Full-field detection of impact damage in CFRP aircraft by electric

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Full-field detection of impact damage in CFRP aircraft by electric
Full-field detection of impact damage in CFRP aircraft by electric heating
Yoshiro Suzukio, Dept of Mech. Eng., Tokyo Institute of Technology, [email protected]
Akira Todoroki, Dept of Mech. Eng., Tokyo Institute of Technology, [email protected]
Yoshihiro Mizutani, Dept of Mech. Eng., Tokyo Institute of Technology, [email protected]
Ryosuke Matsuzaki, Dept of Mech. Eng., Tokyo University of Science, [email protected]
1. INTRODUCTION
Carbon fiber reinforced polymer (CFRP) materials
have been widely used in aerospace because of their
high mechanical properties. However, the
interlaminar strength of the laminated composites is
a weakness with these materials. The electrical
resistance change method (ERCM) has been
proposed as an inspection method(1). For the ERCM,
carbon fibers as structural reinforcement of the
structure can be used as sensors by making use of
their electrical conductivity. The electrical resistance
between electrodes mounted on the CFRP structure’s
surface can be used to evaluate the structural
integrity of the composite. Because we only have to
measure electrical resistance for the inspection, the
ERCM could be applied to inspection of large
structures in a short time. However, there are still
problems for putting the ERCM to actual use. These
problems include reduced accuracy because of
fluctuations in the electrical properties of the
composites caused by air disturbances. The
applicability of this method to commercial aircraft is
also hindered by the large number of segments that
measurements need to be conducted on and the large
amount of wiring required to enhance the sensitivity
and accuracy of damage detection.
2. PLINCIPAL OF THE INSPECTION
To overcome these problems, a new diagnostic
method was proposed in the previous study(2). This
method was based on increases in the fiber–fiber
contact at the interlaminar interface and the electrical
conductivity through the thickness of the composite
after indentation damage from an impact load. As
shown in Fig. 1, the two outer layers of a continuous
graphite fiber laminate have the fibers orientated at
different angles (orthogonal each other). The lower
section of the laminate does not have the first 0°
layer on it, so that direct electrical contacts can be
placed on the second 90° layer. When an electrical
current is applied between the two electrodes on the
two layers, the maximum current density occurs
around an indentation caused by an impact load
Fig. 1 System for detecting electrical resistance
decrease caused by an indentation by resistive heating.
because the indentation has a much higher electrical
conductivity than the undamaged areas (Table 1).
The current concentration increases the temperature
around the indentation compared to the intact areas.
However, if a large structure is heated using a
simple electrode layout like in Fig. 1, a large amount
of the electrical current is concentrated in the area
close to the two electrodes, which leads to a
non-uniform temperature distribution. This might
make it difficult to detect localized temperature
differences around an indentation. Therefore, a new
electrode layout as shown in Fig. 2 was developed so
that many electrical contacts are distributed
uniformly on the structure surface. Metal strips,
which provide conductive paths for lightning strikes,
are used as the electrical contacts and their wiring.
Furthermore, if an indentation can be detected
using electrical resistance change caused by the
temperature change, then thermography will not be
required. With the proposed inspection method, the
electrical resistance of each area of the structure can
be measured using the lightning protection strips as
shown in the magnified image in Fig. 2. The metal
mesh is divided into orthogonally oriented metal
strips that are not electrically connected to each other
but do have electrical contacts with the CFRP surface.
When voltage is applied between strips B and 4,
most of the electrical current flows towards strip B
like white arrows. Consequently, the resistance of the
area around the intersection of strips B and 4 can be
measured by connecting both the stripes to a
resistance meter.
3. ANALYTICAL VERIFICATION
Coupled thermal-electrical analyses were used to
verify the practicality of the impact-damage
inspection system using lightning protection strips.
The temperature distribution of a large CFRP
laminate with an indentation was calculated using
analysis software (ANSYS). The properties used for
the analysis were the same as those used in our other
work(3) (Table 1). In the analysis, both the shape and
size of the indentation were constant and only the
Fig. 2 Impact-damage inspection system without
thermography. Resistance change is measured after
resistive heating using the lightning protection shield.
