Long-term testing of composite through-thickness properties

The use of composite materials offshore has increased over the last years. Long term performance of a component is an important consideration for users and authorities when evaluating a composite solution. Long term data of composites are widely available for many material combinations. Data in sea water environment are more difficult to find. However, most of the available long-term data is related to fibre dominated properties. The argument for measuring these properties is that composites are designed with fibres running in the main load bearing path. Composite structures where fibres do not carry the main load should be avoided. Practical applications have shown that it is sometimes impossible to carry all significant loads through the fibres. This is particularly the case for the design of joints. The loads are often transferred through the matrix in the joint. The lack of data in long term performance of matrix dominated properties has been a considerable problem when evaluating such constructions. This study investigates the matrix dominated long-term behaviour of a typical glass polyester marine laminate. Cyclic fatigue and stress rupture (static fatigue) data were measured to provide the basis for the evaluation of long term performance. Characteristic long-term performance curves were calculated with a 97.5% tolerance and 95% confidence based on the procedures given in the DNV offshore standard for composite components, DNV-OS-C501. The influence of sea water on short and long-term properties of the laminate was negligible for this laminate. Both static and cyclic fatigue cause a significant reduction of interlaminar shear strength with time, about 35% reduction after 20 years and 80% reduction after 106 cycles. This reduction should be considered when designing joints or other components where interlaminar shear strength is critical.