The visualisation of acoustic waves by schlieren and photoelastic techniques

Abstract

Visualisation of acoustic waves in transparent media is described using schlieren and photoelastic principles. Continuous and pulsed waves are examined. Enhanced sensitivity results for both visualisation systems when a thyratron-triggered spark light source is introduced. The schlieren apparatus demonstrates the Fresnel and Fraunhofer fields of acoustic waves in water. Reflection, refraction, diffraction and transmission through thin plates is demonstrated. The schlieren system is applied to the design of a water column-coupled ultrasonic probe array, for rail testing. Photoelastic experiments include methods of construction of the plane and polariscopes. Three methods by which acoustic waves may be identified are described. The suitability of glass for test pieces is emphasised and compared with modern photoelastic materials. Ultrasonic probes are shown invariably to transmit more than one wave type. Explanation of the sources of these waves is given. Studies of the interactions of compressional and shear waves with horizontal cracks and inclined cracks situated at a model rail bolt hole, are illustrated. Experiments to measure the depth of surface-breaking cracks in rail heads, using Rayleigh waves, show the existence of three received scope signals on the ultrasonic display. These are explained when the interactions of waves with cracks are visualised. An improved testing method is suggested. The circular polariscope is employed to identify the types of stress associated with compressional and shear waves in glass test pieces. The magnitude of static stresses in a cantilever beam is first obtained by Tardy compensation. Further development produces equations enabling stress magnitudes to be calculated using measured optical retardations. A linear relationship is experimentally derived between the probe excitation voltage and the induced stress magnitude for both compressional and shear waves. Stress measurements are verified by comparison with amplitude measurements of received signals expressed logarithmically.