JEC COMPOSITES MAGAZINE - Issue #96 - April 2015 - 57

small coupons to large composite structures runs into practical issues.
We introduced criteria for the determination of the optimal CNT
quantity required to ensure damage detectability at any region of
large composite structures. One of them was the uniformity of CNT
distribution, which gives rise to uniformity in electrical conductivity,
and another one was sensitivity to change in electrical resistance [4,
6]. The SHM technique offers a number of original aspects described
below.
Producing intelligent fibre/epoxy/CNT composite structures
First, the epoxy resin (Epon 862) and curing agent (EPIKURE
W, 26.4 wt%) were mixed. Then, 0.30 wt% multiwalled carbon
nanotubes (MWCNT), as the optimal quantity [4, 6], were added
to the epoxy matrix. The mixture was processed on a three-roll mill
(EXAKT 80E, EXAKT Technologies Inc.) to disperse the MWCNT
within the matrix. The modified matrix was dispersed between
non-conductive plain weave glass fibres and conductive carbon fibres
by hand lay-up to produce glass fibre/epoxy/MWCNT composite
plates and carbon fibre/epoxy/MWCNT composite plates. The
plates were cured in an autoclave. The manufacturing process used to
produce the intelligent composite structure is shown in Figure 2.

Fig. 2: Schematic illustration of the fabrication of intelligent composite structures

Electrical property measurement strategy
- Non-conductive fibre/epoxy/CNT composite structures
For non-conductive fibre/epoxy/CNT composites, a grid of forty
conductive contact points made from silver-epoxy paste spaced
3 inches (76 mm) apart was mounted on the surface of non-conductive fibre/epoxy/CNT plates (22×13 in., 560x330 mm2) (see
Figure 3a). Electrical wires were attached to the contact points to
make electrodes. It is very important to have a wide spacing between
the electrodes in order to avoid using a large number of grid points.
Electrical resistance measurement (ERM) was carried out by the
two-probe method. Electrical resistance change (ERC) is defined as
Rf,i,j - RI ,i,j
∆R(%) =
×100, where RI,i,j and Rf,i,j are the
RI,i,j
initial and final resistance values, respectively.
- Conductive fibre/epoxy/CNT composite structures
For conductive carbon fibre/epoxy/CNT composites, a new electric
potential measurement (EPM) strategy was proposed, as shown in
Figure 3b. Two sets of grid points were mounted on the surface of the
plate. Each set consisted of forty contact points. Each contact point
was made from silver-epoxy paste. Electrodes were made by attaching
wires to the grid points. The first set of grid electrodes, labelled from
1I to 40I and spaced 3 inches (76 mm) apart, was used to apply a
constant electric current. The second set of grid electrodes, labelled
from 1V to 40V and spaced 3 inches (76 mm) apart, was diagonally
shifted by 0.2 inches (5 mm) with respect to the first set to measure
electric potentials. EPM was performed by the four-probe method,
where a constant current was directly applied to first grid electrodes
and then the potential across the second grid electrodes was measured. Electric potential change (EPC) is expressed by:

Fig. 3: Schematic illustration of a) ERM strategy for non-conductive fibre/epoxy/
CNT composite plates, b) EPM strategy for conductive fibre/epoxy/CNT plates

No96 April 2015 /

jec composites magazine 57

JEC COMPOSITES MAGAZINE - Issue #96 - April 2015

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