The communication present results from work on development and evaluation of new polymeric carbon fiber composites with extreme temperature performance: Tg up to 360°C is targeted. The anticipated use of such composites is found in aeroengine-applications. In the work we are exploring a new and tailored phenyl ethynyl terminated imide (PETI) formulation, specially developed for the program. The formulation utilizes crosslinkers of the Nexamide" type (from Nexam Chemical AB, Sweden). The resins are initially evaluated from a processing and property perspective. Both DSC-measurements and rheology characterization are utilized in the development. Suitable RTM-processing schemes are investigated from a viscosity point of view. The schemes are used in the composite sample manufacturing. Besides a processing perspective the study also present the first results on physical behavior of the polymers and their composites.

A race towards a more sustainable society is going on worldwide and decreasing dependence on fossil resources in energy and transport sectors is main goals. One path to decreased oil usage is development of lightweight materials from renewable resources like bio-based composites. However these new bio-based materials have not only to compete in mechanical performance, they also have to restrain environmental loads like moisture and temperature over time. In this study two bio-based composites have been compared to an oil-based composite in terms of long-term properties and water absorption behaviour. The long-term behaviour is determined by dynamic mechanical thermal analysis, DMTA, and time temperature superposition, TTSP. The water uptake is determined by submersion of specimens into water and tracking their weight change over time. The moisture influence is characterised in form of water uptake and change in the master curves created by TTSP procedure. The results show that there is a significant difference in long-term performance between the bio-based and oil-based composites. It is realized that the bio-based composites can be a good alternative for some applications especially when taking their eco-friendly nature into account.

In this study, three different types of bio-based resins are compared to a conventional oil-based epoxy in terms of moisture uptake, long-term properties and its influence of moisture and glass transition temperature, T _g. Moisture uptake is determined by means of gravimetric method, time temperature superposition (TTSP), and T _g data obtained in dynamic mechanical thermal analysis (DMTA). Moisture uptake show Fickian diffuison behavour for all resins, saturation level and diffusion coefficient however differ. The long-term properties is characterised by creep compliance master curves created by means of TTSP. The examined bio-based resins are compatible to the reference epoxy in term of stability up to 3–10 years. Comparison between master curves for virgin, wet, and dried material show that moisture present in the specimen increases creep rate, and that some of this increase remains after drying of samples. T _g measurements show that moisture inside the specimen decreases T _g; this is anticipated because of the plasticizing effect of water. The overall conclusions are that the bio-based resins of polyester, and epoxy type are comparable in performance with oil-based epoxy, LY556 and they can be used to develop high-performance composites.

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.

Composite materials are an important and growing material group in a large number of industries such as aeronautics, marine, automotive, energy production and infrastructure. The most common composite materials is manufactured from fibres such as glass fibres and carbon fibres together with oil based resins i.e. polyester, vinyl ester and epoxy. An increasing use calls for alternative environmental friendly, biobased, constituents. The new biobased materials have not only to compete in mechanical properties but it also has to restrain environmental loads like moisture and temperature over time. In the present work are predictions of the long term properties of biobased resins made. The work also presents the influence of moisture, comparison with creep test data and comparison with oil based resin. The long term property prediction is made by using dynamic mechanical thermal analysis, DMTA, measurements and time temperature superposition, TTSP, [1,2]. The procedure is to make DMTA measurements in 3p-bening mode of the storage modulus at different frequencies at increasing temperature. The method used in this case was to measure the modulus at 0.1, 0.3, 1, 3 and 10 Hz at temperatures 25 – 175°C with 5°C intervals. The frequencies where then transformed to time by Eq. (1) below t=2/πω (1)where ω is the frequency. 25°C were chosen as reference temperature where as all other curves where shifted horizontally creating a continuous master curve. The modulus time master curve was then inverted creating creep compliance time master curve, this curve is compared to creep test. The moisture behaviour is characterised in form of water uptake, change in glass transition temperature, Tg, and change in dynamic response. The water uptake is determined by submersion into water and tracking the weight change over time. The glass transition temperature is determined in DMTA by conducting a temperature sweep at constant frequency. The change in Tg is used as a knock down factor and a vertical shift of the master curve. An attempt to determine the dynamic response of wet samples has also been made with the insight that higher temperatures will dry the sample during testing leading to a non homogenous moisture distribution within the sample. The results from the study show on differences between TTSP and creep data, it also shows the differences between different biobased resins and a comparison to oil based epoxy. REFERENCES1. Christensen R.M., “Theory of viscoelasticity, an introduction” 2nd edition, Academic Press, 1982.2. Ward I.M. and Hadley D.W., “An Introduction to the mechanical properties of solid polymers” 1st edition, John Wiley & Sons, 1993.

