City Research Online

Abstract

Direct ultrasonic visualisation of defects (DUVD) is a method of producing an instantaneous optical image of acoustic discontinuities inside a material. The material is insonified with repetitive ultrasonic pulses, reflected pulses are focused by a sonic lens system into a transparent medium,and the acoustic image is visualised using a stroboscopic schlieren system. This thesis describes further development of the technique, with the main aims of improving the sensitivity and resolution beyond those apparent in previous crude demonstrations.

A theoretical analysis of schlieren visualisation of ultrasonic pulses leads to the identification of the most sensitive visualising media - liquids with up to ten times more sensitivity, theoretically, than water. The sensitivity of selected liquids is measured.

The choice of materials for the sonic focusing system is discussed, measurements of the acoustic properties of some plastics being given. The designs of certain systems of plastic lenses in liquid media are optimised for the best resolution by a ray-tracing method.

An experimental sonic system of polystyrene lenses in water is described; it is easy to make and convenient to use. Visualisation is by a compact mirror schlieren system of 60 mm aperture. Resolution of 1 mm over a 25 x 25 mm field is demonstrated, and the performance is a considerable improvement over the previous practical systems.

Schlieren observations of the behaviour of short pulses, which confirm recent theoretical predictions, are presented. Observations in the experimental sonic lens system show that pulses are focused in the
expected fashion.

It is concluded that the DUVD technique is capable of good resolution, but its principal limitation is the poor sensitivity of schlieren visualisation. General applications in medicine and to metal inspection are not feasible; possible uses lie in the rapid examination of materials where great sensitivity is not required.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > Q Science (General) Q Science > QC Physics T Technology > TA Engineering (General). Civil engineering (General)
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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