FATIGUE CRACK DETECTION BY ACOUSTIC EMISSION MONITORING
IN THE COURSE OF LABORATORY STRENGTH TEST
J.Běhal
Aircraft Strength division
Aeronautical Research and Test Institute
Beranových 130, Prague, Czech Republic
Abstract
All-metal aircraft structure was inspected by acoustic emission method in the course of laboratory fatigue test. Acoustic
emission method takes a part in modern range of non-destructive testing applications. The acoustic emission monitoring and
appropriate analyses of experimental data are objects of research tasks today. The aim of this research is to formulate
methodology, according which will be possible to reliably evaluate the material cracking. This study is derived on the base of
laboratory experiments, where the service conditions are simulated on the aircraft frame and its model specimen.
Introduction
Non-Destructive Testing (NDT) is an integral part of modern industry. The safe-life philosophy secures an aircraft safety in
service by a scatter factor related to experimental strength test. NDT is required in the course of full-scale fatigue test for
establishing the fatigue life of the structure. Much more attention has been focused on NDT through damage tolerance
approach. It can be define as the ability of aircraft structure to sustain expected limit loads in the presence of fatigue, corrosion
or accidental damage until such damage is detected through inspections and repaired. The damage tolerance approach
requires that damage should be detected before reaching the critical size under maintenance inspection schedule, so this
approach forces periodic NDT inspection or permanent health monitoring of structural significant items in service.
Aeronautical Research and Test Institute is the centre for research, development and testing in the Czech Republic. The
Institute successfully fulfils orders from the Czech as well as foreign industries comprising both civil and military sectors. The
Experimental Strength Department performs experimental task connected with certification strength tests of primary aircraft
structure, their components and models. In order to meet present and future requirements for fatigue related NDT, there are
studied



choice of appropriate inspection method and technique
inspection reliability
development of inspection systems
Airframe inspections are applied under difficult conditions, e.g., composites, steel/aluminium boundary, large curved surfaces,
complex and unknown structures. Visual method, especially remote techniques, is the most common one, but its reliability is
limited by human factor and surface conditions. Sophisticated instruments give better results and are used in selected points.
Acoustic emission is permanently monitored and signs of fatigue cracking are checked in frequency domain. One of the
advantages compared to other NDT methods is the possibility to observe damage processes during the entire load history
without any disturbance to the specimen. Common problem for every inspection method is to difference between fatigue
cracking and background noise, e.g. scratches for visual, geometric shape for ultrasound, material imperfection for eddy
current, friction for acoustic emission.
Figure 1. Monitoring of acoustic emission
A traditional approach is to combine statistical and empirical description, thereby establishing a statistical correlation between
measurements and generalising the features. An alternative scheme is to train a neural network to extract the required
material properties using a reference set of specimens and measurements. However, both of these approaches depend on
acquiring a specific set of test data under controlled conditions. The data is then useful for estimating the properties of new
samples whose treatment conditions and material quality are consistent with those of the reference set.
Parallel to loading in the course of fatigue test, there is composed a health monitoring system suitable for service maintenance
scheduling. Methods of non-destructive testing are used through crack detection and documentation of test specimen
continuous degradation. Experience and validated practice are transfer from laboratory environment to service condition. Final
surface treatment and outfit installation should be reflected for service usage. The different reliability and capability movement
in the field condition are established by experimental test of full-equipped critical point model.
Aluminium became an essential metal in aircraft industry. Although the integral structures using composites are validated, the
most planes are made of metal today. Significant fraction of airframe structure consists of stiffened panels. The main loads are
transferred through beam elements of the spars. According to fatigue point of view, the critical points of wing structure are
present in the lower flange of main spar, Figure 2. Because of customer request a fatigue crack detection as well as possible,
the periodic inspections by tradition techniques (visual, eddy current, ultrasound) were supported by acoustic emission
monitoring.
Figure 2. Inner area of airframe structure
Instrument Set-up
Multi-channel monitoring system, Figure 3, is linked with PC. Piezoceramic sensors use external amplifiers. Frequency range
varied between 100 and 600 kHz.
Figure 3. System of acoustic emission monitoring
Full-scale Fatigue Test
In the course of fatigue test of full-scale airframe, the most of measured emission events were not related to material cracking.
There was plenty of friction between structure parts together and temporary buckling of the skin. There are several possibilities
to detect the cracking by data analyses:

localization through time differences of signal arrival

shape of emission sample (duration, amplitude, …)

