Durability of Composites in a Marine Environment

This chapter presents an overview of key considerations for the successful application of fibre reinforced composites in the marine environment. It is intended to complement and update an earlier text Searle and Summerscales (Effect of Water Absorption on Time–Temperature Dependent Strength of Unidirectional CFRP). After consideration of factors affecting the environmental resistance of conventional composites, the potential for natural fibre reinforced polymer composites is briefly discussed. Finally it is argued that Quantitative Life Cycle Assessment is essential to establish the “sustainability” of any system.

Water sorption is a key issue in assessing the durability of polymer matrix composites. In fact absorbed water can adversely affect mechanical properties of the matrix and fibre-matrix interface integrity. In this contribution the general issue of water sorption thermodynamics in polymers is addressed from the experimental and theoretical point of view. The case of both rubbery and glassy polymers is considered modelling thermodynamics of water-polymer systems using lattice fluid theories accounting also for the occurrence of possible self- and cross-hydrogen bonding interactions. Outcomes of theoretical analyses are compared to experimental results obtained by vibrational spectroscopy and gravimetric measurements.

In this chapter, several aspects of the ageing phenomena induced by water in organic matrix composites are examined, essentially from the physico-chemical point of view. It is first important to recognize that there are two main categories of humid ageing. First there are physical processes, mainly linked to the stress state induced by matrix swelling and sometimes matrix plasticization. This kind of ageing can occur in matrices of relatively high hydrophilicity (affinity with water). Highly crosslinked amine cured epoxies are typical examples of this behavior. The second category of humid ageing involves a chemical reaction (hydrolysis) between the material and water. Unsaturated polyesters are typical examples of this category. They display a low to moderate hydrophilicity, swelling and plasticization have minor effects, but hydrolysis induces a deep polymer embrittlement and, eventually, osmotic cracking. Whatever the ageing mechanism, it needs the water to penetrate into the material and depends on the water concentration and its distribution in the sample thickness. This is the reason why the first and second sections are respectively dedicated to water solubility and diffusivity in matrices, interphases and composites. In each case, the elementary processes are distinguished, to examine the effects of temperature and stress state and to establish structure–property relationships. It is shown that, in most of these aspects, research remains largely open. The last section is devoted to hydrolysis, its kinetic modeling, including the case of diffusion controlled hydrolysis, and its consequences on polymer properties. Structure reactivity relationships are briefly presented. The very important case of osmotic cracking, which can be considered as a consequence of hydrolysis, is also examined.

The present contribution investigates the effects related to the plasticization of the polymer matrix occurring during the water sorption process on the internal mechanical state profiles, at the constituent and ply scales. Then, two multi-physics models are considered to account that the moisture sorption depends on the internal mechanical states: the free volume theory and a thermodynamic approach.

Moisture absorption in carbon fiber vinyl ester reinforced composite facings used for marine composite sandwich structures can be approximated by the Fickian diffusion model. Comparative study of water uptake for VARTM based polymer composites for exposure to tap, distilled, and sea water is presented. Evaluating moisture diffusion coefficient based on one-dimensional Fickian model usually involves errors resulting from large scatter of weight gain data at early times of exposure due to very small changes in moisture content of polymer composites. A novel measurement technique is proposed here for precisely measuring moisture absorption–desorption curves with improved precision to evaluate the diffusion coefficient. Mechanical properties of carbon fiber composite facings in terms of modulus and failure stress and associated degradation due to long-term sea water exposure are also summarized corresponding to different ply lay-up orientations. The matrix dominated lay-up orientations show considerable degradation in mechanical properties due to sea environment. Notably, tension–tension fatigue on matrix dominated composites after sea water saturation resulted in 30 % degradation of fatigue life.

The effects of marine environmental exposure on moisture absorption and the mechanical properties of vinylester resins (VE510A and VE8084) and unidirectional carbon fiber/VE510A composites have been experimentally investigated. Two carbon fiber sizings (F and G) were examined. Neat resin specimens were exposed to humid air at 50 °C and seawater at 40 °C until saturation. The composite materials were immersed in seawater at 40 °C. The resin specimens absorbed small amounts of moisture. The composite specimens absorbed more moisture than the resins. Both the neat resins and composites displayed Fickian diffusion behavior. Mass balance analysis of moisture absorption of the composites was performed which shows that moisture up-take is dominated by the fiber/matrix interface region. The moisture absorption was found to depend on the fiber sizing. Dry and moisture saturated neat resin and composite specimens were tested in tension, compression, and shear. Moisture absorption slightly improved the ductility of the neat resin specimens. Composites with F-sized carbon fibers displayed higher strengths than those with G-sized fibers at both dry and moisture saturated conditions. Moisture absorption reduced the in-plane and interlaminar shear strengths of the composites.

