Conference PaperPDF Available

Improving flexural modulus of interleaved composites using reinforced thermoplastic interleaves

Authors:
  • TCKT - Transfercenter für Kunststofftechnik GmbH

Abstract

Interleaving the plies of carbon fibre reinforced epoxy composites with thermoplastic interleaves have previously been shown to enable these composites to display controllable stiffness and shape memory properties. However, the incorporation of unreinforced thermoplastic interleaves leads to a decrease in flexural modulus of the interleaved composites. In this study, the flexural modulus of composites with reinforced polystyrene interleaves was investigated. The reinforcements used in this study were: (1) stainless steel mesh (SS), (2) unidirectional carbon fabric (UD), (3) woven carbon fabric, (4) woven carbon fabric with epoxy coating and (5) non-woven short carbon fibre mesh. The flexural moduli of the interleaved composites with reinforced interleaves were predicted theoretically and determined experimentally. Among these composites, significant increases in the flexural modulus were achieved in the interleaves with UD, woven and woven+epoxy reinforcements. Additionally, these interleaved composites were shown to retain their controllable stiffness and shape memory properties.
SAMPE Europe Conference 2021 Baden/Zürich - Switzerland
1
IMPROVING FLEXURAL MODULUS OF INTERLEAVED
COMPOSITES USING REINFORCED THERMOPLASTIC
INTERLEAVES
Gokul Ganesh Murali1, Paul Robinson1, Alexander Bismarck2, and Christoph Burgstaller3
1 The Composites Centre, Department of Aeronautics, Imperial College London, South
Kensington Campus, London SW7 2AZ, United Kingdom. Email: g.murali@imperial.ac.uk,
p.robinson@imperial.ac.uk
2 Polymer and Composite Engineering (PaCE) group, Department of Material Chemistry,
University of Vienna, Waehringer Strasse 42, A-1090 Wien, Austria. Email:
alexander.bismarck@univie.ac.at
3 Transfercenter für Kunststofftechnik (TCKT), Franz-Fritsch-Straße 11, A-4600 Wels,
Austria. Email: christoph.burgstaller@tckt.at
ABSTRACT
Interleaving the plies of carbon fibre reinforced epoxy composites with thermoplastic
interleaves have previously been shown to enable these composites to display controllable
stiffness and shape memory properties. However, the incorporation of unreinforced
thermoplastic interleaves leads to a decrease in flexural modulus of the interleaved composites.
In this study, the flexural modulus of composites with reinforced polystyrene interleaves was
investigated. The reinforcements used in this study were: (1) stainless steel mesh (SS), (2)
unidirectional carbon fabric (UD), (3) woven carbon fabric, (4) woven carbon fabric with
epoxy coating and (5) non-woven short carbon fibre mesh. The flexural moduli of the
interleaved composites with reinforced interleaves were predicted theoretically and determined
experimentally. Among these composites, significant increases in the flexural modulus were
achieved in the interleaves with UD, woven and woven+epoxy reinforcements. Additionally,
these interleaved composites were shown to retain their controllable stiffness and shape
memory properties.
1. INTRODUCTION
In an attempt to impart controllable stiffness and shape memory behaviour in composites, a
novel technique of interleaving has previously been explored [1]. The interleaved composites
consist of standard thermoset (TS) based carbon fibre reinforced polymer (CFRP) plies
interleaved with thermoplastic (TP) layers (Figure 1). When these composites are heated from
a temperature T<Tg-TP<Tg-TS to Tg-TP<T<Tg-TS (where, Tg-TP - glass transition temperature of
TP, and Tg-TS - glass transition temperature of TS), the CFRP plies can slip relative to each
other and, as a consequence, the flexural stiffness of the composite is greatly reduced. When
the temperature is restored to T<Tg-TP<Tg-TS, the TP layers regain their full stiffnesses, ply slip
between the TS layers is no longer possible. Thus, by transitioning between these temperatures,
controllable stiffness can be achieved.
