Overview of the failure mechanisms of
sandwich panels with foam or woven cores
Ruben Geerinck
Supervisor(s): Wim Van Paepegem
Abstract – Now a days, a lot of sandwich panels are used in
automotive, aerospace, high speed trains and many other
applications. The main advantage is the high bending strength
and stiffness for its weight. A lot of research has been conducted
to the failure mechanism and properties of sandwich panels. This
paper gives an overview of the failure mechanisms of sandwich
panels with foam or woven cores in bending, compression, shear
and impact. Woven sandwich panel prove to be a good
alternative for sandwich panels with foam. Of course, the
problems with these panels needs to be further investigated.
Keywords – Sandwich panels, foam, woven core, bending,
compression, shear, impact
I. INTRODUCTION
Sandwich panels are materials consisting of thin strong face
sheets or skins separated with a light weight core. This
combination leads to an element with very high bending
stiffness and strength to weight ratio [3]. The skin of the
sandwich constructions carries the in-plane tensile and
compressive stresses. While the function of the light weight
core is dual. It has to resist and transmit the shear forces and
keep the two skins apart without any relative movement to
each other [3-5]. Sandwich structures are often used in
transportation application as automotive, maritime and
aerospace for their light weight.
Their light weight core is a hot topic. It can consist of
foams, honeycombs, woven cores, pyramidal cores, lattice
cores, foldcores and other cores [6-9]. The main problems are
failure due to debonding of the face sheets (except for woven
cores) or shear failure of the core [3]. This paper will give an
overview of the failure mechanisms of foam cores and woven
cores. The main advantages of woven cores are the physical
connection between the skins and core and the fact that the
weight and cost of the foam is omitted [10]. The sandwich
constructions with woven cores can be divided in two groups
(a) with pile yarns (see Figure 2) and (b) with woven
intermediate layers (see Figure 1). A lot of research has been
conducted to improve the delamination resistance of sandwich
panels with foam cores. A couple of examples are stitching
[11] or inserting shear keys [12].
II.
BENDING
The failure mechanism in a three-point bending test of a
sandwich panel with a foam core is highly dependent on the
properties of the foam and skins. The load-deflection curves
are similar but with UD skins the failure is characterized by
delamination between the compressed skin and core, then,
shearing of the foam and eventually loss of cohesion of the
lower skin and foam. For cross-laminated skins the fracture is
obtained at the upper skin by compression near the indentor
[13].
In a three-point bending test a sandwich panel with woven
core (pile yarns) fails in warp direction due to cracking of the
local skins supported by two neighboring piles. In the elastic
stage in weft direction the piles tilted and the transverse
support to the upper skin was lost which led to crippling of the
upper skin that reduced the shear resistance. The upper skins
then wedged into the core. The plastic hinge mechanism
enables the panels to possess a long deflection plateau after
the peak load. The main shortcoming of the panels is the weak
mechanical properties of the woven skins. For good properties
in bending the skin should be strengthened or a multilayered
skin should be used which is preferably woven in one piece
with the core to avoid delamination [1].
For sandwich panels with woven interconnections the walls
in weft direction will be compressed to the buckling strength
during a three-point bending test. Under the indentor the panel
will be thinning and when the core is compressed, the skins
will directly bent accompanied with fracture of the bottom
skin. In warp direction the core will fail in shear followed by
abrupt skin fracture as is shown in Figure 1 [2].
Figure 1 Bending of sandwich panel with woven intermediate layers
(warp) [2]
III. COMPRESSION
Edgewise compression of a foam core sandwich panel is
determent by the material combinations of the core and skins.
Mamalis et al. [14] described three different failure modes for
the combinations of four foams and two different layups op
the skins. Where the most important factor is the stiffness and
strength of the foam. The most sandwich panels fail by
unstable column buckling. However with a specific foam core
(Herex C70.90 PVC) a stable progressive crushing mode was
observed which indicates that the crash energy absorption is
higher. During the compression the first observed failure is
often debonding of the skins. A lot of research has been done
to improve the delamination resistance of these sandwich
panels like stitching [11] or inserting shear keys [12].
Woven sandwich panels with pile yarns on the other hand
display mainly bending and buckling of the pile yarns due to
the 8-shape of the fibres in vertical compression [1]. In the
elastic stage bending and buckling of the fibres out of the piles
was observed (see Figure 2a). After the peak load, the
buckling of piles led to a load drop. As the pile yarns have an
initial tilt, the failure mode is a coupled failure mode of
compression and shear. If the deformation of the panel
continues, the tilt angle increases and most of the compression
load shifts to the bending resistance of the joints linking the
pile yarns to the skins (see Figure 2b). The coupled failure
mechanism leads to a larger deformation plateau. Therefor it
is likely that the sandwich panels have a high capability to
absorb energy in impact. The higher the pile length, the low
the compression strength and stiffness are. Larger pile length
leads to a lower critical buckling load [10]. Also stretching of
the pile yarns increases the compressive strength. The strength
of a curved beam indeed increases with decreasing curvature
according to the general theory of column bending and
buckling [10].