frequency spectra of emission event
In the example, the spar flange was a beam shape and the sensors were situated in the line. Lower flange of the wing spar
was the critical element. The critical point of Al-alloy flange was hidden by another steel part near the wing attachment.
Because of customer specification, it was not possible to dismount the row of bolted joints for inspection. The detection of
fatigue crack as soon as possible was emphasised, due to later possibility of consideration the structure as damage tolerance.
Structure conception and material differences did not allow inspections by traditional method of non-destructive testing, which
generally required the direct access to the critical point. Acoustic emission sensors were placed along the spar flange and
emission sources are localized linearly. The emissions of whole structure take effect in the post processing of measured data,
Figure 4. Events in “A” place may be the respond of fatigue. The “B” place responds the rib joining in this area and an entrance
of emission of remote sources.
event counts
A
B
Figure 4. Acoustic emission localized in the lower flange of wing spar
Model Test Specimen
crack length [mm]
Draft of the model experiment is illustrated in the Figure 5. Al-alloy plate of 2 mm thickness is uniaxial loaded by harmonic
cycling up to 100 MPa. Monitored emission is documented in the Figure 6. Crack growth in early stage of propagating as well
as acoustic wave dispersion is observed. Common and characteristic features should be recognized for reliable identification
of emission source.
30
activity crack length
origin about 1 mm
25
20
15
10
5
0
0
20 000
40 000
60 000
life [cycles]
Figure 5. Technological sample and crack growth curve
80 000
100 000
Figure 6. Cumulative counts of acoustic emission events
For the wave speed about c=4000 m/s and the frequency varied between 100 and 400 kHz, the wave length is
min 
max 
c
f max
c
f min

4  103
 0,01[m] ,
400  103

4  103
 0,04[m] ,
100  103
For object thickness about 2 mm, there are
t  2mm  min  10mm
and the wave dispersion should be supposed.
It seems to prove the inspection reliability, when material cracking is identified through signal features recognition, Figure 7.
Neural network is useful instrument for the reliability quantification.
2400
1200
0
0
-1200
max: 375 kHz
15
10
500
1000
1500
2000
2500
5
0
-2400
200
400
Figure 7. Analyses of acoustic emission events in frequency domain
Discussion
The task of acoustic emission monitoring is related to airframe inspection reliability. Specific airframe characteristics, strict
demands for meeting the service safety, requirements resulted from service condition to support systems and emphases to
service efficiency evoke to take care of areas, which hold some level of result uncertainty. Acoustic emission method take a
part in modern range of non-destructive testing applications and it appears to be perspective for next research in an aircraft
industry. The acoustic emission monitoring and consequent analyses of experimental data are objects of several R&D projects
today. The aim of next studies is concerning to find and formulate the decision criteria, according which will be possible to
reliably evaluate if the material cracking is present or not. The methodology should be derived on the base of laboratory
experiments where the material fatigue may be objectively described.
Traditional non-destructive testing and monitoring methods all have specific drawbacks when applied to complex structures.
However, future non-linear methods have great detection potential and are more sensitive to common and hidden defects. The
outcome of the feasibility tests looks promising. The non-linear method may be implemented for detecting flaws in local
relatively simple elements, because structure behaviour must be understudied for right signal interpretation. The Aeronautical
Research and Test Institute is involved in national research programs, e.g. STRATECH, PROGRES and TANDEM, and
European “Health Monitoring of Aircraft by Nonlinear Elastic Wave Spectroscopy“, where completely new system for non-linear
NDT is developed.
Aerospace industry is one of the most advanced and important fields of NDT applications. Tradition non-destructive inspection
for structural defects is a vital component of maintenance. Non-destructive evaluation is opening the door to precise
presentation of fatigue test results. Except quality inspection of a test specimen, non-destructive testing is widely used in the
case of fatigue evaluation of aircraft structure. During the term of months or years, when the structure is loaded in laboratory,
early detection of crack allows repairing just a little area and the fatigue test continues, as all the critical points of structure
must be recognized.
With aging aircraft the increase in damage detection is an important and difficult task. Also new is the issue of crack growth
management. The effort to increase fatigue life of aircraft structures leads to an advanced design philosophy, which permits a
fatigue crack being initiated during service. A schedule of structure inspections is established for aircraft in service and critical
points are checked by NDT as maintenance task.
Conclusion
Experimental tasks are important for improving designer knowledge and choosing smaller scatter factor of proposed structure,
decreasing structure weight and increasing service reliability. An optimization of design is the key tool especially in aircraft
industry. Acoustic emission method is very useful tool for NDT inspection. The full-scale aircraft frames are loaded during
simulation of service condition. Appropriate service maintenance for critical point is established and physical phenomena of
inspection techniques are studied too.
Acknowledgments
This study was supported by the Ministry of Industry and Trade of the Czech Republic, project no. FT-TA/026.
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