RESEARCHING THE PROCESSES LEADING TO THE FAILURE OF
COMPOSITE SANDWICH STRUCTURES
A. Fergusson, A. Puri and Dr. J. Dear
Department of Mechanical Engineering
Imperial College London
SW7 2AZ
UK
[email protected]; [email protected];
[email protected]
A. Morris
Technical Head Integrity
E.ON UK, Power Technology
Ratcliffe-on-Soar
Nottingham NG11 0EE
UK
[email protected]
ABSTRACT
There is much interest in reducing the propensity for onset of failure processes in composite sandwich materials. Already,
composite sandwich materials are increasingly finding new applications and this will be more so if their risk of failure can be
reduced. Initial onset of a failure can sometimes be attributed to inherent shortcomings in the design and manufacture of the
composite sandwich panels. Mostly, other failures can be attributed to the way the sandwich panels are used in the structure.
The interest in this study is to simulate loading conditions that can be generated in composite sandwich panels when
employed in different engineering structures and to investigate causes for failure processes to be initiated in these materials.
The main aim of the research reported on in this paper was to study the development and distribution of strain in composite
sandwich panels when subjected to four-point flexure and the failure processes as they occur. In addition to this, another aim
was to develop the use of Digital Image Correlation (DIC) to assist in evaluating the integrity of different types of composite
sandwich structure and also for in-service monitoring of these panels. The overall aim of the research is to relate the onset of
failure processes in producing delamination and other types of failure.
Introduction
Composite sandwich materials are susceptible to different damage and defects depending on their design, manufacture and
use in structures. Also, the causes for the damage are many. Damage in wind turbine blades, for example, can occur from
events such as dropped tools during maintenance, or debris impacting during periods of high wind as well as long service
fatigue. Once initiated, damage can develop into delaminations, core disbonds and cracks at lower stress levels. The resulting
damage is often invisible to the naked eye and is mostly on the interior facings of the composite skins. Inflicted damage to
composite sandwich components by events such as mentioned above can also develop existing manufactured defects. These
manufactured defects and shortcomings can take the form of resin dry spots, skin-core disbonds and such features. Whilst the
cause and nature of defects are wide ranging, their effect can be similar. Namely, these often well-hidden defects can locally
reduce the stiffness of these structures and can be detrimental, to different degrees, to the functional integrity of the
component.
The experimental method used for this study was four-point flexure as this would theoretically produce a constant bending
moment between the two central rollers. It was important to have a central region of the specimen subjected to constant
flexural loading so that the effect of defects on flexure in sandwich panels could be examined consistently. Testing was initially
performed to appropriate standards [1-2], but subsequently the loading arrangement was changed to show more clearly
deformation processes [3]. Also, four-point loading allowed for observation of the area of the panels between the central two
rollers using DIC [4].
As with speckle interferometry, DIC uses a series of digital images of a surface under various levels of load, upon which a
monochromatic paint pattern has been applied [5]. The software divides the image into squares of pixels known as facets, from
which the inter-facet displacements can be calculated between load levels, as shown in Figure 1 below. For greater accuracy
the GOM ARAMIS software, [4], uses an averaging method based on a group of facets. The evaluated displacement matrix
can be differentiated to produce a strain map of the surface, as shown in Figure 2.
a
b
c
Figure 1. Example of DIC solution method: a, b and c represent progressive deformation levels [6].
Figure 2. Example of strain map produced as specimen is loaded, overlaid onto the specimen raw image.
Four Point Loading
Initial tests on foam core glass-fibre panels showed that conforming to ASTM C-393 test geometry produced high levels of
compressive strain underneath the inner rollers due to indentation. In order to combat this, a deviation from the standard was
taken by bringing the inner-rollers closer together, thus creating an increased bending moment for the same load level. The
specimen dimensions are given below in Figure 3. These were cut from a larger Autoclaved carbon-fibre pre-preg foam core
sandwich panel. Specifically the core consisted of Alcan Airex C70-90 foam, upon which the Hexcel Hexply® T300/913
unidirectional carbon-fibre pre-preg was laid, [7].
