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j.compstruct.2018.08.024

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Accepted Manuscript
Rapid guided wave inspection of complex stiffened composite structural components using non-contact air-coupled ultrasound
Rabi Sankar Panda, Prabhu Rajagopal, Krishnan Balasubramaniam
PII:
DOI:
Reference:
S0263-8223(18)32130-5
https://doi.org/10.1016/j.compstruct.2018.08.024
COST 10074
To appear in:
Composite Structures
Received Date:
Revised Date:
Accepted Date:
12 June 2018
30 July 2018
14 August 2018
Please cite this article as: Panda, R.S., Rajagopal, P., Balasubramaniam, K., Rapid guided wave inspection of
complex stiffened composite structural components using non-contact air-coupled ultrasound, Composite
Structures (2018), doi: https://doi.org/10.1016/j.compstruct.2018.08.024
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Rapid guided wave inspection of complex stiffened composite structural
components using non-contact air-coupled ultrasound
Rabi Sankar Panda*, Prabhu Rajagopal, Krishnan Balasubramaniam
Centre for Nondestructive Evaluation, Department of Mechanical Engineering, Indian
Institute of Technology Madras, Chennai-600036, Tamil Nadu, India.
*Email: rabisankarpanda@gmail.com
Abstract
This article demonstrates a rapid, fully non-contact inspection technique for a full-scale
complex composite structural component using air-coupled ultrasonic guided waves. The
presence of different features such as stiffeners, stringers and geometric variations in skinstiffened structures makes the received guided wave signal cumbersome and difficult to
interpret. Experiments, supported by three-dimensional finite element models, are used to
demonstrate the physics of guided wave interaction with complex features and defect
configurations. B-scans are used to detect geometric variations in skin, and also disbonds in
the
skin-stiffener
interface.
Correlation
between the
numerically
simulated
and
experimentally obtained B-scans is established. Different regions in the B-scan images could
be used to locate and identify the defects and geometric variations in the test sample. The size
of the disbond can also be computed from the B-scan.
Keywords: Stiffened composite structure; disbond; B-scan; air-coupled ultrasound; guided
waves; finite element method
1
1. Introduction
Composite structures are used extensively in various applications especially in aerospace, and
both civil and military. The presence of different sub-components makes these structures
more complex, introducing hidden/inaccessible regions making their inspection challenging.
This study focuses on the health monitoring of a full-scale stiffened composite structure used
in the wing assembly of an aircraft, shown in Fig. 1. The component was sourced from an
Indian aircraft industry.
Fig. 1. Photograph showing different features of the test specimen (a) outer view; and (b) inner view.
The test specimen has different features such as skin, bends and stiffeners. Often, the
stiffeners (usually I-, T-, or hat shaped) are co-cured or co-bonded to the skins for lightweighting and achieving rigidity in bending. However, these structural features are highly
susceptible to interface defects such as disbonds or delaminations which may occur during
manufacturing or due to in-service loads such as impact or fatigue. Disbonds/delaminations
considerably reduce the strength and stiffness of these structures. The major concerns for
these structures are the initiation and growth of failure in the form of a disbond between
adherends in bonded joints and delaminations within the skin laminates, which may not be
visible and accessible for inspection from the skin side of the panel.
Thus, nondestructive evaluation (NDE) methods are needed for detailed inspection of
these structures to detect hidden damages efficiently. The ultrasonic technique is one of the
2
most widely used damage detection technique for composite inspection because of its
sensitivity to small damages and also capable of penetrating into internal laminates. The
existing ultrasonic NDE methods are time-consuming involving a point-by-point scan using
bulk waves and sometimes are not feasible where critical inner regions are inaccessible for
probes. Hence, a rapid screening and inspection technique is preferable to overcome these
shortcomings. Ultrasonic non-contact guided wave techniques are promising for rapid
inspection of composite structures [1,2].
Lamb waves are guided ultrasonic waves which propagate in plate-like structures and
have been found suitable for rapid damage detection in composite structures [3–7]. These
waves provide information about the integrity of the structure along their path of propagation.
