Manufacturing Thermoplastic Fibre Metal Laminates by the InSitu Polymerisation Route

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Manufacturing Thermoplastic Fibre Metal Laminates by the InSitu Polymerisation Route

Host Institution: University of Edinburgh

Lead Investigator: Conchur O’Bradaigh, Dimitrios Mamalis, Dipa Roy, Vasileios Koutsos

Aims

Recently, there has been a growing interest to combine composite layers and traditional metal alloys developing the hybrid materials which are also called fibre-metal laminates (FMLs). The FMLs combine the superior properties of both the constituents. They offer improved fatigue resistance and enhanced damage tolerance under impact in comparison to the individual monolithic metallic alloys or fibre reinforced polymer composites. Thus, FMLs are promising lightweight materials for future application in various industries including transport, construction, renewable energy etc. However, the ultimate performance of the FMLs is not only determined by the two constituent materials, but is also influenced to a large extent by the interface formed between them.

The interfacial bonding between the metal sheets and fiberglass plies is significantly affected by their surface roughness and surface energy characteristics. Therefore, surface treatment of the metal, prior to bonding with the composite layer, is a critical step in the bonding process which controls the mechanical performance of the FMLs.

The aim of the project is to develop a new generation of thermoplastic composite (fibre)-metal hybrid laminates using inexpensive resin infusion route. These laminates are likely to be thermoformable and recyclable/reusable.

This will allow manufacturing of hybrid laminates at a lower cost in industrial scale with enhanced properties. Once the properties are validated, these laminates have a potential to find application in transport, construction, renewable energy industries. Representatives from the relevant industries have been contacted and meetings will be held soon to get their inputs on the project findings.

This project involves three major steps:

  1. To prepare the surface of the metal alloy sheets with suitable chemical or physical (atmospheric plasma) treatments to achieve an acceptable level of interfacial bonding with the composite layer.
  2. To manufacture FMLs using vacuum assisted resin infusion route.
  3. To investigate the properties of the FMLs in comparison to an equivalent reference laminate with no metal interlayer.

In the first step of the project, different chemical and physical (atmospheric plasma) surface treatment methods were evaluated. Trial FMLs were manufactured with the treated metal layers. The fibre-metal bondings were qualitatively tested to downselect the optimum treatment processes for the next step of FML manufacturing.

In the second step, novel thermoplastic composite (fibre)-metal laminates (FML) were successfully manufactured with infusible thermoplastic liquid resin using Vacuum Assisted Resin Infusion technique. Few micron thickness of an organic coating was applied at the interface of the surface treated metal and the composite layer to promote bonding through in-situ polymerisation with the matrix resin.

The overall objective of the project is to develop thermoplastic composite (fibre)-metal hybrid laminates by low-cost VARTM route. The laminates will be thermoformable and recyclable with enhanced mechanical properties.

Progress

  • Al alloy sheets (6082-T6) were prepared for manufacturing fibre-metal laminates (FMLs) i.e. hole mapping for resin infusion process.
  • Surface treatments were done with chemicals i.e. alkaline and acid etching of the Al alloy sheets. The effects of different parameters such as treatment time, concentration and mixing of the chemical solutions on the Al alloy surface were studied.
  • Surface modifications of the Al alloy sheets were also done using atmospheric plasma with varying conditions such as distance of the plasma nozzle, scan speed and number of scans.
  • Chemically and plasma treated Al alloy sheets were examined via optical microscopy, surface profilometry and contact angle goniometry.
  • FMLs were manufactured by vacuum assisted resin infusion process (VARTM) with thermoplastic resin/glass fibres and Al alloy sheets.
  • Mechanical characterisation of the fabricated FMLs such as short beam shear test (interlaminar shear strength) and flexural tests (3-point bend test) were performed.
  • The interfacial bond strengths between the Al alloy sheets and the thermoplastic composite layers with the use of two types of coatings were tested qualitatively.

Key Achievements

  • Down-selection of the optimum chemical treatments and atmospheric plasma treatment conditions for Al alloy sheets
  • Successful VARTM manufacturing of fullywetted FMLs with liquid thermoplastic resin
  • Successful coupon extraction from the laminates for testing without any debonding at the thermoplastic composite-metal interface
  • Mechanical characterisations: 1. Flexure strength- Reference (no metal) 500 MPa and FML-463 MPa 2. Flexural Modulus-Reference (no metal) 24 GPa and FML-28 GPa.
  • Bonding achieved between Al alloy sheets and thermoplastic composite layers through in-situ polymerisation, which is first of its kind and has never been reported before
  • New generation of thermoplastic fibre metal laminates were produced with low cost VARTM route using thermoplastic liquid resin and Al alloy sheets which can offer enhanced fatigue resistance and improved damage tolerance in comparison to the monolithic metallic alloys or composites. Such FMLs can fulfil a large demand in industry to have strong, lightweight, recyclable and durable structures.
  • Interfacial bonding achieved between the fibre and the metal layers via surface treatment and an organic coating through in-situ polymerisation with the matrix is novel and can have a significant impact on the FRP industry.
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