Composites- Engineering Basics

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Composites- Engineering Basics. Paul H. Miller, D.Eng., PE Naval Architecture Program United States Naval Academy. Outline. Blended properties/carpet plots Advanced analysis Tests to get properties or verify a structure Basic loads Safety factors An example Guides and references.

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Composites- Engineering Basics

Paul H. Miller, D.Eng., PE

Naval Architecture Program

IBEX 2004 - Session 102

Outline
• Blended properties/carpet plots
• Tests to get properties or verify a structure
• Safety factors
• An example
• Guides and references

IBEX 2004 - Session 102

Blending Properties
• Taking a composite and treating it like an isotropic material (like aluminum)

Two Basic Approaches

• Approximate methods based on common laminates. Carpet plots, design data sheets, approximate equations.
• Classical Lamination Theory (CLT) - Matrix math using unidirectional material data to come up with an equivalent stiffness and strength. This is much more accurate!

IBEX 2004 - Session 102

CLT Properties

E = 5,530,000

Str= 54,000

Efy= 2,130,000

Efx= 8,910,000

Flex Str x= 20,800

Flex Str y= 87,000

Blended Props

E = 5,000,000

Str= 50,000

Ef= 5,000,000

Flex Str= 50,000

Comparison of Blended and CLT For a 2 ply knitted (90/0/0/90) 0.25” Laminate With Unidirectional PropertiesEx=10,000,000 Ey=100,000 Str=100,000

IBEX 2004 - Session 102

My Suggestions
• You can use blended methods for wovens and mats in balanced or near balanced laminates (about the same amount of material on each side of the core)
• You MUST use CLT for uni’s or unbalanced or asymmetric laminates
• You should use CLT for knits

IBEX 2004 - Session 102

More Suggestions

The “best” solution is to:

• Test for uni properties
• Preliminary design with CLT
• Make a test laminate
• Analyze the structure with finite element analysis
• Prototype test

#4 and possibly #5 if it is a “risky” design!

IBEX 2004 - Session 102

Determines which way the load goes in a structure

Can use either blended (bad idea) or CLT (good idea) to analyze composites.

Finite Element Analysis

IBEX 2004 - Session 102

FEA Example

IBEX 2004 - Session 102

Realities of FEA
• Not for everyone! It takes practice.
• It might be cheaper and faster and more accurate to test a prototype!
• When you need a really accurate prediction, it is the only way to go.

IBEX 2004 - Session 102

Testing 1-2-3
• First, determine if you need basic material properties. If so, use standard tests.
• Second, determine if you need to test for something special (fatigue, impact, high heels)? Design an appropriate test.
• Third, do you need a prototype test?

IBEX 2004 - Session 102

Standard Test Methods
• Primary Tests
• Tensile- ASTM D3039
• Compressive – C364/D3410
• Shear – D4255
• Bending – C393/D790
• Secondary Tests
• Peel, Void Content, Resin Content

IBEX 2004 - Session 102

D3039 Tensile

C393 Flex

Standard Tests

IBEX 2004 - Session 102

Impact – A hull panel

IBEX 2004 - Session 102

Prototype – a deck panel

IBEX 2004 - Session 102

Safety Factors

• Example:
• 3/4” three strand nylon has a breaking strength of 16,700 pounds
• The design load for anchoring a 110’ boat in 60 knots is 3,500 pounds
• The Safety Factor is 16,700/3,500 = 4.8

IBEX 2004 - Session 102

Safety Factors
• Overall Approach – All calculations use the same FOS.
• “For Composite Materials Use FOS=4”
• Is 4 too conservative?
• Does one value fit all conditions?
• Do you know the loads and strengths?
• What if you went lower and guessed wrong?

IBEX 2004 - Session 102

IBEX 2004 - Session 102

Safety Factors
• Based on Materials:
• “For Polyester Laminates use 4”
• “For Epoxy or Vinyl Ester use 3”
• Why?
• Epoxy and VE have greater strains to failure and therefore higher fatigue endurance limits (longer fatigue lives)

IBEX 2004 - Session 102

• If loads are static rather than dynamic, reduce SF by 33%.
• If short lived, reduce by 15%.
• Are boats ever static?

IBEX 2004 - Session 102

• Decrease by 15% if using CLT
• Decrease by 30% if using FEA (and the operator knows how to use it!)
• Increase by 50% if the builder is not intimately familiar with the materials!
• Increase by 25% if the material properties are not well known.
• Increase by 10-200% if the loads are unknown!

IBEX 2004 - Session 102

• A standard boat laminate of polyester will have a SF = 4
• An epoxy laminate designed using FEA for a competitive race boat could have a SF as low as:

3 x (1-0.3)x(1-0.15) = 1.8

A 55% Reduction!

That’s a lot less weight!

And no resale value!

IBEX 2004 - Session 102

• Tensile
• Compression
• Buckling
• Shear
• Bending

IBEX 2004 - Session 102

A Bending Example2’ x 3’ Hatch Cover WR/Poly

Load is a 250 pound person in the middle.

Might use a 2.0 dynamic factor if it could be jumped on!

Safety Factor is 4.

Material strength is 40,000 psi

Moment is (WxL)/4 = 1500 in-lb

Bending Stress is (M*t/2)/(bt3/12) and must be no more than material strength/safety factor.

So t needs to be at least 0.26” (~8 plies 18 oz) (“blended”)

With CLT it is ~7 plies and with FEA ~6 plies

But it could be stiffness driven!

IBEX 2004 - Session 102

Guides and References

• Everything you need to know about basic structural analysis?
• My most-used reference!

IBEX 2004 - Session 102

Guides and References
• The classic
• Structural details
• Basic properties
• Carpet plots
• OK if building to 60’s technology

IBEX 2004 - Session 102

Guides and References

• A good, basic reference from the 70’s for low tech construction
• A good alternative to Marine Design Manual

IBEX 2004 - Session 102

Guides and References

• The most up-to-date and comprehensive of the basic guides
• Great materials section!
• My second most-used reference!

IBEX 2004 - Session 102

Guides and References

• In case you want to really learn composites analysis!
• Presents CLT!
• Review your matrix math, then take the short course!

IBEX 2004 - Session 102

Guides and References

• A \$35 program that uses CLT.
• My most-used composites software!
• Works great with a spreadsheet.

IBEX 2004 - Session 102

If you want it done right…
• Do it yourself!