|The company IFC Composite GmbH fabricates
more than 1000 glass fiber reinforced leaf springs
per day for the minivans Mercedes Sprinter and
Volkswagen Crafter. Besides of destructive testing
there is a need for non-destructive testing for
production surveillance. The approximate size of
the leaf springs is 1400 mm x 70mm x 30 mm.
Unidirectional glasfiber prepregs are used. The
complete volume of the leaf spring is to be tested
Here the prototype of a test system for this purpose
is described, see figure 1.
Figure 1: Photograph of the Microwave-Based NDT System for GFRP Leaf Springs.
A microwave-based test procedure was chosen
because traditional test procedures fail (ultrasonic
testing) or are too expensive (X-ray, radiography).
For microwaves the leaf spring is almost
transparent; using microwave procedures local
variations of the dielectric constant become visible
as it is well-known from refractive index variations
in optics. A frequency of 24 GHz was chosen which
is allowed for industrial purposes (ISM frequency
band). Furthermore this frequency is high enough
for a moderate spacial resolution. The system uses a
transmission procedure. The transmitter side
consists of a linear array of 30 microwave channels,
each with one antenna. Opposite there is a linear
receiver array, also with 30 channels and receiving
antennas. These arrays are oriented across the leaf
spring, i.e. they extend over about 70 mm. When
scanning, the microwave modul is moved along the
leaf spring over a length of about 1400 mm. The
scan time is about 40 seconds for a leaf spring, the
following time for data processing needs about 20
seconds. Therefore the complete testing time for
one leaf spring is about 60 seconds. Figure 2 shows C-scans of leaf springs with artificial defects.
Figure 2: Microwave Scans of Leaf Springs with Artificial Defects.
a) Tranvsers Bore Holes b) Grooves. c) Metal Filament. d) Plastic Foil
Figure 2a shows the C-scan of a leaf spring with
transversal bore holes on half height, i.e. 15 mm
from lower side and top side. They are recognizable
down to diameters of 1 mm.
Figure 2b shows indications of grooves which
extend over the whole width of the leaf spring.
They are recognizable down to 0.5 mm depth.
Figure 2c shows the C-scan of a leaf spring with an
oblique metal filament of 0.25 mm diameter at the
bottom, i.e. at the receiver side of the microwave
Figure 2d shows the indication of a plastic foil of
0.6 mm thickness. All these artificial defects are
This application note shows an example for the use
of microwave-based non-destructive testing in a
production line. We would like to answer your
questions regarding avaible microwave-based
measurement and test equipment which can also be
made customer-specific and application-specific as
well as regarding further use of microwave-based
FI Test- und Messtechnik GmbH,
D-39144 Magdeburg, Germany
FI Test- und Messtechnik GmbH