A TS-TP interleaved composite in its low stiffness state (Tg-TP<T<Tg-TS), is capable of large
flexural deformations at low applied forces, compared to what is possible in its high stiffness
state. This is due to the lower stresses developed in the TS layers as they are uncoupled from
one another in the low stiffness state. When the composite is brought back to a temperature
T<Tg-TP<Tg-TS in the deformed state, the composite returns to its full flexural stiffness and, on
the removal of the deformation force, the deformed shape is almost fully retained. There is a
small springback due to partial relaxation of the internal stresses developed in the CFRP plies
SAMPE Europe Conference 2021 Baden/Zürich - Switzerland
2
during high-temperature deformation (see Table 1). When the unconstrained composite is
heated again to temperature Tg-TP<T<Tg-TS, the stored stresses are released as the composite
(now in its low stiffness state) returns to its original shape, thus making the composite exhibit
shape memory property, as seen in Table 1 [2].
Figure 1 Ply slip in
a variable stiffness
composite made of
TS and TP plies
Description Manufactured
composite
Composite shape
modified by
force F at
Tg-TP<T<Tg-TS
(reduced
s
tiffness)
Composite shape
set at T<Tg-TP and
removal of force
F (restored
stiffness)
Shape recovery
at
Tg-TP<T<Tg-TS
(reduced
stiffness)
Interleaved
composite
Temperature T<Tg-TP<Tg-TS Tg-TP<T<Tg-TS T<Tg-TP<Tg-TS Tg-TP<T<Tg-TS
Internal stress
(assuming
composite
cross-section
as seen in
Figure 1)
Table 1 Schematic of the shape memory process,
where TS
-
Thermoset layer and TP
-
Thermoplastic layer
In conventional CFRP composites, the carbon fibres act as one of the main load-bearing
structures. The introduction of non-reinforced TP interleaves leads to a decrease in the fraction
of carbon fibres in the composite cross-section and results in a drop in the flexural stiffness of
the interleaved composites [1]. Such a characteristic occurs because the thermoplastic
interleaves used in these composites have a low Young’s modulus (compared to the CFRP
plies). To offset this loss, in this study, a variety of reinforcements have been introduced in the
thermoplastic interleaves. In addition to the flexural stiffness, the controllable stiffness and
shape memory properties of these composites have also been studied to understand the
influence of interleaf reinforcements.
2. EXPERIMENTATION
2.1 Materials
Unidirectional TS300/914 carbon epoxy prepreg (Hexcel UK), which has a curing temperature
of 175°C, a cured thickness of 125 m and a flexural modulus of 120 GPa, was selected as the
carbon fibre reinforced polymer (CFRP) part of the interleaved composite. Empera 124N
polystyrene film, produced at Transfercenter für Kunststofftechnik (TCKT) by film extrusion
+
σ
TS
TP
TS
+
σ
SAMPE Europe Conference 2021 Baden/Zürich - Switzerland
3
method, with a thickness of 100 m was selected as the TP part of the interleaved composite.
The materials used as reinforcements in TP interleaves were,
(i) Stainless steel mesh (SS) - SS304 grade woven wire mesh with an aperture of 82 m, wire
diameter of 28 m and areal weight of around 90 gsm, procured from The Mesh Company
(Warrington) Ltd, UK
(ii) Unidirectional carbon fibre fabric (UD) - Pyrofil TR50S 15K unidirectional carbon fibre
fabric of thickness 100 m and areal weight of 100 gsm, procured from Easy Composites
Ltd, UK. The unidirectional fibres are supported by a heat-bonded polypropylene net.
(iii) Woven carbon fibre fabric (Woven) - Plain weave carbon fibre fabric (weft and warp
fibres: Toray T300 1K, Weave: 1/1 Plain) of thickness 100 m and areal weight of 90 gsm,
procured from Easy Composites Ltd, UK
(iv) Woven carbon fibre fabric with epoxy powder coating (Woven+Epoxy) - Plain weave
carbon fibre fabric (weft and warp fibres: Toray T300 1K, Weave: 1/1 Plain) of thickness
100 m, areal weight of 90 gsm and with epoxy powder coating, procured from Easy
Composites Ltd, UK
(v) Non-woven carbon fibre fabric (Non-woven) - Non-woven carbon fibre fabric of
thickness 100 m, areal weight of 10 gsm and with epoxy binder, procured from Nanjing
Koptech New Material Co Ltd, China.
2.2 Sample preparation
To produce the reinforced TP films, the reinforcement fabric was placed between two PS layers
and covered with a release film and placed in vacuum using a heated vacuum table (Global
vacuum heat press, Nabuurs Developing S.L.). Then, the reinforcement fabric and the PS films
were heated to 120°C and held for 30 minutes to allow impregnation of PS into the
reinforcement fabric.
