JEC COMPOSITES MAGAZINE - Issue #33 - June 2007 - 71

A new formula to predict the structural life of composite pipes Three aggressive media A special solvent AND Infrastructure applications require solutions that are cost competitive and durable. Over the years, the composites industry has done well in cutting costs and improving its competitive position. The answer to the issue of durability, however, is still largely empirical. This paper introduces a new equation to predict the long-term structural life of composite pipes subjected to tensile, flexural and cyclic loadings. ANTONIO CARVALHO, REICHHOLD DO BRASIL CARLOS MARQUES, POLYPLASTER LTDA We define as aggressive any medium that lessens the ability of composites to perform. Aggressive media can be grouped in three categories: 1- Non-penetrating agents act only on the surface of the laminates. An example of such medium is the UV radiation that wears away the surface of the composite while leaving the inner layers undisturbed. 2- Semi-penetrating agents have small penetrating power and their effect is limited to the layers next to the surface. Examples of such media are the many chemicals found in industrial applications. 3- Penetrating media pervade the whole laminate and reach all layers. Examples of such agents are some solvents, especially water. The structural life of the composite is determined by the penetrating agents. Solvents have the capability to fully penetrate the laminate, but since they are generally not reactive with the resin or the glass fibres, their damage is (a) non-progressive and (b) can be evaluated within a short time of exposure. The structural life of composites in solvents is a go/no-go situation in which full destruction takes place either in a short time or not at all. However, there is one kind of solvent that is reactive and capable of causing progressive long-term damage to composites. That special solvent is water, which can be harmful in two ways. Regression lines First, like all solvents, it swells the laminate and causes a reversible decrease in mechanical properties. Second, water progressively dissolves and destroys the glass fibres in a process known as “strain corrosion”. The swelling effect of water is reversible and easy to quantify. The slow and continued “strain corrosion”, however, is irreversible and leads to the rupture of the composite in the long term. The above justifies the two following hypotheses. 1st hypothesis - Semi-penetrating chemicals have no effect on the structural life of a composite. The surface damage that they do causes a loss of thickness and affects the service life, but has no effect on the structural life. 2nd hypothesis - Water is the only agent that penetrates and degrades the mechanical properties of laminates. The structural life of a composite is determined by water. The above hypotheses separate short-term service life – determined by chemicals – from long-term structural life – determined by water. Service life is useful to schedule shutdowns for maintenance. Structural life is useful to design long-term infrastructure applications. The recognition that water is the only agent affecting structural life is a tremendous simplification to this otherwise intractable problem. This paper addresses the long-term structural life of composites. Short-term service life is addressed elsewhere (Reference 1). This section focuses on the significance of the regression lines used to assess the long-term structural life of composites. The structural failure of glass-reinforced composites is caused by the attack of water primarily on the glass fibres and secondarily, on the resin-fibre interface. Water attack on the resin itself is small and can be ignored. No33 June 2007 / JEC Composites Magazine

JEC COMPOSITES MAGAZINE - Issue #33 - June 2007

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