Debonding detection in CFRP-reinforced
steel structures using anti-symmetrical
guided waves
Jingrong Li, Ye Lu*, Yu Fung Lee
Department of Civil Engineering, Monash University, Clayton VIC 3800, Australia
*Corresponding author
Abstract
Carbon fibre reinforced polymer (CFRP) has proved to be effective in strengthening
ageing steel structures, due to its outstanding properties. However, debonding problems
which can occur at the interface severely impair the performance of retrofitted
structures. In this paper, a guided wave based structural health monitoring (SHM)
method is employed to detect debonding in CFRP-reinforced steel structures. The
proposed samples are CFRP-reinforced steel plates with different sizes of debonding.
Due to its sensitivity to interfacial damage, the anti-symmetrical Lamb wave mode is
chosen as the incident wave and surface-mounted piezoelectric (PZT) wafers are
employed to excite and collect the signals. Theoretical derivation is introduced to find
the relationship between the time-of-flight (ToF) of the first arrival wave package in the
received signals and the extent of debonding damage. A linear relationship is obtained,
in that the ToF decreases linearly with the increase in the size of debonding. When
corresponding numerical simulation and experimental study are performed to verify the
relationship, similar trends are observed.
Keywords: Guided Waves, Finite Element Method, CFRP-reinforced Steel Structures,
Debonding Detection, Structural Health Monitoring
1. Introduction
Steel is one of the most popular construction materials for buildings and bridges
due to its high strength-to-weight ratio, ductility and variety [1]. When degradations
appear in steel structures after long-term service, instead of replacing the entire
deteriorated part, the chosen response is usually to repair or reinforce such structures in
situ, which is much more economical and efficient. In this respect, the bonding of
carbon fibre reinforced polymer (CFRP) materials has proved considerably successful
in strengthening steel structures because of its unique advantages, namely, high
strength-to-weight ratio, impressive stiffness and durability, outstanding corrosion
resistance, easy installation and tiny alteration to the dimensions of the structure [2, 3].
On the other hand, however, debonding of externally bonded CFRP laminates, which
can be triggered by improper cure of the epoxy adhesive during the installation process
or the harsh environment, as well as stress concentration during the service life, can
dramatically affect the transfer of load through the interfacial bond and result in
premature failure [4-6]. Consequently, debonding has been recognised as a dominant
failure mode in CFRP-strengthened steel structures, and long-term monitoring is
required to ensure the functionality and integrity of retrofitted structures [7].
Debonding is very difficult to inspect, since it is concealed. The development of
appropriate non-destructive testing (NDT) methods is therefore necessary, to monitor
the initiation and growth of debonding in CFRP-reinforced structures before
catastrophic failure occurs. In recent years, the ultrasonic guided wave based SHM
technique has attracted much attention for the detection of damages, including cracks,
delaminations, corrosion, holes and debonding, in various types of civil structures [812]. Compared with other debonding detection techniques, such as ultrasonic testing,
acoustic emission, electromechanical impedance (EMI) and infrared thermography, it
has the advantages of cheap set-up, fast and accurate flaw determination, large area
scanning, and continuous and permanent monitoring [13-16].
Guided waves are mechanical waves that propagate in a structure with boundaries.
They can be generated or sensed by several kinds of transducer, among which, the
surface-mounted piezoelectrical (PZT) wafer, which has remarkable properties
including portability, affordability, wide frequency response and low power
consumption, is most commonly employed in practice [17]. After a diagnostic signal is
sent out by the actuator, if there are defects on or near the wave propagation path, the
wave pulses are reflected or scattered. Based on the abnormalities in the collected signal,
information about the damage can be identified.
Corresponding to different waveguide configurations, guided waves can be classed
in several forms. The wave that propagates in thin plate-like structures is called Lamb
wave, which is a combination of reflected longitudinal wave and shear wave [18]. Lamb
wave is a good candidate for SHM of thin-wall structures and is gaining increasing
prominence due to the following merits: (1) it can travel over a long distance without
losing much energy; (2) its inspection covers through-thickness; (3) it is economical
and its implementation is efficient; and (4) it has high sensitivity to small-scale defects
[7, 19].
