Composite Materials inAerospace Design

Soviet Advanced Composites Technology Series

Series editors: I .N. Fridlyander, Russian Academy ofSciences, Moscow, Russ iaI.H. Marshall, University of Paisley,Paisley, UK

This series forms a unique record of research, development and applica­tion of composite materials and components in the former Soviet Union.The material presented in each volume, much of it previously unpublishedand classified until recently, gives the reader a detailed insight into thetheory and methodology employed and the results achieved by the SovietUnion's top scientists and engineers in relation to this versatile class ofmaterials.

Titles in the series

1. Composite Manufacturing TechnologyEditors: A.G. Bratukhin and V.5. Bogolyubov

2. Ceramic- and Carbon-matrix CompositesEditor: V.I. Trefilov

3. Metal Matrix CompositesEditor: I.N. Fridlyander

4. Polymer Matrix CompositesEditor: R.E. Shalin

5. Fibre Science and TechnologyEditor: V.I. Kostikov

6. Composite Materials in Aerospace DesignEditors: G.I. Zagainov and G.E. Lozino -Lozinski

CompositeMaterials inAerospace Design

Edited by

G.I. ZagainovDirector of the Central Aero-hydrodynamic InstituteZh ukovskiMoscow RegiotlRussia

and

G.B. Lozino-LozinskyNPO MolniyaMoscowRussia

imiSPRINGER-SCIENCE+BUSINESS MEDIA, B.V .

;('j Springer Science+Business Media DordrechtSoftcover reprint ofthe hardcover 1st edition 1996Originally published by Chapman & Hali

Typeset in Palatino 10/12 pt by Thomson Press (1) Ltd ., New Delhi

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The publisher mak es no repr esentation , express or implied , w ith regardto the accuracy of the information contained in this book and cannotaccept any legal resp onsibility or liabili ty for any erro rs or omissions thatmaybe made.

A catalogue record for this book is available from the British Library

r>,

~: Print ed on acid-free text pa per, manufactu red in accord ance withANSI/NISO Z39.48-1992 (Permanence of Paper) .

ISBN 978-94-010-4254-3 I eISBN 978-94-011-0575-0 (eBook) DOI 10.1007/978-94-011-0575-0

Contents

List of contributors ix

Series preface xi

Preface xiii

1 Specific features of composite-material structural design 1 V.F. Kutyinov and AA Ionov 1.1 Introduction 1 1.2 Strength requirements of the airworthiness standards 8 1.3 Design of composite constructions and elements 18 1.4 Experimental studies of composite structures 81 1.5 Validation of strength computations 113

References 116

2 Analysis of stiffness, strength and fatigue characteristics of multilayer composites 118 G.P. Sukhobokova and Yu.P. Trunin 2.1 Introduction 118 2.2 Analysis of laminate stiffness 119 2.3 Analysis of laminated-composite strength 126 2.4 Fatigue and cyclic crack resistance of composites 140

References 155

3 Methods of composite structural strength analysis 156 V.M. Andrienko, K.M. Ierusalimsky, AA Ionov, AL. Rubina, G.P. Sukhobokova, AA Dudchenko and AN. Yelpatyevsky 3.1 Stability analysis of composite laminates 156 3.2 Strength analysis of composite rods 169 3.3 Analysis of sandwich cylindrical panels 183 3.4 Analysis of wafer panels 189 3.5 Analysis of stiffened stringer panels 196 3.6 Stability analysis of thin conic and cylindrical shells 225 3.7 Stability analysis of sandwich conic and cylindrical shells 242 3.8 Analysis of panels with cut-outs 260

vi Contents

3.9 Analysis of beam structures 273 References 291

4 Methods for experimental and analytical evaluation of the residual strength of composite structures with stress concentration 295 Yu.P. Trunin, A.E. Ushakov and S.A. Lurie 4.1 Experimental procedures for investigating the stress

concentration effect on strength of composites 295 4.2 Model of static fracture toughness and fracture criteria 304 4.3 Residual strength of damaged structural elements 315 4.4 Methods for increasing the residual strength of damaged

structural elements 323 4.5 Fracture of a flat specimen with delamination under

compression 329 References 341

5 Methods of design and analysis of joints 343 A.A. Ionov, V.F. Kutyinovand Yu.P. Trunin 5.1 Analysis of mechanical joints 343 5.2 Analysis of adhesive joints 351

References 370

6 Application of the finite-element method to the structural analysis of composite structures 372 A.S. Dzuba, A.A. Ionov and V.F. Kutyinov 6.1 Introduction 372 6.2 Method of analysis of complex load-carrying structures

made of composite materials 372 6.3 Mathematical models of typical fragments of structures

made of composite materials 376 6.4 Results of analysis of stress-strain state and strength of

composite structures using the example of cargo compartment doors 379 References 387

