RECENT ADVANCES IN STRUCTURAL JOINTS AND REPAIRS FOR COMPOSITE MATERIALS

Transcription

1 RECENT ADVANCES IN STRUCTURAL JOINTS AND REPAIRS FOR COMPOSITE MATERIALS

2 Recent Advances in Structural Joints and Repairs for Composite Materials Edited by LIYONGTONG School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney, Australia and COSTAS SOUTIS Department of Aeronautics, Imperial College, London, U.K. SPRINGER-SCIENCE+BUSINESS MEDIA B.V.

3 A C.I.P. Catalogue record for this book is available from the Library of Congress. ISBN ISBN (ebook) DOI / Printed on acid-free paper All Rights Reserved 2003 Springer Science+Business Media Dordrecht Originally published by Kluwer Academic Publishers in 2003 Softcover reprint of the hardcover 1st edition 2003 No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work.

4 PREFACE Joints in components or structures incur a weight penalty, are a source of failure and cause manufacturing problems; therefore, whenever possible, a designer will avoid using them. Unfortunately it is rarely possible to produce a construction without joints due to limitations on material size, convenience and cost in manufacture or transportation and the need for access in order to inspect or repair the structure. Such considerations apply equally to joints between metallic components or between composite components. Thus, an important requirement for the complete design of practical structures is the development of attachment methods, joint designs, and the problem ofload introduction in the composite assembly. Without properly thought and designed joints, we are unable to take full advantage of the high stiffness and strength oflaminated composites. Basically, there are two types of joint commonly employed with fibre-reinforced materials: adhesive bonded joints and mechanically fastened joints. Welding is also a possibility for thermoplastic composites, but this technique is not well developed for load-carrying joints. In the following paragraphs, the first two types of joint are discussed. Adhesive joints are natural to consider for polymeric matrix composite materials because many matrix resins are also good adhesives. When the matrix material of the laminate is also used as the adhesive in the joint, excellent adhesion can result. However, even with the excellent adhesion, the joint does represent a discontinuity in the material, and resulting high stresses often initiate joint failure. Therefore, knowledge of the stresses in joints is vital if we are to understand the failures that occur in practice and hence improve designs and predict strength. Chapters 1, 2, 6 and 7 cover the topics related to stress analysis, strength determination and design of adhesive bonded joints. In structures where parts are removed for inspection or maintenance bolted joints (Mechanically fastened joints) will be required. The behaviour of bolted connections, for composite laminates made from unidirectional pre-preg material has been extensively examined by several workers, who investigated a wide range of variables such as lay-up, fastener type (screw, rivet, and bolt), friction effects, clearance and their influence on the failure mode. A full theoretical description of the stresses in such a joint must include their three-dimensional nature, a fact that has limited the analytical treatment given to such connections. The prediction of failure loads, is for the moment at least, only semi-empirical at best and any improvement will depend on the development of failure criteria that are more generally applicable, together with an easy-to-use three-dimensional stress analysis. Chapters 3, 4 and 7 describe stress analysis, strength prediction and design of mechanically fastened joints. The maintenance and repair of composite structures are always of main concern to the end-users as well as the manufactures. On an aircraft, the need to repair a component can arise from physical damage due to accidental impact, bird strike, hailstones and lightning strike or from deterioration caused by the absorption of moisture or hydraulic fluid. Once a defect has been found and it falls outside approved allowable limits, one of the following actions should be taken according to its extent and the constraints of operational conditions: temporary repair, permanent repair or v

