37.3 Conclusion Using mechanical testing to assess if ageing has taken place in composite materials is a good tool when associated with other analytical methods. It appears that ILSS in combination with flexure testing and visual assessment can give an insight in degradation. This can be associated to other techniques such as a study of the evolution of glass transition temperature with exposure time. Although ageing can be characterised in particular conditions, extrapolating the long term behaviour of composites does not appear to be possible at the moment. The different ageing mechanisms at play may need to be isolated and understood before being combined in FEA models to have a better picture of degradation mechanisms and extent. In addition to ageing, Oil & Gas conditions present further challenges for composites. These materials, in contact with high solvation power fluids such as supercritical carbon dioxide may sustain rapid gas decompression damage. When equilibrium is reached between external gas pressure and dissolved gas concentration within a composite, the rapid release of external pressure—called rapid gas decompression (RGD)—triggers material expansion and may lead to the formation of cracks and delamination, which could help promote ageing and loss of structural properties. The avoidance of RGD in service situations is often not possible since slow depressurisation over a long period—one way to prevent fracture development—is not viable. Moreover, uncontrolled gas decompression may accidently occur. Composites in contact with fluids which may induce RGD damage (also called blistering) have to be assessed in conditions close to that of the field. Interestingly, some material may present good RGD resistance in their dry unaged state, but fair poorly when subjected to pressure cycling after exposure to fluids which may have affected the bond between fibres and resin or resin and/or fibre integrity. There are currently no standards available to define RGD resistance for composites in oilfield fluids. Fig. 37.7 SEM micrographs: surface state of unaged samples and specimens having been exposed to seawater for 245 days at 60, 80 and 100 C (140, 176 and 212 F) 326 S. Munch et al.
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