Hence, these waves can be used for NDE as well as structural health monitoring (SHM) of
composite structures [8–16]. However, in a stiffened composite panel, the occurrence of
multiple Lamb modes becomes unavoidable with different levels of complexities in the
structure. Also, the wave attenuation is more due to material damping as well as the damping
caused by stiffeners, spars, ribs and geometric discontinuities (such as thickness variations,
bends, etc.). So, this makes the interpretation of received Lamb wave signal more difficult.
Therefore, it is necessary to understand the interaction of Lamb waves with various damages
and discontinuities in these structures. Several ultrasonic non-contact damage detection
techniques are available, and among these, the air-coupled ultrasonic testing (AUT) technique
has shown potential for non-contact inspection of built-up structures [17–19]. The advantages
of AUT include rapid automated scanning, excitation of a pure Lamb wave mode [11,20,21]
inspection of irregular and inaccessible regions. Also, it can be rapidly deployed in the field
and applicable to structures under harsh environments.
The following literature is relevant to understanding the overall nature of the work
presented in this paper. Ramadas et al. [22,23] studied the interaction of the anti-symmetric
3
A0 mode with a structural discontinuity in a co-cured T-joint section composite structure.
They also observed the generation of a mode converted S0 mode and “turning mode” at the
junction. Padiyar et al. [24] investigated the propagation of incident S0 Lamb mode in a
typical composite T-joint with delamination between the flange and skin interface. They
established a quantitative B-scan imaging method for characterization of delamination in the
T-joint. Flynn et al. [25] showed the local wavenumber estimation could detect local impact
delamination in a complicated composite component. Tian et al. [26,27] presented
wavenumber-based methods to quantify impact induced delamination damage which can be
comparable to the shape and size results obtained from ultrasonic C-scan imaging. Ricci et al.
[28] proposed a damage identification technique that uses bonded PZT patches to generate
and record responses in a stiffened composite panel. They presented an analysis of the
reflection and transmission of guided waves when they interact with discontinuities such as
delamination and disbond. Kudela et al. [29] studied the interactions of guided waves with
delamination in composite panels, a panel with a honeycomb core, and real stiffened
composite panel from the aircraft. The authors proposed a new signal processing algorithm
for the damage detection, and that was limited to panels of uniform thicknesses. Philibert et
al. [30] studied the wave propagation in a composite T-joint using dispersion and tuning
curves, and also detected low energy impact damages with bonded PZT crystals by
comparing Lamb wave signals between the pristine structure and the damaged structure.
The studies mentioned above either use bonded PZT patches or contact probes as
transmitter/receiver and mostly a single beam scanning laser Doppler vibrometry (SLDV) as
a receiver. The data collection using SLDV requires high precision and also slow point-bypoint data collection. The bonded PZTs may deteriorate over time. The signal processing
algorithms were limited to uniform thickness panels. This paper is set in the context of the
need for a rapid, automated, and fully non-contact damage detection technique.
4
The goal of this paper is to demonstrate a rapid, fully non-contact technique for
inspection of realistic complex composite components, using air-coupled guided ultrasound.
The preliminary work is reported in previous work by the authors group; see Panda et al. [31]
and Dileep et al. [32]. The experimental results are supported by finite element (FE)
simulation studies, which provide insight on guided wave interaction with complex defect
configurations. The advantage of this technique is that it does not require any sensor
installations, which are expensive, labor-intensive and also deteriorate over time. Structural
damage in each region of the test specimen is proposed to be detected acquiring a B-scan map
[13,18,24,33,34] obtained in the pitch-catch arrangement. The studies are carried out in the
time domain, and frequency domain (through fast Fourier transform, 2D-FFT [35]) to extract
features and are compared for healthy and damaged regions. Low-frequency, dispersive
fundamental anti-symmetric (A0) Lamb wave mode is used for damage detection. Results
demonstrating Lamb wave interaction with various regions of the test specimen are presented
in terms of B-scan images. Three-dimensional (3D) FE simulation models, validated by
experiments are used to study the Lamb wave interaction with different geometric
complexities in the skin region and the bonding between skin and stiffeners.
This paper is organised as follows. Section 2 gives a brief theoretical background to the
method used, along with an introduction to the problem studied. The details of the methods
such as experimental descriptions for inspecting the skin and skin-stiffener bonding, and 3D
FE simulations are given in Section 3. Experimental results are presented in section 4. The
behaviour of guided wave interaction with geometric variations in skin region as well as with
the disbond in the skin-stiffener interface is discussed using 3D FE analysis in section 5. The
paper concludes with directions for further work in section 6.
