Bridging in co-cured composite joints

Further deployment of advanced polymer-based composites in critical structures requires, among others, new progress in adhesive bonding solutions from processing to performance. Among others, the cost and time intensity of manufacturing can be mitigated through co-curing of composite elements using a thermoplastic film, typically a Polyetherimide (PEI) film. However, the use of PEI films in resin transfer moulding leads to low toughness (<500 J/m(2)). The objective of this work is to improve the toughness of co-cured composite joints by inserting UHMPE filaments between a PEI film and the composite panels in order to trigger crack bridging and a significant R-effect. The influence of the spacing and of the tensile strength of the filaments has been determined experimentally, resulting in an increase of the toughness by up to a factor 3. In addition, the influence of these parameters has been further investigated with a closed-form crack propagation model. Decreasing the spacing and increasing the strength of the filaments enhances the joint toughness. Increasing the tensile elongation of the filaments results in a less steep R-effect, requiring a longer crack propagation to attain of the maximum fracture toughness.