The non-linear and time-dependent stress–strain response of NCF [+-45]s laminates in tension is studied. Testing methodology is suggested to separate and quantify the effect of damage development, non-linear viscoelastic effects, and viscoplasticity on the inelastic response. This is achieved by decomposition of viscoelastic and viscoplastic response, both of them being affected by microdamage accumulated during the service life. Material model based on Schapery’s work on viscoelasticity and Zapas viscoplastic function with added damage terms is presented and used. Simulation is performed and validated with constant stress rate tensile tests, identifying the non-linear viscoelasticity and viscoplasticity as the major sources of the non-linear response.

Vacuum Infusion is a closed mould manufacturing process that can replace open mould processes. To improve resin flow it is common to add flow enhancement layers to the reinforcement stack. This paper is aimed to analyze water uptake in these materials and its effect on mechanical performance. Ageing data for 3000 h in 90ºC water are presented for GF NCF/VE composite without flow layer and three composites with it. Tensile tests demonstrate degradation of mechanical properties and increasing viscoelastic and viscoplastic strains in wet composites.

Vacuum Infusion is a closed mould manufacturing process that can replace open mould processes. To improve resin flow it is common to add flow enhancement layers to the reinforcement stack. The basic feature for all flow enhancements is a coarse microstructure with regions of low fibre content. Degradation of mechanical properties due to environmental loading such as temperature and moisture is very important. A lot of research has been done on this area but earlier work has been done on materials without internal flow layer. The question is how well vacuum infused laminates, based on reinforcements containing internal flow layer, resist environmental ageing. Present work presents 3000 h 90 degrees C water ageing data and differences between the material types in the rate of water uptake, T-g, mechanical properties and density was investigated. The results show that the relative weight gain is an increasing function of time. From the T-g measurements we see that T-g is increasing for the immersion aged materials. Tensile tests after 3000 h of immersion ageing show a decrease in Young's modulus, tensile strength and failure strain for the immersion aged material. Density measurements show that the average density has increased for the immersion aged materials

Polymer composites have low specific weight and high specific strength and stiffness. They are therefore a competitive material when designing mechanical applications. The mechanical advantages and the fact that they also resist corrosion well are reasons why polymer composites have a long history of use in marine vessels, piping, corrosion equipment, and underground storage tanks. Industrial experience show that they can be designed to have long service lives in contact with moisture and aqueous solutions. Moisture diffuses into all organic polymers, including polymer composites. Absorbed water or solutions can lead to changes in the thermomechanical, physical and chemical characteristics of the material. For a composite most effects of moisture and solutions are related to degradation of strength. The changes in modulus are in most cases small. Based on bad experiences with stress rupture in the early days of composite applications, industry now use factors of safety between 4 to 10 for boats and storage tanks. Since these factors seldom are chosen based on tests and modelling of the actual in-service conditions the designs commonly are very conservative, but in some cases they may also be too weak which can give catastrophic consequences. Paper A presents an attempt for service life assessment of a composite spring subject to multiple mechanical and environmental loads. Based on preliminary tests the polymer composite solution promised the best performance for the device, but to select that solution it first had to be made probable that the composite spring would survive the design life of 10 years. Due to the time-frame of the overall development project the life assessment had to be made in very short time. The spring is pre-stressed and the main functional requirement is that the reaction load may not decline by more than 10% over 10 years. A rapid, but possibly unconservative, methodology was used for the life assessment. To account for degradation of the material due to the environmental, static fatigue and cyclic fatigue, we defined partial material safety coefficients. These coefficients are then used to derive design allowables that should be low enough to ensure that macroscopic damage do not occur during the design life of the component. Linear visco-elastic material properties are then determined by short-term testing of the neat resin and micromechanics. The visco-elastic stress relaxation for the spring is then predicted by FE-analysis. Both relaxation over the design life and relaxation at accelerated conditions are predicted. To validate the analysis the predicted relaxation at accelerated conditions is compared to results from short-term component tests. Paper B presents experiments to study the effect of the reinforcement structure and manufacturing method on the sorption kinetics and ageing of E- glass/vinyl ester composites. The results show that the microstructure, in particular the void content, has a major effect on the equilibrium value for moisture as well as on the diffusion coefficient and ageing characteristics. The results imply that the effect of the voids can be modeled by rather simple means by accounting for the possibility for humidity to condense in the voids. How suitable the micromechanical methods are to determine service live for composite materials are studied in Paper C. Paper C presents a micromechanical approach for accelerated testing based on the single fibre fragmentation and bundle tests which we believe will overcome several of the shortcomings of today's methods. Conditioning of very thin polymer specimens with single fibres embedded will shorten the timescale for diffusion from years to days to enable isolated studies of ageing. The micromechanical tests will enable us to monitor both fibre degradation and interface degradation separately.