5
2. Theoretical background
2.1. Selection of frequency, and mode of excitation
Lamb wave interaction with various types of defects has been investigated by many
researchers [1,10,11,25,26,28]. The choice of the excitation frequency and mode selection
plays a vital role in damage detection. Lamb waves are generated and recorded in the
structure using non-contact air-coupled transducers. Low-frequency excitation of 100 kHz
was considered in the study, to minimize the number of modes and wave attenuation in the
received signal. In this frequency range, only the fundamental symmetric (S0) and
antisymmetric (A0) modes exist. Here, we consider the antisymmetric, A0 mode for damage
identification because of its shorter wavelength, permitting the study of smaller features and
also can be easily generated and received by air-coupled transducers. The optimal incident
angle θ for generating and receiving the A0 mode was calculated from the dispersion curves
[36] and Snell’s law. It was found to be approximately 170 at the center frequency of 100
kHz.
 cair 

 c A0 
  sin -1 
where
cair
is the phase velocity of the wave in air and
(1)
c A0
is the phase velocity of the
selected A0 mode, m/s.
2.2. Problem studied
The authors are interested in the inspection of the real-scale composite structures with
varying levels of complexities as illustrated, for example, in Fig. 1. For the studies in this
paper, a real full-scale component of the wing assembly of aircraft was used. Experimental
results show the integrity of the whole structure including bonding between skin and
stiffeners, bend region, and body of the stiffeners, which are not accessible from outside.
Finite element (FE) simulation validated by experiments, gives an insight into the Lamb wave
6
propagation and interaction with discontinuities in the skin and skin-stiffener bond regions.
The basis for the study is based on the comparison of the wave signal actuator-receiver
detected on a pristine condition/healthy region of the component with the signal detected
after damage/defective region of the test sample. Differences in the two signal intensities in
the time domain are used to denote the presence of the defect in the actuator-receiver path.
This technique is used to approximately locate and characterise delamination, or debonding
in a stiffened composite test specimen. More details of this study are given in Results and
Discussion sections.
3. Methods
3.1. Experiments
3.1.1. Specimen description
The test specimen was a single carbon/epoxy composite panel, as shown in Fig. 1, with upper
and lower skins, supported by I-shaped stiffeners. The panel has a total length of 1260 mm, a
width of 480 mm and varying stiffener pitches of 120 mm (between 5th & 6th, 6th & 7th), 145
mm (1st & 2nd, 2nd & 3rd, 3rd & 4th), and 270 mm (4th & 5th), respectively. The positions of the
stiffeners are marked on the specimen. The skins are of non-uniform thickness, varied from
2.4 mm to 5 mm, as shown in Fig. 1. The stiffeners are of 2.4 mm thick, tapered in nature
with 45 mm height at the open end of the panel and 20 mm at the closed end (i.e. at the bend).
The stiffeners are bonded to the skins with two 40 mm wide upper and lower flanges. The
bend is of 20 mm width and runs along the length of the panel.
3.1.2. Procedure for experiments
A photographic representation of the experimental setup used for the study is shown in Fig. 2.
The experiments were performed in a custom-made C-scan like setup. The experimental
setup consists of a custom-built 3-axis scanner, a pulser-receiver, a 50-MHz A/D card, a
motion controller and a personal computer. Two air-coupled transducers were used to inspect
7
the specimen. The transmitter and receiver (NCG-100-D25, the Ultran Group, New York,
USA) were employed in pitch-catch configuration to generate and receive A0 mode Lamb
wave. Both the transducers have a circular active area with a diameter of 25 mm and a central
frequency of 100 kHz. The air-gap between each transducer and the sample was
approximately 10 mm, corresponding to the best signal amplitude of the received A0 wave
mode.
Fig. 2. Photograph of the experimental setup used for the study.
The excitation pulse was a five-cycle toneburst modulated with Hanning window.
