Inter-ply friction behaviour in the temperature assisted forming of magnesium/glass fibre reinforced thermoplastic polymer laminates

The paper investigates the metal/prepreg inter-ply friction behaviour in thermoplastic fibre metal laminates (FMLs) based on magnesium alloy and glass fibre-reinforced polymer (GFRP) under a range of parameters (normal pressure, relative sliding velocity, temperature) that are typical of forming processes conducted at elevated temperature on FMLs. The surface roughness of metal sheets was also varied and its influence on the inter-ply friction behaviour was evaluated. Pull-through tests with a stop-start controlling strategy were designed and carried out for identifying the inter-ply friction coefficient at varying conditions. To support the quantitative data from the tests microscopic observations were performed on the residues of the prepregs left on the tested metal sheets. The obtained results proved that the differences in the friction coefficient are strongly dependent on the process conditions and the metal surface roughness. The increase in the relative sliding displacement enables the transition from the hydrodynamic to the mixed lubrication mode according to the Stribeck theory, which further rises the friction coefficient. The inter-ply friction coefficient is observed to increase at decreasing temperature and normal pressure, and at increasing sliding velocity and metal surface roughness.