SMA-Tufted CFRP Composites with Enhanced Interlaminar Toughness and Damage Detection Capabilities

<p dir="ltr">Carbon Fibre-reinforced polymer (CFRP) composites are widely used in high-performance structural applications due to their superior in-plane mechanical properties, lightweight design, and corrosion resistance. However, these laminated composites remain susceptible to delamination under out-of-plane or cyclic loading, which undermines their long-term damage tolerance. Delamination is also difficult to detect and may grow undetected under fatigue loading until it reaches a critical size, resulting in structural failure. Through-thickness reinforcement techniques, such as z-pinning, tufting, and hybridisation, have emerged as effective strategies to arrest delamination growth and improve interlaminar fracture toughness.</p><p dir="ltr">This PhD research investigates the multifunctional properties of composite laminates reinforced orthogonally (through-thickness) using Shape Memory Alloy (SMA) wire tufts. The study explores the influence of tuft material, geometry, and distribution on interlaminar fracture toughness and fatigue behaviour under mode I and mode II loading. The multifunctional role of SMA tufts in delamination crack closure and damage detection is also investigated. Furthermore, a comparative study is performed to quantify the level of improvement in fracture and fatigue resistance achieved with SMA tufts compared to other conventional tufting materials.</p><p dir="ltr">Composite laminates were fabricated using vacuum-assisted resin infusion, integrating carbon, copper, and shape memory alloy (SMA) tufts using a manual tufting process. Reinforcement parameters, including areal content, tuft density, angle, and filament morphology, were systematically varied. Standardised test procedures were followed to evaluate mode I and mode II fracture toughness, traction-separation property, and mode I fatigue crack growth resistance. Microstructural and fractographic analyses were conducted using optical microscopy, SEM, and X-ray computed tomography (CT) to identify the key toughening and strengthening mechanisms.</p><p dir="ltr">The second technical chapter (CHAPTER 4) evaluated the effect of SMA tuft wire diameter, specifically thin (0.15 mm) and thick (0.25 mm) at 0.30% areal content, on the interlaminar fracture toughness of carbon fibre-reinforced polymer (CFRP) laminates. Mode I double cantilever beam (DCB) and mode II end-notched flexure (ENF) tests demonstrated that SMA tufts significantly improved delamination resistance in both modes, with mode I fracture toughness enhanced by approximately 14-fold. CT analysis revealed the formation of extensive crack-bridging zones as the dominant toughening mechanism.</p><p dir="ltr">The third technical chapter (Chapter 5) presents a comparative assessment of CFRP composites tufted with carbon tow, copper, and SMA wires at a constant areal content of 0.3%. The results showed that both carbon and SMA tufts achieved similar improvements in mode I and mode II fracture toughness, exceeding nine-fold compared to untufted laminates. Chapter 6 extends this comparative analysis to mode I fatigue performance. SMA tufts provided comparable improvements to carbon tufts, while copper tufts offered the least enhancement due to lower strength and limited traction resistance. The performance differences were linked to the crack bridging efficiency of each material under cyclic loading.</p><p dir="ltr">Chapter 7 focused on the multifunctional performance of SMA tufts in enabling delamination crack closure and in-situ damage detection. Thermal activation tests confirmed that thicker SMA tufts enabled higher levels of crack closure (35–50%) due to enhanced stiffness and buckling resistance. The crack closure function remained repeatable across four thermal cycles. When used for damage sensing, the SMA tufts enabled electrical resistance-based monitoring of delamination propagation under both mode I and mode II loading. Mode I sensing showed higher gauge factors (~14.6) compared to mode II (~0.5), with a measurable lag due to the development of bridging zones.</p><p dir="ltr">The final chapter introduces a finite element modelling (FEM) framework to simulate mode I crack propagation in tufted composites. Developed in Abaqus/CAE, the model used a cohesive zone modelling combined with a traction-separation law for the tufts derived from the mode I bridging traction results for the tufts. The FEM results accurately predicted R-curve and steady-state toughness for carbon, copper, and SMA tufts at 0.30% areal content. The model was further used to predict performance at 0.15% and 0.45% areal content, revealing a nearly linear relationship between areal density and mode I fracture toughness.</p><p dir="ltr">The findings from this thesis demonstrate that SMA tufts offer a viable solution for enhancing delamination resistance, delivering crack closure functionality, and enabling in-situ damage detection in composite structures. These contributions support the development of next-generation smart composites with integrated sensing and damage-tolerant capabilities.</p>