Adhesion-cohesion balance of prepreg tack in thermoset automated fiber placement. Part 2: Ply-ply cohesion through contact formation and autohesion

Contact formation and autohesion with respect to their role as the major mechanisms governing the tack between thermoset prepregs in automated fiber placement were explored. Therefore, a novel 90 degrees peel test with strictly separated and individually controllable compaction and debonding phases was employed for experimental tack characterization in a rheometer. Variation of compaction pressure, dwell time and temperature enabled the experimental isolation of contact formation and autohesion influences. The experimentally determined tack, plyply contact area and resin viscoelastic characteristics were used to parametrize simplified semi-empirical bond strength sub-models that have originally been developed for thermoplastic composite manufacturing techniques. The model prediction was validated successfully within the experimentally reproducible parameter range. Eventually, manufacturing scenarios for thermoset automated fiber placement (AFP) respecting different lay-up velocities (up to 1 m s-1), compaction pressures (up to 10 N mm-2) and both lay-up and mold temperatures (20-60 degrees C) were assessed in terms of estimated prepreg tack. The implication of both mechanisms, contact formation and autohesion, in the evolution of prepreg tackiness was found to be able to replicate the bell-shaped tack curves proposed by the adhesion-cohesion balance.