Adhesive bonding performance of out-of-autoclave thermoset and thermoplastic composite joints: Experimental and numerical investigation

Out-of-autoclave (OoA) thermoset and thermoplastic composites are increasingly being adopted for lightweight structures due to their lower manufacturing costs, reduced energy consumption, and improved sustainability. However, the adhesive bonding performance of these materials, particularly for hybrid thermoset–thermoplastic joints, remains poorly understood. To address this gap, this work presents a systematic experimental and numerical investigation of adhesively bonded joints manufactured using three OoA composite technologies: heated vacuum-assisted resin transfer moulding, thermoset hot pressing, and thermoplastic hot stamping. Material and joint behaviour were characterised through bulk tensile, thick-adherend shear (TAST), double cantilever beam (DCB), end-notched flexure (ENF), and single-lap joint (SLJ) tests, considering both homogeneous and hybrid joint configurations. The thermoset hot-pressed composites exhibited the highest apparent shear strength (25.55 MPa), whereas the hybrid thermoset–thermoplastic joints reached up to 22.56 MPa. In contrast, the weakest hybrid configuration achieved only 9.45 MPa. Thermoset joints predominantly exhibited cohesive failure, while thermoplastic-containing joints failed mainly at the adhesive interface. A three-dimensional cohesive zone model reproduced the experimental maximum loads and qualitatively captured the dominant damage locations observed experimentally. Overall, this work provides a systematic comparison of adhesively bonded joints manufactured using three OoA composite technologies, including homogeneous and hybrid thermoset–thermoplastic configurations, providing valuable guidance for the design of lightweight composite structures.