Composites - Composites In Design

Figure 1 – Interactive Design Phase

Steve Goddard

Composite Materials – Assignment 2Composites in Design

Q1

a) Describe the main design features to be taken into account when incorporating a composite material into a new or modified assembly.

Composite materials can be engineered in a numbers of different ways but in general this type of material offers the following benefits: - Low weight- High strength and stiffness- Corrosion resistance- Chemical resistance- Excellent fatigue properties- Electrical insulation- Design freedom - Low maintenance

Designing with composite materials differs substantially from working with materials such as metals. In composite design, there is a much stronger interaction between Product requirement, Process and Material during the design phase. It can generally be described as in Figure 1. As metals are isotropic (the same properties in all directions), the mechanical properties in composite materials are strongly depending on the fiber direction and fiber volume. Another mayor difference from other materials is the fact that the composite material and the product are manufactured at the same time. Compare this with metal where you take the bulk material, i.e. sheet steel, aluminium bar etc, and shape it into a product. In this process the basic properties of the materials are normally left unchanged. In composite processing, fiber and matrix are put together giving a cured composite part. The properties and characteristics of the composite material are created at the same time as the product is formed. Thus, designing with composites is to a great extent dependent on the manufacturing method and the raw materials to be used.

To understand composite materials and processes is very important when designing an optimized and cost effective product. Developing a composite product must address product requirements, material and fabrication process during the design phase. A designer has the responsibility to take these different elements into account and must therefore be skilled in composite materials and manufacturing techniques as indicated, in Figure 1 above.

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A design process consists generally of the following steps:

1. Definition of product requirements

2. Conceptual design

3. Detailed design

4. Prototype manufacturing

5. Testing and evaluation

6. Design adjustments

7. Production set-up

The steps are in principle sequential but since it is a very dynamic process the design phase may have to start all over again in order to find an effective design solution.

1. Definition of product requirements

It is very important to establish a list and specify all requirements of the product in the beginning of the design process. Depending of the application the list looks differently but the following aspects may be specified:

Mechanical requirements

- What kinds of strength and stiffness must the component withstand?

Environment

- Indoor or outdoor application, subjected to water, chemicals, service temperature, maximum temperature or other environmental conditions

Surface requirement

- A visible or non-visible surface, surface quality, colour etc.

Weight requirements & Cost Target

Production volumes and production rates

- Critical issues in selecting manufacturing method

Codes and standards

- Must the product meet and be designed according to certain codes and standards

If possible all items shall be quantified in figures in order to avoid misunderstandings between the team members. This will also force, or give the opportunity to, the buyer of the development work to really think through the requirements for the application. Requirements as ¨stiff enough¨ or ¨as low weight as possible¨ do not help the designer to select a proper material combination. This first step has been proven to be quite difficult, especially if a current component made of some other construction material is to be replaced by a composite solution.

2. Conceptual design

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Now the evaluation of all parameters and other information from Step 1 can be evaluated and a preliminary design concept can start. Quite often the type of work to be done in this step is referred to as a pre-study or feasibility study, especially if composite materials are evaluated for completely now applications and you would like to get a feeling for cost, investments, possible materials etc.

During this step, two or three possible solutions will be defined with suggestions for choice of materials and manufacturing method. In composite design it is normal to end up with different solutions fulfilling the requirements. The solutions will have differences with both advantages and drawbacks.

In close co-operation with the buyer of the project (external or internal client) and other project members, the differences must be considered and a final solution has to be found. In some cases, two or more possible solutions are developed in parallel. A finial decision can then be taken when prototype manufacturing and a complete evaluation is performed.

3. Detailed Design

In this step, a detailed design will be performed. With CAD and FE-analyses (figure 2) the designer can draw and calculate the performance of the part. Normally a number of iterations is needed before a design is completed. A very important consideration during this step is also the chosen manufacturing method. Each composite process has characteristics that must be considered in establishing the final part geometry insuring successful manufacturing. Normally this work is performed in different activities where a number of people are involved contributing with special competences. The complexity of the development work will determine the size of the project team.

Figure 2 – Finite Element Analysis of a Carbon Fibre Reinforced Plastic (CFRP) Panel

For detailed calculation, drawings, process simulations, cost calculations, etc. a number of software is available to assist the design work. The use of this software requires skilled and trained personnel with a fundamental understanding of the composite materials.

