D3: Lamina - Form Flashcards
What are the 2 types of lamina? What applications would you use each for?
- Unidirectional and woven
- Unidirectional for high loading, high strength and stiffness, and compression ability. Woven for small curvature/complex shapes.
How do designers and manufacturers classify lamina?
- Designers tend to use volume fractions, manufacturers tend to use weight fractions.
What are the 2 theoretical fibre packing arrangements, along with their Vfs?
- Square, 75%
- Hexagonal, 90%
What is the range of Vfs typical in aerospace applications? What is the reason for this?
- 45-60%
- Higher than this would result in gaps between fibres that are too small, causing stress concentrations.
What is an acceptable amount of voidage in aerospace composites?
Below 1-2%
What issues do composites suffer from related to temperature, and how can we design to limit their effects?
Issues:
- High temperature: low matrix limits
- Low temperature: microcracking
- Temperature cycling: degrades matrix & interface
- Softening: reduced matrix buckling support of fibres
Design:
- Careful selection of TS and TP
- Confirm by exposure tests
What are the 5 main environmental issues that composites are affected by?
- Temperature
- Moisture
- Chemical
- Erosion
- Electrical
What issues do composites suffer from related to moisture, and how can we design to limit their effects?
Issues:
- Matrix swelling/expansion
- Matrix softening
- Degraded matrix/interface-dominated properties
- Reduced matrix buckling support of fibres
Design:
- Use coatings and shields
- Careful selection of TS and TP
- Confirm by exposure tests
In what conditions do composites perform at their worst?
Hot and wet, particularly in compression.
What issues do composites suffer from related to chemical exposure, and how can we design to limit their effects?
Issues:
- Compatibility with fuel and hydraulic fluid etc.
- Galvanic corrosion of carbon in the presence of Al or steel
- Flammability/smoke toxicity, particularly for internal applications
Design:
- Perform soak tests for compatibility with chemicals
- Test flammability and bi-products
- Screen CF from Al and steel, or use Ti or stainless steel
What issues do composites suffer from related to erosion, and how can we design to limit their effects?
Issues:
- TS epoxies have poor erosion resistance
- Dust, sand, and rain result in surface wear and moisture ingress
Design:
- Use corrosion shields and coatings, e.g. Ti strips
What issues do composites suffer from related to electricity, and how can we design to limit their effects?
Issues:
- Composites have low electrical and thermal conductivity
- Poor energy dissipation
- Susceptible to lightning strike damage
Design:
- Use metallic mesh, strips, foil, or braids to conduct heat and electricity in the structure.
- Use EMF screening to protect sensitive electrical equipment
- Use earth returns, unipole aerials
What are the general rules when applying statistical techniques to composite experimental results?
- Use a large sample size (ideally 20-30 specimens) to allow for variability
- Use lowest value and extreme value statistics (e.g. fit data to Weibull distribution)
- Normalise values to an equivalent Vf when comparing similar laminates.
Why must you be aware of transverse moduli and how they’ve been measured?
Because the modulus works assuming linear elasticity, but stress/strain behaviour for transverse and shear can be significantly non-linear in composite lamina.
What shape of specimen should be used in tension tests and why?
Long gauges, to allow uniform stress distribution and reduce grip effects.
