Failure modes of CF-PEEK overmoulded onto unidirectionally reinforced CF-LM-PAEK: Multi-scale pull-off testing and microscopy investigation of rib stiffened geometries

Overmoulding is a manufacturing technique used to create structural composite components with complex shapes, combining regions of continuous and short fibre reinforcement. It is possible to produce structural parts at high rates, whilst minimising materials costs. The interface between the regions of continuous and short fibre material is key to the performance of the structure.This work investigates pull-off testing as a direct method for measuring rib-on-plate bonding strength, a common structural feature of overmoulding. Different rib joint types (Butt, Radius, Step) and specimen sizes (1, 5, 20 mm) are used to investigate the fidelity of the test, when applied to unidirectionally-reinforced low melting point (LM)-PAEK bonded to PEEK ribs.The main findings are that the Butt joint appears to provide the most reliable pull-off test data in terms of determining the interface strength. While the Radius and Step configurations improve local strain and moulding consistency respectively, they tend to promote failure through the ply structure rather than the interface. Recommendations are made in relation to specimen length, ply structure and allowable deformation during manufacture. • Step joints give the most consistent out of plane deflection and pull-off strength • Butt joints provide the most consistent interface pathway failure mode • Shorter specimen lengths result in more through-ply failure modes • High out of plane deflection increases pull-off strength • Ply layup near the surface significantly affects pull-off strength