Figure 3 – Future Lynx Design Offices at AgustaWestland

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4. Prototype manufacturing

A recommendable way to verify a design is to manufacture prototypes and test panels and then perform evaluation on critical parameters. It is still much more cost effective to evaluate prototypes and adjust the design if required compared to adjust the final manufacturing procedure including tooling, fixtures and other handling equipment.

To verify the structural behaviour the prototype must represent the geometry and material properties of the ultimate part. The best way to achieve this is if it can be included in the production process. Prototypes for hand lay-up, spray-up and vacuum infusion are relatively easy and non-costly to produce once the surface is defined. Also prototypes for filament winding is fairly easy to make.

Prototypes where matched tooling is required are more difficult to fabricate. Metal tools for compression moulding and injection moulding have normally long lead times and are quite expensive. Alternative solutions include composite tooling and cast aluminium. Despite the fairly high cost of this type of tooling, it is still relevant and time saving compared to design changes in the production tool.

Figure 4 - Prototype Berp IV blade

5. Testing and evaluation

Testing of prototypes is an important step in the design process. Structural behaviour, functionality and other critical parameters of the component will be verified against the product requirement – as defined in Step 1 in the design process. To evaluate the functionality, the prototypes can be made of other substitute materials, as long as the geometry is correct. For structural testing the prototype must consist of proper material lay-up.

Mechanical evaluation can be extensive depending on the product. Fatigue, internal pressure, testing of joints - the list of possible tests is endless. Most important is that a representative prototype is manufactured and tested against specified requirements. Remember that most dedicated testing also requires special fixtures and other equipment, also to be included in the cost and time table.

6. Design adjustments

Allow corrective design changes in your timetable. After evaluation of prototypes, test panels etc. there are normally changes to be incorporated in the design before the product can be

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put into production. When changes are made, sometimes new prototypes must be made to confirm the properties.

7. Production set-up

A successful design process ends with production of the composite part. Preparation prior to a production start must be carefully planned. This is in most plants handled by the production department. The activities involve purchasing of tooling and handling equipment, writing of special operation instructions and education of operators. Quality documents are also prepared.

Design limitations

As with all materials, there is no exception with composites – design limits exist. Restrictions to keep in mind are thick sections, significant thickness differences, sharp corners, etc. Joining also requires some thinking. Gluing and mechanical joints as rivets and bolts will give good properties provided the joints are carefully designed.

b) Determine the longitudinal tensile strength of an aligned glass fibre epoxy matrix composite in which the average fibre diameter and length are 0.010mm and 2.5mm respectively and the volume fraction is 0.4. Assume that:

i. The fibre matrix bond strength is 75 Mpa.ii. The fracture strength of the fibres is 3500 Mpa.iii. The matrix stress at fibre failure is 8 MPa.

Average Dia. = 0.010 mm.

Average Fibre length = 2.5 mm.

Volume fraction = 0.4

Fibre matrix bond strength = 75 Mpa.

Fraction strength of fibres = 3500 Mpa.

Matrix stress at Fibre failure = 8 Mpa.

Some critical fiber length is necessary for effective strengthening and stiffening of the

composite material. This critical length is dependant on the fiber diameter and its

ultimate or tensile strength , and on the fiber-matrix bond strength according to the

following equation:

For a discontinuous and aligned fiber composite having a uniform distribution of fibers and in

which , the longitudinal strength is given by the following equation:

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where represents the fracture strength of the fiber and represents the stress in the

matrix when the composite fails.

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Q2

Explain the significance of Griffith Crack Theory in relation to CMC’s and describe suitable methods for improving the fracture toughness of such materials.

The Griffiths equation describes the relationship between applied nominal stress and crack length at fracture, i.e. when it becomes energetically favourable for a crack to grow. Griffith was concerned with the energetics of fracture, and considered the energy changes associated with incremental crack extension.