The interleaved composites were then laid up in a sequence of [(0°/TPR)7/0°] (where, -
unidirectional CFRP layer and TPR - reinforced thermoplastic layer) and cured for 1 hour at
175°C. To study the behaviour of unreinforced TP interleaves, composites were laid up with a
sequence of [(0°/TP)7/0°] and cured following the CFRP manufacturer’s guidelines. For
controllable stiffness and shape memory tests, samples were cut from the composite plates with
0° direction in the longitudinal direction.
2.3 Controllable stiffness studies
Experimentally, the apparent flexural Young’s modulus (E) of a composite can be measured
from the force-displacement curve of a 3-point-bend (3PB) test using Eq 1. For the controllable
stiffness (CS) study, 3PB tests were performed in a three-point bending fixture according to
ASTM D7264M standards. The studies were carried out on an Instron universal test machine
with a 50 kN load cell. In the tests, the support and loading rollers had a diameter of 6 mm, the
span between the supports varied depending on the thickness of the sample (approximately 32
times the thickness of the sample) and the crosshead speed was 1 mm/min.
𝐸
=
𝑚
𝑙
48
𝐼
Eq 1
where,
m
ratio of applied force to mid-span displacement,
𝑙
test span and
𝐼
– second moment of inertia of specimen cross-section
SAMPE Europe Conference 2021 Baden/Zürich - Switzerland
4
The composite specimens were tested in their high stiffness state (T23°C; T<Tg-TP< Tg-TS),
then in the low stiffness state (T=120°C; Tg-TP<T<Tg-TS) and finally again at high stiffness state
in order to check reversibility of their controllable stiffness. The flexural moduli of composites
at these three different conditions are referred to as ERT1, EHT1 and ERT2 respectively. An Instron
environment chamber mounted on the test machine was used to provide these test temperature
conditions.
2.3.1. Theoretical prediction of flexural moduli
By weighing the samples and using the areal weight and density of the reinforcements, the fibre
volume fraction of reinforcements in the interleaf was calculated and the theoretical modulus
of reinforced thermoplastic films (ERTP in Eq 2) was predicted using the mosaic model
discussed by Byström et al [3].
𝐸

=
𝑐
𝑉
𝐸
+
(
𝑐
𝑉
𝐸
)
+
(
1
𝑐
)
𝑉
𝐸
+
𝑉
𝐸

Eq 2
where,
𝐸
– Young’s modulus of reinforced thermoplastic film,
𝑉
– Fibre volume fraction,
𝑉
– Matrix volume fraction,
𝐸
– Modulus of fibre,
𝐸
– Modulus of fibre and
c – Constant corresponding to fabric weave pattern (c=1 for UD fabric and c=0.5 for
plain weave fabric).
Reinforcement
materials
Volume fraction Individual moduli
components (GPa)
Apparent flexural
moduli of
composites (GPa)
Fibre
Matrix
Fibre
Matrix
E
RTP
E
RT
E
HT
None (Control)
0.000
1.000
0.0
3.2 at
RT,
0 at
HT
3.2
76.0
0.325
SS
0.085
0.915
193.0
16.7
73.2
0.248
UD
0.451
0.549
240.0
110.1
115.5
0.211
Woven
0.312
0.688
230.0
46.2
83.5
0.164
Woven+Epoxy
0.311
0.689
230.0
46.2
83.0
0.160
Non
-
woven
0.047
0.953
~0
3.1
70.2
0.245
Table 2 Theoretical modulus prediction of reinforced films
and interleaved composites
Using the simple beam theory technique presented by Maples et al [4] but including the
stiffness of the interleaf for the control case at room temperature, the apparent flexural moduli
of the interleaved composites in RT and HT states were predicted, and are listed in Table 2.
2.4 Shape memory studies
For the shape memory (SM) investigation, each initially flat composite specimens was re-
shaped to a nominally 90°-bend using the 3PB test setup at high temperature (T = 120°C; Tg-
TP<T<Tg-TS). (This stage is the deformation stage of the SM study.) The 90°-bend shape was
then fixed by lowering the temperature of the specimen (T 25°C; T<Tg-TP< Tg-TS). Next, the
load was removed and springback of the specimen was observed. (This stage is the springback
stage of the SM study.) Finally, the specimen was brought to high temperature (T = 120°C; Tg-
TP<T<Tg-TS) and the resulting recovery in shape was observed, as shown in Figure 2. (This
stage is the recovery stage of the SM study.) During the different stages of the investigation,
the shape profiles of the composites were measured optically using UVX Flexi video
SAMPE Europe Conference 2021 Baden/Zürich - Switzerland
5
extensometer (iMetrum Ltd, UK). An Instron environment chamber mounted on the test
machine was used to provide the test temperature conditions.