Figure 2 Pile (a) buckling and (b) tilting under compression. [1]
If these panels are stacked together as a multilayer woven
sandwich panel a higher energy absorption can be reached.
The failure mechanism in compression in the different layers
is the same as for an individual layer: pile buckling controls
the failure mode. The layers fail one by one as the load is
transmitted to the other layers when a layer is completely
crushed. The compressive strength of a multilayer sandwich
panel in comparable to a monolayer sandwich panel but the
deformation is multiplied hence the greater energy absorption
[15].
With woven interconnections the out-of-plane compression
strength and stiffness is dependent on the buckling behavior of
the woven core. In the elastic deformation stage the walls will
bend and at a peak loading the walls will buckle which leads
to an abrupt load drop followed by a deformation plateau.
During this deformation plateau the woven walls get steeper
and the core gets thinner but the sandwich panel keeps a
relative strong anti-crushing resistance [2].
IV. SHEAR
A sandwich panel with a foam core under shear loading will
fail due to delamination between the skin and the foam core. It
is observed that a thin layer of foam is still adhered to the skin
which suggests that, in this case, the adhesive bond is stronger
than the shear strength of the foam core [3].
The mainly vertical orientation of the pile yarns causes a
rather low transverse shear resistance for sandwich panels
with a woven core [10]. For most applications an acceptable
shear strength can be reached. As in compression the core
shear modulus decreases with increasing pile length. The most
important effect is the introduction of 45° piles which
considerably increase the shear modulus in warp direction. A
higher resin content will increase the shear resistance and
compression properties, especially if the pile density is
sufficient. Connections will be formed between the pile yarns
and a wall-like structure is obtained. This, of course, increases
the price and core weight. The usage of a foam in the core
drastically improves the core properties (shear and
compression) as the foam prevents the pile yarns from
bending and buckling and supports the top skin [10, 16]. Here
also the weight of the core and the price will increase.
If the interconnections between the two skins are woven
layers instead of just pile yarns, the shear resistance is more
anisotropic. The shear strength and stiffness in warp direction
are significantly less than in weft direction. In warp direction
one half of the walls are compressed and the other stretched.
The compressed walls will buckle at the peak load. No
delamination is observed which indicates that the shear
resistance in warp is smaller than the debonding resistance. In
weft direction the shear strength is higher than the bonding
strength as delamination occurs as failure mechanism [2].
V. IMPACT
An impact, for example accidental tool dropping, on a
sandwich panel with a foam core (high modulus) can cause
severe delamination [17]. To improve this Potlurie et al. [11]
used a Kevlar threat to stitch-bond the skins and core in z
direction. Due to this the interlaminar properties of the
composite where increased to such an extent that the
delamination resistance of the panels increased significantly
with increasing stich density.
Impact on a sandwich panel with a woven core with pile
yarns causes less damage and no severe delamination. During
impact (32 J) the pile yarns will bend until a critical buckling
load is reached. The pile yarns just below the point of impact
are broken and the top skin is penetrated. As discussed in
compression (III) the critical buckling load decreases with
increasing pile length. Due to this fact less energy can be
dissipated in the top skin and core resulting in higher damage
in the bottom skin. For a larger impact (48 J) there was severe
damage for a small core thickness, even damage in the bottom
skin. Sandwich panels with a higher thickness show no
complete perforation of the top skin. The impact energy can
be dissipated as core shear damage, since they have weaker
cores. The damage and deformation was spread over a wider
area [16].
Introducing a foam into the core drastically improves the
core properties as discussed in shear (IV). This also means
that the absorbed energy is higher with foamed sandwich
panels [16].
VI. CONCLUSIONS
This paper discussed the failure mechanisms of sandwich
panels with foam and woven cores in bending, compression,
shear and impact. It is clear that sandwich panels with woven
cores whether it is with pile yarns or woven interconnections
can be a good alternative for sandwich panels with woven
cores. There is no need for foam which is beneficial for
weight and price of the panels. The main problems are the
thickness and properties of the skins and the shear resistance.
The thickness and properties of the skins can be improved by
using a multilayer woven fabric which is preferably woven in
one piece with the core to avoid delamination. To improve the
shear resistance the pile yarns or woven interconnections can
be woven in 45 degree angle.
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