Sensing of shock-wave velocity and pressure using shock-wave induced mechanoluminescence of crystals

When a projectile moving nearly at 1 km/s makes impact on target, the shock waves travelling at velocity of nearly 10 km/s in target produces intense mechanoluminescence (ML) pulse of microsecond duration, both in compression and post-compression conditions. Piezoelectric charges produced on surfaces of fractured target causes emission of energetic electrons which excite luminescence. Expressions explored for the characteristics of shock-induced ML are able to explain satisfactorily the experimental results. The ML appears after a delay time t(th) whose value decreases with increasing value of the applied pressure. Initially, the ML intensity increases with the shock pressure because of the creation of more surfaces; however, for higher values of the shock pressure, the ML intensity tends to attain a saturation value because of the hardening of the crystals due to the creation of small crystallites in which the creation of new surfaces becomes difficult. The ML emission in the uncompressed condition takes place due to the movement of blocked cracks in the compressed condition, when the pressure is relaxed. The ratio between peak ML intensity in the uncompressed region and the maximum ML intensity in the compressed region decreases with increasing pressure because more defects produced at high pressure generate higher barrier for the relaxation of blocked cracks under compression. Thus, the shock-wave induced ML provides a new optical technique for the studies of materials under high pressure. Shock-wave velocity, shock pressure, transit time, and decay time of surface charges, can be sensed independently by the shock-induced ML, and therefore, no other devices may be required for their measurement. (C) 2015 Elsevier B.V. All rights reserved.