Shearographic detection of delaminations in sandwich structures:
investigation on various excitation modes
G.Kalogiannakis1,2, B. Sarens2, D. Van Hemelrijck1 and C. Glorieux2
1
Dept of Mechanics of Materials and Constructions,
Vrije Universiteit Brussel, Belgium
2
Laboratory of Acoustics and Thermal Physics,
Dept of Physics and Astronomy,
Katholieke Universiteit Leuven, Belgium
ABSTRACT
Sandwich structures are increasingly used in engineering constructions wherever the design specifications demand the
combination of light structures with high bending stiffness. The most common defects of such structures are the so-called
delaminations, which occur between the core and the skins and originate from existing manufacturing flaws or the loading
conditions. This study was focused on the detection of such defects with digital shearography in a dynamic mode compared to
competitive nondestructive testing (NDT) techniques. The material under investigation was Polymethacrylamide rigid foam
used as core reinforced with woven as well as unidirectional (UD) Carbon/Epoxy layers.
Introduction
Sandwich materials are light weight structures, which are especially oriented to satisfy design requirements for high bending
stiffness. To this end, the core, a typically rather thick material of low density and strength, is combined with thin (and therefore
low bending stiffness) skins of high tensile strength [1]. It is evident that a critical factor for the performance of the sandwich
structure is the quality of the adhesion between the skins and the core. Manufacturing flaws and/or compressive or impact
loads, which may increase dramatically the interlaminar shear stresses, may often initiate and result in the local debonding of
the layers. Such defects are very common in sandwich structures and they are so-called delaminations. The timely detection
becomes critical for the evaluation of the residual strength of the structural component as this defect may lead to local or
global buckling and eventually failure of the structure [2-5].
Consequently, various NDT methods are dedicated to the detection and characterization of delaminations. The methods are
generally divided in full-field and scanning depending on whether we can obtain directly (in a single step) the image of the
object under investigation or we have to scan in a raster-like manner over the surface. It is reasonable that, in the recent
years, the full-field techniques gain increasingly ground due to their efficiency and operation speed, especially for large
structures. Full-field techniques are basically transient [6-7], lock-in [8] and ultrasound lock-in thermography [9-10], as well as
the speckle pattern interferometric techniques [11-13]. In principle, the thermographic techniques make use of an IR camera to
detect the perturbations in the temperature field due to the disruption in the heat flow by the embedded delaminations while
the object is thermally excited. In the first two cases, the heat is generated through the surface by means of a laser or halogen
lamps and in the case of ultrasound lock-in thermography the heat is generated locally around the defects due to hysteresis
and friction by means of an ultrasonic welder or a shaker.
In contrast, speckle techniques, namely electronic speckle pattern interferometry (ESPI) and shearography make use of a
normal charge-coupled device (CCD camera) to record an image of the object before and after loading. They are based on the
speckle effect, which is a light interference phenomenon occurring when coherent light like a divergent laser beam is diffusely
scattered on an optically rough surface (roughness of the order of the optical wavelength). In classical interferometry, a laser
beam is passing through a beam splitter and one part is reflected on a fixed reference plate, while the other is reflected on the
object under investigation and the two beams are superposed on an optical detector. When the object is displaced along the
direction of the optical path, the phase difference between the two laser beams is accordingly changed resulting in a change of
the light intensity on the detector. In speckle techniques, the so-called objective speckles are formations of random optical
interference in space from light rays coming from different places on the object, and their size and intensity change with the
distance from the object. Such a speckled image pattern of the object under investigation is recorded using a camera with
each individual pixel acting as a separate optical detector, which probes the displacement of a respective area on the object.
Adjusting the aperture and the distance of the lens from the object controls the size of the recorded so-called subjective
speckle and therefore the size of the respective area on the sample. Subtraction of successive images before and after
loading results in a fringe pattern which is directly associated to the change of the optical path and therefore the absolute
displacement of the object under investigation.
The interferometric setup of shearography in particular consists of a rotating mirror. As a result the recorded image is the
superposition of two images slightly sheared with one another. Each pixel represents thus the relative position of two points
(slope) on the object before and after loading. Therefore, shearography is suited to measure deformations and strains rather
than absolute displacements. The advantage is that the technique has no need for a reference beam and the sensitivity to rigid
body motions is considerably suppressed. This is why the method is widely appreciated and well established in the industry.
