Handout 7: Composites Flashcards
Name different types of composite fibres and their properties.
- E-Glass - Relatively inexpensive. Density ρ 1/3 that of steel; specific modulus (E/ρ) similar; specific strength (σf/ρ) 10x greater. May be used as single or multi-filaments, or woven into cloth Often chopped to form short fibres (so can use standard polymer forming technologies such as extrusion).
- Carbon Fibre - Very expensive. High strength; very high specific modulus (E/ρ); high resistance to corrosion, creep and fatigue. Impact resistance less than glass or aramid; poor in compression.
- Aramid - e.g. Kevlar,Very expensive. Yellow fibres containing highly aligned polymer chains. High strength, low density. Extremely tough; excellent impact resistance. Can be UV sensitive.
Why are composite fibres woven?
For ease of handling.
These are then infiltrated with the matrix polymer in liquid form and cured to form the composite:
Name and explain the different types of matrix materials.
- Thermosets - By far the most common matrix for PMCs. They are cheaper than thermoplastics, and the higher stiffness is advantageous.
- Polyesters - Unsaturated polyesters will cross-link in the presence of accelerators to form a rigid structure. Low performance so generally only used with cheaper reinforcement such as glass.
- Vinyl esters - Much better mechanical properties and environmental resistance than polyesters. More expensive.
- Epoxy resin - Usually made by mixing two liquid components resin + ‘hardener’ . Polymer cures by forming chemical cross-links between polymer chains. Good mechanical properties (strength, toughness, high modulus), good environmental resistance. May be more than twice as expensive as polyesters or vinyl esters. Used principally with carbon, glass and aramid fibres.
- Thermoplastics - Advantages over thermosets: Short-fibre composites can be made by conventional thermoplastic processing Greater potential for high-grade recycling Tougher matrix Parts can be joined by welding (using heat and pressure)
Explain spray lay-up, give advantages/disadvantages.
- Chopped fibre (glass) and polyester resin + catalyst mixed in a hand-held gun and sprayed directly into the mould or on to the structure. Gives a random 2-D fibre array. Quality depends on operator skill.
- Advantages: Cheap, well-characterised, versatile, quite foolproof.
- Disadvantages: Resin-rich laminates; only materials available are glass fibre and polyester; health hazards from styrene monomer. Good surface finish on one side (in contact with mould), rough on other. Poor dimensional accuracy and repeatability. Applications: Bathtubs, shower trays, small boats.
- Good for one-off jobs.
Explain the process wet lay-up, hand lay-up,
give advantages/disadvantages and applications.
- Resins impregnated by hand (using rollers) into fibres .
- Only suitable for low-viscosity resins. Left to cure at room temperature. Roller used to spread resin and remove bubbles; gel coat allows release from mould and gives smooth surface finish.
- Advantages: Reasonably cheap, technique easily learned, versatile. Suitable for a wide range of fibres and resins. Good for one-off jobs.
- Disadvantages: Quality of composite very dependent on skill of operator. High fibre volume fractions difficult to achieve. Health hazards from low viscosity resins and monomers.
- Applications: Wind-turbine blades, boats, architectural mouldings.
Explain the process vacuum bagging, give advantages/disadvantages and applications.
- An extension of wet lay up, but quality improved by using a vacuum to apply a uniform pressure to the moulding before and during curing. Heat and pressure can also be applied using an autoclave (pressurized oven) when making articles.
- Advantages: High fibre contents, lower porosity, better process control.
- Disadvantages: More costly, greater operator skill needed.
- Applications: Wind turbine blades, large boat hulls, aircraft structures, racing car components. Can also be used for in-situ repairs, e.g. on boat hulls.
Explain the process filament winding, give the advantages/disadvantages and application.
- Generally used for hollow circular components, though large curved sheets can also be made by carving these up after winding. Continuous fibres are passed through a resin bath before being wound onto a mandrel in a variety of orientations.
- Advantages: Can be very fast and economical. Resin content carefully controlled.
- Disadvantages: Limited to convex components. Fibres cannot be laid exactly along the length of a component. Fibre feeding mechanism and mandrel can be expensive. Suitable for low viscosity resins only.
