IMAGING OF FATIGUE DAMAGE IN CFRP COMPOSITE
LAMINATES USING NONLINEAR HARMONIC GENERATION
Christophe Mattel and Pierre Marty
CSM Materialteknik AB, Saab Aerospace Group, Box 1340, 581 13 Linkoping, Sweden
ABSTRACT. In this paper, experimental evidence is presented that suggests a strong nonlinear
interaction between acoustic wave and micro-structural damage before the onset of delaminations in
fatigued CFRP samples. Sample used were 32 plies quasi-isotropic graphite/epoxy laminate
fatigued with a four point bending fatigue. First harmonic images were constructed from the
amplitude of the first harmonic normalized by the amplitude of the fundamental. Harmonic imaging
technique (HIT) shows a much higher sensitivity to micro-damage than amplitude C-scan.
Correlations are established between the image zone where the nonlinear parameter is high and the
region where a high density of micro-delamination and matrix cracks is observed.
INTRODUCTION
The increasing use of composite material in demanding loading and environmental
conditions calls for nondestructive techniques that can be instrumental in evaluating the
structure's residual life. As Graphite/Epoxy composite materials are known for their good
response to fatigue, they also present very few measurable sign of damage before the onset
of delaminations. In the framework of a study on the influence of environmental
degradation on the long-term performances of carbon fibre reinforced plastic laminates, it
has been shown that the combined effects of humidity and thermal loading can reduce up
to 40% the residual life of composite laminate under bending fatigue cycling [1]. However
traditional health monitoring techniques such as Acoustic Emission or Ultrasonics do not
show any significant sign of degradation under the first 90% of the fatigue life which
makes the task of assessing the effect of fatigue and predicting the residual life of a
composite structure very difficult. The objective of this work is to evaluate the
performances of the Harmonic Imaging Technique (HIT) for detection and monitoring of
early fatigue damage such as microcracks and microdelaminations in composite laminate
under fatigue cycling. The technique, based on the measurement of the amount of
distortion of an ultrasonic signal after propagation in the material, is commonly used in the
medical field. However, HIT has seldom be used for NDE applications and the results
reported [2] focused mainly on geometrical defects in metals. In this paper, experimental
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
989
evidence is presented that suggests a strong nonlinear interaction between acoustic wave
and micro-structural damage before the onset of delaminations in fatigued CFRP laminate.
HARMONIC IMAGING OF FATIGUE DAMAGE
Background
Ultrasonic nonlinearities associated with fatigue damage have recently received a lot of
attention. Harmonic generation due to of fatigue cracks in metals [3], Nonlinear effects in
bondlines under fatigue [4-6] and microstructural changes in inhomogeneous materials [7]
have been reported. One commonly referred explanation for the origin of the nonlinear
interaction between acoustic waves and fatigue damage is the so-called crack-closure
effect [8,9]. This effect refers to the local dependency of the elastic moduli in the region of
a fatigue crack on the stress induces by ultrasonic wave. Under compressive stresses, the
crack becomes tightly closed and the local modulus approaches that of the matrix. Under
tension stresses, the crack fully opens inducing a decrease the local modulus. This stress
dependency of the material stiffness translates into a nonlinear stress-strain relationship
that induces the distortion of the ultrasonic wave as it propagates into the material. The
detection of harmonic generation using spectral analysis of narrow banded signals is one of
the techniques used to characterize the non-linearity of a solid medium. The aim of this
work was to experimentally determine whether microdamage in fatigued composite can
induce this type of acoustic nonlinearity and if this effect is strong enough to be detected
without the use of high amplitude ultrasonic.
Experimental Set-Up
The experimental configuration used in this study is a typical through-transmission setup in immersion as shown in figure 1. A function generator was used to produce narrowbanded tone-burst amplified with a RF amplifier. The signal is sent to a focused transducer
with a central frequency response optimised for the operating frequency. The signal is
detected on the other side of the composite laminate by a second transducer. The system is
position indexed so that imaging of the sample is made possible.
FIGURE 1: Experimental set-up for harmonic imaging of composite laminates.
990
Input
Output
Imaging
C-Scan imaging (linear UT)
F=18MHz
Harmonic Imaging (Nonlinear UT)
F=9 MHz
FFT of time domain output
9MHz 18MHz
FIGURE 2: Input and output signals used for imaging of fatigue damage in composite laminate.
Conventional amplitude C-scans and Harmonic Imaging are performed on each sample.
In order to compare the sensitivity of harmonic imaging with conventional UT, two sets of
measurement were performed on the composite laminate as shown in figure 2. First a
conventional amplitude C-scan was performed using a tone-burst signal with a frequency
of 18 MHz. Harmonic imaging was then performed using an 9 Mhz tone burst signal. The
Fast Fourier Transform of the output signal was computed and the amplitude of the first
harmonic (at 18 Mhz) normalized by the fundamental (at 9 Mhz) was determined at each
point of the scan. Tone-burst frequency and length were optimised in order to avoid
through-thickness resonances.
FATIGUE CYCLING OF COMPOSITE LAMINATE
The samples used for the study are 32 plies AS4/8552 laminate with a [(0,90,45,+45)4]S32 lay-up. The size of the sample was 130mm*13mm*4mm. Fatigue cycling was
performed using a four-point bending system. A picture of the experimental set-up for
fatigue cycling of composite is shown in figure 3. This type of loading introduces shear
and bending stresses in the laminate. Shear stress is expected to induce microdelaminations
between plies and bending stress induces mainly matrice cracks running through the outer
plies. A critical aspect of the fatigue procedure was to stop the cycling before delamination
onset in the sample for evaluation of the harmonic imaging technique. In order to monitor
the fatigue damage acoustic emission monitoring was used.
