Development of composite materials modified with bis-maleimide polymer containing carbon nanoparticles: Investigation of fracture toughness properties and healing capabilities

• Κύριος συγγραφέας: Ρούβαλης, Κωνσταντίνος
• Άλλοι συγγραφείς: Rouvalis, Constantinos
• Γλώσσα: English
• Έκδοση: 2021
• Θέματα: Composites; Self-healing; Electrospinning; Fiber bridging; Cohesive zone model; Σύνθετα; Αυτοΐαση; Ηλεκτροϊνοποίηση; Γεφύρωση ινών; Μοντέλα συνεκτικής ζώνης

In the present thesis, an investigation was conducted on the fracture toughness properties and the Self-Healing (SH) capability of a Carbon Fiber Reinforced Polymer (CFRP), which was modified with interleaves of a self-healable Bis-Maleimide (BMI) polymer – based on the reversible Diels-Alder (DA) reaction – that contained carbon nanoparticles. The formation and distribution of BMI interleaves (SH Agent - SHA) on the CFRP plies was performed locally before curing, only in interlaminar regions of interest, by the Solution Electrospinning Process (SEP). Namely, reference (unmodified) and BMI-modified CFRPs – either without or with one of two nanoparticle types, viz. Multi-Walled Carbon Nanotubes (MWCNTs) and Graphene Nanoplatelets (GNPs) – were fabricated and tested under Three-Point Bending (3PB) and Mode I Delamination Fracture conditions. The experiments showed that the SHA-modification had only a small negative effect on the flexural mechanical properties of the CFRPs and their interlaminar fracture toughness properties were enhanced, mostly due to the fiber bridging phenomenon (caused by the presence of SHA), with the BMI & GNP-modified case exhibiting the best toughening performance. Subsequently, a healing process took place, but the repetition of Mode I fracture tests indicated that low recovery of mechanical and fracture properties was achieved. Based on the first Mode I Fracture experiments, Finite Element Models (FEM) were developed, including Cohesive Zone Models (CZM) with Multilinear Cohesive Laws to account for the Interlaminar Mode I Fracture response of the specimens. The suitable CZM parameters were in agreement with the experimental conclusions, regarding the superiority of the BMI & GNP-modified CFRP and the principal role of fiber bridging to the toughening effect. Although the numerical models offered a good approach around the maximum load of the fracture response, they displayed some deviation from the experiments in the propagation part of the force – displacement curves. This was mainly attributed to the insufficient approximation of the stress field around the delamination crack by the utilized plane strain finite elements and secondarily to the inability of the models to describe possible degradation of the CFRP properties around the crack of the DCB specimen. In the end of the thesis, suggestions are made for development of the implemented SH strategy (to achieve healing) and improvement of the numerical models (to reduce deviation from the experiments).