1
Intelligent Structural Health Monitoring (SHM)
of Composite Aircraft Structures using Acoustic
Emission (AE) sensors
Dirk Aljets, Faculty of Advanced Technology

Abstract— The objective of this paper was to present
initial results from a new project aiming to develop an
intelligent system to monitor the health of carbon fibre
reinforced plastics (CFRP) in aircraft structures using
Acoustic Emission (AE) technology.
Structural Health Monitoring (SHM) is a relatively new
method in the aircraft industry of monitoring the
condition of a structure in real time while the structure is
in service. A SHM system could make some regular
inspections unnecessary and allow maintenance only
when required. Especially for composite aircraft
structures where the incidence of fatigue is low, this
technique could potentially be economical and improve
the safety of the structures.
This paper shows that AE is an appropriate technique to
monitor the health of composite materials online. AE is
sensitive to micro cracks even before most other NonDestructive Testing (NDT) techniques can detect them. It
is possible to determine the location and the type of
defect (delamination, matrix cracks, fibre breaks) in the
structure to a certain degree. Also the overall AE activity
increases significantly towards the end of the life of the
structure.
Index Terms—Acoustic Emission (AE), Composite Materials,
Structural Health Monitoring (SHM)
I. INTRODUCTION
A
IRCRAFT are complex systems that need to be
maintained at regular intervals, the frequency of which
depends to some extent on the experience of the
manufacturer. In determining the best maintenance intervals
the designer will assess the probability of defects, the
importance of different parts of the structure for the safety of
the aircraft, the type of material, the environmental
conditions, the number of service cycles and the age of the
aircraft. SHM is a relatively new method in the aircraft
industry of monitoring the condition of a structure in real
time while the structure is in service. Inspections with NDT
techniques like X-ray or Ultrasonic are time consuming and
Manuscript received February 21, 2008.
This work is supported in part by Airbus/Filton, TWI and Cardiff
University.
Dirk W. Aljets is with the Faculty of Advanced Technology,
Department of Engineering, University of Glamorgan, Pontypridd, Mid
Glamorgan, CF37 1DL, United Kingdom (Corresponding author e-mail:
daljets@glam.ac.uk).
therefore expensive for airlines. Reducing the downtime for
inspections lowers the cost of maintenance [1]. A SHM
system could make some regular inspections unnecessary
and allow maintenance only when required. Especially for
composite aircraft structures where the incidence of fatigue
is low, this technique could be very economical and improve
the safety of the structures.
II. METHOD AND APPLICATION
Acoustic Emission (AE) is a phenomenon of all materials
that when forces are applied stress waves are propagated
through the material structure, which are measurable with
suitable sensors. AE sensors are piezo-electric elements in
most cases. They transform the stress waves into a voltage,
which can be analysed with a suitable system. The frequency
response of the sensors must be suitable for the frequency
range to be detected.
AE stress wave sources are associated with breaks in
molecular structure, i.e. in polymers between main-chain
linkages or weak secondary linkages. The waves have a high
frequency content (100 kHz – 2 MHz) which makes this
technique insensitive to mechanical vibrations usually
generated by the engines and other aircraft parts. As a crack
propagates AE is generated and so, particularly for
composite materials, the growth of flaws like delamination
or cracks in the matrix or fibres can be detected before they
become dangerous. Early damage detection in CFRP is
important since gradual damage accumulation can abruptly
change into rapid damage growth which ends with a
catastrophic failure [2]. Unnthorsson et al. [2] reported that
the AE technique is highly sensitive and detects many
damages earlier than other NDT techniques.
Damages in CFRP include delaminations (separation of
plies), cracks in the matrix (i.e. resin) and fibre breaks, as
already mentioned above. Almost all defects can be
attributed to these three types of flaw. In the majority of
cases impact damage is a combination of all three flaw
types, whereas fatigue starts with small cracks in the matrix
which propagate and can continue to delamination [3]. Fibre
fracture occurs last and would most probably lead to the
total failure of the structure.
Tests were carried out on a carbon fibre composite plate
using one AE sensor. The plate dimensions were
381x403x3 mm. The plate consisted of 16 layers with
alternating fibre orientations in the 0° and 90° directions. A
Pencil Lead Break (PLB) test was performed to set up the
test rig and investigate the wave propagation in this
2
specimen. The PLB Test is a standard test to characterise
AE systems. A pencil lead is broken on the specimen and
this generates AE waves which travel through the structure
and can be detected by an AE sensor. Fig. 1 shows the AE
signal of a PLB test picked up by the AE sensor. The results
correlated well with those reported in the literature. Also it is
possible to observe that reflections from the sides of the
specimen and superpositions of these reflections with the
original signal have an effect on the overall waveform
picked up by the sensor.
Other techniques to determine the actual damage in the
structure are the wave energy, amplitude and the duration of
the event. Matrix cracks for instance have normally low
amplitude and short duration while fibre breaks are
associated with high amplitude and long duration signals [5].
In addition Bussiba et al. [6] reported a specific dominating
frequency for the different flaw types. For example fibre
breaks have a higher frequency than matrix crack
propagation.
The actual location of damage in a structure can be
identified by the time delay of the AE signal at different
sensors. At least three sensors are necessary to identify the
flaw location in a composite plate. Nevertheless the more
complex the structure the more sensors are necessary. The
maximum distance between the sensors is limited by the
attenuation of the AE wave while it propagates through the
structure and the accuracy the system should have to detect
small flaws.
III. CONCLUSION AND FUTURE WORK
Fig. 1: AE signal of a Pencil Lead Break Test and data reduction
with a threshold
Most signal analysis applications in the literature [2]-[6]
use a threshold to get rid of the background noise and reduce
the data (see Fig. 1). A ‘hit’ is said to be detected if the
threshold was surpassed. The threshold can be fixed or
floating and depends on the signal-to-noise ratio, structural
size and test setup. According to Unnthorsson et al. [2] the
main problems to set the threshold are that the same type of
damage can emit AE signals with different amplitudes and
that the signals in composites suffer from high attenuation.
This means that signals close to the transducer are stronger
and more likely to be detected than those generated farther
away. In order to obtain information about the actual
structural health the AE hits can be counted. Bourchak et al.
[4] investigated the AE response to variable amplitude
loadings and observed an increase of AE activity with the
actual load amplitude. Furthermore the authors discovered a
“wear-in” effect after long duration of high load cycles.
After these cycles the AE activity took some time before
inactivity resumed. In general the number of hits increased
towards the end of the structural lifetime independent of
whether this was due to fatigue or a tensile test (Fig. 2).
The initial results show that AE patterns in composites
depend on the type of flaw and that the wave propagation is
suitable to determine the location of its source. AE technique
requires significant preliminary analysis and calibration for
each system; composed of material, geometry and type of
loading to distinguish among the various types of damage
and failure mechanisms [7].
Research is in progress to determine a suitable AE sensor
and the best way to attach these sensors to the composite
structure on an aircraft. Furthermore the wave propagation
for specific composite structures will be investigated. An
artificial intelligence based signal processing methodology
will be developed to allow real-time interpretation of the
signals.
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Fig. 2: Increasing AE activity with load (tensile test) [3]
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