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, in particular interlaminar shear strength, has been a considerable problem when evaluating such constructions. The measurements of long term interlaminar shear properties in this study showed that the influence of sea water 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. This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording or otherwise) without the prior written permission of the copyright owner.