Experimental and Numerical Assessment of Adhesively Bonded Non-Crimp Fabric Carbon Fiber/Epoxy Composite Joints
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To achieve a reduction in the contribution of greenhouse gas emissions from the transportation sector, automotive manufacturers have shifted towards utilization of advanced lightweight materials for vehicle structures, including fibre reinforced plastic (FRP) composite materials. FRPs exhibit desirable high specific strength, stiffness and energy absorption capabilities; however, they often have relatively high manufacturing costs and duration of part manufacture cycles. The recent development of heavy-tow non-crimp fabric (NCF), rapid curing resins and automated fabrication processes such as high-pressure resin transfer moulding (HP-RTM) enable reduced manufacturing cost and cycle times, which may accelerate the integration of FRP composites into the structures of high-volume production vehicles. A prevailing challenge for FRP composite structures is the design and assessment of robust joining methods. Conventional mechanical fastener joining methods require drilling holes in FRP parts, which affects the continuity of the reinforcing fibres, causes local damage around the periphery of the drilled hole, and ultimately degrades mechanical performance of the composite. On the contrary, owing to lower structural weight, lower fabrication cost, and improved damage tolerance, adhesive bonding is a more widely considered joining method for the assembly of composite structures. The objective of this thesis was to assess the performance and fracture behaviour of adhesively bonded NCF carbon-fibre-reinforced plastic (CFRP) composite joints. During the first part of this study, adhesively bonded NCF-CFRP single lap joints (SLJ) were used to assess the influence of the composite adherend surface treatment and stacking sequence on the joint strength. The highest joint strength (24.7 MPa) was achieved for specimens with adherends treated by abrading the surface with sandpaper compared to acetone degreasing or grit blasting. Furthermore, specimens comprising adherends with a higher effective flexural longitudinal modulus exhibited higher joint strength. Variation in the CFRP adherend stacking sequence also led to distinct failure processes. The intra-ply and inter-ply cracks observed in the adherends were driven by local in-plane and out-of-plane stresses, which was confirmed through corresponding three-dimensional finite element analyses of the SLJ specimens. During the second part of the study, double cantilever beam (DCB) tests were employed to assess the effects of adherend fibre volume fraction, adhesive bond-line thickness and loading rate on Mode I fracture behaviour of adhesively bonded NCF-CFRP joints. Increasing the fibre volume fraction of the CFRP adherend, the bond-line thickness of the adhesive or the loading rate increased the average Mode I critical strain energy release rate (G_Ic). A two-dimensional finite element model was employed to calibrate the Mode I traction-separation law parameters for a cohesive zone model using a commercial finite element software (ABAQUS 6.14-2). The predicted force-displacement response was in good agreement with that of the DCB tests, which demonstrates the capability of the numerical model to capture the macroscopic response of the DCB specimen.
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Ramin Chitsaz Dehaghani (2021). Experimental and Numerical Assessment of Adhesively Bonded Non-Crimp Fabric Carbon Fiber/Epoxy Composite Joints. UWSpace. http://hdl.handle.net/10012/16937