D5: Laminate - Empirical

Question 1

What assumption can we use to simplify the initial design of lamina?

Answer 1

That loads are carried exclusively by plies aligned with them (e.g. transverse loads are carried by 90 deg plies alone)

Question 2

What 2 simple rules should we follow to ensure secondary loading isn’t problematic?

Answer 2

• Ensure no less than 10% of the plies are in each direction (for QI)
• Ensure no more than 4 plies of one orientation in a row (reduce thermal cracking & splitting along fibre direction)

Question 3

How can we avoid warping with the layup?

Answer 3

• Use symmetric layups with balanced 45 plies.

Question 4

When should unsymmetric or unbalanced layups be used?

Answer 4

• When it is unavoidable, or when elastic tailoring is required.

Question 5

Why should 45 plies be placed on the surface?

Answer 5

To protect the primary loaded 0 deg plies, and to provide higher torsional stiffness.

Question 6

What are 3 ways we can design for secondary loading?

Answer 6

• Spirally stack plies to reduce angle between them & maintain a consistent stacking pattern.

Question 7

What are 3 ways we can design for secondary loading?

Answer 7

• Spirally stack plies to reduce angle between them & maintain a consistent stacking pattern.
• Allow for differing thermal expansion in hybrid laminates.
• Select layup for reducing through-thickness stresses.

Question 8

How do the poisson’s ratios of composite laminates differ from metals’? Why is this an issue? How can we minimise this?

Answer 8

• Composites have a larger range of p.r than metals
• P.r. mismatch between components can lead to significant secondary stresses.
• Control the p.r. using 90 deg ply “cross-bracing”

Question 9

What are the 3 empirical laminate analysis methods? What is their limitation?

Answer 9

• Netting rule, rule of mixtures, carpet plots.
• Limited to membrane loading (not much of an issue because that’s mainly what components will be designed to carry)

Question 10

What is the netting rule?

Answer 10

Only the layers with fibres in the direction of loading carry the loading, other layers don’t contribute.

Question 11

Can the netting rule be used to estimate shear strength?

Answer 11

Yes, by estimating “diagonal direct strength”, i.e. interpreting the shear load as an equ. and opp. biaxial diagonal stress.

Question 12

What is the mixture rule?

Answer 12

Empirical estimation of how much each ply direction contributes to a loading. 10% rule.

Question 13

Why are carpet plots expensive to create?

Answer 13

Huge number of tests needed, lots of specimens, done for specific conditions and specimen types that can’t be extrapolated well to others.