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  • The propagating velocity of failure wave in test alumina

    2018-11-12

    The propagating velocity of failure wave in test alumina can be also obtained from the trajectory, which is the slope of the trajectory curve about 5.051 km/s in Fig. 6. This velocity is apparently higher than those of the failure waves observed in the shocked glasses, where they are usually 1–3 km/s [7,10–12]. The formation mechanism of failure waves in shocked glasses is always interpreted as the activation and growth of microcracks on the impact surface [7,15]. From this point of view, the propagating velocity of failure wave should be slower than the limiting growth velocity of crack which is always slower than Rayleigh wave velocity [28]. Once the velocity of cracks reaches a limited value which is much slower than Rayleigh wave velocity, they tend to branch out [29]. The Rayleigh wave velocity in tested alumina can be calculated directly by the shear wave velocity and the Poisson\'s ratio as follows [28] According to the SEM micrographs of alumina sample shown in Fig. 7, it SBI-0206965 is known that the microstructure of alumina consists of alumina grains, pores and intergranular glassy phase. The grains and pores distribute randomly with the diameters of 1–15 µm. Intergranular glassy phase is distinct in a compact area. Pores and glassy phase weaken the mechanical capabilities of alumina, and these heterogeneous microstructures act as the stress concentrators. It has been well known that a high shear stress would be produced due to the large confining stress under the uniaxial strain loading. The localized stress concentrations are expected to arise from the propagation of cracks and flaws at grain boundaries. The failure is proposed to proceed essentially through rapid in situ grain boundary microcracks nucleation and comminution with very limited crack growth after a delay time once a shock wave travels through the sample. As the microcracks in situ nucleate in the stressed alumina and do not need time to transmit from the impact surface, the failure front with lower dynamic impedance in the shocked alumina could be detected much earlier from the rear surface, and it therefore gives a higher observed failure wave velocity. This failure mechanism is different from that in the shocked glass, but similar to that in the shocked rocks [30].
    Summary
    Acknowledgments The project is supported by the Science and Technology Development Foundation of China Academy of Engineering Physics (Grant No. 2014B0101009) and the National Natural Science Foundation of China (Grant No. 11502258, 11272300).
    Introduction A different approach can be realized by a set of geometrically separated channels and an array of slit apertures, in order to prevent multi-exposure. Due to geometrical boundary conditions with respect to the target set-up and safe distances, the number of channels is also limited. Therefore, both methods allow only pseudocinematography of the process to be observed, since the radiographs of several experiments have to be combined in order to get a time-resolved image of the process. However, this requires a high reproducibility of the experiments, which can be difficult to achieve in a series of tests. The lower the reproducibility, the higher is the number of tests needed. For this reason it is desirable to have a flash X-ray system that provides a high-number of radiographs in just one experiment. A system which provides eight flash radiographs at a frame rate of up to 200 kHz has been developed at EMI [1]. This so called flash X-ray cinematography technique was applied in order to study the dwell–penetration transition with small caliber AP projectiles impacting different SiC ceramics on three types of backing. The phenomenon of dwell with small caliber AP projectiles at impact velocities below 1000 m/s was already discovered in the pioneering studies of Wilkins [2], who examined the interaction of 7.62 mm AP projectiles and surrogate steel penetrators with thin ceramic/aluminum targets. Using the classic flash X-ray technique Wilkins observed that the steel projectiles did not penetrate the ceramics during a time interval of about 20 µs after impact. During this phase the projectiles were eroded to about half of their initial length. The phenomenon that a projectile does not (or only very little) penetrate a target over a period of time is designated as dwell. Several studies with small caliber projectiles on the dwell phenomenon have demonstrated that erosion or “wear” of the steel core is one key factor in the energy dissipation of the projectile and, thus, for the ballistic resistance. P.C. den Reijer [3] studied the interaction of steel cylinders with thin Al2O3-ceramic/aluminum targets using a pseudo-cinematography set-up and determined penetration curves and dwell times. Penetration velocities were determined by Gooch et al. [4] for 7.62 mm APM2 projectiles with B4C ceramic, and the dwell and penetration behavior with B4C/aluminum targets was studied by Anderson et al. [5] using two 1 MeV X-ray pulse generators.