Table 1 Electrical resistivity of a CFRP material
(IM600/133, Toho Tenax, Tokyo, Japan)
Direction
Value [Ωm]
Fiber (Undamaged)
2.8874×10–5
90° (Undamaged)
0.82086
Thickness (Undamaged)
Thickness (Indented)
Indentation depth 0.11 mm
Thickness (Indented)
Indentation depth 0.15 mm
308.77
15.7
6.68
Fig. 5 Calculated difference between damaged and
undamaged structures for the resistance change of
each segment after applying electric heat.
Fig. 3 Analytical model for the inspection system.
Fig. 4 Temperature distribution of the surface with
indentations in segment 2 when supplying 10 V.
through-thickness electrical resistivity of the
indentation was changed.
The finite element model of a three-layer laminate
([0/902]T) is shown in Fig. 3. To reduce the scale of
the analysis, only the three outer layers were
modeled for a unit (Fig. 3, middle) that was repeated
at regular intervals. The model was 1256 mm × 80
mm by 0.45 mm thick. The first 0° layer had many
electrical contacts (in yellow) connected to each
other in the 0° direction by conducting lines (in
yellow). Symmetry boundary conditions were
applied to the three surfaces, including the back
surface and both lateral surfaces. Because actual
structures have more internal layers underneath the
three-layer laminate, no boundary conditions were
applied to the bottom surface to prevent heat loss
from the bottom. An indentation (rectangular solid, 2
mm × 2 mm, 0.3 mm thick) in the model was located
at the left-center of segment 2. To perform the
impact-damage inspection for segments 1 through 5,
these segments were heated by resistive heating (10
V, Fig. 3, right). After applying electric heat, the
electrical resistance changes were calculated.
The temperature distribution results for different
indentation loads are shown in Fig. 4. The surface
temperature distribution indicated that the damaged
left-center area of segment 2 was heated more
intensely than undamaged areas. The difference in
the maximum temperature between the undamaged
structure and that has a 0.15 mm depth of indentation
was >13 °C. This was large enough to inspect an
aircraft outside that are subject to thermal
disturbances from ambient air. Figure 5 shows the
difference in the resistance changes compared to
undamaged state. All the results of segment 2 that
has the indentation showed the resistance decrease.
With an indentation depth of 0.15 mm, the resistance
of segment 2 changed by 0.4 m. This was relatively
easy to detect.
4. CONCLUSION
A proposed self-monitoring method was applied to
detect an indentation in a large CFRP structure.
Aircraft lightning protection strips were used as the
electrical contacts and their wiring, which made it
possible to apply a uniform electrical current and
electric heat to a large structure (>1 m in size). In this
case, even small-scale impact damage (12 kN in
indentation load and 0.15 mm in indentation depth)
showed an temperature increase of >13 °C. This was
clearly visible with thermography. A measuring
segment that had the indentation of 0.15 mm depth
showed the resistance decrease of 0.4 m, which
was detectable without thermography. We need to
detect a >0.15 mm depth of indentation that may lead
to a large reduction in the compression after impact
(CAI) strength, so the proposed method is thought to
have enough sensitivity of indentation detection.
REFERENCES
1) Suzuki, Y., Todoroki, A., Matsuzaki, R.,
Mizutani, Y. Impact Damage Detection in
laminated Carbon Fiber Reinforced Polymers by
the SI-F Method Using Resistance-Temperature
Characteristics, JSMME, 5, 1, 33-43, 2011.
2) Suzuki, Y., Todoroki, A., Matsuzaki, R.,
Mizutani, Y. Impact-damage visualization in
CFRP by resistive heating: Development of a
new detection method for indentations caused by
impact loads, Composites A, 2011, in press.
3) Ogasawara, T., Hirano, Y., Yoshimura, A.
Composites A, 41, 97381, 2010.

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