A low temperature acoustic imaging technique is proposed to enhance the sensitivity of defect detection in materials with highly temperature dependent acoustic properties. An acoustic imaging device has been developed where the sample temperature is precisely controlled between 30 and -94°C using a methanol coupler. To illustrate both the feasibility and advantages of this method, a bonded structure using bisphenol ether type epoxy resin was observed; and defects and non-uniform structure not visible at ambient temperature became apparent in 25 MHz acoustic images when the sample was cooled to -30°C. From measurements of velocity and attenuation, this improvement was ascribed primarily to a decrease in the attenuation loss within the epoxy resin. The use of phased arrays and non-destructive evaluation of fibre-reinforced plastics are also discussed.
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