Understanding failure mechanisms in composite injection overmoulded components through multi-scale imaging during pull-off testing

The structural integrity of composite injection overmoulded components is critically dependent on the bond between the overmoulded rib and the thermoplastic organosheet. However, the mechanisms by which these bonded regions fail remain poorly understood, limiting broader industrial adoption. This study investigates the failure behaviour of carbon-fibre poly-ether-ether-ketone (CF-PEEK) ribs injection overmoulded onto carbon-fibre low-melting point poly-aryl-ether-ketone (CF-LM-PAEK) organosheets using real-time, multi-scale imaging during pull-off testing. Results reveal that failure mode is strongly influenced by proximity to the injection gate. Near the gate, elevated thermal energy during moulding promotes strong interfacial bonding. Here, the interfacial bond exceeds the organosheet’s interlaminar strength and the failure mode is via delamination within the substrate. This delamination is driven by inherent porosity in the organosheet, likely introduced during deconsolidation and initial compaction. Conversely, in regions farther (10 cm) from the injection point, reduced melt temperatures result in weaker interfacial bonds. Here, failure preferentially occurs at the interface, governed by process-induced porosity arising from polymer shrinkage during cooling. Importantly, this study demonstrates that pull-off testing does not yield a singular "interfacial strength" value. Instead, it serves as a diagnostic tool for assessing local manufacturing quality in injection overmoulded thermoplastic composites, identifying whether the interface or the organosheet is the limiting factor in structural performance. These insights are essential for developing predictive models and refining manufacturing protocols for thermoplastic composite structures. • Pull-off testing serves as a diagnostic tool for assessing local manufacturing quality • Rib-on-plate joints near the injection gate fail at the organosheet • Rib-on-plate joints distant from the injection gate fail at the overmoulded interface • Process-induced local micro-pores dictate failure modes