5 VI replacement. The temporary repair (field repair or emergency repair) is usually a preferred action when the operational limitations are severe and the temporary repair is a safe option. The limitations are usually time, environment and facilities. Temporary repair aims to restore the mechanical strength required to permit aircraft operation until a permanent repair or replacement can be carried out. Adhesively bonded repair is the most common type of repair carried out with composite materials, because load transfer paths through mechanical fasteners can cause local overloads and damage in the relatively brittle composites. Two types of bonded repairs are mainly used; the highly refined and structurally efficient but expensive flush patch repair and the external metal or composite patch. In the external patch technique, the damaged material is removed by cutting a hole, the parent plate is then cleaned and applied with filler and adhesive materials before the patches are attached. In practice, the repair patch would always be tapered at the edges to reduce peel stresses. This type of repair could also be used for a permanent repair in lightlyloaded and relatively thin structures. Flush, scarf-type, bonded repairs are used on critical, highly loaded, components where load concentration and eccentricities, especially for compressive loading, must be avoided. Thick monolithic structures lend themselves to such repairs since external patch would cause excessive out-of-mouldline thickness and unacceptably high bondline peel and shear stresses. In all these repair methods the main concerns are the quality of surface preparation prior to bonding, the design of the composite patch and the prediction of both strength and durability of the repaired configuration. Chapters 5 and 6 cover stress analysis, strength determination and design of adhesive bonded repmrs. LT is grateful to his postgraduate students, Quan Nguyen and Phu Due Nguyen, for helping with preparation of camera ready copy of the book. L. Tong and C. Soutis

6 Table of Contents Preface 1 Stress Analysis and Failure Assessment of Lap Joints C.H. Wang and L.R.F. Rose 1.1 Introduction 1.2 A critical review of stress analysis and failure assessment 1.3 Ends effect 1.4 Stress concentration in adherends 1.5 Triaxial stresses and plastic yielding 1.6 References 2. Strength Determination of Adhesive Bonded Joints L. Tong, JK. Spelt and G. Fernlund 2.1 Introduction 2.2 Failure of adhesive joints 2.3 Stress analysis of adhesive joints 2.4 Failure criteria and materials characterization 2.5 Strength determination for static loading 2.6 Strength determination for cyclic loading 2.7 Design for creep crack growth 2.8 References 3. Stresses in Mechanical Fastened Joints F.L. Matthews and P.P. Camanho 3.I Experimental behaviour 3.2 Semi-analytical and numerical methods of stress analysis 3.3 Strength prediction methods 3.4 Conclusions 3.5 References 4. Strength Determination of Mechanical Fastened Joints F.-K. Chang and XL. Qing 4.1 Introduction 4.2 Progressive failure analysis 4.3 Numerical prediction 4.4 Verification and comparison 4.5 Effect of the clamping 4.6 Composite joints under combined bearing and axial-bypass loads 4.7 Conclusion 4.8 Acknowledgement 4.9 References v vii

7 V Strength Analysis of Adhesively Bonded Repairs C. Soutis and F.Z. Hu 5.1 Introduction 5.2 External Patch Repairs 5.3 Design ofbondedjoints 5.4 Strength prediction of external patch repair 5.5 Scarf patch repairs 5.6 Discussion and concluding remarks 5. 7 References 6 Adhesively Bonded Joints for Fibrous Composite Structures L.J Hart-Smith 6.1 Introduction 6.2 Adhesively bonded joint configurations 6.3 Adhesive stress-strain curves in shear 6.4 The non-uniformity ofload transfer in adhesively bonded joints 6.5 Double-lap and double-strap joints 6.6 Single-lap joints 6.7 Stepped-lap bonded joints 6.8 Scarf joints 6. 9 Adhesively bonded joints other than at splices 6.10 Flaws in adhesive bonds 6.11 Repair of fibrous composite structures 6.12 Rules ofthumb for designing structurally efficient joints 6.13 Concluding remarks 6.14 Acknowledgement 6.15 References 7 Design and Analysis ofbo1ted and Riveted Joints in Fibrous Composite Structures L.J Hart-Smith 7.1 Introduction 7.2 Single-hole test coupons 7.3 Effects of fibre pattern on strength of bolted composite joints 7.4 Experimental test program Correlation between test and theory 7.6 Joint efficiency charts for designing bolted composite joints 7. 7 Miscellaneous considerations 7.8 Rules of thumb for designing structurally efficient joints 7.9 Concluding remarks 7.10 Acknowledgement 7.11 References