300 mm
Skin thickness:
0.5 mm
11 mm
135 mm
135 mm
P/2
P/2
Figure 3. Four-point specimen loading configuration.
The typical strain plots that were obtained from these experiments are given below in Figure 4, showing the bending strain
(%X) and crushing strain (%Y). Standard beam bending theory suggests that the bending strain for a symmetric sandwich
beam should be distributed with negative strain on the compressive face (bottom), going to zero somewhere near the centre
depending on the compressive behaviour of the faces, and from this point becoming more positive, reaching a maximum at the
outer edge of the tensile face [8-9]. Considering the plots of the bending strain along the specimen centre line (line A-A’ on
Figure 4) at various loading levels shown in Figure 5, it can be seen that the distribution is not as expected, especially at the
higher loads. These graphs are based on data extracted directly from the DIC plot, and they show that the maximum
compressive strain occurs in the core, rather than at the face, and that at the higher loads there is no apparent neutral axis.
B’
A’
0 kN
B
A
1.22 kN
1.72 kN
1.92 kN
1.96 kN
Y
X
%X Strain
%Y Strain
Figure 4. %X and %Y strain increase for carbon-fibre panels for increasing load levels.
Figure 5. Bending strain variation along specimen centre-line as load increases.
The reason for such a distribution is likely to be due to the fact that the skins are much stiffer than the core, and thus through
continuum effects will restrict displacement of the core near the top and bottom. Nearer the centre of the core this effect is less
prominent, so the displacement can be higher, and thus the maximum compressive strain occurs here. In addition to this, the
lack of apparent neutral axis is likely to be due to an interference effect arising from the rollers. This can occur if the rollers
indent into the specimen and thus confine the specimen from lateral motion, as demonstrated in Figure 6, which may also
explain why there is such an imbalance of compressive strain. This increased restriction would also cause the central section
to bow out with increasing force, an effect that is visible in the %X strain maps in Figure 4.
A’
A’
Neutral
Axis
A
A
Standard result from
Monolithic beam incorporating slip of neutral
monolithic beam
axis as a result of restrictive lateral motion
Figure 6. Schematic of bending moment distribution variation along specimen centreline.
What all this effectively signifies is that the specimen is not in pure flexure. To further understand this behaviour the strain data
has been extracted along a line above the roller, indicated as line B-B’ in Figure 4. This data is presented in Figure 7, where
the left hand graph represents the bending strain, and the right hand represents the vertical strain. The bending strain graph
shows an opposite effect to that seen in Figure 5, but this can be explained as being due to Poisson’s effect [10]. This is better
revealed when looking at the %Y plot, which almost mirrors the %X strain.
An interesting aspect regarding the %Y distribution is that the high level of compressive strain near the bottom is due to the
indentation of the rollers, adding grounds to the theory that lateral motion of the beam is restricted. Interestingly it can be seen
that the skin balances this out to a small degree as there is a small tensile load at the bottom. Another interesting aspect from
this graph is that there is a secondary region of high compressive strain near the top face, and it is believed that this is another
effect of the skin restricting the core deformation. This again highlights that pure flexure is not experienced by the beam.
However, one contrary result is shown in Figure 8, where it can be seen that the shear strain is zero in the centre of the panel,
which is a feature of flexural loading, indicating that some aspect of flexure was experienced by the panel. What this figure
also shows that there is no shear experienced by the skins with a strip of zero strain along the top and bottom at all stages,
which would be expected considering it is a very thin part with a high shear modulus across the layers. Further work will
include comparing these results with those obtained from monolithic beams and wholly composite beams to determine exactly
the contribution of the foam core.
Figure 7. %X and %Y strain variation along line B-B’ as load increases (see Figure 4).
0 kN
1.72 kN
1.92 kN
1.96 kN
Zero shear strain
along faces
Figure 8. Shear angle (degrees) distribution as load increases.
Non-Destructive Testing
One of the major benefits of using this optical based system is that it can reveal the effect of the exterior of a component and
from this assess the occurrence and severity of an internal defect. In relation to sandwich panels, an example of a typical
internal defect is a delamination between the skin and core [11]. Such a defect was simulated in an experiment by inserting a
cut just underneath the compressive face into the core of a four-point bend specimen, such that a small rectangular strip of the
compressive face would not be attached to the core, as shown in Figure 9 below. Such a defect will cause local buckling of the
compressive skin and it is this aspect that would be captured by DIC.
