Quasi-static testing of new peel stopper design for sandwich structures
Jakobsen J., Bozhevolnaya E., and Thomsen O.T.
Department of Mechanical Engineering, Aalborg University
Pontoppidanstraede 105, 9220 Aalborg, Denmark
ABSTRACT
In this paper a specially designed peel stopper will be presented [1]. The proposed peel stopper appears as a substructural component, which has to be embedded into the core of the sandwich element during its manufacturing. The peel
stopper works by activating an internal mechanism that prevents the propagation of delamination beyond the peel stopper
boundaries, and thus restricts damage to a limited area where the delamination/peeling was initiated. When
delamination/peeling is locally confined to the allowable and a priori predicted damage areas, the structural integrity of the
sandwich is preserved, and the development of global failure/collapse may efficiently be prevented. Three configurations of
sandwich beams were manufactured and tested in 3 point bending under quasi-static load conditions. The first configuration
had not been furnished with the peel stopper. The second configuration had an insert of the same material and of equal weight
as the implemented peel stopper. The third configuration had the peel stoppers embedded. High-speed video recordings were
performed during loading of the specimens up to failure, and post mortem inspections were subsequently conducted. The
results of the tests showed that only the specimens with peel stoppers were able to arrest the crack propagation, thus ensuring
that the material behind the peel stoppers remained intact and undamaged.
Introduction
A sandwich concept is a layered assembly made of two thin strong face sheets separated by and bonded to a compliant
lightweight core. Such a lightweight structural element provides very high bending stiffness, very high strength and other useful
properties like high thermal insulation, high internal damping, high corrosion resistance, low maintenance costs, etc. This
makes sandwich structures to be well suited for applications in the aerospace, marine, automotive, sustainable energy
industries as well as in civil engineering. However, sandwich elements are only functional with sub-structural internal
components such as various inserts, edge stiffeners, core junctions, etc., which allow an assembling of the sandwich elements
into complete structures and their subsequent practical usage. It is known that introducing the sub-structural components into
sandwiches makes them vulnerable with respect to their structural integrity, because various stress concentrations might arise
in the vicinity of the geometrical and material changes. The stress concentrations might lead to local and possibly global
failures (damages) of the sandwich assembly. Impact, manufacturing flows or fatigue loading might also instigate damage and
failure. General scenario for the sandwich failure is a presence of delamination between the face sheets and the core of the
sandwich structure. Such delaminations may start without prior warning, propagate very fast and eventually lead to a complete
failure and collapse of the sandwich structure.
Initiation and propagation of cracks in sandwich structures are in focus of many reported and ongoing studies. Burman and
Zenkert studied the fatigue behavior of initial damaged and undamaged sandwich structures loaded in four point bending [2,3].
They found that fatigue may start in the core and continue as a face/core delamination. Carlsson et al studied the influences
from face/core debonds on the crack developments in sandwich panels under compressive loads [4-7]. They found that the
debonded area propagated as a face/core delamination when the load reached the buckling/instability level of the sandwich
panel. Furthermore Bozhevolnaya et al studied the influences of core junctions on the structural response [8-12]. They found
that the fatigue life was very dependant on the shape of the particular internal core junctions, and face/core delamination was
commonly observed as the failure mode.
There is a known attempt by Grenestedt [13-14] to introduce a sub-structural element which stops the peeling of the face
during delamination. The technique is shown to be effective but rather manufacture challenging. Therefore there is a need in
developing a peel stopper, which would prevent/limit delamination in the sandwich structures and at the same time would be
easy to implement from the technologic point of view.
Peel Stopper Concept
The basic design of the suggested peel stopper [1] is illustrated in Figure 1. The peel stopper is a sub-structural component
embedded into the sandwich panel (like an insert or edge stiffener), and its main purpose is to arrest face-core interface crack
propagation by rerouting the crack path into a closed/restricted area of the sandwich panel, thus preventing the spreading of
debonding/delamination into the remaining parts of the sandwich structure.
Figure 1 Proposed design of the peel stopper. The case shown displays a crack re-routing angle of 10 degrees.
For the present study, the peel stoppers were manufactured from an elasto-plastic material, with elastic properties close to the
sandwich core properties. Generally, it is recommended that the material of the peel stopper is chosen to be compliant and
with large straining capability (i.e. ductile), and the elastic stiffness of the peel stopper is recommended to be of the same order
as the elastic properties of the main sandwich core (or somewhat higher). Good adhesion properties with respect to both the
core and the faces are required as well.
The peel stoppers may be mounted into a sandwich panel (e.g. a sandwich beam, plate or shell), as shown in Figure 2,
together with other sub-structural components like e.g. structural inserts. Essentially, there are no or only minor manufacturing
difficulties associated with the introduction of the peel stoppers, since they are similar to insert types already widely used in
sandwich structures.