Data acquisition and signal processing program was used to filter and average the captured
data to improve the signal-to-noise ratio of the digitized signal. The responses from each data
point were measured 16 times and averaged in the time domain. A bandpass filter ranged
from 30 to 500 kHz was employed to limit the bandwidth of the output signal. The
transmitter-receiver pair with a fixed distance between them was moved along a line to
collect the data (A-scans) at a resolution of 0.5 mm. The data were then processed using a
MATLAB™ program to generate the B-scan images of the inspected region. In the B-scan
image, the variation of signal amplitude gives the approximate position of the
damage/discontinuity in the structure. The configurations of the transmitter-receiver pair for
inspection of the skin and skin-stiffener bonding are shown below.
3.1.2.1. Inspection of skin
8
Fig. 3 shows the photograph of the setup and schematic illustration for inspection of the skin
of the test specimen. The transmitter-receiver pair was employed in pitch-catch mode along
the length of the skin. The distance between transmitter and receiver was fixed at 300 mm for
full-length scan of the skin; however, it was 150 mm for each sub-region (i.e. between
stiffeners 3rd and 4th, 4th and 5th). The probe separation distance was reduced to half to reduce
the size and total run time of the 3D FE models, as a large number of models are needed in
order to generate a single B-scan. The exciting and receiving angles of the transducers were
set at approximately 170 with the surface of the skin to excite and receive A0 wave mode.
The transmitter is marked as “T” and receiver as “R”. White solid lines correspond to the
position of stiffeners in the sample.
Fig. 3. (a) Photograph; and (b) schematic illustration of the setup for inspection of the skin
region.
3.1.2.2. Inspection of skin-stiffener bonding
Fig. 4 depicts the photograph of the setup and schematic illustration used to inspect the
bonding quality between skin and stiffener. The transmitter-receiver pair was employed in
pitch-catch arrangement and moved along the bondline, along the width of the sample.
Guided waves were generated and transmitted across the bondline. The bondline between
9
each stiffener and skin was scanned separately, and results are then compared to find the
good and defective bonding in skin-stiffener assembly.
Fig. 4. (a) Photograph; and (b) schematic illustration of the setup used for inspection of
bonding between skin and stiffener.
The interaction of guided waves with geometrical features such as a varying thickness
in the skin, skin-stiffener assembly and damages lead the experimental signals difficult to
interpret. Some regions in the B-scan image (shown in ‘Results’ section) cannot be
explainable. In order to identify and understand the guided wave modes interaction with such
features, 3D FE models were used.
3.2. Finite element simulations
3.2.1. Procedure for 3D FE simulations
3D FE simulations were carried out using commercially available package [37] to predict the
A0 Lamb wave propagation behaviour in the test specimen. Since the specimen has different
regions with various levels of complexities, so different numerical models were modelled and
analysed separately. The material used for each model was carbon fabric and epoxy resin as
reinforcement and matrix, respectively. An eight-layer quasi-isotropic laminate with ply
layup [0/45/-45/90]s was considered for modeling each region of the specimen. The
mechanical properties of the materials used in the simulations are extracted from Prabhakaran
10
et al. [38] study and are presented in Table 1. The material for noodle region was taken as
epoxy with properties, elastic modulus, E = 10.1 GPa, Poisson’s ratio,  = 0.31 and density, 
= 1300 kg/m3.
Table 1
Material properties of lamina.
E xx (GPa)
E yy (GPa)
 xz
 yz
G xz (GPa)
 (kg/m3)
147
8.17
0.317
0.317
2.42
1550
Each lamina was modelled individually and properties were assigned depending on
the layup. For simplicity, viscoelastic material properties were not considered in the models.
Different parts of the model were assembled by using the surface-based tie constraints on the
interfaces to enforce the continuity of displacements and stresses. In each case the element
mesh size and time step size were calculated using Ref. [39]. A toneburst of 5-cycle,
modulated with Hanning window pulse, 100 kHz center frequency was used to excite A0
mode in the specimen. The excitation frequency was so chosen, to ensure that only
fundamental modes (A0 and S0) exist. Usually, acousto-elastic coupled field simulations are
used for air-coupled transmitters and receivers [24]. However, direct structural simulations
show results in accordance with the acousto-elastic simulations and takes less run time.
Therefore, out-of-plane nodal displacements were applied to the surface nodes directly to
generate Lamb waves.
3.2.1.1. Finite element simulation of the skin region
The skin of the test specimen has geometric variations between stiffener positions. So, we
have modelled two sections: 1) region between stiffeners 3rd and 4th (number marked in the
specimen), uniform thickness skin; and 2) region between stiffeners 4th and 5th, varying
thickness skin.
