GB2237113A

GB2237113A - Thermographic inspection

Info

Publication number: GB2237113A
Application number: GB9020803A
Authority: GB
Inventors: James Morris Milne; Christopher George Tasker
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1989-09-19
Filing date: 1990-09-18
Publication date: 1991-04-24
Also published as: GB8921160D0; GB9020803D0

Abstract

A thermographic inspection technique enables discontinuities e.g. reinforcing bars or defects to be detected and located in a material e.g. concrete, brick or fibre reinforced plastics. A surface (14) is heated by an array (12) of lamps controlled in response to measurements of surface temperature to maintain a steady surface temperature for several minutes. The heating array is then removed, and the surface temperature distribution observed with an infra-red imaging camera for a period of up to an hour. <IMAGE>

Description

Thermographic Inspection This invention relates to a thermographic inspection technique for poor thermal conductors such as concrete or brick, or fibre reinforced plastics.

The use of a thermographic imaging camera in conjunction with a source of radiant heat for defect detection is known. For example Y.A. Popov et al.

(Defektoskopiya, No 6, 1975) describe a technique for inspecting plastic/metal/plastic laminates for defects, using heating times between 10 seconds and a minute.

EP 0 089 760 (UKAEA) describes a technique used to inspect fibre reinforced plastic materials, wherein heat is provided by a pulse of light of duration about a millisecond. N.A. Bekeshko (Defektoskopiya, No 2, 1987) describes a technique for inspecting concrete, brick and wood, the surface of the object being heated either by an array of infra-red lamps or by a scanned halogen lamp, for a period of 0.5 to 10 minutes.

According to the present invention there is provided a method of inspecting a material, the method comprising illuminating a surface of the material with one or more radiation sources arranged such that the intensity of radiation received by the surface is substantially uniform, measuring the temperature of the surface, controlling the illumination in accordance with the measured temperature so as to heat the surface to a desired temperature and to maintain that desired temperature for at least five minutes, then ceasing illumination of the surface and observing the subsequent surface temperature distribution with a thermographic imaging camera for a prolonged period.

Preferably the surface of the concrete is heated to between about 500C and 800C, and is held at that temperature for about ten minutes before ceasing the illumination. This ensures that an adequate temperature gradient has been created within the material in order to enhance the contrast in the subsequent images of hidden structures and defects. The surface temperatures are then preferably observed for at least ten minutes, more preferably half an hour or longer, to ensure that defects below the surface have time to make their presence evident. The time at which the contrast in the image of a defect is greatest can be related to the depth of the defect below the surface.

The invention will now be further described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows diagrammatically apparatus for illuminating a specimen; Figure 2 shows diagrammatically apparatus for subsequently observing the surface; and Figure 3 shows graphically the variation of temperature with depth during the illuminating step.

Referring to Figure 1, it is desired to inspect a reinforced concrete structure 10 for any sub-surface defects, and to locate the reinforcing bars. A radiant heater panel 12 is set up about 250 mm away from a surface 14 of the concrete structure 10 to heat the surface 14 to 700C. The panel 12 comprises twenty 375 W heavy duty tungsten lamps 16 in a four by five regular array in a reflecting support 18 with reflecting inclined edge portions 20, the panel 12 being about 1.4 m by 1.2 m.

(Such a panel may be obtained from Old Acre Engineering, Reading, England). The panel 12 is connected electrically to a power supply and controller 22, to which are also connected one or more temperature sensors 24 stuck to the surface 14. The power supply and controller 22 is operated in response to the signals from the sensors 24 to raise the temperature of the surface 14 of the concrete structure 10 to 700C and then to hold it at that value.

The surface 14 reaches 700C (about 50K above ambient) after about five minutes, but heating is continued for a further ten minutes to ensure an adequate temperature gradient has been established within the concrete structure 10. The array of lamps 16 and the reflective support 18, 20 ensure the temperature of the surface 14 is substantially uniform. The radiant panel 12 is then removed, and as shown in Figure 2, an infra-red imaging camera 30 set up facing the surface 14. The camera 30 is connected to a video recorder 32 and a television monitor set 34.The camera 30 produces a TV-compatible output signal; 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 300 K, and is sensitive to temperature differences of as little as about 0.1 K near room temperature. (A suitable camera is available fom Rank Taylor Hobson of Leicester, England).

It is preferable to observe the surface temperature distribution for at least 15 minutes, if defects or reinforcing bars up to 50 mm deep are to be reliably detected. Defects such as cavities are of lower thermal conductivity than the concrete, and so appear as hotter regions of the surface 14, while reinforcing bars, if well bonded to the concrete, appear as cooler regions. The temperature distribution may be observed in real time, using the monitor 34, or by using the recorder 32 the changes in temperature may be played back later (possibly faster).

Referring to Figure 3 there is shown the expected temperature distribution in a block of homogeneous concrete, whose surface is held at 800C and with an ambient temperature of 200C, for three different heating times, t.

The temperature distributions depend upon the thermal diffusivity D of concrete (i.e. thermal conductivity divided by the product of density and specific heat capacity), which is typically about 0.5 x 10-6 m2/s. These graphs are given by the equation:

where T is the temperature at a depth x.

It will be observed that for parts of the concrete near the surface (for example x = 20 mm) the temperature gradient decreases as the heating time increases from fifteen to sixty minutes, whereas for greater depths (for example x = 75 mm) the temperature gradient increases as the heating time increases from fifteen to sixty minutes.

At a particular depth the gradient passes through its maximum value at a time given by the square of the depth divided by twice the diffusivity, which for a depth of 50 mm is about forty minutes but which for a depth of 20 mm is only about seven minutes, taking the above value of diffusivity. It has been found that subsurface defects and reinforcing bars at a depth x can be observed with best contast if the total time between switching on the lamps 16 and observing the defects or reinforcing bars is about half that value, ie.

total time = x2 4D with the lamps being on for about half the total time. If the heating time is much less or much more than this value (for example by a factor of ten) then the defects will not be observable, as they will have negligible effect on the surface temperature distribution.

Claims

1. A method of inspecting a material, the method comprising illuminating a surface of the material with one or more radiation sources arranged such that the intensity of radiation received by the surface is substantially uniform, measuring the temperature of the surface, controlling the illumination in accordance with the measured temperature so as to heat the surface to a desired temperature and to maintain that desired temperature for at least five minutes, then ceasing illumination of the surface and observing the subsequent surface temperature distribution with a thermographic imaging camera for a prolonged period.

2. A method as claimed in Claim 1 wherein the radiation sources are light bulbs.

3. A method as claimed in Claim 1 or Claim 2 wherein the desired temperature at which the surface is maintained is between 500C and 800C.

4. A method as claimed in any one of the preceding claims wherein the desired temperature is maintained for at least ten minutes.

5. A method as claimed in any one of Claims 1 to 3 wherein the total time for which the surface is illuminated is approximately equal to the time calculated by: time = x2 8D where x is the expected depth of a non-uniformity and D is the thermal diffusivity of the material.

6. A method of inspecting a material substantially as hereinbefore described with reference to, and as shown in, Figures 1 and 2 of the accompanying drawings.