early obscuration this assessment method would not be expected to yield a reliable measure of fragment velocity.
    Velocity and blast results In total the trial consisted of 9 firings; the first was a PE4 bare charge followed by the eight test charges. Table 5 shows a summary of the fragment velocities recorded by the velocity foils and the peak blast pressures recorded by the first two blast gauges.
    Conclusions The rgs protein     in peak pressures for the Hybrid design of ~33% was very similar to the value observed in the previous bare charge study [1] at ~35%. The similarity in the peak pressure reductions suggested that, when the case fracture is not delayed, both cased and uncased configurations operate in the same manner.
    Recommendations
    Acknowledgments The authors would like to acknowledge the financial support of the Anglo-French Materials and Components for Missiles, Innovation and Technology Partnership (MCM ITP) program jointly funded by UK MoD (Dstl) and DGA.
    Introduction The high strength of armour ceramics [1–3] makes it possible to partially or totally defeat high velocity projectiles directly at the surface of the ceramic material. This phenomenon is called interface defeat or dwell [4–17] and is an important defeat mechanism in, e.g., light armour applications. One limitation when applying this in heavier armour designs is that it appears to be length scale dependent. Replica scaled impact experiments with unconfined ceramic targets show that the transition velocity, i.e., the velocity at which interface defeat ceased and ceramic penetration occurred, decreased as the length scale increased [11]. A probable explanation of the observed scale effect is that although maximum shear strength determines the upper bound for the transition from interface defeat to penetration, it is usually limited by the formation and growth of macroscopic cracks. Since the crack resistance of ceramic materials decreases with increasing length scale, in contrast to the otherwise scale-invariant stress field, the extension of a crack to a critical size will occur at a lower impact velocity in a larger target. An analytical model in [11] for the influence of length scale on the growth of a cone shaped modus I crack in thick unconfined ceramic targets gave reasonable results compared to the replica scaled impact experiments. The model showed that the projectile pressure at transition, i.e., the impact velocity at which the contact pressure exceeds the strength of the ceramic material and penetration initiates, is proportional to one over the square root of the length scale.