3. RESULTS AND DISCUSSION
3.1 Controllable stiffness tests
The specimen dimensions, the span between support rollers and the maximum mid-span
displacement for composite specimens subjected to CS studies are provided in Table 3. Typical
load-displacement curves of various composites in RT1 and HT1 states are provided in Figure
3 (a) and (b). The apparent flexural moduli of different interleaved composites at different
stiffness states are listed in Table 4.
Figure 2 Composite
specimen shapes used in
shape-memory studies
Reinforcement
Specimen dimensions (mm) Mid-span
displace-
ment
(mm)
Materials
Interleaf
thickness
(μm)
Length - l’ Width - w Thickness - t Span - l
None
(Control)
137.1 ± 5.1
79.80 ± 0.21 9.69 ± 0.13 2.22 ± 0.04 70 2
SS
137.7 ± 4.5
79.52 ± 0.09
9.73 ± 0.03
1.94 ± 0.03
64
1.5
UD
152.9 ± 10
79.59 ± 0.05
9.82 ± 0.01
2.07 ± 0.07
66
1.5
Woven
179.1 ±
4.1
79.69 ± 0.12
9.83 ± 0.02
2.25 ± 0.03
66
15
Woven+
Epoxy
181.4 ± 4.8
79.56 ± 0.03 9.82 ± 0.01 2.27 ± 0.03 70 1.5
Non-
woven
138.9 ± 5.6
79.68 ± 0.06 9.80 ± 0.02 1.97 ± 0.04 64 1.5
Table
3
Specifications
of composite specimens subjected to controllable stiffness study
Figure 3 Typical force-displacement graph of reinforced interleaved composites at
(a) RT1 and (b) HT1 states
SAMPE Europe Conference 2021 Baden/Zürich - Switzerland
6
Reinforcement
material
E
RT1
(GPa)
E
HT1
(GPa)
E
RT2
(GPa)
None (Control)
66.33
± 1.14
0.33 ± 0.06
63.28 ± 7.16
SS
66.46
± 1.68
0.66 ± 0.17
67.66 ± 1.85
UD
76.15
± 2.40
0.87 ± 0.28
73.98 ± 2.86
Woven
72.67
± 1.71
0.41 ± 0.15
72.54 ± 1.40
Woven+Epoxy
71.16
± 0.78
0.63 ± 0.07
72.51 ± 0.79
Non
-
woven
67.37
± 1.02
1.44 ± 0.13
68.88 ± 1.15
Table 4 Apparent flexural moduli of reinforced interleaved composites
at RT1, HT1 and RT2 stiffness states.
In addition to the predicted moduli of the composites with reinforced interleaves (as shown in
Table 2), the apparent flexural moduli of the interleaved composites in RT state for different
composites in an unreinforced scenario, but with corresponding interleaf thickness were also
theoretically predicted. The comparison between theoretical predictions and experimental
observations are also shown in Table 5.
3.2 Shape memory tests
The specimen dimensions, the span between support rollers and the maximum mid-span
displacement for composite specimens subjected to SM tests are provided in Table 6. The
angles subtended by the composite specimens at different stages of SM study, as observed by
optical tracking techniques, are provided in Table 7. The optical technique used in this study
was, however, not sensitive enough to measure the small change in angle which occurred
during the springback process.
4. CONCLUSIONS
Reinforcement of thermoplastic interleaf materials in interleaved carbon fibre epoxy
composites has been investigated to improve their flexural stiffness. Tests were also conducted
to assess the controllable stiffness and shape memory properties of these composites.
Reinforcement
materials
Theoretical
(unreinforced
interleaves)
Theoretical
(reinforced interleaves) Experimental
E
(GPa)
E
(GPa)
%
difference
compared to
unreinforced
composites
(theoretical)
ERT1
(GPa)
%
difference
compared to
unreinforced
composites
(theoretical)
None
(Control)
76.0
0
67.51
± 1.
