Mixed-mode I+II fracture behaviour of thermoplastic composites and their adhesive bonded joints

In this work, a comprehensive experimental and numerical investigation of the fracture behaviour of carbon fibre-reinforced polyamide 6 thermoplastic composite materials is presented, considering both bulk laminates and adhesive bonded joints under pure and mixed-mode I+II loading conditions. Five fracture test configurations were employed allowing broad coverage of the G I – G II space. Due to the low surface energy of thermoplastic composites, an optimized two-part acrylic adhesive (3M™ DP8805 NS) was chosen to improve bonding performance. To address unstable crack propagation beyond the loading point observed in some tests, an extended version of the Compliance-Based Beam Method (CBBM) was developed. A finite element model incorporating cohesive zone modelling (CZM) with a trapezoidal law was used for validation procedure. The obtained fracture envelopes revealed that while bulk laminates maintain nearly constant toughness across the entire mode mixity range, bonded joints exhibit strong degradation under mode I influence. • Mixed-mode fracture characterization of TPCs and their bonded joints was performed. • Five different tests were employed to cover the entire range of the mode ratio. • New data reduction analysis based on equivalent crack length is proposed. • Fracture envelopes for bulk laminate and bonded joints were obtained and compared.