- since 1996 -
|NDT.net Issue - 2009-03 - NEWS |
|Microwave testing becomes more and more
important as an NDT method concerning
electrically non-conducting material. Glass fiber
reinforced plastic (GFRP) is a typical example of
this class of material.
This application note describes the detection of debonds in an adhesively bonded connection between two pipe ends and a coupler. A circumferential PTFE film on one of the pipe ends is used to artificially model a debond of kissing bond type, see figure 1.
The pipe ends were of 6'' inner diameter with a wall thickness of about 3 mm. The coupler length was 220 mm, its thickness was about 8 mm.
Figure 2 shows a photograph of the circumferential debond before bonding.
The test setup is shown in figure 3. The microwave system FMH1 of FI Test- und Messtechnik GmbH was used, fixed in a scanning system. The probe was placed about 1 to 2mm above the coupler surface. Scanning was performed primarily in axial direction with additional rotational increments. The raw data are postprocessed by using a special software which allows the user to look specificially for different kinds of defects. Two of such evaluations are shown in the following
Figure 4 shows the C-scan along the cylindrical area in a plane view: the circumferential coordinate as the vertical coordinate extending from 0° to 360° and the axial coordinate horizontally extending over 270 mm. The letters A and B mark the ends of the coupler and the ends of the debond, respectively. The broad white vertical line between B and B clearly gives an image of the debond.
Figure 5 is based on the same raw data, but the postprocessing is adjusted to optimum gap recognition. This gap extends between both letters G and can clearly be recognized. The dark vertical line in the middle of the gap is caused by a circumferential bulge of the adhesive which was verified by visual and tactile inspection. Figure 4 and Figure 5 are only two typical examples of a series of tests on adhesively bonded GFRP pipe couplers.
We want to thank Richard Lee from ESR Technology Ltd. for providing the test samples.
If you would like to know more about microwave testing please do not hesitate to contact us.