TRANSIENT THERMOGRAPHY

Abstract: A non-destructive technique for detecting defects within a body (10) whose surface (13) is capable of converting radiant energy to thermal energy. The technique involves exposing the surface (13) to a precisely determined pulse of radiant energy from a flash tube (12), the distribution of heat within the body (10) subsequently being observed by an infra-red camera (20) producing a T.V. compatible signal.

Description: Transient Thermography This invention relates to non-destructive testing techniques and in particular to transient infra-red thermography techniques, that is to say techniques involving the subjecting of a body to a pulse of heat and the subsequent recording of infra-red radiation emitted by the body. Such techniques can be used to detect internal defects within, for example, laminated materials, or within bonded joints. However application of these techniques has been hindered by the fact that existing methods of heating bodies are imprecise and have relatively slow rise times, thus making these techniques insensitive in comparison to other non-destructive testing techniques. For example Y.A. Popov and A.A. Ketkovich et al. in "Delamination Detection Using a High Speed Thermal Scanner" (Soviet Journal of NDT, 1975, No. 6) describe the use of a 16 frame/sec, 100 line, infra-red scanner to investigate delaminations in a composite material, a pulse of heat being provided by illuminating the surface of the material with lamps for a period of between 10 seconds and 45 seconds. According to the present invention there is provided a method for detecting the presence of discontinuities within a body of a material having a surface which is capable of converting incident radiant energy to thermal energy, comprising the operations of irradiating a region of the surface with a pulse of radiant energy and determining the subsequent distribution of thermal energy by means of an infra-red scanning camera, characterised in that the duration of the pulse is less than the time taken by the camera to scan one frame. The use of such a short duration pulse has the advantage of providing much better spatial resolution than a long pulse. The pulse may be provided by means of a photographic flash tube, and the duration of the pulse is preferably less than 20 milliseconds. The infra-red scanning camera may produce a television compatible signal, and may be used in conjunction with a video recorder. The method thus provides an entirely non-contact detection method for defects and discontinuities within a structure. The material the surface of which is capable of converting optical energy to thermal energy may comprise for example carbon fibre reinforced plastic or glass fibre reinforced plastic. When applying the method to a material the surface of which reflects a substantial proportion of incident radiation an optically absorbent coating such as matt black paint may be applied to the surface. The subsequent distribution of thermal energy may be observed.over the surface of the body which has been irradiated with the pulse of optical energy, or may be observed over a different surface of the body. The invention will now be described, by way of example only, and with reference to the accompanying drawing which is a diagrammatic view of an apparatus for carrying out the method of the invention. As shown in the drawing, a large rectangular sheet 10 about 40cm by 50cm of carbon fibre reinforced plastics material, which is to be checked for the presence of internal delaminations, and whose surface is matt black, is set up with one side 11 facing a 2000 joule photographic flash tube 12, which is connected to a power supply unit 14. An infrared camera 20 which produces a TV compatible output signal is set up facing the side 11 of the sheet 10, and is connected to a video recorder 22 and a television monitor set 24. The camera 20 produces a complete frame in 40 milliseconds, consisting of two interlineated fields, each taking 20 milliseconds. It is sensitive to radiation of wavelength between 8 and 13 micrometres, and so is sensitive to the peak radiation intensity from a black-body radiator at about 300K, i.e. near room temperature, and it is sensitive to temperature differences of about 0.1K near room temperature. A suitable camera is available from Barr and Stroud Ltd., Anniesland, Glasgow, model number It 18 Mark 2. When the power supply unit 14 is energised, the flash tube 12 emits an optical pulse, the duration of which is about one millisecond. Light from the flash tube 12 incident on the surface 13 of the side 11 of the sheet 10 is absorbed and converted to thermal energy within the surface 13 of the side 11 of the sheet 10, providing about 1 joule/cm2 of thermal energy, and raising the surface temperature by about 5K. Heat flow then takes place within the sheet 10 so as to distribute the thermal energy to cooler regions of the sheet 10. Any defects within the sheet 10 will differ in thermal conductivity from neighbouring regions of the sheet 10 in which there are no defects, and so will affect the temperature of the sheet 10 in the vicinity of the defects. At the same time as the power supply unit 14 is energised, the camera 20 and the video recorder 22 are switched on so as to record the temperature distribution over the surface 13 of the sheet 10 and are left on until the temperature of the surface 13 of the side 11 of the sheet 10 has become uniform. The temperature distribution over the surface 13 of the side 11 of the sheet 10 following irradiation subsequently can be determined by playing back the recording taken by the video recorder 22 on the monitor 24, and individual frames can be selected for examination, if required. Defects of different types and at different depths below the surface 13 of the side 11 of the sheet 10 will be seen with best contrast at different times after the absorption the optical pulse, this time depending upon Lr. diffusivity of the material (i.e. thermal conductivity divided by the product of density and specific heat capacity, which for a resin is about 1 x 10-6 m2/s and for metals may be between ten and a hundred times greater). In cases where the defects are seen with best contrast a fraction of a second after the optical pulse, slow replay of the video recording may be used to observe the defects. Although the apparatus has been described as comprising an infra-red camera 20 on the same side of the sheet 10 as the flash tube 12, it will be understood that the camera 20 can be placed so as to view the opposite side of the sheet 10.

Claims: Claims: 1. A method for detecting the presence of discontinuities within a body (10) of a material having a surface (13) which is capable of converting incident radiant energy to thermal energy, comprising the operations of irradiating a region of the surface (13) with a pulse of radiant energy and determining the subsequent distribution of thermal energy by means of an infra-red scanning camera (20), characterised in that the duration of the pulse is less than the time taken by the camera (20) to scan one frame. 2. A method as claimed in Claim 1 wherein the pulse of radiant energy is provided by a photographic flash tube (12). 3. A method as claimed in Claim 1 wherein the camera (20) produces a television compatible signal, and the signal is recorded by a video recorder (22). 4. A method as claimed in any one of the preceding Claims including the initial operation of applying a radiant energy absorbent coating to the region of the surface (13).

Inventors: CHILD, JOHN WILLIAM, MILNE, JAMES MORRIS, REYNOLDS WILLIAM NORMAN
Applicants: UNITED KINGDOM ATOMIC ENERGY AUTHORITY
Application year: 1983
Main classification: G01N25
IPC codes: G01N25, G01J5