Design for continuous fibre composites
Gareth McGrath joined TWI's Engineering Department in 1989. He has been involved with adhesives, composites and polymers for the last six years, after an initial career in metallurgy. His move to composite technology began during a research assistantship at Sheffield City Polytechnic where his thesis was devoted to reclamation prospects for advanced thermoplastic composites. This concluded with a predictive model and component fabrication. During this period he also completed a number of industrial research contracts on polymers.
Since joining TWI, Gareth has been developing techniques necessary to explain composite failure. These have included fracture toughness measurement techniques, impact damage analysis, and study of the time dependent response of joints in thermoplastics and composites. He has recently been appointed to a technical co-ordination function for the expanding adhesives activities at TWI.
Fibre-reinforced composites offer exciting combinations of properties, allowing cost and weight savings in demanding service environments. Gareth McGrath looks at the important factors to consider when designing composite structures.
Materials selection is an integral part of the design process. When choosing materials for load-bearing applications, designers are increasingly considering the advantages of using plastics materials, both thermosets and thermoplastics, rather than traditionally accepted materials, particularly metals. Plastics materials frequently have higher strength-to-weight ratios than aluminium and steel, but lower stiffness-to-weight ratios, giving rise to increased risk of buckling under load. Therefore, when designing with plastics the design is often influenced more by stiffness than by strength.
To improve the mechanical properties of plastics, they are reinforced using ceramic, metallic or polymeric fibres. This very important improvement in properties is easily achieved, and often the composite formed is cheaper than the polymer matrix alone.
The strengthening mechanism depends on the geometry of the reinforcing agent, which may be particulate or fibrous. This article is concerned with design methods for continuous fibre composites.
Designing with advanced composites is fundamentally no different from designing with more conventional materials. The same basic factors must be considered. These exciting materials are, however, more complex, and this complexity requires a basic understanding by all involved - designer, test engineer, stress analyst, manager, etc. Design with datasheets, using methods handed down and not really understood, is a formula for disaster.
The basics of these materials and structures are not difficult to understand. A designer with a commonsense approach and a grasp of the techniques to use and the factors to watch out for can produce good structures.
Composite macrostructure
By macrostructure is meant the orientation of the layers of fibres within a structure.
The orientation of each layer of a laminated structure is simply defined with respect to a control axis. To identify the construction of a laminate it is necessary to indicate the ply orientation with respect to the control axis and the number of plies. Thus 0 4 indicates that four plies are laid up with their axes parallel to the control axis, while 90 4 indicates that the four plies have axes perpendicular to the control axis.
A set of brackets indicates a repeated sequence, the numerical suffix being the number of times that sequence is repeated. A further suffix, S, indicates that the lay-up is symmetrical about the central axis, eg (0, 90) 2S expands to 0, 90, 0, 90, 90, 0, 90, 0.
Symmetry about a central axis is essential when constructing laminates from the anisotropic pre-impregnated tape, or thermal stresses may cause buckling.
In the case of a symmetric lay-up with an odd number of plies, the central ply which is not repeated is indicated by a bar, eg (0, 90,
) S expands to 0, 90, 0, 90, 0.
The quasi-isotropic laminate is popular, having approximately uniform properties in all directions in the plane of the sheet, eg (+45, 90, -45, 0) NS or (0, 90, +45, -45) NS where N is any number.
For convenience a woven cloth is identified as 0/90 or ± 45.
Design
Geometry
The geometry of a structure is usually predefined to some extent by operational requirements such as those of aerodynamics, hydrodynamics, surrounding structures, mechanisms, people, etc. But it is not usually completely predefined, and the design usually requires local geometric re-definition such as panel thickness, curvature, bend radius, and spar location (the position of an undercarriage, for instance).
Stiffness constraints may also require consideration of allowable deflections, natural frequency, aeroelasticity and elastic postbuckling.
Applied loads
Definition of applied loads is always important and errors in load determination, together with inadequate manufacturing quality assurance, are usually at the root of any in-service problems.
These composite materials have inherent planes of weakness, and so second-order loads which might be ignored in metal structures must be considered. The key ones are tensile through-thickness, peeling, and interlaminar shear effects. Areas worthy of special consideration are free edges, bending around tight radii, local offsets and - most important - rapid changes in stiffness.
A good understanding of load distribution is necessary when considering these second-order requirements. Finite element analysis is useful, allowing areas requiring particular attention to be identified.
Cost and weight
When deciding which materials to use, a designer usually finds an interactive balance between cost and weight. Advanced composites are not always selected for weight savings alone. For example, they may be chosen to give increased fatigue life for rotor blades to promote reliability, or for their formability for fairings to reduce parts count and manufacturing costs. Although weight is always a consideration in the assessment, the common factor is normally market advantage.
