Fatigue and Impact Behaviour of Carbon Fibre Composite Bicycle Forks

Abstract

A B S T R A C T The use of carbon fibre-based polymeric composites in bicycle components has dramatically increased in the last few decades. This paper presents the results of fatigue and impact testing on bicycle forks that are known to have quality-related manufacturing defects. In the investigation performed, acoustic emission testing is used to monitor crack growth during fatigue loading and impact. The results show three distinct stages in composite fatigue failure related to initiation, propagation and final growth. Implications of the results on design and testing of composite bicycle components are addressed pointing to a greater need in understanding current design procedures.Keywords acoustic emission of manufacturing defects; bicycle components; fatigue and impact response. N O M E N C L A T U R Ea = crack length l = average delamination length m = material parameter C = material constant N = number of cycles ρ = transverse crack density G = difference between maximum and minimum strain energy release rate K = difference between maximum and minimum stress intensity factor I N T R O D U C T I O NThe world's top performing cyclists have been quick to embrace the advantages of carbon fibre composites. During the last decade, the reduction in costs has resulted in this carbon fibre technology being filtered down to the casual consumer. Today, most retail bicycle shops carry several models of carbon fibre bicycle frames and components catering to cyclists willing to pay a premium for the optimum of lightweight bicycle performance. The use of carbon fibre-based composites is expected to grow as trends continue to show a move towards lower costs of production. Manufacturers typically combine carbon fibres with a thermoset or thermoplastic resin to achieve a structural material with a highly superior-specific strength and stiffness in comparison to metallic alternatives. Compared with other material performance criteria, however, carbon fibre composites possess unique design challenges due to the relatively lower strain to failure and linear stress-strain response. As such they require careful design to increase fatigue performance through careful selection of laminate schedules to provide adequate damage tolerance and performance. Laminated carbon fibre components are significantly different in their properties and behaviour from metals, which are homogeneous. These composites can offer several advantages over metals. The direction and number of layers can be varied to achieve greater strength or stiffness in various directions based on the stresses to be experienced by the design. However, the manufacturing process can produce anomalies that are not necessarily accounted for in testing standards that were originally developed for metallic bicycle components (Fig. 1). These anomalies such as porosity or fibre waviness can impact the mechanical behaviour of the composite components. 1,2 Carbon fibre-based composites also vary from metals in that they lack the ductility and toughness that make metals such an ad...

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