For a loaded brittle body undergoing incremental crack extension, the only contributors to energy changes are the energy of the new fracture surfaces (two surfaces per crack tip) and the change in potential energy in the body. The surface energy term (S) represents energy absorbed in crack growth, while the some stored strain energy (U) is released as the crack extends (due to unloading of regions adjacent to the new fracture surfaces). Surface energy has a constant value per unit area (or unit length for a unit thickness of body) and is therefore a linear function of (crack length), while the stored strain energy released in crack growth is a function of (crack length)2, and is hence parabolic. These changes are indicated in the figure below:

The next step in the development of Griffith's argument was consideration of the rates of energy change with crack extension, because the critical condition corresponds to the maximum point in the total energy curve, i.e. dW/da = 0, where a = a*. For crack lengths greater than this value (under a given applied stress), the body is going to a lower energy state, which is favourable, and hence fast fracture occurs. dW/da = 0 occurs when dS/da = dU/da. The graph below shows these energy rates, or differentials with respect to a.

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R is the resistance to crack growth (= dS/da) and G is the strain energy release rate (= dU/da).

When fracture occurs, R = G and we can define Gcrit as the critical value of strain energy release, and equate this to R. Hence Gcrit represents the fracture toughness of the material. In plane stress the Griffith equation is:

where, to get the fracture stress in MPa (the standard SI engineering unit), the critical strain energy release rate is in N/m, E is in N/m2, and a is in m. This provides an answer in N/m2 (Pa), which needs to be divided by 106 to get the standard engineering unit of MPa. In plane strain:

Suitable methods for improving fracture toughness in CMC’s

Reinforced ceramicsj

Among the strategies for achieving ceramics with improved mechanical properties, especially toughness, some involve the engineering of microstructures that either resist the propagation

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of cracks or absorb energy during the crack propagation process. Both goals can be achieved simultaneously in microstructures with fibrous or interlocked grains. In ceramics produced with such microstructures, cracks are deflected from a straight path, leading to a dramatic increase in crack length; at the same time particles behind the advancing crack tip bridge the crack, tending to hold it closed. Crack deflection and crack bridging also occur in whisker-reinforced and fibre-reinforced ceramic composites. The result is increased fracture surface area and much greater energy absorption.

Micro cracking.

Another mechanism that can lead to increased fracture toughness in ceramics is micro cracking, which occurs in single-phase polycrystalline ceramics whose grains are anisotropic (that is, whose mechanical properties vary with direction) or in intentionally biphasic polycrystalline microstructures. In these materials tiny micro cracks open up to either side of the main crack path ahead of the advancing crack tip. This phenomenon has two effects. First, the energy that goes into the opening of the subsidiary cracks increases the energy needed for propagation of the main crack. Second, as the main crack propagates, micro cracks opening up in the wake or process zone adjacent to the main crack but behind the crack front result in an increase in volume, which tends to close the main crack. The resistance to propagation thus increases the farther the crack propagates.

Transformation toughening.

The most promising toughening mechanism for ceramic materials involves a phase transformation; the method is referred to as transformation toughening and is illustrated in Figure 5. Although other materials such as alumina can be transformation-toughened, zirconia (zirconium dioxide, ZrO2) is the prototype material for this process. Pure zirconia, upon cooling below 1,150° C (2,100° F), undergoes a dramatic 3 percent volume expansion as it transforms from a tetragonal form to a monoclinic form. This expansion can be used to advantage by dispersing extremely fine tetragonal particles in a matrix of cubic zirconia or alumina. The small size of the particles (less than 1 micrometre) and their intimate contact with the matrix induce the tetragonal structure to remain stable at room temperature. Ahead of an advancing crack, however, a stress field triggers the transformation of the embedded tetragonal particles to the monoclinic form. Behind the advancing crack, a process zone forms in which all the tetragonal particles have transformed to the monoclinic form. The cumulative increase in volume exerts a closing force on the advancing crack, as well as a corresponding resistance to crack propagation that increases with crack length. Ceramics such as transformation-toughened zirconia (TTZ) are often referred to as ceramic steel because the strain, or change in dimension, in response to stress behaviour resembles that of steel instead of a brittle ceramic. Also, the underlying phase transformation is called martensitic, after a similar transformation in rapidly quenched steel to a phase known as martensite.

Figure 5: Resistance to cracking in transformation-toughened zirconia. In a ceramic composed of tetragonal zirconia dispersed in a zirconia matrix, the stress field advancing ahead of a

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propagating crack transforms the small tetragonal particles to larger monoclinic particles. The larger particles exert a crack-closing force in the process zone behind the crack tip, effectively

resisting propagation of the crack

Q3

a) Show how the laws of mixtures equation can be used to determine the strength and stiffness of composite materials.