The ultrasonic guided wave based technique can be classified into two groups,
linear and nonlinear [20]. Compared with the nonlinear method, which has higher
sensitivity to micro-damage in its initial stage but is also easily disturbed by noise, the
linear method is much easier to be carried out in practice [21]. Although the linear
method cannot recognise damage that is smaller than the wavelength of the probing
signal, it is sensitive enough to identify gross damage [21]. A number of studies have
been conducted to investigate the linear method, most of which focus on the
characteristics of wave propagation, which mainly include the time of flight (ToF),
wave mode conversion, wave scattering and attenuation [22]. The guided wave based
detection method has been adopted for the debonding inspection of several materials.
For example, Mustapha et al [23] utilised the fundamental anti-symmetric (A0) wave
mode, which was excited at a low frequency, to assess debonding in a sandwich
composite beam by investigating the delay in the ToF and the change in energy
magnitude of wave signals. Okabe et al [24] performed both finite element analysis and
experiment to investigate the detection of delamination in a composite structure by
mode conversion of Lamb waves. Ng et al [25] analysed the scattering characteristics
of the A0 mode propagating in complex structures made of composite laminates with
different debonding sizes and locations. However, investigation of the feasibility of
using guided waves for debonding detection in CFRP-reinforced steel structures is still
limited in current research.
In this paper, debonding evaluation of CFRP-reinforced steel structures was
achieved by analysing the ToF of the guided wave signals. Both three-dimensional
numerical simulation and experimental studies were carried out to investigate the linear
characteristics of guided waves propagating in CFRP-reinforced steel plates with
different sizes of debonding. The paper is organised as follows: Section 2 introduces
the methodology of the study. Section 3 demonstrates the finite element model of Lamb
waves propagating in the proposed structures. An experiment is then conducted to
verify the simulation results in Section 4. Finally, Section 5 presents the conclusions
from the research and introduces proposed future work.
2. Methodology
2.1 Incident wave selection
Lamb wave has inherent properties of multi-mode, dispersion and attenuation
because it is constrained by the media boundary [26]. These complex properties
dramatically increase the complexity of signal processing. Hence, the selection of an
appropriate incident signal is very important in the Lamb wave-based NDT method.
On the basis of different directions of particle motion, Lamb waves can be mainly
divided into two modes: symmetrical (S0, S1, S2, …) modes and anti-symmetrical (A0,
A1, A2, …) modes. Most of the particles in symmetrical wave modes have in-plane
displacement whereas those in anti-symmetrical wave modes mainly have out-of-plane
motion [18]. As a result, anti-symmetrical (A) modes are observed to be more sensitive
to debonding and delamination damage, which is interfacial, while symmetrical (S)
modes are more sensitive to in-plane damage, e.g. through-thickness or open crack [27].
For this reason, the A mode was selected as the incident wave to detect the debonding
damage in this study.
Lamb waves are dispersive, which means that the group and phase velocities of
wave modes depend on the wave frequency (f), the thickness (h) of the medium and the
properties of the media [23]. Usually, a narrow bandwidth signal with a certain number
of cycles can minimise dispersion [18]. Therefore, a 5.5-cycle Hanning-window was
employed to narrow the bandwidth of the selected Lamb wave mode and to increase
the signal-to-noise ratio. The velocities of certain Lamb wave modes propagating under
different excitation frequencies and in different materials are normally obtained from
the dispersion curves, which can be plotted based on calculation using the commercial
software “DISPERSE”. The dispersion curves for Lamb waves propagating in steel
plate, CFRP-reinforced steel plate and CFRP laminate with selected thicknesses are
shown in Figure 1. It can be seen that, compared with the A0 and S0 modes, the A1 mode
travels at a higher group velocity at 250 kHz in the 10 mm thickness steel plate and the
CFRP-reinforced steel plate. Moreover, the dispersion curve of the A1 mode at 250 kHz
is relatively flat, indicating a less dispersive behaviour. On the basis of these
observations, a 5.5-cycle Hanning-windowed sinusoidal tone burst signal with the
central frequency of 250 kHz was employed as the incident wave.
(a)
(b)
(c)
Figure 1. Dispersion curves (group velocity-frequency) for (a) 10-mm thickness steel plate; (b)
CFRP-reinforced steel plate; (c) 1.57-mm CFRP laminate
2.2 Theoretical derivation of ToF
In this study, the samples are CFRP-reinforced steel plates with different sizes of
debonding damage located in the central area and across the whole width of the
structure. The dimensions of the steel plate are 90 mm × 500 mm ×10 mm (W×L×H)
and the detailed configurations of the structure are schematically illustrated in Figure 2.
As can be seen, double-side debonding is introduced by bonding CFRP laminates to
both sides of steel plates with specified debonding sizes. PZT wafers are distributed at
the left and right sides and are employed to excite and receive the signals respectively
in the longitudinal direction.
