Design Fundamentals

Composite Design Fundamentals David Richardson

Contents • A review of the fundamental characteristics of composites – – – –

Stiffness and Strength Anisotropic Role of fibre, matrix and interface Composite failure

• Design – Criteria and Considerations – Aircraft Composite Design Process – Analysis approach - FEA

• Advantages and Disadvantages of Composites • Design Case Studies

Stiffness and Strength • Young’s modulus is a measure of how stiff a material is – How much is stretches under a given load – Measured in GPa or GN/m2 • Steel = 210 GPa, Aluminium = 70 GPa, Polymers = 3 GPa

• Stress is a measure of how strong a material is – Failure stress / strength (MPa) • Aluminium example = 400 MPa

– Yield stress • Aluminium example = 200 MPa

Stress – Strain of Fibre & Matrix

Diagram taken from Harris (1999)

Composites versus Aluminium

Source: Aerocomp Ltd

Isotropic versus Anisotropic • Most materials are isotropic – Meaning that their properties are the same in all directions • Metals, polymers, ceramics

• Composites are anisotropic – Meaning that their properties are (may be) different in different directions • Wood, plywood

Anisotropic Material • Benefits – We can put the material properties where we need them • Strength, stiffness +

– We do not need material where it is not required • Saving in weight

• Disadvantage – We need to understand and determine where and in which direction we need high performance

The Challenge of Composites 10+ fibre options

10+ manufacturing options

30+ matrix options

+ many process variations

- Many combination considerations

- such as tooling and consolidation options

- More complex design due to anisotropic materials, near net shape manufacture & intimate link between manufacture and material properties

Composite Material

Reinforcement Phase - Fibre • Reinforcement – May be particulate, short fibres or continuous fibres – Provides strength and stiffness + – Influences the formability & machinability of the resulting structure

Composite Material Forms Particulate - microballoons (hollow microspheres) - nano particles (sized between 1 and 100 nanometres) Discontinuous Fibre -Chopped strand mat -Chipped fibres for injection moulding (100 μm long) Continuous Fibre

Matrix Phase • The roles of the Matrix – Holds the fibres in position – Protects the fibres – Transfers loads to and from fibres

• The Matrix determines – Transverse mechanical properties • Where fibres do not reinforce structure • Y-plane ? Z-plane?

– – – –

Inter laminar shear characteristics Environmental resistance (moisture, chemical, fire) Temperature resistance Processing/manufacturing routes

The Fibre-Matrix Interface • Has a significant effect on – Shear, transverse, flexural, impact and crack propagation properties

• The bond must have a good shear strength in order to: – transmit load between matrix and fibre – minimise ingress of corrodents – control de-bonding

• There are a number of factors which affect the bond strength including: – – – –

compatibility of resin and fibre imperfections on surface of the fibre finish (or size) applied to the fibre during fibre manufacture length of the fibre

Role & Characteristics of Matrix • Consider following 3 loading conditions: – Axial compressive loads

• The matrix needs to keep the fibres straight to avoid buckling.

– In-plane shear loads

• Adjacent plies attempt to slide over one another. • The matrix transfers these loads, relying on adhesion to the fibre.

– Bending loads

• A combination of compression, tension and shear loads

Composite Failure • Composites tend to fail in a different way to metals – Different failure modes – Brittle fibres in a ductile matrix – Sudden brittle failure – no elasticity – Crazing and matrix cracking may occur – Unseen failure may initiate in the laminate • Hence fear due to BVID in carbon fibre structures • Inter laminar disbonding and damage

When does material fail?

