Compact thermosonic NDE system

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Thermosonics is a novel non-destructive evaluation (NDE) technique that employs an infrared camera to image defects, typically cracks or delaminations, by detecting the heating caused by friction at the surfaces of defects when a part under inspection is vibrated. Typically, a pulse of high power ultrasound in the 20-100 kHz range is applied at one point on the test-piece to generate a high frequency vibration field in the structure. The method is considerably quicker than conventional ultrasonic or eddy current inspection techniques that require point by point scanning. The method is also particularly well suited to the detection of closed cracks that can cause problems with other techniques. Whilst impressive results have been achieved in a number of laboratories worldwide, the system, particularly the excitation, needs engineering. The reliability of the system needs to be improved - there is concern that defects in some locations are missed; this is almost certainly a function of the vibration field that is generated by the exciter. The amplitude of vibration required needs to be defined and assurance that this will not propagate the defects is required. To date a high power ultrasound horn, of the type produced commercially to weld plastics, has been used to excite vibrations. The high power horn has the major disadvantage that it is bulky and is very difficult to couple reproducibly to the structure. This proposal follows an earlier research programme in which the applicants have established a quantitative understanding of the excitation requirements for a successful thermosonic inspection. This programme has shown that for a number of industrially important applications, successful thermosonic inspections can be completed using significantly lower amplitude vibrations than those associated with high power ultrasonic welding horns. An objective of this proposal is to design and produce ultrasonic exciters that are engineered for specific demonstrator applications. These demonstrators will be selected from applications that have been found to be particularly suitable (ie requiring low excitation power) for thermosonic inspection in consultation with the industrial partners who are supporting the project. The lower power requirement and tailored design should make the exciter significantly smaller and lighter than the bulky ultrasonic welding horns in current use. The other part of a thermosonic system is an infrared camera that was, until recently, also a bulky heavy component. However, very small microbolomer array infrared cameras are now available for use in NDE systems. The intention is to produce a compact portable, possibly hand held, thermosonic inspection system incorporating a small microbolometer array camera and a custom engineered vibration exciter. A full vibration analysis of the demonstrator parts will be completed to determine the optimum mode to excitation to ensure the reliability of the inspection. The system will also include a means of monitoring the vibrations to enable the user to check that adequate vibration amplitude is produced in a part to reliably complete a test. The inspection system will be field tested on a selection of demonstrator parts. The overall aim of the project is to take thermosonics, a very promising new NDE technique, out of the laboratory and to introduce it successfully into industry.In addition to producing a prototype testing system, the project will advance scientific understanding of the performance of ultrasonic exciters, and in particular the influence of the coupling between the exciter and the structure. Novel means of non-contacting measurement of the vibration field produced by the exciter will be investigated and these are likely to have other scientific and industrial applications.