7 Characteristics of the certification of composite structures 389 Yu.A. Stuchalkin, A. V. Stewart and A.E. Ushakov 7.1 Introduction 389 7.2 Enlarged scatter of strength properties and additional

safety factor 389 7.3 Evaluation of reliability and safety factors in the case of

short-term strength reduction with subsequent restoration 401 7.4 Damage tolerance evaluation 405

Contents vii

7.5 Specific methods for providing damage tolerance of composite structural elements at the stage of certification 415

7.6 Strength degradation due to climatic exposure 422 References 429

Index 431

Contributors

Vladimir M. Andrienko, TsAGI, Zhukovsky, Moscow Region

Alexander A. Dudchenko, Moscow Aviation Institute, Moscow

Alexander S. Dzuba, TsAGI, Zhukovsky, Moscow Region

Constantine M. lerusalimsky, TsAGI, Zhukovsky, Moscow Region

Alexander A. lonov, TsAGI, Zhukovsky, Moscow Region

Vladimir F. Kutyinov, TsAGI, Zhukovsky, Moscow Region

Sergey A. Lurie, Moscow Aviation Institute, Moscow

Anna L. Rubina, TsAGI, Zhukovsky, Moscow Region

Andrey V. Stewart, TsAGI, Zhukovsky, Moscow Region

Yury A. Stuchalkin, TsAGI, Zhukovsky, Moscow Region

Galina P. Sukhobokova, TsAGI, Zhukovsky, Moscow Region

Yury P. Trunin, TsAGI, Zhukovsky, Moscow Region

Andrey E. Ushakov, TsAGI, Zhukovsky, Moscow Region

Andrey N. Yelpatyevsky, Moscow Aviation Institute, Moscow

Series preface

Some years ago in Paisley (Scotland) the International Conference on Composite Materials, headed by Professor I. Marshall, took place. During the conference, I presented a paper on the manufacturing and properties of the Soviet Union's composite materials.

Soviet industry had made great achievements in the manufacturing of composite materials for aerospace and rocket application. For example, the fraction of composites (predominantly carbon fibre reinforced plastics) in the large passenger aircrafts Tu-204 and 11-86 is 12-15% of the structure weight. The percentage by weight share of composites in military aircraft is greater and the fraction of composites (organic fibre reinforced plastics) used in military helicopters exceeds a half of the total structure weight. The nose parts of most rockets are produced in carbon-carbon materials. In the Soviet spacecraft 'Buran' many fuselage tubes are made of boron-alumin­ium composites. Carbon-aluminium is used for space mirrors and gas turbine blades. These are just a few examples of applications.

Many participants at the Paisley conference suggested that the substan­tial Soviet experience in the field of composite materials should be distilled and presented in the form of a comprehensive reference publication. So the idea of the preparation and publication of a six volume work Soviet Advanced Composites Technology, edited by Professor I. Marshall and me, was born.

Academician J.N. Fridlyander Moscow, May 1994

Preface

The final goal of application of any material is its rational utilization in a structure. The introduction of new materials into structural design is sometimes very expensive, but for the design fields where structural weight saving is the main means to increase effectiveness, this method is very promising. Structural weight saving while retaining the required high reliability is a problem in aircraft engineering. This weight saving is of even greater importance in designing space structures because of the high cost of each kilogram of payload.

The introduction of composite materials into aerospace engineering is very successful owing to the broad range of physical, mechanical and chemical properties and the possibility to vary these properties, which provides the designer with new degrees of freedom for creative rational structural design.

Composite materials based on graphite and boron fibre systems are recognized as the most promising. At present, the use of graphite/epoxy materials enables one to reduce structural weight by 20-25%. Further weight reduction can be attained by increasing the percentage of compos­ites in the total amount of applied materials, as well as by improving the design methodology and fabrication technology used for composite struc­tures. Initially, composites were used in secondary structures, like the interior details and floors; next, they were used in less critical load-bearing components, i.e. the landing gear well doors, doors of the hatches, etc. At present, composite materials are being introduced into such primary structures of the airframe as the wing, fuselage and control surfaces.

It was impossible to use the new degrees of freedom given by composite materials without the development of the corresponding science of com­posite structures. This science has been developed in Russia for many years with the support of government. A small part of this science is presented in this volume, but all major areas are covered: analysis of strength, stiffness and fatigue at the level of composite laminate; methods of strength analysis at the level of composite structure; methods for experimental and analytical evaluation of the residual strength of compos­ite structures with stress concentration; methods of analysis of joints; analysis of composite structures by the method of finite elements; and peculiarities of composite structures certification.

xiv Preface

Chapter 1 can be used as an extended introduction to the other chapters. The most important features of composite structure design are considered as well as the efficiency of composite-materials application in aviation structures.