During the whole process, as shown in Figure 3, Lamb waves travel along areas
containing steel plate, CFRP-reinforced steel plate and a debonding zone. It is
noteworthy that, when the incident wave which first propagates in the bonded area
encounters the boundary of the debonding zone, part of the wave is reflected while the
remainder is transmitted. The transmitted Lamb wave is separated into two parts which
propagate separately in the steel plate and the debonded CFRP laminate. Subsequently,
the transmitted waves which are separated because of debonding arrive at the bonding
area again and further propagate before being sensed by the sensors.
Since the velocities of different wave modes propagating in different materials at
the frequency of 250 kHz are not the same, as summarised in Table 1 referring to the
dispersion curves, the two parts of a separated wave do not arrive at sensors
simultaneously. By comparing the propagation velocities of the S0 mode and the A1
mode, which are respectively the highest for the wave transmitting through the
debonded CFRP laminate and steel plate, it is easy to find that the former is much
greater. As a result, the first wave package to arrive at the sensor is the part of the wave
that propagated through the debonded CFRP laminate, while the other part that
propagated through the steel plate arrives later.
Figure 2. Structure configurations (a) front view; (b) top view; (c) cutaway view
Figure 3. Schematic diagram of wave propagation in debonded CFRP-reinforced steel structure
Table 1. Theoretical velocities for three modes of Lamb waves
Velocity (m/ms)
Pure steel plate
CFRP-steel plate
S0
A0
A1
1.77
2.73
3.18
3.62
3.74
4
13.33
Pure CFRP laminate
5.3
-
ToF is the time for a specific wave package to propagate over a certain distance.
In this study, the ToF of the first arrival wave package which has the highest propagating
velocity in the received signal is analysed. The corresponding ToF can be expressed
based on the distances and velocities as follows:
2L
 L2
(1 −  ) L2
L
L2
2L
L2
ToF = 1 +
+
=( 2 −
) + ( 1 +
)
(1)
vsteel vdebongding vsteel −CFRP vdebonding vsteel −CFRP
vsteel vsteel −CFRP
where L1 is the distance between the centre of the PZT and the edge of the CFRP
laminate, which is 15 mm; L2 is the total length of the CFRP laminate, which is 250
mm, as shown in Figure 3; ∅ is the proportion of the debonding area; vsteel is the velocity
of wave propagating in the 10 mm thick steel plate, which is 3.74 m/ms; vdebonding is the
velocity of the wave propagating in the debonded CFRP laminate, which is 13.33 m/ms;
vsteel-CFRP is the velocity of the wave propagating in the bonded part, which is 4 m/ms.
As can be seen, since all the distances and velocities are known, a linear relationship
between the ToF and the debonding size can be found in Equation (1).
On the basis of that observation, the theoretical ToF values of four debonding
conditions (0% debonding, 25% debonding, 50% debonding, 75% debonding) are
calculated in accordance with the equation and are listed in Table 2. From the table it is
obvious that there is a decrease in ToF with the increase in debonding size, which is
attributed mainly to the fact that more and more waves propagate in the CFRP laminate
with a higher group velocity. Signals travel faster and arrive with a lower ToF in the
samples with larger debonding size. To verify this linear relationship, simulation as well
as experimental studies were conducted.
Table 2. Theoretical results of the ToF values
Debonding size
Debonding length (mm)
ToF(µs)
0%
0
70.52
25%
62.5
59.59
50%
125
48.65
75%
187.5
37.71
3. Finite element simulation
3.1. Finite element model
The commercial finite element package ABAQUS/EXPLICIT was used to
simulate CFRP-reinforced steel plates under the same debonding statuses as those in
the theoretical derivation. The properties of the steel and CFRP sheets are listed in Table
3.
Table 3. The material properties of steel and CFRP sheets
Material
properties
Steel plate
CFRP sheet
Density
(kg/m3)
7850
1900
Young’s
modulus (GPa)
200
177
Poisson
ratio
0.3
0.32
Thickness
(mm)
10
1.57
In the finite element model, the CFRP reinforcement was introduced by tying
CFRP laminates to both sides of steel plates. To simulate the double-side debonding
conditions, corresponding areas of the CFRP sheets and steel plates were untied while
other parts were kept with the ‘tie’ constraint, as depicted in Figure 4. Furthermore, a
sensor network containing 6 PZT wafers, each 10 mm in diameter, was mimicked on
the top of the steel plate to excite and sense signals in a pitch-catch configuration.
