• Kumar, Rajnish

Abstract

Pultruded profiles of unidirectional carbon fibre composites are unique materials with exceptional mechanical properties. For the wind turbine industry, spar caps made with pultruded carbon fibre composite profiles are opening the door to designing stiffer and lighter blades with improved aerodynamic performance and increased power output. However, the required accurate characterisation of the mechanical properties of such high- performance unidirectional composite materials is a challenge.<br/><br/>This thesis aims to improve the characterisation of tensile properties of pultruded profiles of unidirectional carbon fibre composites. Three objectives covering domains, namely, single fibre characterisation, composites characterisation and micro-mechanical analysis, were set to reach the overall research goal.<br/><br/>In the single fibre characterisation domain, tensile properties of a large sample size of single carbon fibres have been characterised at eight different gauge lengths using a semi-automated single fibre testing machine. A second-order polynomial fitting method was used to quantify the non-linear behaviour of the fibres using a so-called stress-strain curvature coefficient “α”. The measured failure stress of the fibres was analysed using the two-parameter Weibull distribution model, leading to an excellent linear fit (R² ≈ 0.99) of the log-log relations for all gauge lengths.<br/><br/>In the composite characterisation domain, advances in tensile testing of unidirectional carbon fibre composites are presented using a novel specimen type and a robust data analysis method. The proposed specimen type of protected X-butterfly is developed by combining the two strategies of specimen geometry and specimen protection. The study’s central finding is that the protected X-butterfly specimens led to an improvement of 17 % in the measured strain to failure and 20 % in failure stress compared to rectangular-shaped specimens based on the widely used ISO 527-5 standard.<br/><br/>In the micro-mechanical analysis domain, the stress-strain behaviour of unidirectional carbon fibre composites is predicted using a modified Curtin model. The fibre input modelling parameters are based on a comprehensive single carbon fibre dataset. The predicted stress-strain curve incorporating the non-linearity of the carbon fibres using the curvature coefficient α agrees well with the experimentally obtained stress-strain curve. Assuming a perfect bonding between the fibre and matrix, the model overestimates the end of the curve, the failure stress and strain to failure by 32 % and 38 %, respectively. For a more realistic bonding scenario, the overestimation of failure stress and strain to failure drops to 22 % and 28 %, respectively.<br/><br/>Altogether, the work of the thesis presents a link between the tensile behaviour of single carbon fibres and the tensile behaviour of unidirectional carbon fibre composites. Learnings from this thesis will help understand and realize the true potential of unidirectional carbon fibre composites in terms of their application in general and in particular for the spar caps in wind turbine blades.

Topics

• impedance spectroscopy
• Carbon
• stress-strain curve
• stress-strain behavior
• composite