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Development of intrinsically heated, interleaved composites with controllable flexural stiffness and shape memory capability

  • TCKT - Transfercenter für Kunststofftechnik GmbH
Development of intrinsically heated, interleaved composites with controllable flexural stiffness and
shape memory capability
Authors: G.G. Murali1, P. Robinson1, A. Bismarck2, and C. Burgstaller3
1 The Composites Centre, Department of Aeronautics, Imperial College London, South Kensington
Campus, London SW7 2AZ, United Kingdom
2 Polymer and Composite Engineering (PaCE) group, Department of Material Chemistry, University of
Vienna, Waehringer Strasse 42, A-1090 Wien, Austria
3 Transfercenter für Kunststofftechnik (TCKT), Franz-Fritsch-Straße 11, A-4600 Wels, Austria
Carbon fibre reinforced epoxy polymer (CFRP) composites interleaved with Polystyrene (PS)
have been shown to exhibit controllable flexural stiffness properties [1]–[3]. A significant reversible
reduction in flexural stiffness is produced when the composite is heated above the glass transition
temperature of PS. At this temperature, the composite can be readily deformed and if cooled in this
deformed state, it will retain the new shape. These composites also display shape memory behaviour;
if the composite is reheated in its new shape, it will return to its original cured shape [4].
In the previous studies investigating the controllable flexural stiffness and shape memory
characteristics, the interleaved composite specimens were heated in an oven. In the current work, we
have explored the use of an embedded SS304 stainless-steel (SS) mesh (plain weave, wire diameter:
0.028 mm, aperture: 0.082 mm) procured from The Mesh Company (UK) as a heating element within
the interleaved composite (Hexcel Fibredux 914/TS/6K/5/34% 0⁰ CFRP plies interleaved with 0.07 mm
Empera 124N PS sheets). The composites were laid up in [0⁰/PS]7/ 0⁰ sequence with (1) Layup A having
no embedded SS mesh, (2) Layup B having 1 SS mesh and (3) Layup C having 3 SS meshes. For Layup
B the SS mesh was embedded in the central PS interleaf (i.e. interleaf layer 4 when counting from the
top). For Layup C the SS meshes were incorporated in interleaf layers 2, 4 and 6.
For layups with embedded SS meshes, the meshes were attached to a DC power supply to
allow Joule heating of the composites. A DC power input of 30 W was chosen as the power input for
Joule heating necessary to reach a surface temperature of around 120°C for controllable stiffness and
shape memory studies (120°C is the temperature used in previous works [1]–[4]). In layup A (which
was heated in an oven) and in layups B and C (subjected to Joule heating), the temporary loss in
flexural stiffness (measured by ASTM D7264-07 standard) due to heating was over 98.5%.
Furthermore, the shape memory capability of these composites was also investigated. Flat
coupons of these composite layups were heated (as described previously), bent to a ~90° angle in a 3-
point bend test setup and then cooled down in the deformed state. The ~90° shape was retained by
the composite and when these coupons were again subjected to heating (using an oven for layup A
and Joule heating for layups B and C) the original flat shape was fully recovered. For layups B and C,
the shape recovery was completed in approximately 1 minute after switching on the Joule heating.
The recovery time for Layup A was around five minutes. Optical microscopy was performed to
investigate any change in the internal structure of the composites after the shape memory trials.
[1] 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.
[2] H. Maples, “Composites with Controllable Stiffness,” Imperial College London, 2014.
[3] H. A. Maples, O. Smith, C. Burgstaller, P. Robinson, and A. Bismarck, “Improving the
ply/interleaf interface in carbon fibre reinforced composites with variable stiffness,” Compos.
Sci. Technol., vol. 128, pp. 185–192, May 2016.
[4] 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, Jun. 2017.
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.
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.
Polystyrene-interleaved carbon fibre reinforced epoxy composites exhibiting controllable stiffness have been manufactured. These composites undergo reductions in flexural stiffness of up to 99% when heated above the glass transition temperature Tg of the interleaf layers. Potential applications for such materials include their use in morphing and deployable structures. Flexural tests at room temperature indicated that improvements in adhesion between the polystyrene and CFRP layers are required to prevent premature failure of the composites at low shear stresses. Here we investigate how modification of the interleaf layer improves the interlaminar shear strength of the laminates without affecting the stiffness loss at elevated temperatures. Two poly(styrene-co-maleic anhydride) (SMA) films with different maleic anhydride content were prepared and used as interleaf films. Thick adherend shear tests showed that the adhesion strength more than doubled, while flexural tests showed that composites containing SMA interleafs had more than twice the apparent flexural strength of composites containing pure polystyrene layers at 25 °C and yet still undergo significant reductions in stiffness at elevated temperature.
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.
Composites with Controllable Stiffness
  • H Maples
H. Maples, "Composites with Controllable Stiffness," Imperial College London, 2014.