3.2.1.1.1. Region between stiffeners 3rd and 4th
11
The schematic layout of the region is shown in Fig. 5. It has two stiffeners (marked 3 and 4 in
Fig. 5) with a 3.2 mm uniform thickness skin. The stiffener pitch and thickness of skin and
stiffener are the same as those in the experimental sample. The length and width are so
chosen that the reflected waves from the model edges can be avoided. The center-line
distance between stiffeners 3rd and 4th is 145 mm, with a 20 mm overlap of stiffener flanges
from both ends. A line excitation and reception of 25 mm each were chosen as a transmitter
(T) and receiver (R), which is similar to the dimension of the probes used in the experiment.
The out-of-plane nodal excitation was given on the transmitter line segment and responses
were recorded on the receiver line segment. The distance between transmitter and receiver
was fixed at 150 mm. A number of models were modelled by changing the position of
transmitter-receiver pair from position (A-B, as in Fig. 5(a)) on stiffener-3 to position (D-C)
on stiffener-4 with 1 mm scan resolution. At each position, the responses were recorded as Ascans and then processed in MATLAB with a customized program to generate B-scan. The
model was discretized using 8-noded linear hexahedron 3D solid brick elements (C3D8R)
having three degrees-of-freedom at each node, except 4-noded linear tetrahedron 3D solid
brick elements (C3D4) in the noodle region. The size of the elements was 0.4 mm in the
thickness direction and 1 mm (≈ λ/13) in both length and width directions. The time step
chosen was 10 nano seconds.
12
Fig. 5. Schematic layout of the region between stiffeners 3rd and 4th: (a) top & front view of the skin;
(b) side view showing stiffeners; and (c) side view showing noodle region in the stiffener (red line).
3.2.1.1.2. Region between stiffeners 4th and 5th
The schematic layout of the region is shown in Fig. 6. It has two stiffeners (marked 4 and 5 in
Fig. 6) with varying thickness from 3.2 mm (thin region in Fig. 6(a)) to 5 mm (thick region in
Fig. 6(a)) thickness skin. The center-line distance between stiffeners 4th and 5th is 270 mm,
with a 20 mm overlap of stiffener flanges from both ends. The other parameters and
procedure are similar as in the case of stiffeners 3rd and 4th.
13
Fig. 6. Schematic layout of the skin region between stiffeners 4th and 5th: (a) top & front view
showing thickness variation; and (b) side view showing stiffeners.
3.2.1.2. Finite element simulation of the skin-stiffener bonding
The schematic layout of the region is shown in Fig. 7. It consists of skin with a stiffener. The
dimensions were long enough to avoid the edge reflections. The skin thickness was 2.4 mm
with 8-layers each of 0.3 mm thickness. Two different cases were considered: (a) model
without disbond, and (b) model with a disbond in the interface between skin and stiffener.
The disbond was of 40 mm length and 100 mm width and positioned as shown in Fig. 7. The
disbond was modelled by disconnecting the surfaces of adjacent layers of skin and stiffener
imitating an air gap. The initial position of the transmitter-receiver pair is shown in Fig. 7.
The distance between transmitter and receiver was fixed at 110 mm. Several models were
modelled and analysed by changing the transmitter-receiver pair from A-D to B-C in Fig. 7
with 1 mm scan resolution. The mesh size, incremental time step and other parameters are
similar as used in section 3.2.1.1.1.
14
Fig. 7. Schematic layout of the skin-stiffener bonded model: (a) top view showing the
disbond position in the skin-stiffener interface; and (b) side view showing the stiffener.
4. Results
This section presents the experimental results for skin region and skin-stiffener bonding of
the test specimen. The results are obtained with fully non-contact air-coupled ultrasonic
technique. The position of the defects and geometric variations in the test specimen are
confirmed by conventional bulk wave immersion C-scan technique.