Debonding area
Bonded area
Figure 4. Simulation of debonding condition
The input signal was simulated by applying in-plane time-varying concentrated
loads with the maximum amplitude of 0.1 N on the circumferential nodes of the actuator
along the radial directions, while the output signals were extracted by averaging the
strain values of the measurement points at the receiver. The mesh size of the element
had to be small enough to obtain adequate accuracy while, to avoid numerical instability,
the integration time step could not be too large [28]. To this end, the composite structure
was meshed by eight-node linear brick elements (C3D8R) with the mesh size of 0.6
mm and the time step was set as 1.2 × 10−8 s.
3.2. Simulation results
The strain distribution of wave propagation in the 75% debonding model is shown
in Figure 5 as an example. It verifies the assumption in the theoretical section that, when
excited waves interacted with the debonding zone, they would be divided into two parts.
The majority of the signals would propagate on the steel plate while the rest would
transmit in the debonded CFRP laminate at much higher velocity. Furthermore, the
strain distributions of wave propagation in CFRP-reinforced steel structures with
different debonding sizes are demonstrated in Figure 6, in which it is apparent that the
wave propagates differently with different debonding sizes.
Bonded area
Debonding area
(a)
Bonded area
Debonding area
(b)
Figure 5. Strain distribution of wave propagation (a) on the debonded CFRP laminate only in the
75% debonding model; (b) on the steel plate only in the 75% debonding model
Bonded area
(a)
Bonded area
Debonding area
(b)
Bonded area
Debonding area
(c)
Figure 6. Strain distribution of wave propagation on the debonded CFRP laminate (a) in the 0%
debonding model; (b) in the 25% debonding model; (c) in the 50% debonding model
The collected signals for each debonding condition are shown in the time domain
in Figure 7, where the signal for 0% debonding (intact structure) functions as the
benchmark signal and other signals are compared with it individually. As can be seen,
there is an obvious trend that ToF decreases with the increase in debonding size, a result
which is consistent with the result of theoretical derivation in Section 2.2. The ToF
values of the first arrival wave package for each debonding case were subsequently
extracted as listed in Table 4 by selecting the first peak point in the wave package, as
shown in the circles in Figure 7.
(a)
(b)
(c)
Figure 7. Simulation results for the time domain signals in FRP-reinforced steel plates with different
debonding conditions (a) 25% debonding; (b) 50% debonding; (c) 75% debonding
Table 4. Simulation results of the ToF values
Debonding size
Debonding length (mm)
ToF(µs)
0%
0
70.31
25%
62.5
58.52
50%
125
52.62
75%
187.5
42.46
4. Experimental test
4.1. Experimental set-up
To confirm the simulation results, experimental testing was subsequently carried
out. Steel plate samples with the same material properties and bonding conditions as
already given were prepared, in which the bond regions were first sandblasted to
roughen the surfaces to achieve a chemically active environment for bonding [29]. The
CFRP sheets, cut into the designated sizes, were bonded onto both sides of the steel
plates using Araldite 420 A/B epoxy adhesive mixed at the weight ratio of 100:40. The
adhesive was applied evenly on the bonding area of the steel plates with brushes, within
two hours after sandblasting. After that, the CFRP sheets were placed properly and a
uniform pressure was imposed on the top of the sample by a steel plate [29], with the
aim of squeezing out superfluous adhesive as well as the bubbles. The pressure was
maintained for more than one day to ensure that the adhesive thickness was as uniform
and regular as possible.
The artificial debonding was introduced during the bonding process by placing
different sized pieces of thin Teflon film at specific positions on the steel plates before
bonding the CFRP laminates [25]. The Teflon films could separate the surfaces of steel
and CFRP layer effectively without influencing the material properties [9]. Preparation
of the other side of the steel plate was done using the same procedure after at least one
day, to achieve double side debonding. Then, the specimen was cured for one week to
achieve full adhesive strength. During the entire process, acetone was used to clean all
the surfaces to remove contaminants such as oil particles.