The mass of a composite is made of the masses of the matrix and the fibres , since

mass is volume ( ) X density ( ), then the equation can be written as:

This can be transposed into:

is the volume fraction that is matrix and is the volume

fraction that is fibre. Thus:

This is the law of mixtures.

Two mathematical expressions have been formulated for the dependence of the elastic modulus on the volume fraction of the constituent phases for a two-phase composite. These rules of mixtures equations predict that the elastic modulus should fall between an upper limit represented by:

And a lower limit:

Where, E represents the elastic modulus, V represents the volume fraction, c = composite, m = matrix, p = particulate.

Essentially what the law says is that if you have a composite of 40% matrix with a modulus of 10GPa, and 60% fibre with a modulus of 100GPa then the elastic modulus of the composite will be equal to each phases volume percentage multiplied by its elastic modulus, as follows:

Strength is more difficult to predict that stiffness because there are many more ways the composite can fail, such as: fibre fail, matrix fail or fibre matrix interface failure.

The matrix has a lower strength that the fibres so it will have a very small volume fraction when the matrix fails, the load has to be carried by the fibres but as there are not many fibres to carry the load complete failure follows. When you have the applied in a direction parallel to the fibres, gives:

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Where is the composite failure stress, is the matrix failure stress and =is the fibre stress when the matrix fails. Thus:

And because there are so few fibres you can normally neglect their contribution and show the equation as simply:

At large fibre volume fractions, the fibres will take most of the load and after matrix failure will still be able to carry a load and determine the strength of the composite. Then you must have:

Where is the fibre failure stress.

As with the stiffness calculations mentions above, the strength depends on the orientation of the fibres relative to the load direction. The graph below shows the effect of fibre orientation on the strength of a typical continuous fibre composite, glass fibre-reinforced epoxy.

b) A composite composed of long parallel fibres of Kevlar in an Epoxy resin matrix, is required to have a tensile strength of 1600 Mpa. If the strengths of the Kevlar and epoxy resin are respectively 2560 Mpa and 80 Mpa. What volume fraction of Kevlar must be used in the composite?

Tensile strength = 1600 MPa

Fibre strength = 2560 MPa

Resin strength = 80 MPa

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Figure 6 - Strength Variation with angle

Angle of Fibres

Strength MPa

Steve Goddard

Therefore there must be at least 61.29% by volume of Kevlar.

Q4

a) Explain the differences between dispersion strengthening in Composite Materials and precipitation hardening in metallic alloys.

Precipitation hardening is a heat treatment technique used to strengthen malleable materials, including most structural alloys of aluminum, magnesium and titanium, and some stainless steels. It relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal's lattice. Since dislocations are often the dominant carriers of plasticity, this serves to harden the material. It relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal's lattice. Since dislocations are often the dominant carriers of plasticity, this serves to harden the material.Precipitation hardening can be described as similar to dispersion strengthened composites, due to the fact that the impurities in the surface of a precipitation hardened metal play the same role as the particle substances in particle-reinforced composite but there are differences. Essentially the only similarity is the way that imperfections in the material are used to prevent the propagation of cracks.

The dispersion strengthening effect is not at major as with precipitation hardening, the strengthening effect is retained at elevated temperatures and for extended periods of time. This is because the dispersed particles are chosen specifically to be un-reactive with the matrix phase.With precipitation hardened metals the increase in strength can fade upon heat treatment as a consequence of precipitate growth of dissolution of the precipitate phase.

b) What effects do voids have on the strengthening mechanisms in PMC’s

Voids in a Polymer Reinforced Composite can occur in three ways; within the fibre, in the matrix or in the matrix/fibre interface. Voids in PMC’s create weak areas and act as stress concentration points. Voids can significantly decrease the inter-laminar shear strength of a composite, and in turn this impacts the compressive strength, impact resistance and fatigue life. The consistent production of void-free composite structures cannot be guaranteed for all

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fibre-resin systems. For this reason the effect of voids on the mechanical properties of composite materials must be considered and understood to the allowable deviations from void-free conditions can be determined and specified when necessary.Voids can be created during manufacture for a number of reasons; poor mixing techniques for the resin, trapping air during wet-lay up. The presence of voids is significantly reduced by using pre-impregnated materials with an autoclave finishing process, as will be discussed in assignment 3. Voids can also develop during service due to degradation of the matrix or fibres caused by the environment the component works in – sunlight, moisture, chemicals, abrasion etc.In the aerospace industry composite materials that are classed as critical parts, such as rotor blades, are inspected using ultrasonic testing. If voids are not found in this method they are normally identified when cracks develop and propagate. Composites can be repaired with this occurrence, but it is also highly likely that the component will fail before the crack is identified.

