Mechanics of Aeronautical Composite Materials

Christophe Bouvet

• Publisher: Wiley-ISTE
• Year: 2017
• Print ISBN: 9781786301147
• eBook ISBN: 9781119459088
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Mechanics

This book presents the principles of composite laminate sizing widely used for composite structures. The focus is on aeronautics in particular, including the concepts of limit loads and ultimate loads.

After a brief overview of the main composite materials used in aeronautics, the basic theory of laminated plates and the associated rupture criteria are given. The author presents two fundamental cases of the sizing of aeronautical composite structures: the calculation of the holed structures and their subsequent multi-bolt joints, and the calculation of the buckling.

The concept of damage tolerance is also explored, with a focus on its application for tolerance to impact damage. These notions are fundamental for understanding the specificities of the sizing of aeronautical composite structures.

The book also contains corrected exercises for the reader to test their understanding of the different topics covered.

1
Presentation of an Aeronautical Unidirectional Composite

1.1. Introduction

As it stands, the two main materials in use in aircraft structures are aluminum and a carbon fiber-based material. These two materials make up approximately 70% of the mass of the structure of a typical commercial plane such as the Boeing 787 (Figure 1.1).

Figure 1.1. Material breakdown of the Boeing 787 (according to http://www.boeing.fr). For a color version of this figure, see www.iste.co.uk/bouvet/aeronautical2.zip

The remaining structural materials are glass fiber-based composites, sandwich structures (a honeycomb core covered with two composite sheets), titanium, steel, etc.

Keep in mind that the weight ratio of composite materials is of over 50% of the overall mass for the Boeing 787 or the Airbus A350, but more standard commercial aircrafts such as the Airbus A320 or A380 are primarily composed of aluminum alloy, which makes up over 60% of its mass.

1.2. Carbon/epoxy composite T300/914

We will now look at a carbon/epoxy composite that is widely used in aircraft structures, called T300/914. The T300 portion of the name refers to a carbon fiber produced by Toray^® [TOR 16], while 914 is a reference to an epoxy resin produced by Hexcel^® [HEX 16]. T300/914 is a first generation carbon/epoxy composite that is 50% (in volume) carbon fiber and 50% epoxy resin. It takes the form of a thin fabric (less than a millimeter thick) that can subsequently be cut and draped to obtain a desired thickness (Figures 1.2 and 1.3).

Figure 1.2. Unidirectional carbon/epoxy laminate

Figure 1.3. Unidirectional and quasi-isotropic laminate

Performing a test along the direction of the fibers, also called the longitudinal direction, we can observe a brittle elastic behavior comparable to the fibers. The elastic limit is obviously lower than that of the fibers, since approximately 50% of the resin has been added, which has a relatively low elastic limit (Figure 1.4). This resin is necessary in order to obtain a less brittle material and to shape it. The carbon fibers are indeed thoroughly interesting elements from the aspect of their mechanical characteristics but cannot be used to adopt a desired geometry. Furthermore, when a crack appears in the material and propagates perpendicularly to the fibers, it will cause a lot of fiber failures and fiber debonding, thus requiring an elevated dissipation of energy; the material will therefore be less brittle (Figure 1.5). In practice, a crack would travel parallel to the fibers if it could, which is why there are plies in other directions, so as to reinforce the material in different orientations of loading (in practice, we can demonstrate that four directions 0°, +45°, −45° and 90° are enough). It is then a case of a composite laminate, as opposed to a composite with fibers facing only one direction, which is called a unidirectional composite (Figure 1.3).

Figure 1.4. Tension along the longitudinal direction of a composite: behavior of fiber, resin and composite. For a color version of this figure, see www.iste.co.uk/bouvet/aeronautical2.zip

Figure 1.5. Tension along the longitudinal direction: damaged area

Comparing Young’s modulus and the strength of the main structural materials according to density, we note that composite materials are very well positioned compared to the metals (Figures 1.6 and 1.7). Ceramic materials are also very interesting but often too brittle for any structural use.

Figure 1.6. Young’s modulus according to density [ASH 00a] CFRP: carbon fiber reinforced plastic; GFRP: glass fiber reinforced plastic. For a color version of this figure, see www.iste.co.uk/bouvet/aeronautical2.zip

Figure 1.7. Strength according to density [ASH 00a] CFRP: carbon fiber reinforced plastic; GFRP: glass fiber reinforced plastic. For a color version of this figure, see www.iste.co.uk/bouvet/aeronautical2.zip

The way to better understand a composite material is to take a close look at its composition and microstructure, in particular for epoxy resin.

1.3. Polymers

The epoxy matrix is part of the polymer family commonly referred to as plastics. The word plastic comes from the mechanical behavior of polymers that present plastic strains, i.e. the deformation does not return to its original point when loading is released.

Polymers are composed, as indicated by the name, of chains of monomers linked together with covalent bonds. In this particular work, we will limit ourselves to organic polymers. Keep in mind that organic matter is created by living creatures (plants, mushro...

Table of contents

• Cover
• Table of Contents
• Title
• Copyright
• Foreword
• Preface
• Introduction
• 1 Presentation of an Aeronautical Unidirectional Composite
• 2 Characteristics of UD Ply
• 3 Characteristics of a UD Ply in a Given Direction
• 4 Fracture of a Laminated Composite
• 5 Fracture Criteria of a UD Ply
• 6 Membrane Behavior of a Laminated Composite Plate
• 7 Bending Behavior of a Laminated Composite Plate
• 8 The Fracture Criterion of a Laminate
• 9 Damage Tolerance
• 10 Interlaminar and Out-of-Plane Shear Stress
• 11 Holed and Bolted Plates
• 12 Buckling
• 13 Miscellaneous Rules for Stacking
• 14 Exercises
• 15 Solutions to the Exercises
• Bibliography
• Index
• End User License Agreement