A Novel Hybrid Toughening Strategy Using Core-shell Rubber Nanoparticles and Micro-fibre Non-woven Veils for Composite Laminates and Their Co-cured Joints

Abstract

Composite laminates are increasingly utilized due to their outstanding mechanical properties. However, out-of-autoclave manufactured composite laminates and their co-cured structures are prone to interlaminar fracture failure (i.e. delamination or debonding) because of their low fracture toughness. Therefore, this thesis investigates cost-effective toughening methods suitable for out-of-autoclave manufacturing to enhance the interlaminar fracture toughness of composite laminates and improve the structural performance of co-cured composite joints. Commercially available core-shell rubber (CSR) nanoparticles and non-woven micro-fibre veils are chosen as tougheners to introduce intrinsic and extrinsic toughening mechanisms. The toughening performance is evaluated using standard tests. First, the effect of binders on the toughening performance of non-woven veils is investigated. The results indicate that the veil binder influences inter-fibre and fibre-matrix adhesion, significantly affecting the toughening performance of non-woven veils, especially for high-modulus carbon fibre veils. Second, the toughening performance of carbon and polyphenylene sulfide (PPS) fibre-hybrid non-woven veils is compared with that of pure carbon veils and pure PPS veils. The fibre-hybrid toughening approach is observed to overcome the drawbacks of pure carbon veils (i.e. unstable crack growth) and pure PPS veils (i.e. falling R-curve) in interlaminar toughening. Third, two-scale hybrid toughening performance with CSR nanoparticles and non-woven micro-fibre veils is evaluated. The two-scale toughening strategy incorporates both intrinsic and extrinsic toughening mechanisms simultaneously, significantly improving interlaminar fracture toughness with a rising R-curve. This approach outperforms non-hybrid toughening methods based on either CSR particles or non-woven micro-fibre veils alone. Compared to hybrid toughening with carbon veils and CSR nanoparticles, hybrid toughening with PPS veils provides stable crack growth, which is more desirable for applications. Therefore, further investigation on the effect of hygrothermal ageing on the hybrid and non-hybrid toughening performance is conducted exclusively with PPS veils and CSR nanoparticles. The results show that the toughening performance of CSR nanoparticles and PPS veils is degraded with moisture ingress and the degradation is irreversible. Although the degradation is more pronounced in the case of hybrid toughening, the degraded fracture energies of hybrid toughened specimens remain considerably higher compared to the fracture energies of the dry untoughened specimens. Finally, co-cured composite single-step joints are manufactured using non-hybrid or hybrid toughening routes with CSR particles and PPS veils. The effect of various toughening methods on the quasi-static behaviour, high-cycle-fatigue behaviour and failure mechanisms are investigated. It is observed that the two-scale hybrid toughening method is more effective in improving the quasi-static strength and fatigue life of co-cured composite joints compared to the single-scale toughening route based on either CSR particles or PPS veils alone. Additionally, strain gauge and 2D-DIC data indicate that the improved fatigue life is due to the superior crack delay ability provided by hybrid toughening. Overall, this research demonstrates that the two-scale hybrid toughening method, utilizing CSR nanoparticles and micro-fibre PPS veils, effectively improves the interlaminar fracture toughness of composite laminates and the structural reliability of co-cured composite single-step joints under both quasi-static and fatigue loadings.

Date of Award	16 Dec 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Kali-Babu Katnam (Main Supervisor), Prasad Potluri (Co Supervisor) & Zhenmin Zou (Co Supervisor)