Surface-mounted PZT wafers 10 mm in diameter were bonded on each end of the
steel plates with superglue. A 5.5-cycle sinusoidal tone burst signal, modulated by a
Hanning-window, was input into a Tektronix AFG3102 arbitrary function generator as
the incident signal. The peak-to-peak voltage of the input signal was then amplified 50
times, from 5 V to 250 V, by a Trek model 2100HF amplifier. After that, the amplified
signal was applied to the PZT wafers as actuators at the left end of the specimen. The
PZT wafers at the right end sensed the signals after the waves propagated through the
specimens. The output signals were collected by a Tektronix DPO4043B digital
phosphor oscilloscope after being averaged 512 times to maximise the signal-to-noise
ratio. The prepared test samples and the set-up of experiment are shown in Figures 8
and 9 respectively.
Figure 8. The prepared test samples
Figure 9. Set-up of experiment
It is understood that received guided wave signals are sensitive not only to
debonding but also to environmental noise. The continuous wavelet transform (CWT)
was used in this study to filter out the noise signals for accurate extraction of signal
features. Mathematically, the CWT provides an overcomplete representation of a signal
by continuously changing the translation and scale parameters of the wavelets [30]. The
CWT of the signal x(t) at the scale a and translational value b can be expressed as:
+
1
t −b
X w (a, b) =
x(t ) * (
)dt
(2)

a
a −
where Xw(a,b) is the CWT coefficient. a is the scale parameter used to control the
stretching or compression of a wavelet, and b is the time shift parameter indicating the
time locality.  (t ) is a continuous function known as the mother wavelet and  * ()
represents the operation of the complex conjugate [31]. In this study, the signals were
denoised by the Daubechies mother wavelet.
4.2. Experimental results
The experimental signals in the time domain, having been reconstructed by CWT,
are depicted in Figure 10, where a ToF variation trend similar to that in the simulation
can be observed with the change in debonding size.
(a)
(b)
(c)
Figure 10. Experimental results for the time domain signals in FRP-reinforced steel plates with
different debonding conditions (a) 25% debonding; (b) 50% debonding; (c) 75% debonding
Following the same procedure, Table 5 lists the experiment results of the ToF with
different debonding sizes. The linear regression equations of the ToF for both
simulation and experiment are plotted, together with the theoretical linear line, in Figure
11, using data shown in Table 2, Table 4 and Table 5, to demonstrate the relationship.
Table 5. Experiment results of ToF values
Debonding size
Debonding length (mm)
ToF(µs)
0%
0
60.88
25%
62.5
49.48
50%
125
40.76
75%
187.5
34.88
Figure 11. Fitted results between the debonding size and the ToF for theoretical derivation,
simulation analysis and experimental testing
From Figure 11, it can be seen that all three lines share a similar linear trend, in
particular, for the experimental and simulation results, which show similar gradients.
However, there are still some differences, which may be caused by the following factors:
(1) Debonding size and profile prepared during the experimental testing could not
be exactly the same as that in simulation and theoretical analysis, because of the
unavoidable spread of epoxy adhesive which could yield to irregularity of the
boundaries of debonding areas;
(2) Adhesive thickness which is not even in the experiments was not taken into
consideration in the theoretical and simulation processes, which could result in
differences in the values of propagation velocity used in the ToF calculation;
(3) The dispersion curve generated by the software “DISPERSE” is based on
numerical iteration and might not precisely reflect the feature of actual wave
propagation in CFRP-reinforced steel structures as composite systems.
Nevertheless, it can be observed that all the correlation coefficients exceeded 0.98,
clearly revealing a linear relationship between the ToF of the first arrival wave package
and the debonding size. It can thus be concluded that the anti-symmetrical mode is
sensitive to debonding occurring in CFRP-reinforced steel structures.
5. Conclusions and future work
In this paper, a linear guided wave based technique using anti-symmetrical Lamb
wave mode was proved to be effective in assessing the debonding of CFRP-reinforced
steel structures. The theoretical derivation was established to obtain the explicit
relationship between the ToF of the first arrival wave package and the debonding size.
Simulation and experimental tests were then conducted to verify the theoretical
deduction. Surface-mounted PZT wafers were used to excite and collect the wave
signals, while debonding in the shape of strips was created in the middle part of the
specimen.
The ToF of the first arrival wave package was extracted from the time domain in
both simulation and experiment. From the results, it was concluded that the ToF
decreased linearly with the increase in debonding size, which was consistent with the
theoretical derivation. In future study, more attention will be paid to the detection of the
real-world micro fatigue-induced debonding by investigating the nonlinear acoustic
phenomena in the signals, such as higher harmonic generation and mixed frequency
response. A reliable NDT method based on the guided wave technique will be
developed for continuous detection of the initiation and growth of actual debonding in
FRP-reinforced steel structures.
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