56
-
1
1
SS
70.5
76.2
8
66.46
± 1.68
-
6
UD
67.5
115.5
7
1
76.15
± 2.40
1
3
Woven
62.8
83.9
3
4
72.67
± 1.71
1
6
Woven+Epoxy
62.4
83.6
3
4
71.16
± 0.78
1
4
Non
-
woven
70.2
70.2
0
67.37
± 1.02
-
4
Table 5 Experimentally measured and theoretically predicted
apparent flexural
m
odulus of interleaved composites in high stiffness state
SAMPE Europe Conference 2021 Baden/Zürich - Switzerland
7
Reinforcement
materials
Specimen dimensions
(mm)
SM deformation
(mm)
length –
l’
width –
w
thickness –
t
Specimen
span
l
Midspan
displacement
δ
None (Control)
79.80 ± 0.21
9.69 ± 0.13
2.22 ± 0.04
40
10.5
SS
79.55 ± 0.06
9.74 ± 0.02
1.96 ± 0.03
65 18.72
UD
79.66 ± 0.16
9.82 ± 0.02
2.09 ± 0.06
Woven
79.65 ±
0.04
9.80 ± 0.03
2.25 ± 0.03
Woven+Epoxy
79.63 ± 0.10
9.82 ± 0.01
2.22 ± 0.06
Non
-
woven
79.70 ± 0.04
9
.
77 ± 0.02
1.92 ± 0.08
Table
6
Specifications of composite specimens subjected to shape memory tests
Reinforcement
materials
Angle
subtended
by the composite (°)
Initial
After
deformation
After
springback
After
2 min
shape
recovery
After
5 min
shape
recovery
None
(Control)
180
98.9 98.3 175.6 178.6
SS
89.3
90.0
168.8
176.6
UD
96.3
96.6
160.3
172.2
Woven
96.7
97.0
164.4
177.7
Woven+Epoxy
99.5
99.7
163.7
173.5
Non
-
woven
91.1
91.3
165.8
173.6
Table
7
Angles
subtended
by the composites at different stages of SM study
It has been shown that the apparent flexural modulus of the interleaved composites can be
improved by introducing reinforcements. In the cases examined in this study, high fibre volume
fractions were achieved in the interleaves with unidirectional carbon fibre, woven carbon fibre
and woven carbon fibre with epoxy powder coating reinforcements. Their resultant composites
showed 13%, 16% and 14% increase in flexural modulus respectively in high stiffness state,
as shown in Table 5. However, in these composites, the bond between polystyrene and the
reinforcements was probably intermittent and so the full benefits of stiffening were not
achieved. This may be the reason why the theoretically predicted modulus was not achieved in
these composites. Compared to theoretical predictions, the modulus of control composites and
composites with SS and non-woven reinforcements in experimental studies were lower (-11%,
-6% and -4% respectively). These discrepancies are currently being investigated.
In the low stiffness state, the measured loads in the 3PB setup appear to be too low to be
measured effectively by a 50 kN load cell, thus leading to a large noise, as seen in Figure 3. It
is expected that using a more sensitive load cell could decrease this noise. Furthermore, the
flexural modulus of composites in the low stiffness state is higher than prediction probably due
to the residual stiffness of the reinforced interleaves which was not considered in the
predictions. This has to be confirmed with the HT tests that are conducted using a more
sensitive load cell.
The addition of reinforcements to the PS interleaves also does not seem to noticeably influence
the controllable stiffness behaviour of the interleaved composites. In all the composites, over
SAMPE Europe Conference 2021 Baden/Zürich - Switzerland
8
97.5% loss in flexural modulus was observed as a result of heating. The modulus was almost
completely restored upon cooling the composites back to a high stiffness state. Similarly, all
the composites showed shape retention in the deformed condition and almost full shape
recovery in the SM studies, thus confirming that composites with reinforced TP interleaves
exhibit good shape memory behaviour.
Overall, this study has showed that reinforcing the interleaves could lead to an increased
flexural modulus in interleaved composites that are capable of effective controllable stiffness
and shape memory behaviour. Poor bonding between the reinforcement and the PS interleaves
or poor impregnation of PS into the fibre bundles of the reinforcements may be the reason
behind the flexural modulus of the composites in their high stiffness state be not as high as the
theoretical predictions, but this requires further investigation. It is expected that the intermittent
bonding issue could be resolved through surface treatment of the reinforcement fabric and
using a more compatible thermoplastic interleaf material that is capable of forming better
secondary bonds with the reinforcement fabric. Also, a more optimised fabric impregnation
technique could also be explored.