In Chapter 2 the main relationships of elastic theory of laminated composites are given. The stiffness characteristics as well as stress-strain state are derived depending on the monolayer properties and layup arrangement at normal and elevated temperature. Analysis of laminate strength is performed taking into account the anisotropic behaviour of lamina. Fatigue and cyclic crack resistance of composites is investigated in traditional manner, like for metals. So, the effect of loading rate, effect of amplitude and mean cycle stress and effect of complex loading on the fatigue of composites are discussed. Special consideration is given to the fatigue resistance of hybrid composites.

Chapter 3 is dedicated to static stability analysis and numerical methods for different structural elements such as beams, rods (torsion bars), plates, panels, shells and trusses, for different boundary conditions and under different loading conditions. The specific features of aerospace structures like the presence of cut-outs, specific shapes of structure and structural asymmetric of composites are included. This chapter gives a detailed review of the state of the art in Russian strength/stiffness analysis of composite structures. The main assumptions of the theory and the specific assumptions of each application are discussed in detail as well as the drawbacks and limitations of the methods. The optimization procedures for stiffened and sandwich panels are described. The experimental verifi­cation of methods is described in some topics. On the basis of several investigations, the comparison of typical structural decisions is made and corresponding recommendations are worked out.

In Chapter 4 methods for experimental and analytical evaluation of the residual strength of composite structures with stress concentration are reviewed. As a rule this concentration results from in-service impact damage. For the case of a damaged composite structure, the empirical two-parameter model of fracture is used, which is based on linear elastic fracture mechanics. The experimental procedures used for obtaining the parameters of the model are recommended, including the condition of impact tests, inspections and strength tests. In the case of delamination the simplified theory of crack propagation is given. Crack stoppers are con­sidered as an effective means for increasing the post-impact strength of composite structures. Different types of stopper are compared.

Chapter 5 provides an extensive review of joints used for composite structure assembly. Two basic types of joints are considered, i.e. mechan­ically fastened and adhesive joints. The method of strength analysis of mechanically fastened joints is based on linear fracture mechanics rela­tions with correction for the cracking zone. Bearing stress in joints is

Preface xv

discussed. The results of strength analysis and test results are presented for different designs. The extensive examination of adhesive joints is provided with regard to stress-strain behaviour. The features of different types of joints are discussed.

In Chapter 6 the application of the method of finite elements of complex load-carrying composite structures is discussed. The basic concept used here is the step-by-step comparison of data obtained from multilevel analysis on the FEM models and the results of laboratory tests. The test measurements are made to update/verify mathematical models. This makes it possible to elaborate the posterior models with required accuracy in points where measurements are available and to optimize the methodol­ogy of designing the prior mathematical models.

In Chapter 7 some features of composite structure certification are discussed. Composite structures exhibit a number of intrinsic differences from traditional ones, which should be taken into account during aircaft certification. There is a considerable difference between the Russian and well known Western approaches to establishing certification strength requirements for composite structures. In the Western approach the con­cept of constant safety factor is used, while the design allowable material properties (A-value, B-value) depend on the scatter of these properties, strength degradation and damage-tolerance criteria. In the Russian ap­proach the concept of additional safety factor is used. This factor is established from probability consideration depending on the scatter of the strength parameters of composite structures and the scatter of the maxi­mum load expected in operation. Thus the mean values of strength characteristics are used. The Russian approach looks more complicated. The application of this approach is not so clear. But it permits one to combine all uncertainties in a probabilistic manner and to account for scale effects. Chapter 7 clarifies some details of this approach.

All the reviewed results were obtained before the end of the 1980s, when the reduction of Government expenditure made subsequent fast progress in this area impossible. In this respect the editors and other participants of this volume wish to thank the publishers for granting them the opportun­ity to complete their investigations of composite materials by publishing the main results for an international readership.

The editors of this volume express their gratitude to the Central Aero­hydrodynamics Institute (TsAGI) for granting them, the authors and the translators of the volume the possibility to participate in this work and for providing them the necessary assistance. The editors are indebted to Academician LN. Fridlyander for his invitation to participate in this treatise as editors. The editors wish to thank the authors of the chapters for their enthusiasm in completing our cooperative work. The editors also wish to thank the translators of the volume, G. Alekseev (Chapter 1), A. lonov (Chapters 3 and 6), S. Paryshev (Chapters 2 and 5) and A. Stewart

xvi Preface

(Chapters 2,3,4 and 7), who have understood and translated correctly and rapidly the ideas of the authors, and in particular to Dr Stewart who managed to read and correct all the translations.

Gleb E. Lozino-Lozinsky German L. Zagainov