Figure 2 Suggested implementation of the proposed peel stoppers in a sandwich plate. The grid type pattern will confine
damage to the grid mesh.
Failure is often initiated by crack formation in the interior core parts of the sandwich structure due to fatigue load conditions,
impact/shock (dynamic) loads or from manufacturing imperfections. Under such circumstances the core crack will often
propagate towards a core-face interface, from where it usually proceeds as a delamination along the face-core interface. The
principle idea of the new peel stopper concept is that propagation of delamination/debonding is prevented to spread beyond
the boundaries of the proposed peel stoppers. This inhibition is due to a rerouting of the crack along the internal curve of the
peel stopper (arrowed line) instead of propagation along the interface (dashed line), as shown schematically in Figure 3. This
technique will confine the debonding/delamination to a limited area of a single grid of the peel stoppers (cf. Figure 2).
Furthermore, the proposed peel stopper will allow the debonded sandwich face be kept attached to the sandwich component
(contrary to the method described in [18,19]). This will retain some structural load carrying capability of the debonded
structure after delamination/debonding, especially under in-plane tensile loading.
Figure 3. The basic idea of the peel stopper is to force the crack to propagate along the stopper-core interface (internal curve arrowed line) and not along the face-core interface (dashed line).
Test Specimens
Verification and functionality tests of the proposed peel stopper design were performed by means of comparing three sandwich
beam test configurations, denoted as (a), (b) and (c) in Figure 4.
Figure 4 Three test configurations with Rohacell® foams and carbon fibre reinforced composite faces for the quasi-static
validation of the peel stopper concept.
These beams were manufactured with PMI Rohacell® cores. For each configuration (a, b, and c) two specimens were
manufactured and tested quasi-statically in a three-point bending loading, as illustrated schematically in Fig. 4.
The main purpose of the experiments was to induce a shear failure in the softer core of the sandwich beam, followed by crack
propagation and crack-kinking towards the face-core interfaces and finally delamination along the interfaces. This allowed a
detailed study of the influence of the presence of the peel stoppers on the propagation of completely developed delaminations.
Each test specimen had a total length of 500mm and a beam span between the supports of 460mm. The beam configurations
o
o o
(a), (b) and (c) were manufactured with a 1mm thick carbon fibre laminate face sheets. The lay up was (0 ,90 ,0 ) of both top
and bottom faces. The core of these specimens consisted of two 25mm thick PMI foam core parts from Rohacell® with
different densities (51WF and 200WF). The stiffer core, 200WF, was located at the edges of the beams, and an araldite
diaphragm was placed in the beam centre to avoid indentation failure due to the external loading. In beam configurations (b)
and (c), polyurethane inserts were embedded between the two cores as shown in Fig. 4. A conventional butt insert was used
in configuration (b), and the proposed peel stopper was used in configuration (c). The material data are specified in Table 2.
Table 1 Mechanical properties of the tested beams.
Materials
Test configurations (a), (b), (c)
T700 UD/SE 84LV[23] – face
Rohacell® 200WF[24] – edge core
Rohacell® 51WF[24] – main core
Test configurations (b) and (c)
PERMAlock 40496 (PU)[25] – peel stopper
All Test Configurations
Araldite 2011 [27] – adhesive
Araldite B30[26] – diaphragm
E-Modulus
[MPa]
Tensile
Strength
[MPa]
Compress.
Strength
[MPa]
Elongation
at failure
[%]
129,200
350
75
2844
6.8
1.6
1187
9.0
0.8
3.5
3.0
100
10
-
25
3700
Shear lap strength 26MPa
60
100
5-6
Configurations (b) and (c) were designed to have equal mass and identical material composition. Configuration (a), which
represents a realistic design configuration, was considered as a reference to evaluate the two other configurations against.
Every test configuration shown in Figure 4 was manufactured by assembling and bonding the core prior to prepreg/face
lamination using a vacuum bagging technique. The sandwich panel was cured for six hours at 100ºC, and afterwards postcured for 48 hours at room temperature. Finally, the sandwich panel was cut into separate beams with a final width of 58mm.
All the tested configurations (a)-(c) were geometrically similar, and the only difference was the choice of material composition.
The mechanical properties of the sandwich beams constituents are given in Table 1.
Test Results
The three-point bending scheme was chosen, as it provided a controlled shear cracking of the core in the bulk of the weaker
foams. The shear cracks propagated towards the face sheets, where crack kinking occurred followed by crack propagation in a
delamination mode along the face-core interfaces.
