Multi-scale observations and 3D modelling of inkjet printing modified carbon fibre composite

Thesis

Carbon fibre reinforced polymer (CFRP) composite is in high demand in industries such as wind turbine, automobile, and aerospace due to its exceptional strength and stiffness with low weight. However, its internal defects can lead to catastrophic failure by delamination. Therefore, enhancing interfacial resistance is critically important. Inkjet printing of polymethyl methacrylate (PMMA) or polyethylene glycol (PEG) between prepregs has shown promise in increasing fracture toughness with minor weight penalty and negligible compromise of other mechanical properties. To obtain the insight into the toughening mechanism, a series of characterizations as well as 3D finite element models were applied in this thesis.

This project began with a description of manufacturing process for inkjet printing modified CFRP. The mechanical performance was then evaluated using Double Cantilever Beam (DCB) test, which verified the reported toughening effect. A series of in situ observations were conducted on the laminates to study the Mode I fracture propagation behaviour: Dual X-ray imaging and diffraction analyses were employed with the aim of monitoring the lattice strain in carbon fibres via X-ray diffraction (XRD), while tracking crack tip position through radiographs processed with Digital Image Correlation (DIC). In-situ synchrotron X ray computed tomography was used to image the 3D evolution of damage propagation. Post processing using Digital Volume Correlation (DVC), the displacement fields were resolved, and larger critical crack openings were observed for the printed specimens. The displacement field was imported into a finite element model as boundary conditions to evaluate J-integral and quantify the different modes of stress intensity factor (K), confirming the enhancement in fracture energy post printing. The fracture surface of laminates was observed by multiple characterization methods, and larger surface unevenness was observed to correspond with higher fracture toughness. Confocal microscopy revealed that PEG diffused into the epoxy matrix during curing, whereas PMMA remained separate. Fractography observed micropores and fractured particles in the PMMA-printed sample. It was deduced that the PMMA deposits facilitated crack deflection and cavitation formation, while PEG improved the matrix adhesion. Both printed laminates exhibited increased active functional groups on the fracture surfaces, suggesting enhanced bonding activity. A finite- element microstructure meshfree (FEMME) model was then applied to simulate DCB experiments with varying interfacial strength. The model replicated the wider crack opening in printed samples, consistent with XCT observations.

Authors: Lu, Q

• DOI: 10.5287/ora-6rwxq4r5z
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: carbon fibre composite; finite element analysis; x-ray computed tomography
• Subjects: Materials science