Position of inner rollers
15mm
d
a
h
g
i
j
e
f
Y
c
b
Region captured in photograph: efgh
30mm
Delaminated region: abcd
X
Figure 9. Photograph of delaminated four point sandwich panel specimen.
Camera
Field of view
Specimen
a.
b.
Figure 10. a) Cross-section view of camera orientation; b) Custom inner rollers to allow camera view.
The material used for this experiment was a Hexcel Fibrelam® Grade 1 [12], a mass production panel with 0/90 glass-fibre
faces with a medium density Aramid honeycomb core, of which the cell size is 3mm. With a skin thickness of 0.38mm, the thin
faces would encourage buckling. Unlike the previous testing, this panel was loaded as per the ASTM standard [1], with a span
of 100mm, and the inner rollers 50mm apart. The specimen width was 50mm, total thickness is 10.16mm and the total length
was 150mm.
%X -Strain
In this situation the concept of non-destructively testing requires that images were captured of the surface rather than cross
section, thus the pattern was applied to the compressive skin, as shown in Figure 9. The set up of the camera is shown in
Figure 10a, alongside the set of custom inner rollers that were produced to allow such a camera angle, Figure 10b. An
important aspect of this setting is that the angle produces an out-of-plane view, thus all the results presented are relative rather
than absolute. Regardless of this, the strain maps produced show that the DIC can clearly pick out the buckling effect of the
skin as compared to a control specimen without a delamination. This is shown in Figure 11, which gives both the %X and %Y
strain. Although there is a clear difference in both pairs, the buckling effect is most clearly highlighted in the %Y plots where in
the region of the delamination there is a tensile and compressive region separated by a section of zero strain. The graphs of
relative Y displacement in Figure 12 support these results, where the data has been extracted from the DIC results. As
expected the wavelength of this buckle is approximately the length of the inserted delamination, a result that was visible to the
eye at the highest loads. Interestingly Figure 12 also shows that the left hand side of the buckle is undergoing reduced
displacement, a result which is probably due to it touching the core, which is not restrictive for the right hand side.
Delaminated Region
%Y -Strain
Non-delaminated
Figure 11. DIC %X and %Y strain plots of non-delaminated and delaminated specimens, at 1.16kN.
Figure 12. Y-axis relative displacements along line i-j for increasing load levels (see Figure 9).
In addition to the strain maps presented in Figure 11, the results at the lower levels of load help to show that this mode of
testing is valuable. This can be seen from Figure 13 where the relative Y displacements are plotted for both the control and
delaminated specimens at the two lowest load levels. Firstly the graph shows that the delaminated specimen is experiencing
periodic wave-like displacement, whereas the control is not. Furthermore it can be seen that there is a significant difference
between the displacements for the two load levels for the delaminated specimen, as compared to the control.
The implication of these results is that the DIC surface strain measurement system could be used during controlled load tests
on composite sandwich structures, such as wind turbine blades, to give early indication of internal damage. Furthermore,
pattern recognition software could be used on collated data, such as that given in Figures 12 and 13, to detect the defect at
lower loads. In this way DIC can become an effective tool for non-destructive testing.
Figure 13. Low load level comparison between delaminated and non-delaminated displacement results.
Conclusions
The four-point bend experiments reveal well the bending characteristics of foam-filled composite beams and how these differ
from monolithic beam materials. A result is that the failure processes of the foam-filled composite beams are different as are
the strain patterns that can result in initiation of failures. This needs to be taken into account when considering fatigue, impact
and other loading conditions.
Depending on the type of structured composite laminate, so delamination effects can vary. This is as to the onset of
delamination and its growth affecting the overall retention of integrity of the material at different stages of its failure. Data
obtained by DIC from the surface of a material, needs very careful interpretation as to the damaging processes in the core of
the material as is the case with any surface measurements. However, DIC techniques have the advantage of being able to
explore in much detail the changing strain patterns on the surface of a material that are affected by internal failure of the
material.
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