A major reason for the large testing effort for documentation of long-term properties is the lack of quantitative understanding of how degradation happens in composite laminates. For example, stressed glass fibers exposed directly to water Fig. 9 Comparison of measured and calculated damage in a plate exposed to impact.
aDelamination-numerical, b Delamination-experimental
Integrating Durability in Marine Composite Certification 191
suffer badly from stress corrosion. Stressed glass fibers in dry air show only a slight reduction of strength with time. A stressed glass fiber epoxy laminate exposed to water shows a strength reduction between the two extremes, because the matrix protects the fibers. The protection works, because less water gets to the fibers and the reaction products from the glass degradation process cannot move easily away from the glass fibers. The water may, however, travel more rapidly along the fiber matrix interface than through the bulk matrix [33]. It is difficult to describe all these effects as quantitative predictive models.
All long-term testing described before shall be done in the relevant environ-ments for the application. This ensures that the effect of the environment is con-sidered during the test. Testing is typically done at high stresses or strains and data are extrapolated to longer times for lower stresses/strains. Increasing temperature may also allow extrapolation to longer times at lower temperatures, if an Arrhenius type relationship can be found. All extrapolations with time require that the material does not go through a phase change. Such an evaluation should be made.
It should mainly be evaluated whether some chemical reactions may happen during the lifetime and whether some additives such as antioxydants are being depleted. Chemical degradation is described to some extend for liner materials used in flexible risers [34]. Chemical degradation of polyamide liners is well described [35]. For most other materials chemical degradation needs to be eval-uated without the direct guidance of standards.
Molecular dynamics (e.g. [36–38]) may be a method allowing the prediction of degradation of polymers on a molecular level and converting the local degradation to global engineering constants. Figure10 shows a picture of polymer chains under stress [39]. Due to simplifications in the models the simulations give good qualitative results today, but with ever increasing computer power, they may one day also give the right quantitative answers. This would allow prediction of degradation from first principles on the molecular level. Eventually this approach could also work for composites.
If such models could be developed for fiber degradation and all the other failure mechanisms, as well as environmental conditions, durability could be more easily treated in the certification process.
Fig. 10 Molecular dynamic simulation of a stressed polyethylene polymer
8 Conclusions
Durability of marine composites is only indirectly adressed in most design standards today. Some standards for pipes, pressure vessels and the DNV offshore standard for composite components address long term properties of composites directly. The long term performance and credit for good durability are established through fairly extensive test programs. A better quantitative understanding of the effects of temperature and chemicals on the degradation processes could help to reduce the testing efforts and improve the design standards.
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