Host Institution: University of Edinburgh
Lead Investigator: Dipa Roy
Fibre Metal hybrid Laminates (FMLs), where thin metal sheets and fibre-reinforced composites are integrated, offer the superior characteristics of the composites combined with the ductility of the metals. Hybrid composites containing continuous fibre-reinforced plies and metal layers offer unique mechanical properties. Typical examples of such composites include GLARE (glass fibre/aluminium), ARALL (aramid fibre/aluminium) and CALL (carbon fibre/aluminium) which are attracting interest from a wide range of engineering sectors. The mechanical properties of epoxy-based fibre–metal laminates, manufactured mostly by prepreg/autoclave technology, have been investigated in a number of studies.
The experimental results indicated that the damage threshold energy of such hybrid laminates is significantly higher than the values offered by traditional engineering materials. In recent times, out-of-autoclave Vacuum Assisted Resin Transfer Moulding (VARTM) has been proved to be a successful technology to manufacture FMLs with thermoset epoxy resins. In spite of superior properties, such thermoset FMLs possess some distinct drawbacks such as poor interlaminar shear strength and problems associated with repair and non-recyclability. Thermoplastic FMLs can offer some advantages over thermoset FMLs such as higher interlaminar fracture toughness, easier repairability and recyclability. Most of the research carried out in the field of thermoplastic-metal composites has been on GF/PP (thermoplastic composite or TPC) and metal where an adhesive layer is used to bond the TPC and the metal layers.
Hybrid laminates are frequently subjected to thermal and mechanical fatigue loading. Along with mechanical loading, thermal effects are an important factor which determines the stress distribution in composite materials. During the curing process, when adhesively bonded composite/metal laminate structures are held at elevated temperatures over 120 ◦C and cooled to room temperature, very high residual stresses can build up because of the difference in coefficients of thermal expansion (CTE) between metals and polymers. This thermal mismatch results in delamination or debonding of hybrid composite materials, which facilitates fatigue crack growth in the polymer/metal interface. To decrease the thermal residual stresses in the structure, the curing cycles of high temperature adhesives are currently modified to cure at temperatures lower than standard conditions. Room temperature or low temperature curing might be beneficial in this aspect.
Improving the reliability of hybrid composite materials depends on enhancing the polymer/metal interface bonding, which will also dictate the long-term durability of the interface between the composites and the substrate structure. In this work, thermoplastic FMLs will be manufactured with an in-situ polymerisable infusible thermoplastic resin Elium® 4 and glass fibre reinforcement. In- situ polymerisation will be used as the key step to bond surface-treated metal with the polymer, without the use of an adhesive layer. As the bonding layer will be thermoplastic, it will be reversible in nature and will be suitable for recycling and re-use. Any damage or crack in the laminates will be repairable by injecting the pre-catalysed liquid resin. This could be advantageous for on-site repairing of products.
This study will support the research priority areas of the EPSRC Future Composites Manufacturing Hub 'Manufacturing for multifunctional composites and integrated structures' and 'Recycling and re-use'. Use of a commercially available liquid thermoplastic system and conventional thermoset-like composite manufacturing processes will enable industrial scale manufacturing of such hybrid laminates once the concept is proven. The advantage of in-situ polymerisation will be utilised to introduce a strong bond at the metal/TPC interface and scientific investigations will be carried out to analyse the bond strength and to measure the hybrid laminate properties.