Compact thermosonic NDE system

Lead Research Organisation: Imperial College London
Department Name: Dept of Mechanical Engineering

Abstract

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.

Publications


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Barden T (2007) Detection of impact damage in CFRP composites by thermosonics in Nondestructive Testing and Evaluation
M Morbidini (2009) A CALIBRATION PROCEDURE FOR SONIC IR NDE in J Applied Physics
M Morbidini (2009) Improved Reliability of Sonic IR Inspection in Materials Evaluation
Morbidini M (2009) The detectability of cracks using sonic IR in Journal of Applied Physics
 
Description Thermosonics (also called Sonic IR) 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 vibration produced in the component during a thermosonic test is typically chaotic and is therefore inherently non-reproducible so there has been concern about the test reliability, and in particular the confidence that can be placed in a negative (defect free) test. One approach would be to make the vibration reproducible but during the project researchers around the world developed a consensus that a chaotic vibration field improves crack detectability and helps to ensure that all locations in the component are covered. If this approach is followed it is necessary to check that sufficient vibration amplitude has been applied to deetct any defects that are present. This project developed the use of a Heating Index based on a measurement of the component vibration to give a measure of the potential of the vibration generated in a particular test to generate sufficient heat for any defects of interest to be detected. The procedure was tested on a variety of simple beam-like components and also on a set of turbine blades with known defects, the vibration being measured with a simple high frequency microphone. The correlation between the Heating Index and temperature rise at the crack location was very good and a calibration procedure for practical inspection was developed. The project showed that the Heating Index provides a real-time measure of whether sufficient excitation has been applied to detect the defects of interest if they are present in the test component. The technique promises to significantly improve the reliability of thermosonic inspection and is now being implemented by the industrial partners.
Exploitation Route Use as industrial NDE tool - being implemented by project partners. Direct use by project partners
Sectors Manufacturing/ including Industrial Biotechology
 
Description AIRBUS OPERATIONS LIMITED 
Organisation Airbus Group
Country France, French Republic 
Sector Private 
Start Year 2007
 
Description British Nuclear Fuels plc 
Organisation British Nuclear Fuels Limited (BNFL)
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Public 
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Description Rolls-Royce plc 
Organisation Rolls Royce Group Plc
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Private 
Start Year 2007