Processing of polymer composites Flashcards
What are PMC’s usually reinforced with and what are the advantages of these fibres
Glass - Inexpensive
low density
Similar specific modulus
Specific strength 10x greater
Carbon fibre
very expenisve and high strength
high corrosion and creep resistnace
Aramid eg kevlar
high stength low density
high impact resistance
What are the most common thermosets?
Polyesters
Unsaturated polyesters will cross-link in the presence of accelerators to form a rigid structure. Some restrictions on their use as styrene can be released during curing. 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.
What advantages to thermoplastics have 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)
What is spray lay up?
Chopped fibre (glass) and resin + catalyst ( i.e. polyester) mixed in a hand-held gun and sprayed directly into the mould or on to the structure. Gives a random 2-D fibre array.
Advantages: Cheap, well-characterised, versatile. “Cheap and cheerful”; also quite foolproof.
Disadvantages: Resin-rich laminates; only materials available are glass fibre and polyester; health hazards from styrene monomer.
Applications: Bathtubs, shower trays, small boats. Good for one-off jobs.
Explain FIlament windings
Generally used for hollow (circular or oval sectioned) components, though large curved sheets can also be made by carving these up after winding.
Continuous fibres (‘continuous rovings’ in the figure below) 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. Composite structures can be designed precisely to support the anticipated stresses.
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 lowviscosity resins only.
Applications: Chemical storage tanks and pipes, boat masts, wind turbine blades, gas cylinders, other pressure vessels
What is pultrusion?
Can be used either to process fibre bundles in a form which can be used for subsequent lay-up processes (pre-pregs, avoiding the need to deal with resin separately at this stage), or can be used to produce composite material articles in final form.
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 (unidirectional: fibres all parallel - NB low lateral strength), good structural control. Good range of compositions.
Disadvantages: Costly (particularly with heated dies), limited to constant-section components.
Applications: Beams and girders, bridges, ladders.
What is needed for good mechanical properties of PMC’s?
For good properties, need strong, high-modulus fibres to which load is transferred from the weaker, low-modulus matrix. Load transfer happens via shear stress at the fibre-matrix interface. Bonding at the interface is therefore important.
What is fibre pullout
Continuous brittle fibre (e.g. carbon, glass) composites: maximum strength is proportional to the volume fraction of fibres. If load increased enough to exceed fibre fracture stress, then fibres break up into shorter lengths.
Fracture resisted by fibre pullout
Explain the fracture mechanism for brittle fibres
In tension: cracking generally starts with a break in a fibre (more brittle than the matrix). Toughness is increased if cracks running normal to fibres can be blunted:
Brittle fibre has cracked. High elastic stresses in matrix at ends of crack. There are two ways to blunt the cracks:
(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)
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.
Explain the design of continous fibre composites
Used for apllication requiring optimum strength and stiffness
accomodate loads by positioning the fibres along directions of maximum stress
simple structures can use prepegged lay up sheets more complex use filament winding
As material deforms anisotropically, internal stresses are created in composite as it is loaded
To avoid out of plane distortions, lay up in a balanced way e.g.
o +ø -ø, 90 ,-90
Explain the design of extruded short- fibre composites
thermoplastics can be extruded using scew extrution and injection moulding
fibres however increase melt viscosity, this could place limits on max volume fraction of fibre
fibres alligned in one direction due to extrusion
good strength one way
weak in transverse direction
Explain design of joints in composite structures
long fibre composites, strength normal to fibres is low
leads to delamination which is bad for joints
SOLUTIONS
Try to avoiud joints all together
or use compressive joints with bulky couplings to minimuse stresses
If joints must be used ensure that sress maxima are internal
Internal channels will not provide channels for wicking of fluids in from exterior