An experimental set-up was designed and manufactured on the basis of a 100kN servo hydraulic Schenk Hydropuls® testing
machine. The testing machine has four test regions, where the lowest region was used for this particular test setup. The upper
load limit in this test region is 12.5kN, which gave a discrepancy between the input and output load signal of less than 3%. A
load controlled mode was used during loading, which was performed with a load rate of 0.02kN/sec. The central deflection of
all specimens was recorded via the displacement of the cross head. High-speed video recording of specimen failure was
enabled with a frame rate of 6000 frames/sec.
The observed flexural load vs. central deflection responses of configurations (a)–(c) shown in Figure 5 were very similar as
expected. This is an indication that the overall structural stiffness of the sandwich beam was not affected by the introduction of
the peel stoppers.
Figure 5 Applied force vs. central displacement for the specimens of configurations (a)-(c).
configuration were tested.
Two specimens of each
The failure characteristics of all six test specimens are summarized in Figure 5. The recorded maximum loads and maximum
central deflections of the beams at failure were quite close for three different configurations. In addition the failure
characteristic of the six specimens is given I tabular form in Table 2.
Specimen
and
configuration
Failure
load
[N]
a1
2307
a2
2332
b1
2094
b2
2106
c1
2185
c2
2246
Table 2 Failure characteristics of the tested specimens.
Cross head
Avg. cross head
Avg.
displacement
displacement
Location of
failure load
at failure
at failure
failure initiation
[N]
[mm]
[mm]
Compliant core
11.58
(51WF)
2320
11.78
Compliant core
11.97
(51WF)
Compliant core
10.11
(51WF)
2100
9.96
Compliant core
9.80
(51WF)
Compliant core
10.43
(51WF)
2216
10.83
Compliant core
11.22
(51WF)
Completed
delamination
Yes
Yes
Yes
Yes
No
No
The average failure load measured for the specimens of configuration (c) was 2216N. This load level is within 5% of the
average failure loads of configurations (a) and (b). Additionally, the central deflection at failure for configuration (c) is observed
to be between the central deflections observed for configurations (a) and (b). This difference is estimated to be around 9%. In
this connection it should be mentioned, that the short length of the weak core compared to the total length of the edge
stiffeners and peel stoppers is the reason for the difference in central deflections observed for the improved (c) and
conventional (a), (b) edge stiffeners. If the length of the pure core part of the sandwich structure was larger, compared to the
total length of the embedded sub-structures, which would be the case for realistic design configurations, the difference in the
ultimate displacements, and thus the influence of the peel stoppers on the overall structural stiffness of the sandwich, would be
much smaller.
Damage initiation and development of failure occurred according to the predicted scenario, as shown in Figures 6-8. High
speed video recordings were used to identify the location of failure initiation and its progression in the sandwich specimens. In
all cases, failure initiated as a shear crack in the centre of the weak core, the crack tip kinked towards the faces and continued
as a delamination along face-core interfaces. Notice that a full delamination of the face occurs for the cases of conventional
edge stiffeners (Figure 6and Figure 7), while the peel stopper in Figure 8 clearly confines the crack inside the weak core, and,
moreover, lets the sandwich face still be attached to the sandwich beam edge.
Figure 6 Failure of test specimen a1
Figure 7 Failure of test specimen b2
Figure 8 Failure of test specimen c2
The sandwich beams with embedded peel stoppers were subsequently loaded in the 3-point bending fixture in order to inspect
the damage zone in the vicinity of the peel stoppers as seen in Figure 9. All beams of configurations (a) and (b) ended up with
completely delaminated face sheets, while peel stoppers in configuration (c) effectively stopped delamination.
Figure 9 Post mortem inspection of the cracked part of test specimen c2.
Conclusions
Three beam configurations were tested in a three point load condition and configuration (c) was equipped with the proposed
peel stopper. Configuration (a) was considered due to its common practical design and configuration (b) and (c) had an
Polyurethane insert embedded in its core. The only difference between configuration (b) and (c) was the particular shape of
this embedded insert. The shape (internal curved boundary) of the polyurethane insert in configuration (c) was designed to
arrest face-core peeling.
Regarding the three considered configurations only configuration (c) was able to arrest face-core peeling and then confine the
damage to an area in between the proposed peel stoppers.
The presented concept may also be adapted to other types of structural sandwich elements (i.e. plates and shells) and then
add security to future sandwich components.
Acknowledgments
The work presented was supported by the Danish Research Council for Technology and Production Sciences; Grant N 26-040160, “Structural Grading - a novel concept for design of sandwich sub-structures”, and the Innovation Consortium “Integrated
Design and Processing of Lightweight Composite and Sandwich Structures” (abbreviated “KOMPOSAND”) funded by the
Danish Ministry of Science, Technology and Development. The support received is gratefully acknowledged.
The authors also acknowledge Degussa Röhm GmbH (Germany) for supplying the sandwich core materials used in this
investigation.
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