Static strengths for four typical directions of unidirectional CFRP were measured under various temperatures at a single loading rate for dry and wet specimens. The four directions were longitudinal tension and bending, transverse bending, and compression. Water absorption effects on temperature dependence of these static strengths of unidirectional CFRP were assessed. Results show that the static strengths in these four directions of unidirectional CFRP as well as the viscoelastic coefficient of matrix resin decrease concomitantly with increasing temperature and water absorption. Each of four static strengths of CFRP laminates is determined uniquely by the viscoelastic coefficient of matrix resin. Therefore, the master curves of four static strengths against the reduced time at a reference temperature can be constructed and formulated using the time–temperature shift factor for the viscoelastic coefficient of matrix resin based on the time–temperature superposition principle. These master curves of four static strengths clarify that the long-term lives for four directions decrease with water absorption.

Polymer matrix fibre reinforced composites have been employed in marine applications for over 50 years, and there is considerable experience of their long term behaviour. However, the recent development of systems designed to recover ocean energy, such as tidal turbines and wave energy generators, imposes much more severe constraints on materials than traditional structures. The requirements in terms of sea water aging and fatigue resistance require specific test programmes; this presentation will describe some of these applications and the tests needed to guarantee long term behavior of composites for these structures. Some results from studies performed in this area at Ifremer over the last 5 years will be discussed.

A brief overview is given on how design standards are written. Static strength, fatigue, stress rupture, damage tolerance and the effect of the environment are addressed. Examples are given to show how durability is or can be addressed in design standards. Extensive testing is required today to certify long-term properties. Suggestions for research topics to reduce the test effort are discussed.

Deep sea applications of composite materials are increasing rapidly (Choqueuse D and Davies P in Ageing of Composites Woodhead, 2008). Light weight is critical for submarines structures, in order to facilitate their underwater deployment and increase pay-load, and various specific properties (buoyancy, thermal insulation, remarkable behavior with respect to contact with water,…) strongly favor the use of composite materials. Currently the three main sectors concerned are: the offshore oil and gas industry, the navy, and oceanographic equipment. Important developments are ongoing in the oil and gas offshore sector in particular for riser application (Airbone, 2012). In the oceanographic field, a composite pressure capsule was recently developed for a manned submarine to reach the deepest point of the oceans (Black S, High performance composites, 2010). For submarine housings the first point to be solved is the capability to withstand the hydrostatic pressure, which is the main design factor. Safety factors applied are generally high in order to take into account the difficulty in designing composite structures subjected to compression loads. Generally the parameters affecting durability (creep, fatigue, water absorption) can be neglected and are integrated in the static safety factor. Nevertheless, considering that long life times are expected (nominally 25 years) the durability of underwater composite structures has to be considered, and the main question marks, specific for underwater applications, still globally unsolved are the following: (1) Fatigue in sea water, for riser applications; (2) Creep-fatigue interactions and high compressive strains; (3) Effect of pressure on water uptake kinetics and resulting degradation.

This chapter describes the design of racing yachts with composite materials at HDS, an SME specialized in marine design and calculation. Three case studies are presented to illustrate the company expertise. First, a study of keel flutter is described. The development of an analytical tool is discussed and validation by comparison with numerical modeling is presented. Then wave impact is discussed, a regular source of damage in fast craft, and the development of a specific test to evaluate material systems is described. Finally, adhesive bonding is discussed and a specific application to mast tracks is detailed. These three studies underline the importance of a detailed understanding of composite mechanics in developing durable marine structures.

Water diffusing into the bulk of glass reinforced plastics (GRP) can, over time, degrade the mechanical properties of the polymer matrix and the fibre/matrix interface. Residual strength and modulus measurements on samples taken from aged minehunter hulls in 1995 enabled the life of the Hunt Class vessels to be extended. Because no methodology existed for accurately predicting mechanical property degradation through the whole of life, a programme was carried out by QinetiQ in collaboration with DCN CESMAN which applied accelerated ageing at 40 and 60 °C to glass/polyester laminates and correlated the results with real-time data generated for the Hunt Class mid-life update. This chapter presents results and through-life predictive models plus Nuclear Magnetic Resonance (NMR) images of water uptake from that study, and describes the effects of water immersion on fatigue life, fatigue limit and damage development, from another study.