4.1. Water immersion C-scan
To ascertain the integrity of the test specimen, it was inspected using ultrasonic C-scan
through water immersion technique. Bulk waves were excited in the pulse-echo configuration
of the Panametrics probe with 15 MHz center frequency and with Olympus Focus LT module
for data acquisition. Scanning was performed at 250 mm/s rate. The obtained results
corresponding to the scan area of the skin region is shown in Fig. 8. As seen in the C-scan
result in Fig. 8(b), the signal response in stiffeners-1 and 2 is better (see color scale) than the
other stiffener regions of the test specimen. Since the scan was carried out with pulse-echo
configuration, at stiffeners-1 and 2, much of the energy was reflected back to the probe from
15
the skin-stiffener interface conforming the position of disbond. These disbonds may be due to
poor curing of the adhesive or improper surface preparation. The skin thickness variations
and the position of stiffeners can be seen from the color variations in the C-scan result.
Fig. 8. (a) Photograph of the skin region of the test specimen; and (b) water immersion C-scan result.
4.2. Non-contact air-coupled inspection
4.2.1. Inspection of the skin
The fundamental A0 mode was used for the study and was selectively excited at the incident
angle of 170 of the air-coupled probes in the pitch-catch arrangement. The group velocity of
the A0 mode in the regular thin-skin region was 1250 m/s. The B-scan result of the skin
region obtained by scanning the sample across its length is shown in Fig. 9. The positions of
the stiffeners can be found in the B-scan result (numbered in Fig. 9(b)). The arrival time of
the selected A0 mode was calculated and is marked with a rectangular box in Fig. 9(b). The
A0 mode arrival time is approximately 300 μs for a probe separation of 300 mm. The signal
response in the skin region is much higher than the stiffener regions due to the less thickened
skin compared to stiffener regions. Also in stiffener region the signal leaks into stiffener from
16
the skin which makes the signal response still feeble. In contrast to healthy stiffeners, the
stiffener-1 (encircled in Fig. 9(c)) with disbond produces a higher response, which indicates
that the damage is present in the wave path. The higher response is due to the disbond
between the skin-stiffener interface, for which most of the signal is coming directly to the
receiver on the skin instead of going into the stiffener. A small shift in time-of-flight of the
A0 mode can be observed which is due to the uneven surface of the skin and also a slight
misalignment of the probes.
(a)
(b)
17
(c)
(d)
Fig. 9. (a) Photograph of the skin region with the position of transmitter-receiver
pair, scan length in red arrow; (b) B-scan result; (c) zoomed view of the rectangular
region in Fig. 9(b) for A0 mode arrival; and (d) energy variation across the length of
the skin.
The energy variation across the length of the skin is shown in Fig. 9(d). The energy
was calculated by the square of the maximum amplitude of the wave across the length of the
skin. The disbonded stiffener-1 and the region with thickness change has better energy
compared to other stiffeners. From the B-scan result, it is clear that the skin has geometric
variation regions. The wave signatures are different at each region of the skin. The interaction
of A0 mode Lamb wave with these geometric variation regions are also studied, and the
results are given in the below sections.
18
4.2.1.1. Region between stiffeners 3rd and 4th
This region of the skin has a uniform thickness of 3.2 mm. The B-scan result corresponding
to this region is shown in Fig. 10. There is an amplitude change across the length of the skin.
In spite of the uniform thickness throughout, there are regions of low amplitude and a slight
time change observed in the B-scan result. The reason is explained in the ‘Discussion’
section. In both stiffener regions, less amplitude is due to the combined thickness of the skin
and stiffener.
Fig. 10. B-scan result of the skin region between stiffeners 3rd and 4th.
4.2.1.2. Region between stiffeners 4th and 5th
The thickness of the skin varies from 3.2 mm to 5 mm in this region. The scan result is shown
in Fig. 11. As mentioned in Fig. 11, the thickness of the thin region is 3.2 mm, the thick
region is 5 mm, and it uniformly varies from 3.2 mm to 5 mm in the transition region. Since
the thickness varies, the change in wave amplitude and velocity are expected and can be seen
in Fig. 11. The amplitude in the thin region is more because the probes are set to excite and
record the maximum amplitude according to the thickness of the thin region. The amplitude
in the thickness transition region is minimal and is explained in the ‘Discussion’ section. The
velocity variation can be explained from the time lag by considering the reference lines. It is
known that the A0 mode velocity increases as the thickness increases. The wavefront bends
19
as it moves from thick to the thin region and vice versa. As the probe unit moves from thick
(stiffener region) to thin skin region, A0 mode velocity is less in the thin region as compared
to its velocity in the thick region. Reduced wave velocity in the thin skin region caused A0
mode to take more time to reach the receiver as compared to that in the thick stiffener region.
