Non-linear behaviour of polymer composites, moisture effects and accelerated testing

Abstract: The society of today is moving fast and the demands on almost everything are getting higher and higher. This can also be seen in the field of material science, where lightweight, high performance materials are gaining increased interest in order to be able to develop for example high performing vehicles with low fuel consumption or wind turbines with higher efficiency. Polymeric composites are a material group, where stiff fibers are embedded in an adhesive thermosetting matrix, which can fulfill these demands. Fibers and matrix can be combined in numerous ways to meet various design criteria’s. The most common matrices are fossil oil based and most fibers such as glass, carbon and aramid are also made from non-renewable feedstock. The use of fossil oil-based resins and non- renewable fibres seems to be in contrast to the world wide ambitions towards a sustainable society whereas the relatively new material group of bio-based composites is both renewable and do not use up valuable fossil recourses. On the other hand, if lightweight structural composites made from non-renewable resources can have a very long service life and can contribute to reduced fuel consumption in the transport sector etc, these could be a more sustainable option than other competing materials for example steel. However, the properties of both non-renewable and renewable composites have similar time- and temperature dependency. This has to be analyzed and understood in order to ensure their long-term reliability as well as their sustainability and improve their competiveness. Research in this area is therefore of high importance. However, long term testing in service conditions is both time consuming and expensive. It is therefore easy to understand and apprehend the need for accelerated test methods (ATM). An ATM uses an accelerated mechanism to increase the rate of degradation. The scope of this thesis and the presented papers are to contribute in the field of time dependence and life assessment for composite materials, both oil-based and bio-based, under different environmental loads. The discussed topics are in the field of viscoelasticity and viscoplasticity, moisture sorption behavior and the moisture influence on mechanical properties and stress relaxation. The method of single fibre fragmentation has been evaluated as a method to finding the constituent’s properties that in turn can be used for prediction of structural behavior. Finally the work focuses on the competiveness of bio-based resins and composites against oil-based alternatives in the field of long-term properties, moisture absorption behavior and moisture influence on mechanical properties. The results indicate that oil-based composite materials retain their properties for a number of decades even with the influence of water. The results also show that bio-based composites are competitive for service life of many years. From the studies and results is it concluded that composite materials has a bright future when it comes to construction materials due to their properties and ability to restrain environmental loads and harsh environments.