991
Four point
bending fixture
AE Transducer
Composite
sample
FIGURE 3: 4 point bending holder and AE emission sensors used cycling and monitoring of the fatigue
damage.
Modal analysis of AE events allowed a precise detection of delamination onset and
allowed the definition of a AE criteria that was then used to stop fatigue cycling before
onset of delamination.
EXPERIMENTAL RESULTS
Conventional amplitude C-scans and harmonic imaging were performed on the pristine
samples and at different stage of the fatigue cycling allowing a comparison of the
sensitivity of the methods. At each stage the samples were unmounted from the fatigue
holder and scanned in an immersion tank. The same experimental conditions were
maintained for each scan. Figure 4 shows the results obtained for the same sample before
cycling, at 15000 cycles and 17 000 cycles. At this stage acoustic emission have been
recorded but no sign of delamination is found yet. This is confirmed by the amplitude Cscan (left side in figure 4) that shows no clear discontinuities at the three fatigue stages. No
clear increase in attenuation is detected. On the right hand side in figure 4, Harmonic
Imaging shows an increase of the nonlinear parameter (amplitude of the first harmonic
normalized by the fundamental) with fatigue cycling. The 17 000 cycles Harmonic image
shows high harmonic generation near the edges of the sample. Variations from 0.1 to 0.3
for the non-linear parameter are recorded over the sample area (i.e. a 300% relative
contrast). Figure 5 shows examples of time domain signal and spectra acquired during the
Harmonic scan. The signals on the left side are extracted from area with high nonlinearity.
Visible signal distortion can be seen on the time domain signal that translates into a high
harmonic component in the spectrum (lower right hand side in figure 5).
After 17000 cycles, fatigue cycling was terminated and the sample was cut and polished
in two cross-sections along the length of the sample. Microdelamination mainly near the
edge and matrix cracks in the center of the sample were clearly observed with an optical
microscope. Figure 6 shows two micrographs of typical damage found in the sample.
992
Classical Ultrasonic C-scan
Harmonic Imaging Technique
FIGURE 4: Amplitude C-scans (left side) and harmonic imaging (right side) of the same sample after 0,
15000 and 17000 cycles.
FIGURE 5: Example of time domain signal and respective spectra in areas with high and low nonlinear
response.
993
Micro delaminations : mainly between +90
and +45 plies, up to 20 mm long, 10 to 20
jLtm "open.
Matrix cracks : mainly in 90 and +45 plies
Up to 200 jum long, 10 to 20 /im "open
FIGURE 6: Micrographs of fatigue damage observed in composite laminate sample after cross-section cutting
and polishing in zones with high nonlinear response.
Microdelaminations were running mainly between the +90 and +45 plies, up to 20 mm
long and lOjim to 20jLim open. Matrix cracks were found in the outer layers as expected
with bending loads. Crack opening is also estimated between 10 and 20 jim. A good
correlation between regions with high crack density and high nonlinearities was found.
CONCLUSION
Harmonic imaging of fatigued CFRP laminate samples was performed. The technique
shows much higher sensitivity to microstructural damage such as matrix cracks and microdelamination then conventional amplitude C-scans. The increase in the nonlinear response
due to fatigue is such that the use of finite amplitude waves were not needed and
conventional ultrasonic transducers and amplifiers were sufficient. A good correlation
between the harmonic imaging and destructive visual inspection within cross-sections was
found.
ACKNOWLEDGEMENT
This work was supported by Saab Aerospace and the Swedish Defence Administration
(FMV)
994
REFERENCES
1. Hyllengren, F., Technical report TEK01-0022, CSM Materialteknik AB, (2001).
2. Wu, P. and Stepinski, T., "Ultrasonic harmonic Imaging in non-destructive evaluation:
Preliminary experimental study," Proceedings of the 2000 IEEE Ultrasonics
symposium, 801 (2000).
3. Ogi, T. et al, "Contactless monitoring of surface wave attenuation and nonlinearity for
evaluating remaining life of fatigued steels," Proceedings of the 15th WCNDT, Roma,
2000.
4. Nagy, P. McGowan, P. and Adler L., "Acoustic nonlinearities in adhesive joints,"
Review of Progress in QNDE, , eds. D. O. Thompson and D. E. Chimenti, Plenum,
New York, Vol.9, 1685-1692, 1990.
5. Achenbach, J. D.et al., "Ultrasonic detection of nonlinear mechanical behaviour of
adhesive joints," Review of Progress in QNDE, , eds. D. O. Thompson and D. E.
Chimenti, Plenum, New York, Vol.lOB, pp. 1837-11844, 1991.
6. Fassbender, S. U. and Arnold, W., " Measurements of adhesion strength of bonds
using nonlinear acoustics," Review of Progress in QNDE, , eds. D. O. Thompson and
D. E. Chimenti, Plenum, New York, Vol.15, pp. 1321-1327, 1996
7. Van den Abeele, et al.,"NEWS: technique to discern material damage. Part I:
Nonlinear wave modulation Spectroscopy," Res. Nondestr. Eval, vol.12, 17-30, 2000.
8. Donskoy, D., Sutin, A. Ekimov, A. "Nonlinear acoustic interaction on contact
interfaces and its use for non-destructive testing,", NDT&E International, 34,231
(2001).
9. Peccorari, C., "An extension of the spring model to nonlinear interfaces," Review of
Progress in QNDE, eds. D. O. Thompson and D. E. Chimenti, Plenum, New York,
(these proceedings).
995