Process Parameters and Fatigue Properties of High Modulus Composites

Abstract

The climate change challenge of today requires construction materials that are strong, light and durable. This is true for the Wind Turbine Industry, where the length of wind turbine blades are in excess of 100 m, but also for applications in personal transport and in the aerospace industry. Fibre reinforced polymers are often the material of choice in these applications, either because they offer excellent strength and weight saving potentials over conventional materials, or because they can be tailored to sustain high magnitude cyclic loadings for long durations at a reasonable cost level. Modern wind turbine blades have almost exclusively been manufactured from glass fibre composite materials, and glass fibre composites is still the most important material for most blade manufacturers. The reason that glass fibre is so widely used in the wind turbine industry can be found in the exceptionally good fatigue performance of the material combined with a relatively low cost. The load carrying elements of a wind turbine blade is the spar cap, and most conventional designs uses a special class of unidirectional glass fibre composites in these regions. Furthermore, unidirectional composites are known for excellent strength and fatigue properties. The manufacturing process for unidirectional composites are mainly based on non-crimp fabrics made of fibre rowing stitched together. Despite being called unidirectional, non-crimp fabrics will typically include around 10% fibres oriented off-axis or transverse to the main direction. These fibres are included to fixate and control the fibre material during manufacturing, and to provide a small stiffness and strength contribution in the transverse direction of the composite material. The current research has shown that the off-axis or transverse oriented fibre rowingcan have a significant impact on the fatigue properties of the composite materials through introduction of small fatigue cracks which later will grow into the load carrying unidirectional layers. This is even the case for very thin backing layers build up of separate fibre rowing. The current work have used bending fatigue combined with optical microscopy to quantify the damage in the load carrying layers. A quantifiable link between the transverse cracks in the thin backing layers and the damage in the load carrying bundles have been shown to exist. The manufacturing process of composites influences the performance of the end product material. Prompted by recent findings of a link between the curing conditions and fatigue performance of unidirectional composites, the current research have investigated ways to modify epoxy cure cycles with the aim of lowering residual stresses in epoxy-based composites. The results show clearly that two-step cure cycles can produce compositeswith significantly lower residual stresses. The current research shows that residual stresses negatively impact the fatigue performance of unidirectional composites. Methods provided in the research have in some cases been shown capable of increasing the service life of a composite by a factor of 5. The increased service life is the result of applying the aforementioned two step cure cycles. The research also shows that the negative impact from high magnitudes of residual stresses can be superseded by the negative impact of high fibre volume fractions, if the fibre volume fractions exceeds a certain threshold.