Non-destructive testing of composite structures.

The present invention relates to a method for non-destructive testing of fiber reinforced polymer material, comprising external infrared inspection by an infrared detector and evaluating the results therefrom.

In the prior art generally two methods are known for non-destructive testing of fiber reinforced polymer material. The first and most accurate one is ultrasonic scanning. However, this method has the drawback of the extensive time and labor involved. Inspecting has to be effected on a point to point base. R. Steinberger as.: "Infrared thermographic techniques testing" in

International Journal of Fatigue, Butterworth Scientific Ltd., Guildford, G.B., 28 (2006), 1340-1347 discloses a non-destructive testing method wherein the related sample is fatigue tested and during hysteretic movement heat is generated in the sample which is used for thermography. WO-01/29545 discloses a second approach for infrared thermography wherein the related article is externally heated during observation by the infrared camera.

Although thermal imaging techniques are simpler than the ultrasonic scanning techniques it is much more difficult to distinguish defects.

It is the aim of subject invention to provide a method which can effect inspection area wise and not point to point but on the other hand gives a clear indication if defects are present in the fiber reinforced polymer material.

According to the invention this is realized in that before or during said infrared inspection said fiber reinforced polymer material is internally electrically heated.

According to the invention internal heating is used. This in contrast to the prior art wherein external heating has been practized. Through the use of internal heating the depth over which inspection can be effected is much larger resulting in better images making detection of defects more easily.

According to the invention internal heating is electrically effected. To this end resistance wires extend into the fiber reinforced polymer material and are connected to a voltage source. Several heating wires can be connected in parallel and current can be put to bus bars being in common.

In case there is a defect in the fiber reinforced polymer material this will have a result on the heat conduction from the electrical heating elements to the outside of the

related article. This difference in heating conduction relative to the surroundings or where there is no defect, is clearly distinguishable in an image generated by an infrared sensor.

If the material comprises a fiber reinforced polymer, which will be useable at all high strength application, preferable the electrical wiring is incorporated in the fibre reinforcement. However, it is also possible to provide such a wiring as a separate item. In all cases the material of the electrical wiring is selected such that the required heat generation is produced. Dependent on the requirements the material for the resistive fibres can be chosen. If a regular check is necessary and the article is in a corrosive surrounding it might be preferable to use a stainless steel wiring. The diameter of the individual wires can be chosen as desired such as between 50 and 200 μm and as an example a value of about 100 μm diameter for the wiring is mentioned.

The wiring can be in any position in the thickness of the fiber reinforced polymer material. However preferably this is arranged near the side which is remote from the infrared sensor such as an infrared camera. In this way there will be no interference of the electrically resistive fibre on the image and an optimal view over the thickness of the material is obtained by the infrared sensor.

Sensitivity can be improved by using lock-in techniques to enhance sensitivity and to filter non-relevant noise. The resistive fibres can comprise both a metallic or organic conductive material.

There are many fields wherein the invention can be used. As a non-restrictive example inspecting of aerodynamic profiles such as aircraft structures is mentioned. On a regular base with relative simple means highly loaded components can be inspected through the simple use of a voltage source and an infrared camera. The related wiring can be permanently connected with the voltage source so that by simple switching and positioning of the camera inspection can be realized. An example is the leading edge of a wing.

An other field is in piping. More particular where high pressure piping is made from fibre reinforced material regular inspection might be required. An other application is the production of such piping on site. For some applications it is advantageous to produce piping in the position of use to keep transport cost as low as possible and to obtain large lengths. However, if pipes or other fiber reinforced polymer articles are produced on the site thorough inspection is necessary. To that end

the method as described above can simply be used. For example a length of 1-5 m can be inspected in a single step after being provided during the production thereof with the resistance wiring for internal heating of the pipe produced.

It has been found that the power requirement is relatively low so that during heating there is no danger to damage the related fiber reinforced polymer material.

A further field of the invention is inspecting the condition of welds (in particular of fusion bonding of polymers) between thermoplastic composite materials are obtained through local heating by an electrical resistance heating element. The same element can be used for later inspection of the material or for immediate inspection of the weld and/or progression of the welding.

The invention will be further elucidated referring to the drawings wherein: Fig. Ia, b schematically shows the method according to the invention; Fig. 2 schematically shows an electrical array according to the invention; Fig. 3 shows schematically a pipe produced with defects; Fig. 4 shows several positions on the electrical wiring;

Fig. 5 shows a photograph of the several layers shown in fig. 4, Fig. 6 shows the position of several defects in a flat panel; and Fig. 7 shows the results of the inspection of the panel according to Fig. 6. In fig. 1 a fiber reinforced polymer article to be tested is generally referred to by 1. A defect has reference number 2 while this fiber reinforced polymer article is provided with a heat emitting layer 3. In fig. Ia the effect of propagation of the heat is shown by arrows 4 and 5. It is clear that at the location of the defect 2 heat will not as easily be conducted to adjacent parts of article 1. In fig. Ib an infrared camera 6 is shown making an image of the related article. It will be clear that at the surface of the article which is monitored by the camera near the location of the defect the rise in temperature in time will be less than at other locations resulting in a clear indication in the image obtained about a defect being present.

Fig. 2 shows an example for a heat emitting layer 3. Two current collectors 7 and 8 are provided between which electrical conductive wires extend. These can have a diameter smaller than 100 μm and can for example comprise stainless steel. Voltage is applied through a voltage source 10. Electrically conductive wires can be incorporated in reinforcement fibres such as glass fibres.

Fig. 3 shows schematically a pipe 12 comprising three layers 13-15 of fibre reinforcing material. In layers 13, 14 and 15 defects A-C are present.

Fig. 4 shows incorporation of the array of electrically conductive resistive wires 31 as shown in fig. 1 in the fig. 3 embodiment . Also the position of the defects A-C is shown. Fig. 5 shows the images obtained via camera 6 directed to the outside of pipe 12 of the fig. 3 and 4.

Fig. 5A shows defect A, whilst figures 5B and 5C show defect B and C respectively.

Fig. 6 shows a laminate comprising three layers A, B and C as well as an array 3 of electrical resistance wires in the bottom of the laminate.

Fig. 7 shows the corresponding pictures obtained of the layers A-C. It will be understood that other articles can be inspected with the method according to the invention.

Example

Referring to figs. 3-5 a 95 mm internal layer diameter was filament wounded over a steel mandrel. Glass fibre rovings were used with an epoxy matrix to create a pipe with a wall thickness of 8.25 mm. A series of Teflon inserts (0.5 mm thick) with dimensions varying between 20 x 20 mm and 5 x 5 m were placed at different depths over the part during the winding process to simulate defects.

The method of the invention with internal heating and more particular internal pulse heating was used for observing the related defects. This was effected by an E030-3 Delta Elektronika power supply with a range of 0-3 A en 0-30V. The electrical resistive wiring comprised stainless steel metallic wires. Voltage and current were adjusted for adequate heat-up of the samples. A single heating pulse is sufficient for good defect detection using the infrared camera. Modulated thermography was used to enhance sensitivity of the camera to sense more easily defects. A phase correction analysis is performed with either a sub-phase or harmonic appro xymation method to produce the images. All phase images are produced from the transient heat-up image sequences. The result obtained corresponds to what has been shown in Fig. 7.

Although the invention as been described above referring to preferred embodiments, it will be immediately clear for the person skilled in the art that the

invention has many other applications wherein non-destructive testing is essential. These are in the range of the appended claims.