This time lag can clearly be seen in the form of inclined lines sloping towards the right in half
portion of the thin region as shown in B-scan. The maximum time lag was found to occur
when the probe unit was in the middle of the thin skin region. As the probe unit traversed
from thin skin region towards thickness transition region, the time lag changes and it is
minimum in the thick region.
Fig. 11. B-scan result of the skin region between stiffeners 4th and 5th.
4.2.2. Inspection of skin-stiffener bonding
The bonding between skin and stiffener was also checked using the scans along the stiffener
length. The position of the transmitter-receiver pair is shown in Fig. 12(a). The red arrow
shows the scan direction. Guided waves were generated and propagated across the skinstiffener bondline. The B-scan results showing the acoustic response of the healthy stiffener
(stiffener-3) and disbonded stiffener (stiffener-1) are presented in Fig. 12. It can be noticed
from the B-scan results that the signal response of the A0 mode in the disbonded stiffener is
better than the healthy stiffener.
20
(a)
(b)
(c)
Fig. 12. (a) Photograph of the skin region with the position of transmitter-receiver pair, scan
length in red arrow; B-scan results: (b) without disbond; and (c) with disbond.
21
5. Discussion
Thus for the paper discussed the A0 mode Lamb wave propagation and interaction with
disbonds in the stiffened test specimen. In this section, the physics of the mode interaction
with geometric variations and disbonds is studied in more detail using finite element analysis
results.
5.1. Skin region
5.1.1. Region between stiffeners 3rd and 4th
The B-scan result obtained for the skin region between stiffeners 3 rd and 4th is shown in Fig.
13(b) corresponding to the scan region, red arrow line in Fig. 13(a).
Fig. 13. (a) Snapshot of the FE model between stiffeners 3 rd and 4th; and (b) B-scan result
corresponding to the red arrow line.
The result shows the wave amplitude variation in the skin region. The less amplitude
in stiffener regions shows that the wave leaks into the stiffener. Also, there is a significant
change in amplitude (low) in some areas, approximately at 50 mm and 95 mm from the
22
stiffener-3 as shown in the B-scan result. We believe that this could be due to the destructive
interferences between the incident wave and the reflected wave from stiffeners. The
amplitude variation can be more clearly seen in Fig. 14. Also, there is bending of wavefronts
in the skin region towards the stiffeners, and this is because of the thickness change as the
probe unit moves from thick to thin and thin to the thick region. This can be seen in the form
of inclined lines sloping towards the right near stiffener-3 and towards left near stiffener-4.
Fig. 14 shows the comparison of the experimental and numerical normalized B-scan
results obtained for the skin region between stiffeners 3 rd and 4th. They are in good
agreement, thus validating the numerical model. The slight variation in the results is perhaps
due to the model assumptions such as not considering the viscoelastic properties and also in
experimental measurements.
Fig. 14. Comparison of experimental and numerical results for the skin region between stiffeners 3rd
and 4th.
23
5.1.2. Region between stiffeners 4th and 5th
Fig. 15 delineates the B-scan result obtained for the skin region between stiffeners 4th and 5th.
It can be seen from the B-scan result that the signal response varies with the thickness. In the
thickness transition (uniformly varied from 3.2 mm to 5 mm) region, low amplitude zones are
observed that could be due to the destructive interferences between the incident wave and
reflected waves from the stiffeners. There is also velocity and phase change observed across
the skin due to thickness variation which causes the time lag at the receiver location. The
wavefront bending was also observed and explained in Section 4.2.1.2.
Fig. 15. (a) Snapshot of the FE model between stiffeners 4th & 5th; and (b) B-scan result
corresponding to the red arrow line.
Since there is geometric variation in this region, to understand the modes present in
the propagated wave packet, the 2D-FFT technique was used. For different thickness (3.2 mm
- 5 mm) regions in the skin, the 2D-FFT technique was applied separately. The transmitter
24
was fixed, and multiple receivers were taken as shown in Fig. 16(a) to plot 2D-FFT. The
calculated wavenumber-frequency spectra are shown in Fig. 16 and compared with predicted
dispersion curves of plate modes obtained using DISPERSE software [36].
