What is meant by intermediates?
Most manufacturing routes for composites make use of intermediates. It is a stage created to facilitate handling following processing stages. Dry fibres have intermediate stages as mats or fabrics, which are combined with a matrix during moulding. A sheet of fibres is easier to handle than lots of loose fibres, but every processing stage adds cost to the final product.
Alternatively, fibres and matrix can be made as an intermediate as prepregs or moulding compounds.
What are the different types of fibre bundle and mat intermediates?
- Fibre bundles: Multiple filaments can be combined in a fibre bundle. These can be aligned or twisted, which reduces the risk of the fibres falling apart or breaking but reduces axial properties and adds cost.
- Non-woven mats: Mats can be made from fibre strands at random orientations. Typically glass fibres, either chopped or continuous. A polymer binder is applied for stabilisation, and they are frequently combined with polyester resins.
- Chopped Strand Mat (CSM): Fibre strands are chopped and deposited on a conveyor, and a polymer binder is sprayed on. The binder is soluble in styrene, and it is typically processed in hand laminating.
- Continuous Filament Mat (CFM): Similar production to CSM, except fibres aren’t chopped and are produced directly from melt. Typically processed in liquid composite moulding processes or hand laminating.
What are textile intermediates?
In textile reinforcements, tows/yarns are arranged in a defined pattern (referred to as a architecture). These are multi-scale structures, and fibre volume fractions >50% can be obtained in finished components.
They result in anisotropic material properties, typically orthotropic. They are frequently processed in liquid composite moulding, hand laminating, and also conversion into prepregs.
What are unit cells?
A unit cell is the smallest repetitive unit allowing a macroscopic textile patch to be generated through translation along weft and warp direction.
How are woven fabrics made and what are the different weave patterns?
What is crimp
Standard textile looms are used to interlace 0/90 degree yarns by lifting warp yarns up and down. The weft yarn is drawn through the gaps.
* Plain weaves refer to 1x1 weave patterns
* Twill weaves refer to 2x2 or 3x1 weave patterns
* Satin weaves refer to 5x1 (5 harness)
Crimp refers to the waviness of yarns. It improves through-thickness properties of fabric and increases yarn fixation. However, the mechanical properties in-plane are reduced compared to straight fibres.
List other types of textile reinforcements
- Spread tow fabrics: These are produced by weaving flattened tows, achieving a thinner laminate. This lower crimp reduces the amount of excess plastic, minimizing weight but increasing the strength.
- Non-crimp fabrics (NCF): Layers of aligned straight tows are stitch-bonded to form multiaxial reinforcement. This results in superior in-plane properties compared to woven fabrics since tows are almost straight, and interlaminar properties
- Knitted fabrics: Fabrics formed through interlocking yarn loops. These conform very well to complex geometries, but unless fabric is fully stretched the fibres are not straight and cannot be aligned with any load direction. Mechanical properties typically poor as stretched fabric results in small bending radius leading to fibre breakage.
Picture frame shear test
What test is used to see how far a fabric can be sheared before wrinkling occurs?
The picture frame shear test. This has a shear frame mounted on a universal testing machine. A cruciform specimen is clamped in the framme, and the cross-head moves upwards at a constant speed.
The shear force is measured as a function of shear angle.
This test works for bi-directional fabrics, but not multiaxial fabrics or random mats.
Fabric friction test
Which test can be used to see how easily fabric layers slide relative to each other?
A fabric friction test. An aluminium table is attached to a universal testing machine and a the fabric is placed on top. A brick shaped steel sled is placed on top with defined mass and pulled at constant speed. The coefficient of fabric is recorded from ratio of tangential force and normal force.
Fabric compression test
What test can be used to see what compaction pressure needs to be applied to achieve a certain target fibre volume fraction?
A fabric compression test. Fabric is placed between two flat steel compression plates, and the compression force is recorded as a function of plate displacement.
What are thermoset prepregs? What are the production methods?
Thermoset prepres are unidirectional tape or fabric pre-impregnated with partially cured (B-stage) resin. They are frequently made with carbon fibres and epoxy resin, and stored at low temperature to delay resin cure.
They have the advantage of having resin/fibre content being controlled accurately and are suitable for demanding applications, but are expensive.
* Solvent dip process - Solvent added to resin to reduce viscosity, and the fibres are dipped in for impregnation. Solvent evaporated in drying oven.
* Hot melt process - A resin film is made from liquid resin and backing film. This resin film is deposited on a dry reinforcement to make prepreg.
Prepregs with fabric reinforcement are typically used for smaller components with higher geometrical complexity. Prepregs with unidirectional reinforcement are used for large components with low geometrical complexity.
What are thermoplastic prepregs? What are the production methods?
Thermoplastic prepregs are unidirectional tape or fabric impregnated with thermoplastic polymer. It is fully impregranted with low void content, with no more chemical reaction to take place. It has unlimited storage life and the sheets are stiff and cannot be formed at room temperature.
It needs to be heated above melt temperature of the matrix and then moulded.
Dry reinforcement is sandwiched between two polymer sheets and it is heated. Pressure is applied which impregnates the reinforcement with the molten polymer, and then it is cooled and cut into sheets.
This type of prepreg is used in compression moulding.
Moulding compounds
- Bulk moulding compounds - Matrix based on thermoset resinnn reinforced with discontinuous fibres. Usually polyester resin with glass fibres.
- Sheet moulding compunds - Same
