Tailoring interlaminar shear and mode-I fracture behavior in fiber-composites via soft self-healing thermoplastic inclusions

The hierarchical microstructure of fiber-reinforced composites (FRC) enables lightweight materials with exceptional mechanical properties. However, their layered architecture is prone to interfacial damage, notably delamination. An effective strategy to mitigate delamination is by integrating thermoplastic interlayers, which not only enhance FRC resistance to interfacial fracture, but also enable self-repair of cracks through thermal mending. In this study, we demonstrate for the first time, repeated in situ self-healing of FRC laminates under both mode-I fracture (via the double cantilever beam) and 3-point flexure (employing short-beam shear). Remarkably, we achieve nearly complete restoration over ten consecutive healing cycles from thermal remending of 3D-printed poly(ethylene-co-methacrylic acid) (EMAA) interlayer inclusions. To understand the mechanical effects of such soft inclusions, we conduct a comprehensive experimental and numerical investigation. Our research findings reveal: (i) Markedly different strain states in short-beam shear with soft inclusions compared to FRC without. (ii) The necessity of incorporating contact algorithms for accurate finite element (FE) simulation of local stress/strain fields and global structural responses. (iii) Adjustments in the density and layer placement of printed EMAA domains can tailor both interlaminar shear strength (ILSS) and mode-I fracture resistance (GIC). This research offers newfound insights into realizing self-healing in actual structures, reliable and efficient simulation strategies for modelers, and advancements towards more modern design motifs and suitable materials testing protocols.