Lecture 3 Flashcards
Fibres in bundle can be:
- aligned (glass roving, carbon tow),
- twisted (yarn)
What are non woven mats?
Mats from fibre strands at random orientation
*Flat sheets, typically from glass fibre.
Fibres are either chopped (all fibres have same length) or continuous.
*Polymer binder applied for stabilisation of fibre structure.
*Typically isotropic properties, although some degree of fibre orientation can be obtained during production.
*Frequently combined with Polyester resin.
*Obtainable fibre volume fractions in finished component < 40 %.
Explain the difference between Chopped strand mats (CSM) and continuous filament mats (CFM)
CSM: Fibre strands are chopped and deposited on conveyer at defined rate, Typically processed in hand laminating
CFM: Same process as CSM but continuous fibres are produced directly from melt. Typically processed in Liquid Composite Moulding or hand laminating
What is a unit cell?
A unit cell is the smallest repetitive unit composing a macroscopic textile patch
*Properties of a unit cell are representative of macroscopic
material properties.
*Analysis of a unit cell is sufficient to determine fabric
properties.
What is fabric crimp?
Crimp refers to waviness of yarns, i.e. sections of yarns orientated out-of-plane.
- Crimp is related to interlacing of yarns which results in cohesion of yarn layers, improving throughthickness properties of fabric;
- mechanical in-plane properties of fabric are reduced compared to straight fibres;
- crimp varies with weave style;
Explain the difference in weave architectures from high crimp to low crimp.
High crimp: high level of crimp, strong yarn fixation.
increasing stability of fabric, easier to handle.
Low crimp: low level of crimp, weak yarn fixation
increasing formability; improved mechanical properties
What are non crimp fabrics? And what are their benefits?
Layers of aligned straight tows are stitch-bonded (multiaxial reinforcement)
- superior in-plane properties compared to woven
fabrics since tows have no crimp - interlaminar properties (delamination resistance,
impact strength) better than for separate UD layers
because of through-thickness fixation
What are Thermoset prepregs?
Unidirectional tape or fabric pre-impregnated with partially cured (B-stage) resin system
Frequently made from carbon fibre and epoxy resin.
Needs to be stored at low temperature, typically -18 °C, to delay any further progression of resin cure
What are the Advantages and disadvantages of Thermoset prepregs?
Main advantages:
* Resin/fibre content can be controlled accurately during
prepreg manufacture.
* High fibre contents can be achieved.
* Void content (detrimental to mechanical properties) is
typically low.
Main disadvantage:
Material cost is high
What are thermoplastic prepregs?
- Unidirectional tape or fabric impregnated with
thermoplastic polymer. - Typically flat organosheets with fabric reinforcement or
continuous tapes with unidirectional reinforcement. - Reinforcement is fully impregnated and consolidated
What are some characteristics of thermoplastic prepregs?
- Low void content.
- Matrix has its final properties, i.e. no more chemical reaction.
- Unlimited storage life at room temperature.
- Surfaces do not need to be covered (material is not tacky).
- Sheets/tapes are stiff and cannot be formed at room
temperature. - Needs to be heated above melt temperature of matrix and can then be moulded.
What are bulk molding compounds? (BMC)
- Matrix based on thermoset resin reinforced with discontinuous fibres.
- Fibre volume fraction is typically low.
- Can be processed using compression moulding or “thermoset injection moulding”.
- Heat to induce resin cure
What are sheet moulding compounds?
Matrix based on thermoset resin reinforced with discontinuous fibres. Similar to prepreg (produced in sheets), but for less demanding applications. Typically higher fibre volume fraction than in BMC.
