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    • The uncertainty in the measurement of the

    2018-11-12

    The uncertainty in the measurement of the projectile tip position is mainly caused by the resolution of the radiographical and optical system and the fact that, at material edges and interfaces, a gradual transition of gray shades occurs. These influences led to an uncertainty in the position measurement of ±1 mm. The uncertainty in the time measurement of the X-ray flashes can be neglected. The application of error propagation calculation yielded the uncertainty in the point of time of the intersection of the straight lines describing the penetration during and after dwell. The uncertainty of the dwell times was determined as ±2 µs, which allows for discrimination between different ceramic/backing combinations. The uncertainty of the penetration velocity after the dwell phase was ±10 m/s in the case described above (Fig. 5). The projectile–target interaction in the case of the ceramic EKasic F on aluminum backing is illustrated in Fig. 6, which shows eight flash radiographs recorded between 5 µs and 41 µs after impact. Due to the similar X-ray SBI-0206965 Supplier in the SiC and the aluminum, the contrast between the two materials in the radiographs was only weak. The aluminum backing appeared only slightly darker than the SiC. In some of the radiographs recorded later than 15 µs, it can be recognized that a small gap opened between the ceramic and the backing, which appeared as a bright line in the radiographs. The projectile did not significantly penetrate the ceramic for more than 20 µs. The position–time curve in Fig. 7 illustrates the course of the projectile–target interaction during the three tests with this material combination. Three sections could be distinguished in the penetration curve. The first phase without penetration (up to 9 µs) was followed by a phase with a low average penetration velocity of 41 m/s, during which the projectile penetrated only about 1 mm of the ceramic. The dwell phase lasted up to 24 µs and then the projectile penetrated the ceramic at an average velocity of 327 m/s. The projectile–target interaction in case of a sandwich panel between the ceramic and the aluminum backing is shown in Fig. 8. Due to its low X-ray absorbing power the sandwich panel appeared as a bright zone between the ceramic and the aluminum backing. The length of the projectile was also reduced significantly in this configuration, but penetration started earlier and the penetration velocity was higher compared to the other targets. After 20 µs a bulge could be recognized at the back side of the ceramic. Since the brittle ceramic cannot withstand such strong deformations without failure, it can be assumed that the ceramic is strongly fragmented in the region of the bulge. However, neither the spatial resolution nor the difference in density of the fragmented ceramic compared to the intact ceramic is sufficient to allow for a distinction between failed and intact materials in the radiographs. The recordings between 30 and 40 µs after impact show that the fragmented ceramic material was pushed into the sandwich panel by the penetrating projectile. The corresponding position–time curve for the projectile–ceramic interface is presented in Fig. 9 along with the data of the second test with this target configuration. The penetration curve can be approximated by two linear sections with different slopes. Penetration started shortly after projectile impact at an average velocity of 165 m/s. After 15 µs the penetration velocity increased up to an average value of 333 m/s and remained constant during the time interval of observation (50 µs).
    Discussion The objective of the conducted test series was to reveal the possible influences of the type of ceramic and backing on the duration of the dwell phase, the penetration velocity and the projectile erosion. The analysis of the radiographs demonstrated that, during the first phase of the projectile–target interaction, the penetration velocity was not zero, but very low. Depending on the target configuration the penetration velocity increased after 10–25 µs up to 200–500 m/s. In order to determine the duration of the dwell phase a criterion had to be defined. Due to the scatter in the position data a criterion based on penetration velocity was preferred. In order to determine the penetration velocity a curve had to be fitted to the position–time data. The best fits were achieved either with a third order polynomial or a section wise linear regression. When using a section wise linear approximation it is assumed that there is an abrupt transition from dwell to penetration occurring within a short time interval of a few microseconds. Whereas an approximation with a higher order polynomial is more representative for a smooth transition from dwell to penetration, which takes about 5–10 µs. Experimental and numerical investigations of the dwell–penetration transition with B4C–aluminum targets by Anderson and Walker [6] indicated a quick transition, i.e. a rapid increase in the penetration velocity. Therefore, a sectionwise linear fit was chosen and the time when the penetration velocity exceeded 100 m/s was defined as the end of the dwell phase. The results for the dwell times according SBI-0206965 Supplier to this criterion are summarized in Fig. 10a and b.