Source: Gurit: Guide to Composites

Attractive Properties of Composites • • • • •

Stiffness Strength Low mass Part count reduction Low cost production of complex shapes • Low attenuation to X-rays • Radar transparent • Inherent excellent FST properties

• Corrosion resistance • Good fatigue performance • EMI/RFI screening • Electrical insulation • Ballistic performance • Low CTE • Good Ablative properties (resistance to erosive processes)

Key Design Considerations • • • • •

Material Selection Processing/Fabrication Methods Structural Considerations Environmental Effects & Protection Sandwich Construction

Design/Material Selection Options • Vary proportion of fibre • Vary angles of fibres • Vary consolidation of laminate (fibre volume fraction) • Vary types of fibres • Vary type of matrix • Vary fibre-matrix interface • Vary manufacturing process – Near net shape manufacturing – Quality of resulting laminate/product

Aircraft Composite Design Process 1. 2. 3. 4.

Determine requirements and loads Select structural configuration Select material, fabric, thickness, style, ply sequence Calculate laminate properties •

5.

Calculate stress induced by loads •

6.

Go back to 3 if stress × 1.5 >1

Evaluate cost versus weight •

7.

Strength, stiffness, strain to failure, etc

Go back to 2 if high cost or weight

Build & test prototype – final design Taken from Composite Aircraft Design, Martin Hollmann

Select Structural Configuration • Important to have a thorough knowledge of the advantages and disadvantages of the various fabrication / manufacturing techniques – Design for Manufacture

• Usually a specific structural configuration is selected for – Ease of construction – Low tooling and fabrication costs – Lightweight

Preliminary Structural Sizing • Once the type of composite structure has been selected = preliminary structural sizing of the components and laminates can proceed – Using standard structural analytical techniques – Together with simple optimization techniques and equations

Design Mechanical Properties • Determine mechanical properties of single oriented lamina or ply – From testing in the longitudinal and transverse directions – Average measured properties minus two standard deviations – Not from material suppliers published data • Because suppliers publish most optimistic data • May be difficult to repeat in laboratory • Not design data

Properties in other directions • The properties of the ply can be calculated in other directions using a range of possible methods – Efficiency or Krenchel factor – Hart-Smith 10% rule – Classical Laminate Analysis

Design and Analysis of Structures • Analysis of composite components is difficult • Dynamic loads are especially hard to consider • Design tools are less developed than those for conventional materials • Testing is still widely used to validate design and analysis models

Analysis Approach • Analysis only as good as the weakest element • Usually begins with approximations • Simplified approach for initial sizing – Rule of Mixtures, experience, empirical data

• Refine analysis – Use of computational tools

• Analysis should always be verifiable in some way (test)

Finite Element Analysis • Lots of FEA codes offer composite capability – However, analysis is not as simple as with isotropic materials

• Models have to be tuned • Verification is essential • Main strength in comparison of materials options/arrangements

Advantages of Composites • • • • • • • • •

Tailor capability (directional properties) Lower density (lower weight) High strength and stiffness Fatigue performance Corrosion resistance Wear resistance Low heat transmission Good electrical insulation Low sound transmission

Further Advantages of Composites • • • • • • • •

Textured surfaces Self colouring Integration of parts Economy of scale Moulding direct to final dimensions Efficient use of materials Durability Lifetime costing attractive

Disadvantages of Composites • • • • • • • • • • •

Environmental degradation of resin dominated properties Notch sensitivity Impact damage Poor through thickness properties Variability Properties not established until manufactured Limited availability of design data Reinforcement incorrectly located Lack of codes and standards Recycling not easy Fire, smoke and toxicity performance

References • Composite Materials - UWE E-learning resource – David Richardson, John Burns, Aerocomp Ltd.

• Composite Aircraft Design – Martin Hollmann

• Design with Reinforced Plastics, a guide for engineers and designers – Rayner M Mayer – The Design Council – Published in 1993 by Bourne Press Ltd, Bournemouth

Contact Details – – – – – – – – – –

Dr David Richardson Room 1N22 Faculty of Engineering and Technology University of the West of England Frenchay Campus Coldharbour Lane Bristol BS16 1QY Tel: 0117 328 2223 Email: [email protected]