Material properties
As with good load distribution, a good understanding of material properties is important to achieve enhanced structural performance. This is emphasised by consideration of specific shortcomings in particular materials: epoxies are sensitive to moisture and temperature, Kevlar is susceptible to ultraviolet attack, amorphous thermoplastics are affected by solvents, and thermoplastics generally suffer from a reduction in properties around their glass transition temperature.
The operational environment must therefore be defined and the relevant allowable conditions identified. For example, performance at room temperature in a dry environment is of no relevance to the leading edge of the wing of a supersonic fighter operating in the tropics. It is therefore vital to know the operating environment and potential mode of failure.
Manufacturing
Design should never be undertaken in isolation from manufacturing considerations. Instead, the design and manufacturing routes should be planned in parallel from the outset, the most sensible approach to achieving a reliable structure at minimum cost. The reliability of the manufacturing process also needs to be assessed when decisions are made on safety factors. Poor housekeeping is always detrimental for a reliable, high-performance structure.
Joining
Fig. 1. Common engineering adhesive joints
Although the parts count is normally very much reduced when a design is based on advanced composite materials, certain joining operations are necessary, and need very careful consideration if the advantages of a particular design are not to be nullified.
A particular area for concern is the joining of thermoplastics. Because thermoplastics are often chosen for their increased resistance to impact damage, moisture uptake and attack by specific solvents, any introduction of another interface will normally offset these improvements. Use of epoxy adhesive systems to assemble thermoplastic composite structures should therefore be avoided and the sub-components joined wherever possible by welding. However, optimum welding processes have not yet been established, and knowledge of weldability in the composite industry is limited.
Several factors are relevant when considering joining, which involves adhesive or mechanical joints in most thermoset composite structures. Attention must be given to the details of the joint; some common engineering adhesive joints are illustrated in Figure 1.
Design sequence
When the importance of the factors above has been firmly established, the design process can continue. Typically it may include several stages as follows:
- The objectives of the design programme require definition. Objectives may vary depending upon the sector of industry concerned. Some of the major considerations are market advantages, cost, weight savings, life expectancy, size and shape, and the possibility of coloration and texturing during processing.
- Any limiting constraints need to be identified. There may be problems associated with necessary certifications and any corresponding rules. Sometimes care must be taken to avoid certain conditions such as solvent attack of the matrix and excessive through-thickness loads.
- For the preliminary stages it is necessary to employ coarse assumptions recognising the key factors, and so arrive at a rough design to meet the prime constraints. A feasibility study will then identify the possible composite materials and a manufacturing route. At this stage the possibilities in terms of the initial constraints and any necessary modifications must be assessed. If the problems cannot be solved, the design study should be terminated.
- The detailed design analysis must follow. The first step in this analysis should be to obtain a good load distribution from the preliminary design; finite element analysis will provide such information. The configurations, joints and laminates can now be redesigned and the loadings analysed iteratively until convergence is achieved. Each iteration must contain a reference to the manufacturing step, to ensure that the component or structure can still be manufactured. Some of the aspects which must be considered when designing a laminate are discussed below.
- The next stage is to build and test a prototype to check both the structure and the manufacturing route. Any necessary modifications can now be made.
- Certification of the structure often involves new constraints, encountered in other manufacturing or operating conditions and further modifications of structure and manufacturing route may be necessary.
- The structure can now enter production, but it is vital to obtain operational feedback.
This procedure is summarised in Figure 2.
Fig. 2. Composite design route
Laminate design
In design of the majority of complex composite structures, difficulties are encountered with the low 'secondary' or matrix-dominated properties of these anisotropic materials. Typical unidirectional properties for composite systems provide a quantitative indication of the magnitude of the differences between fibre-dominated and matrix-dominated properties.
With respect to in-plane directions, a compromise may be reached by suitable orientation of the fibres, eg quasi-isotropic laminates. However, no practicable improvement can be made to enhance through-thickness properties. Difficulties are compounded by multidirectional in-plane arrangements that create free edge effects. The interlaminar shear strength is impaired, relative to unidirectional values, by the edge effects in multidirectional fibre systems. This anisotropy of composites produces coupling problems, which need to be understood for structures to be successful.
Coupling mechanisms
If a metal plate is the subject of an in-plane tensile force, it elongates in the direction of that force and contracts in the transverse direction due to the 'Poisson effect'. However, if a similar plate of multidirectional fibre-reinforced composite is subjected to an in-plane tensile force it may respond differently - elongate and pinch, deform in shear, or bend and twist.
