Creation of ‘over-moulded’ PEEK samples for interface bond strength predictions

The interfaces between thermoplastic (TP) over-moulded features and thermoplastic composite substrates are strongly affected by the process history and resulting crystallinity. However, there are no established tests to reliably examine these interfaces and the relation between process parameters and strength. To address this issue, we propose a new test, which adapts the ASTM 6415 Curved Beam Strength to explore the quality of the weld between neat TP (PEEK) and TP composites (PEEK/CF). The test optimisation was performed using Finite Element Analysis and aimed to deliver the right failure mode in the right location – the interface. This is challenging as the contrast in material properties creates the risk of premature polymer fracture or the failure plane shifting to the bulk of the composite. The study departed from the parametric exploration of material thicknesses and identified high hydrostatic stresses in the TP for the optimal thickness distribution found. Further modifications of the sample configuration, consisting of terminating the length of the TP layer, yielded a “corner with a hat” configuration. This new configuration inhibited the hydrostatic stresses while maintaining the maximum through-thickness stresses at the interface but resulted in the most probable failure mode becoming the σ11 fibre failure in the arms. Both the bi-material and ‘hat’ configurations can be a realistic and straightforward design to explore a range of processing conditions.