Temperature-dependent fracture characterization of methacrylate adhesive-bonded glass fibre-reinforced reactive thermoplastic composites

The Mode I and Mode II fracture behaviour of glass fibre-reinforced reactive thermoplastics bonded with a methyl methacrylate adhesive was characterized under room temperature (RT) and low temperature (LT; –40 °C) conditions via double cantilever beam (DCB) and end-notch flexural (ENF) tests at varying bond-line thicknesses. RT-DCB specimens exhibited a higher peak force, lower elastic stiffness, and higher initiation fracture toughness (77 % increase for thickest bond line) compared to LT-DCB specimens. Mode I initiation fracture toughness was insensitive to bond-line thickness at RT but decreased with increasing bond-line thickness at LT. RT-DCB specimens exhibited stable ductile cohesive failure. Conversely, LT-DCB specimens exhibited unstable oscillatory crack propagation between the substrates which was influenced by the substrate microstructure and caused by thermally-induced residual stresses in the adhesive layer. RT-ENF specimens exhibited higher peak force and a positive correlation with bond-line thickness, while LT-ENF specimens showed a negative correlation with bond-line thickness. All RT-ENF specimens were characterized by ductile cohesive fracture with consistent hackle formations, while LT-ENF specimens experienced brittle failure within the substrate characterized by fibre pullout and shear cusps between fibres. The fracture characterization results presented can contribute to optimizing the damage tolerance of corresponding adhesive-bonded components.