The process of the evaluation of the fringe pattern (interferogram), although usually not required, is depicted in Figure 1. In
order to determine the absolute or relative phase difference of the different gray intensity levels, one has to apply the so-called
phase stepping. By applying successive changes of the optical path of the probing beam, one can evaluate the dependence of
the intensity on the relative phase. The resulting phase image has to be filtered and normalized before the demodulation
process where by taking a reference the phase is unwrapped. The result is integrated to give the deformation in one or two
directions.
Figure 1. The quantification process of the speckled fringe pattern.
In what follows, we shall demonstrate how the method was used in a dynamic mode in order to detect artificial delaminations
in composite sandwich structures and present the advantages as well as the disadvantages with respect to the competitive
lock-in thermography.
Material
Sandwich plates were manufactured using woven (Hexply 920CX-793-50%) as well as unidirectional (Hexcel - Fibredux
920CX-TS-5-42) Carbon/Epoxy prepregs for the skins and polymethacrylimide (PMI – Rohacell 71) as core. This type of
material was selected as it is typically used in advanced applications where any material failure is critical for the safety of
human lives. Regarding the manufacturing process, a lot of effort has been put recently in order to optimize the curing
procedure [14] based on the thermal and mechanical properties and their variance.
(a)
(b)
Figure 1. Specimens with embedded defects –dimensions are given in mm-.(a) sandwich plate (plate A) with circular
delaminations of different diameters, and (b) a plate with square defects of different materials (plate B).
Artificial delaminations of different geometries were created in the plates by placing thin Teflon films (thickness of 10μm)
between the core and the skins in order to evaluate the possibility to characterize the defects. Other types of defects like
inclusion of Al or St foils (thickness of 15μm) were also induced so as to compare the probability of detection for the different
methods under consideration. Since the plates undergo the curing procedure under high temperature and uniform pressure
the resulting surface is smooth and there is no externally visible indication of the delaminations. The exact dimensions of the
plates and the embedded defects under investigation are shown in Figure 1. The plate on the left had a single woven layer
while the one on the right had two UD layers symmetrically placed on either side.
Experiments
Experiments were carried out using two different setups related to shearography and lock-in thermography. The former setup
is depicted in Figure 2. In principle, for the implementation of stroboscopic shearography, the object under investigation is
illuminated at particular instances with a successive increment of the phase difference with respect to the initial point of the
excitation. By evaluating the deformation at each step with respect to the reference, one can thereby reconstruct the full
harmonic motion of the vibrating object. The setup also provides the flexibility for comparison with a scanning laser Doppler
vibrometer. It also allows for the simultaneous excitation with a shaker (Brüel & Kjær) and a piezoelectric transducer so as to
investigate nonclassical acoustic nonlinearities.
Figure 2. Shearography
Figure 3. Lock-in thermography
The setup which was used for the experiments of lock-in thermography is shown in Figure 3. A powerful (30W) diode laser
(λ=810nm – Coherent FAP I) is electro-optically modulated by means of an electrical signal that we provide through a data
acquisition card (DAQ NI PCI 6024E). The same signal is used as a reference for the lock-in procedure of the infrared images
which are recorded. In the alternative approach of ultrasound lock-in thermography the same signal can be used to modulate
the high frequency wave of an ultrasonic transducer.
Results
The composite sandwich plates were examined dynamically with shearography in a stroboscopic mode. Linear as well as
nonlinear approaches were used in order to evaluate the detectability of the defects, analyze the differences and compare the
results. In Figure 4, the results are shown for the excitation of plate A by means of the piezoelectric transducer at 10, 20 and
50kHz. The image is zoomed around the delamination of 28mm, with the circle indicating the underlying delamination. The
feature on the right hand side of the lower part is the piezoelectric transducer. To visualize defects in the linear regime, one
has to excite the object under investigation at one of the resonance frequencies of the thin upper layer lying over the
delamination. In Figure 4, it is apparent that for 10kHz the delamination is hardly visible while for 20 and 50kHz we obtain the
two first mode shapes which clearly indicate the position of the delamination.
(a)
(b)
(c)
Figure 4. Fringe patterns of the sandwich plate obtained with shearography. Excitation is carried out with a piezoelectric
transducer at (a)10, (b)20 and (c)50kHz.