The particular combination or coupling of these deformations depends upon the lay-up of the plate, ie the orientation of the individual plies.
When there is no coupling between two modes of deformation, that lay-up is said to be balanced with respect to that coupling. Generally there are three types of coupling in fibre-reinforced composites:
- The coupling between in-plane and bending stresses in the laminate, ie in-plane stresses causing bending of the lay-up and vice versa. This type of coupling is particularly important when considering the release of high temperature cured components from tooling.
- The coupling between in-plane stresses and in-plane shear deformations, and vice versa.
- The coupling between twisting and bending of the laminate: bending causes twisting and twisting causes bending.
Variation of the laminate lay-up is used to balance these couplings, as explained below.
Bending-in-plane coupling
A composite lay-up is said to be balanced when there is no coupling between in-plane and bending stresses and strains. By analysis of two lay-ups it is possible to appreciate some of the problems encountered in design of the structure:
(+45, -45, 0, 0, -45, +45) (1)
(+45, -45, 0, 0, +45, -45) (2)
It is necessary to make two assumptions so that conditions are available for the analysis:
- The transverse coefficient of expansion for the unidirectional ply is zero;
- Both lay-ups have identical cure cycles. Thus upon cooling all plies contract equally in the fibre direction and a pure inplane stress system is created.
For lay-up (1), take moments about the centreline. All stress moments are balanced by equal stress moments on the other side of the centreline; there is therefore no net moment across the centreline and bending does not result. This is said to be a balanced lay-up.
Application of the same analysis to lay-up (2) indicates a net moment, wherein a tension-compression gradient is set up across the centreline and the laminate bends. This type of unbalanced lay-up is therefore impossible to lay flat with preimpregnated tape.
The type of coupling occurring here is that generally referred to when a balanced lay-up is considered.
In-plane coupling
Fig. 3. The result of an axial force on a lay-up of (+45, +45) 2S, a shear deformation plus extension
In-plane coupling exists between axial and shear actions in a lay-up of type:
(+45, +45, +45, +45, +45, +45, +45, +45) (3)
This is 'balanced' in terms of in-plane bending coupling, but when the plate is subjected to an axial force the result is as shown in Figure 3. This is avoided only if the principal axes of loading are coincident with the orthotropic axes of symmetry of the lay-up. Off-axis load generally results in a shear deformation.
Bending coupling
In bending coupling, applied bending moments result in twist of the lay-up and vice versa.
The twisting component of each ply has an l 3 dependency, where l is the distance of the ply from the centreline.
For example, consider:
(-45, +45, -45, +45, +45, -45, +45, -45)
This lay-up will twist with bending moments, the major contribution being from the outer layers. To offset such consequences, alternative plies can be introduced away from the centreline which have an opposite effect; eg to produce half the twist:
(-45, +45, +45, -45, -45, +45, +45, -45)
The concept is utilised further by manipulation of the value of l, via the introduction of longitudinal plies, eg:
(+45, -45, -45, 0, +45, 0, 0, +45, 0,-45, -45, +45)
The lay-up is uncoupled in twisting-bending.
All these lay-ups are balanced with respect to in-plane bending and in-plane coupling.
It is therefore relatively straightforward to produce balance in the first two instances, but balancing of bending-twisting movements requires appropriate lay-ups.
Caution needed
There is ample scope for mistakes and misjudgements in composite design, as follows:
- Misinterpretation of elastic constants and incorrect use of Poisson's ratio;
- Failure to understand the mechanics of torsional deformation, ie which shear modulus is most significant;
- Lack of understanding of the importance of poor transverse properties in the design of components, ie interlaminar shear and tension;
- Failure to appreciate the difference between stress and strain distributions in anisotropic materials;
- Lack of understanding of the effect of lay-up in behaviour under static loading, and in buckling and vibration;
- Underestimation of the influence of the coefficient of thermal expansion on the thermal distortion and stress distribution in laminates.
Current applications
Continuous fibre composite components are used extensively at present, particularly in the aircraft industry in such planes as the Airbus A310 and Boeing 757 and 767, which use about 1350kg each, and smaller planes such as the 737-300 which uses about 680kg. On the 737-300, composites are used for control surfaces, fairings, and nacelle components, as presented in Figure 4, and account for about 3% of the total structural weight of the aircraft.
Fig. 4. Composite application on the Boeing 737-300
Individual composite parts are up to 30% lighter than their conventional counterparts, a considerable weight saving which could be an advantage in other industries.
Summary
Composite structures are challenging concepts, with every new design involving evaluation of new parameters. This promotes higher performance in the application and understanding of composites as a structural alternative. However, design should not be a straightforward substitution of a composite for a metal component, as this never produces optimum conditions.
