|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.
Figure 1: Principle of coupler with artificial debond
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
Figure 2: Photograph of one pipe end with artifical debond
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 3: Test set up for microwave inspection of the bonded coupler
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 4: C-scan of coupler area. Postprocessing optimized for
Figure 5: C-scan of coupler area. Postprocessing optimized for
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.
FI Test- und Messtechnik GmbH,
D-39114 Magdeburg, Germany
FI Test- und Messtechnik GmbH