The introduction of reinforcements inside the interleaves also provides an opportunity to
intrinsically heat the composites by resistive heating to generate the thermal energy necessary
for CS and SM behaviours. Such an option could further allow out-of-oven morphing of such
interleaved composites.
5. REFERENCES
[1] H. Maples, “Composites with Controllable Stiffness,” Imperial College London, 2014.
[2] P. Robinson, A. Bismarck, B. Zhang, and H. A. Maples, “Deployable, shape memory
carbon fibre composites without shape memory constituents,” Compos. Sci. Technol.,
vol. 145, pp. 96–104, 2017, doi: 10.1016/j.compscitech.2017.02.024.
[3] J. Byström, N. Jekabsons, and J. Varna, “Evaluation of different models for prediction
of elastic properties of woven composites,” Compos. Part B Eng., vol. 31, no. 1, pp. 7–
20, 2000, doi: 10.1016/S1359-8368(99)00061-X.
[4] H. A. Maples, S. Wakefield, P. Robinson, and A. Bismarck, “High performance carbon
fibre reinforced epoxy composites with controllable stiffness,” Compos. Sci. Technol.,
vol. 105, pp. 134–143, 2014, doi: 10.1016/j.compscitech.2014.09.008.
[5] G. M. Spinks, H. R. Brown, and Z. Liu, “Indentation testing of polystyrene through the
glass transition,” Polym. Test., vol. 25, no. 7, pp. 868–872, Oct. 2006, doi:
10.1016/j.polymertesting.2006.05.012.
6. ACKNOWLEDGEMENT
The research leading to these results has been performed within the framework of the HyFiSyn
project and has received funding from the European Union’s Horizon 2020 research and
innovation programme under the Marie Skłodowska-Curie grant agreement No 765881.
ResearchGate has not been able to resolve any citations for this publication.
Article
Trials have been conducted to investigate the shape memory capability of an interleaved composite consisting of carbon fibre reinforced epoxy laminae and polystyrene interleaf layers. It has been shown that the composite can be readily re-shaped by deforming it at an elevated temperature and then cooling the composite in the deformed state. On re-heating, the composite almost fully returns to its original shape. One potential application of the shape memory capability of the interleaved composite is in deployable structures and a simple structure has been manufactured to demonstrate this possibility.
Article
The mechanical properties of polystyrene-interleaved carbon fibre reinforced epoxy composites, which exhibit controllable stiffness, have been investigated. DMTA and flexural tests showed that the storage modulus and flexural stiffness of these composites could be reduced by up to 98% when heated from 20 °C to 120 °C and the stiffness was fully recoverable on cooling. The flexural stiffness of the interleaved composites at room and elevated temperatures were predicted using simple beam theory and were found to be in good agreement with the measured values. Compressive and tensile properties were significantly reduced at 120 °C due to the presence of the softened polystyrene interleaves. Flexural strength tests at 20 °C indicate that there is a need for improvement of the adhesion between polystyrene and carbon fibre reinforced epoxy plies.
Article
A critical analysis of two simple and convenient analytical models for calculation of elastic properties of woven fabric composites is performed. Predictions of these models are compared with results obtained using the method of reiterated homogenization and with experimental data for plain weave glass fiber and carbon fiber polyester composites. Three different scales are identified in the analysis. The first scale predictions, which are the tow properties (obtained by applying Hashin's concentric cylinder model, the Halpin–Tsai expressions or mathematical homogenization technique), are the most critical because they form the input information for woven composite modeling. It appears that the uncertainty in this information causes larger differences in predictions than the deviations between models of different degree of accuracy. This fact sets limits on the required accuracy of the models. Model comparisons reveal that the woven composite model based on isostrain assumption in the composite plane and isostress assumption for out-of-plane components is in very good agreement with both experimental data and the reiterated homogenization method, whereas the modified mosaic parallel model fails to describe composites with large interlaced regions.
Article
The determination of the thermomechanical properties of polymers is vitally important in their characterization. Indentation methods are especially attractive for polymer coatings, since the coatings do not need to be removed from the substrate. In this study, bulk polystyrene has been analysed by conventional cantilever testing and by spherical indentation at different temperatures. It was found that reasonable estimates of the glassy modulus, rubbery modulus and glass transition temperature could be obtained from simple load/unload indentation.
Composites with Controllable Stiffness
  • H Maples
H. Maples, "Composites with Controllable Stiffness," Imperial College London, 2014.