- Applications: Chemical storage tanks and pipes, boat masts, wind turbine blades, gas cylinders, other pressure vessels.
Explain the process pultrusion, give the advantages/disadvantages and applications.
- Can be used either to process fibre bundles for lay-up processes later or can make final form composite.
- Fibres pulled through a resin bath and then through a die. If the composite is being produced in final form, the die is heated to cure the resin.
- Pultruded product may be small bundles or tapes of multiple fibres for subsequent processing, sheets (laminae, which are used for lay-up processes) or any extruded sections (e.g. rods, I-beams)
- Advantages: Fast, excellent fibre alignment,good structural control. Good range of compositions.
- Disadvantages: Costly (particularly with heated dies), limited to constant-section components.
- Applications: Beams and girders, bridges, ladders.
Explain extruded short fibre composites, give the advantages/disadvantages.
- PMCs containing short fibres in a thermoplastic matrix (e.g. glass-fibre filled nylon or polypropylene) can be extruded using standard polymer technology (screw extrusion), and also injection moulded.
- Advantages: High toughness; thermoplastic matrix gives potential for recyclability.
- Disadvantages: Fibres increase the melt viscosity. This may place limits on maximum volume fraction of filler.
- The extrusion process leads to significant alignment of the fibres as well as aligning the polymer molecules giving anisotrophic modulus and strength. This can be a good or bad thing.
- Applications: Currently comparatively limited, but applications in automotive and defense industries are increasing.
Give the equations that let you calculate the composite elastic modulus parallel and perpendicular to fibre orientation.
What mechanism makes composites tough?
Draw and describe the different scenarious that occur when brittle fibre has cracked in a composite.
- (a)Brittle fibre has cracked. High elastic stresses in matrix at ends of crack. There are two ways to blunt the cracks, illustrated in (b) and (c).
- (b) Ductile matrix: cracks can be blunted by plastic deformation (maybe shear yielding)
- (c) If the fibre-matrix interface is weak cracks can be diverted to run along the fibre. (But if it is too weak we lose the load transfer properties. Fibre-matrix bond strength must be carefully controlled)
- (d) If fibre-matrix bond is too strong, the crack is not blunted and propagates through both fibre and matrix. Leads to low toughness (i.e. brittleness)
Why are composites weak in compression?
In compression: composites tend to have inferior properties. Fibres buckle and fail by kinking at a much lower stress than in tension. Carbon fibres crush easily, so it is particularly important that CRFP (carbon fibre reinforced polymer) is used only in tension.
What is delamination?
Many composite structures are made from layers of fibre+matrix pre-preg. Layers may be sheets of uniaxial fibres (i.e. all fibres in same direction), or may be woven cloth.
Bonding between the layers is achieved by resin alone, so strength and modulus are low in this direction (often called “secondary properties”). Stress normal to the layers is liable to cause cracking and delamination.
Draw and explain the lay-up orientation.
Explain issues with the stresss produced in them.
- For critical applications (requiring optimum strength and stiffness), continuous fibre composites are used (more expensive than short-fibre).
- Because the material deforms anisotropically, internal stresses are created in a composite as it is loaded.
- However, the two laminae are bonded together, so they exert stresses on each other.
- The results are:
- Strain of the composite block is intermediate between the strains for the two laminae loaded in isolation;
- High stress at the interface between the laminae (implications for delamination failure; environmental degradation by ‘wicking’ of liquid such as water between laminae)
- Composite block will suffer out-of-plane distortions, as shown below (mainly because of Poisson ratio effects).
Explain how anticlastic distortion occurs and how to prevent it.
An important principle in composite design is to make sure that these elastic distortions are minimised by making multiply composites symmetric, giving balanced layup. e.g. instead of 0°-90° , use 0°-90°-0° by introducing a third ply at the bottom.
Explain and draw how composite sections change from 6 plies down to 2.
When composite sections need to change (e.g. 6 plies down to 2, as below), there will inevitably be out-of-plane stresses.
- In the example on the left, the stress concentrations in the plies leads to delamination at the free surface.
- The effect can be reduced by ensuring that stress maxima are internal. This has two benefits: - interior delamination will not provide channels for wicking of fluids in from the exterior – the continuous surface layers help to maintain integrity.
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