Experiments On Transient Waves In A Periodic Stack

Abstract

Transient waves were initiated by allowing a thick PMMA disk to strike a periodic stack of ceramic layers bonded by thin weak silicone rubber layers. Pressure at interfaces of ceramic and bond layers was measured by carbon gauges along the centerline of the stack. Comparison of experimental histories with those from a 1-D analysis (JASA 94(1), 1993) and a 2-D axisymmetric analysis reveals that the waves propagate two-dimensionally and that flexure of the ceramic layers controls attenuation and shape of compressive wave of first arrival. Visco-elasticity of the bond material sharply reduces tensile stresses. 1 Experiment Transient stress waves were initiated in a weakly coupled periodic stack of square Aluminum Nitride (A1N) ceramic tiles 10.16 cm in size, bonded by a thin weak silicone rubber, ME625, Tham[l]. In one case, tile and bond thicknesses were 1.27 cm and 0.3 mm; and in the second case, 2.54 cm and 0.6 mm. Piezo-resistive carbon gauges 0.08 mm thick of the type C300-50EKRTE from Dynasen Inc. were inserted at the bottom of the first four tiles along the center line of a stack with 10 tiles. Each gauge served as the active resistor of a Wheatstone quarter-bridge circuit. Just before the passage of the wave, the bridge was supplied by a 45 V pulse with a 300 ps duration. The initially balanced bridge was unbalanced by the change in resistance of the gauge from applied pressure. The output voltage was recorded by a 200 Mhz transient recorder. Given the specific gauge calibration and non-linearity of the bridge, pressure histories are valid up to 300 |is from impact. The high electric current in the system produced an inevitable temperature drift of the gauges Transactions on the Built Environment vol 22, © 1996 WIT Press, www.witpress.com, ISSN 1743-3509 544 Structures Under Shock And Impact yielding a fictitious pressure rise of the order of 1.75 MPa per 100 ps. The stack was placed in a metal casing facing the muzzle of a 7 cm diameter compressed air gun at the Ernst Mach Institute, Freiburg, Germany. The stack was struck in its centre by a PMMA disk 5.75 cm in diameter and 2 cm thick, launched by a compressed air accelerator. Velocity of the disk at impact ranged from 14 m/sec to 58 m/sec. Velocities were kept low to avoid damaging the struck ceramic tile. A laser beam in the axis of the launch tube reflecting from the top surface of the first ceramic tile allowed the alignment of stack and disk axes. This procedure resulted in a tilt smaller than 2 mrad. The experimental set up of the stack is shown in Figure 1. A typical digitized output of normal stress o^ in a stack of 10 (10.16 x 10.16 x 1.27 cm) ceramic tiles is shown in Figure 2. The broken horizontal line labeled p^ defines the computed 1-D pressure exerted by the PMMA disk upon the ceramic tile according to