Delamination is a critical failure mode of composite laminates, which is typically investigated though fracture tests on standardized beam-like specimens, such as the Double Cantilever Beam. However, in-service components may experience delamination under constrained conditions that are not typically considered in standard fracture testing. Studies on two-dimensional delamination of central defects in composite laminates have highlighted that significant membrane stresses arise during delamination due to the geometrical constraints, and that their presence may affect the delamination behavior. The influence of membrane stresses on mode I delamination behavior has however not been systematically investigated. In this work, a novel Constrained Double Cantilever Beam (CDCB) specimen was developed and tested to investigate mode I delamination under constrained conditions. The results provide a first experimental assessment of the CDCB configuration, with emphasis on the evolution of in-plane stretching and its influence on crack propagation and specimen stiffness.
Thin-ply unidirectional (UD) carbon-fibre reinforced polymer (CFRP) tapes enable improved mechanical performance in advanced aerospace structures but remain highly sensitive to manufacturing-induced defects. Reliable monitoring at the tape level is therefore essential to ensure consistent fibre distribution and prevent defect propagation into subsequent laminate assemblies. This study evaluates the feasibility of high-frequency Eddy-Current (EC) sensing for through-thickness inspection of individual carbon-fibre thermoplastic (CFTP) tapes. A differential transmit–receive coil pair was designed, manufactured, and optimised for the constrained geometry of UD tapes, enabling through-thickness interrogation in high resolution. UD-CFRP tapes were produced using a controlled powder-based dispersion slurry and calandering process, in which fibre tension, calander pressure, and line speed were varied, from which representative fibre-distribution anomalies could be detected. Twenty-one samples were scanned using a motorised system operating near a 50 MHz resonance, and spatially resolved maps of transfer magnitude and phase were obtained. The sensor exhibited sensitivity to variations in fibre packing density, tow spacing and structure, and local conductivity, with signal-to-noise ratios reaching up to 30 dB for the most distinct features. While some defect classes produced lower contrast, the results indicate that the through-thickness measurement configuration can detect prominent tape-level structural variations. These findings demonstrate the feasibility of inductive EC sensing as an evaluation tool for thin-ply tape production. The approach provides a foundational step toward developing future process monitoring strategies and improving the reliability of next-generation composite manufacturing.
Developing thermoplastic polyurethane (TPU) composites with superior flame retardancy, smoke suppression, and mechanical performance remains a significant challenge. In this work, multifunctional lanthanum/phosphorus-decorated, boron/nitrogen-doped graphitic nanotubes (La@P-BCN-GNTs, abbreviated as LPBN) were successfully fabricated and incorporated into TPU via melt blending. The introduction of phosphorus-containing groups and lanthanum species not only improved the interfacial compatibility between BCN nanotubes and the TPU matrix but also imparted intrinsic flame-retardant functionality. Owing to the synergistic effects of structural reinforcement, catalytic charring, and barrier protection, the TPU/LPBN composites exhibited significantly improved fire safety. At an LPBN loading of 15 wt%, the composite achieved a vertical burning (UL-94) V-0 rating and a limiting oxygen index (LOI) of 29.1%. Meanwhile, the peak heat release rate (pHRR), peak smoke production rate (PSPR), and total smoke production (TSP) were reduced by 65.5%, 60.5%, and 40.9%, respectively, compared with neat TPU. Notably, the composite exhibited a tensile strength of 28.2 MPa while maintaining excellent flame-retardant performance. The synergistic effects of catalytic carbonization, enhanced graphitization, and nanotube-reinforced barrier protection effectively suppressed heat and smoke release during combustion. This work presents a promising strategy for designing high fire-safety TPU composites by multifunctional hybrid nanofiller engineering.