A recent advancement [15] in shearographic visualization of defects is depicted in Figure 5. The so-called cross-modulation
effect due to the contact acoustic nonlinearity [16-20] is visualized in a single shot. The plate A was excited simultaneously by
the piezoelectric transducer as well as a shaker at 50 and 5 kHz respectively. The image is zoomed around to the
delaminations with diameters of 34 and 28mm. In principle, a strong low frequency wave acts as a pump opening and closing
the defect. It becomes thus a carrier for the high frequency wave, which is amplitude modulated. This is an effect which occurs
in such a case only above the defects and is a so-called nonclassical acoustic nonlinearity.
The piezoelectric transducer has the double size of the one related to Figure 4 (diameter of 4cm) and is located at the upper
right hand side. In the figure we observe that at 50kHz the defects are hardly visible while at 5kHz the defects have a certain
influence on the distortion of the fringe pattern giving an indication about the location of the delaminations. When we apply
both excitations at the same time and look at the images obtained for the sum or the difference of the frequencies, the
resulting patterns correspond directly to the underlying defects. What is clearly apparent from the image of the difference is
that the adhesion of the skin and the core between the two artificial delaminations is inadequate. This claim was examined and
confirmed with the laser Doppler vibrometer.
(a)
(b)
(c)
Figure 5. Fringe patterns of the sandwich plate obtained while it is excited simultaneously by a shaker and the piezoelectric
transducer at 5kHz and at 50kHz respectively. (a)image obtained with only 50kHz excitation, (b) image obtained with only
5kHz excitation and (c) image obtained with both excitations active and shearography locked at their difference.
A similar result with two frequencies which are closer to one another, 20 and 25kHz, is shown in Figure 6. The image is taken
with a reference signal at 45kHz. Apart from the two delaminations, one can see again the slight distortion of the fringe pattern
between them, which indicates the lack of good adhesion between the skins and the core.
Figure 6. Fringe pattern of the sandwich plate excited with a piezoelectric transducer at 25kHz and a shaker at 20kHz.
In order to compare the results presented above with other established techniques, we performed experiments with lock-in
thermography on the plate B. A thermal wave is generated by means of uniform heat excitation (laser) which is highly
attenuated. The penetration depth or thermal diffusion length is proportional to the thermal diffusivity of the material and
inversely proportional to the frequency of excitation. The contrast originates from the difference in the thermal resistance of the
inclusion with respect to the bulk material.
In Figure 7, one can see the images obtained at three different frequencies: 0.1, 0.068 and 0.05Hz. Both amplitude and phase
are depicted although for practical applications, phase angle imaging is preferable due to its insensitivity to variable surface
emissivity and non uniformity of heating and/or optical absorption. Evidently and expectedly, the results show that the metallic
inclusions provide higher contrast due to the great difference of its thermal properties with respect to the ones of the PMI and
the composite. On the other hand, poor contrast is obtained in such a case for the pure, closed or open air gaps or Teflon
pieces. This is normal as the thermal resistance of Teflon is very similar to the one of PMI and/or air.
(a)
(b)
(c)
Figure 7. Amplitude (upper) and phase (lower) images of the thermal wave at (a) 0.1, (b) 0.068 and (c) 0.05Hz.
Conclusions-Discussion
The increasing demands of the industrial world for efficient and fast (full-field) NDT methods led us to investigate and compare
the results obtained with two competitive methods. Both methods, shearography and lock-in thermography are highly
applicable in the industrial field due to their compactness and ease of use.
The images obtained with shearography are rather more difficult to interpret but the outcome of the investigation is that the two
methods have different potential. While shearography is more suited to evaluate delaminations, even when they are closed, in
the linear or nonlinear regime, lock-in thermography is rather better in the detection of inclusions with high thermal resistance
contrast. In the latter case, shearography typically provides no result because (depending of course on the dimensions of the
inclusion) the influence on the vibration characteristics is too small. On the other hand, lock-in thermography is less sensitive
to delaminations between the skin and the core of a sandwich material due to the similarities of the thermal diffusivity of air
and the underlying foam.
In the same framework, various nonlinear approaches using multiple excitations were investigated in the contrast creation and
enhancement of dynamic shearography. It has been shown that cross-modulation effects create better contrast and are easier
to interpret the fringe patterns so as to locate the defects.
In the near future, work has been scheduled to compare the results with ultrasound lock-in thermography as well as nonlinear
laser Doppler vibrometry so as to have a global overview of each NDT method potential and investigate possible optimization.
Acknowledgments
The authors would like to express their gratitude to the Flemish Institute for Science and Technology (IWT) for funding this
research.
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