Fig. 16. (a) Schematic of the skin showing different positions (thickness variations) for 2DFFT plot; wavenumber-frequency spectra of the propagated wave field at (b) position P1; (c)
position P2; and (d) position P3.
It can be observed that the A0 mode is the dominant wave mode present in the wave
packet. The velocity of the A0 mode for different thickness regions was calculated by
obtaining the wavenumbers separately. The velocity variation is given in Table 2.
Table 2
Velocity variation with thickness.
Position
Thickness (mm)
P1
3.2
P2
4.1
P3
5.0
Wavenumber (mm-1)
0.08182
0.08091
0.07307
Velocity (m/s)
1222
1235
1369
25
The comparison of the experimental and numerical B-scan results obtained for the
skin region between stiffeners 4th and 5th is shown in Fig. 17. There is a good agreement
between measured and predicted results. The variation in the results could be due to the
material properties considered for the numerical model, slight geometric variation between
the actual test sample and the numerical model, and also in experimental measurements.
Fig. 17. Comparison of experimental and numerical results for the skin region between stiffeners 4th
and 5th.
5.2. Skin-stiffener bonding
The interaction of A0 mode Lamb wave with skin-stiffener bonding was studied for two
cases. In the first case, the skin-stiffener bonding was intact, and a disbond of 40 × 100 mm2
in the skin-stiffener interface was considered in the second case. The schematic of the
numerical model is shown in Fig. 7. The B-scan results for intact and disbonded stiffeners are
presented in Fig. 18.
26
(a)
(b)
Fig. 18. B-scan results (a) intact stiffener; and (b) disbonded stiffener.
It can be seen from the B-scan result for intact stiffener that there is a trailing mode
present along with the excited A0 mode, which is absent in the disbonded stiffener. This
mode generates when the incident wave which propagates on skin region comes in contact
with the stiffener. With the presence of disbond, the skin is separated from the stiffener at the
disbond region. So, the incident wave propagates only in the skin region and the same was
received at the receiver location. Also at the start and end of the disbond, bending of the
wavefronts was observed due to thickness change between the healthy and disbonded region.
This zone is marked with dotted lines in Fig. 18(b), which gives approximately the size of the
disbond. The amplitude and wavelength change can also be noticed in the disbond region.
27
6. Conclusions
This article presents the results achieved by a guided wave inspection system for features in
complex shaped composite structures. The system is based on the acquisitions of information
through a fully non-contact air-coupled scanning technique. The aim was to develop a
screening tool for rapid interrogation of the complex structures. The test specimen had
different levels of structural changes and subcomponents. Different configurations of aircoupled ultrasonic transducers in the pitch-catch mode were used to inspect the skin region
and the skin-stiffener bonding. The fundamental A0 mode was selectively excited by
adjusting the incident angle of the transducers, and data were collected as B-scans to speedup the process. The A0 mode interaction with geometric changes in skin region and disbonds
in the skin-stiffener interface was investigated using 3D FE simulations, and validated by
experimental measurements. Frequency-wavenumber domain analysis (2D-FFT plots) was
used to understand the Lamb wave propagation in complicated geometries and to identify the
modes present. The B-scan results in skin region show that the signal responses such as
amplitude and velocity are different for different regions which can be used to determine the
geometric variations. The wave signals for disbonded stiffener were significantly different
from those recorded for an intact stiffener. Observations from numerical simulations are in
good agreement with experimental measurements, which validates the proposed guided wave
technique. Thus, the proposed technique can be attractive for detection of hidden skinstiffener disbonds in complex structural components through guided ultrasonic waves.
Ongoing research is examining for defect detection and characterization in other regions of
the test specimen and also the development of better signal processing algorithms to enhance
the results.
28
Acknowledgment
The authors would like to gratefully acknowledge the support received from the NPMASS
program managed by The Aeronautical Development Agency (ADA) Bangalore, India.
Additionally, the authors are grateful to Mr Dileep Koodalil, Mr Siddharth G of IIT Madras,
India and Mr Oleksii Karpenko, Prof Lalita Udpa and Dr Mahmoodul Haq from Michigan
State University, USA for the helpful discussions.
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