JEC COMPOSITES MAGAZINE - Issue #117 - November/December 2017 - 62

TECHNOLOGY NDT

Fig. 3: Light microscopy observation of printed paths after testing

Fig. 2: Force and resistance change of conductive paths versus displacement

er 90° layers. The test speed was set to 2 mm/min. Both the loading pin
and supporting pins (further denoted as cylinders) of the three-point
bending test setup consisted of aluminium oxide (Al2O3) to ensure
electric insulation of the specimens during testing. The specimen geometry and the locations of the conductive paths are shown in Figure
1 (a). Two paths were printed on the bottom side of the specimen. To
measure the DC resistance over the length of the conductive paths, the
ends of the paths were connected to a digital multimeter. Figure 1 (b)
shows a schematic of the test setup.
Figure 2 shows representative results of the mechanical and
electrical measurements. The resistance change (∆R) was
specified as the difference of the measured resistance without (R0) and during loading (R). The force increased to a
maximum at a displacement of 14.8 mm and fracture of the
specimen occurred at 18.7 mm. In addition to the mechanical
response, the change of electrical resistance is shown for the
two conductive paths, measured from one end to the other of
the same path, respectively. Both the force-displacement curve
and the resistance change-displacement curves contain several
discontinuities indicated by circles in the diagram. The dis-

continuities correlate perfectly with the steps in the resistance
change. Discontinuities in the measured force are present due
to damages in the material. Since the dominating failure modes
in cross-ply laminates subjected to bending and the failures observed in the tested specimens are inter-fibre failure and delamination, these two failure modes lead to the discontinuities
of the measured force and resistance signals. All the failures
detected by the measured force, and therefore all the inter-fibre failures and delaminations exceeding a certain size, can be
detected by resistance changes. The resistance increase at the
end of the test (up to unmeasurable values) indicates the final
fracture of the specimen. Therefore, all the failures that are relevant for the integrity of the specimen as well as the final fracture can be detected reliably. Light microscopy observations
of the damaged printed paths show that inter-fibre failures at
the surface cause interruptions as well as flaking of the paths in
proximity to the cracks (see Figure 3).

Electrical measurements through
the material for delamination detection
To be able to measure the resistance through the material in
the thickness direction, specimens with printed paths on both
sides were produced. The design of the conductive paths for
measuring through the material is shown in Figure 4 (a). The
DC resistance was measured from the top electrode to each
of the four electrodes on the bottom side of the specimens.
Figure 4 (b) shows the test setup and the connected electrodes
for the four measured resistances.

Fig. 4: (a) Specimen geometry,
design of conductive paths for
measuring resistance through the
material, (b) Three-point bending
test setup with four-channel electrical resistance measurement through
the material

jec composites magazine / N°117 November - December 2017

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #117 - November/December 2017