Q5

Describe the main failure modes and mechanisms in composite materials. How do these modes differ from those in conventional materials?

Compressive Loading

Composites are designed to have high strength and elastic modulus in particular directions, these properties are given by both the matrix and reinforcement material. However in compressive loading the fibres cannot bear a portion of the load and so the compressive strength is dependant on the matrix alone. Excessive compressive loading, that is loads higher than the compressive strength of the matrix, will cause the matrix/reinforcement bond to weaken and fail and can also cause fibres to fracture.

Corrosion

Corrosion in composite materials can be caused by chemicals, like oil or solvents, UV light, moisture etc. The corrosion can weaken the matrix material and sometimes penetrate to the fibres and cause degradation in them also. This can cause the fatigue strength to be reduced, as the component is put under strength and flexed it can increase voids in the composite allowing the corrosion so penetrate further into the component. Corrosion in conventional materials such as metals occurs slightly differently. With metals the presence of oxygen or water causes oxidization on the surface of the component, essentially this is the loss of electrons in the surface. In metals corrosion decreases the minimum metal condition and creates areas of stress concentration.

Creep

There are a number differences between structural metal and fiber-reinforced composites, in terms of creep. For example, metals in general exhibit yielding and plastic deformation whereas most fiber-reinforced composites are elastic in their tensile stress-strain

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characteristics. However, the dissimilar nature of these materials provides mechanisms for high-energy absorption on a microscopic scale comparable to the yielding process. Depending on the type and severity of external loads, a composite laminate may exhibit gradual deterioration in properties but usually would not fail in catastrophic manner. Mechanisms of damage development and growth in metal and composite structure are also quite different.

Delamination

Delamination occurs when the matrix-reinforcement bond weakens and causes them to separate. Energy is released which drives this process. Delamination of a structural panel can significantly weaken the component, and more often than not will cause it to become unserviceable. This failure mode does not occur in conventional materials.

Fatigue Fatigue is the cyclic loading and unloading of the component, with loads below the maximum tensile strength of the component. This causes failure by introducing cracks in the component followed by crack propagation. This then causes softening because the stress and strain are out of phase.Metals are also susceptible to fatigue. However the way that both are affected by fatigue differs.Fatigue basically means that with each cycle of load applied the components maximum tensile strength is decreased, even if the load applied is significantly lower than the maximum strength. Therefore materials are tested to determine how many cycles they can withstand before failure. For metals this can relatively easily be estimated with testing of components and extrapolation of the graph. However composite materials can with stand fatigue a lot better that metals and economic testing and analysis is not as easy.

Fracture Toughness

Composites depend on fibre reinforcement for fracture toughness and in some cases fracture of the matrix does not necessarily result in fibre failure or total failure. With conventional materials fracture toughness is dependant on the bond between atoms in the material structure.Impact Damage

Composite materials are easily damaged by impact. Impacts can cause fibres to shear and weaken the matrix, therefore significantly weakening the component. With metals, however, impacts do not cause as much damage and they still retain structural integrity.

Shear Loading

Shear loading can affect composites worse than metallic components, metals are isotropic – they have similar strengths in all directions which means the loads can be absorbed. Composites rely on the matrix to protect the fibres from shear loading which is why they are more susceptible to this failure mode.

Temperature

With composite materials the two phases will react to temperatures in different ways, one may be stable and the other not at a certain temperature. Different expansions between the two phases can cause delamination. With conventional materials the structure is affected in its properties and dimensions.

Tensile Loading

Tensile failure occurs in both metals and composites and is caused by the maximum tensile strength being exceeded. Tensile loading can cause fractures/cracks and the component will fail.

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Bibliography

Prototype BERP IV Rotorblade - http://www.cartercopters.com/images/weekly_update/2008-10-13_rotor_blade_skin.jpg

http://www.compositesiq.com/

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