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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

United States Naval Academy

IBEX 2004 - Session 102


Outline

  • Blended properties/carpet plots

  • Advanced analysis

  • Tests to get properties or verify a structure

  • Basic loads

  • 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


“You've got to ask yourself one question: 'Do I feel lucky?‘”

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


What about loads?

  • Assume the worst case loading!

  • 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


Safety Factor Adjustments

  • 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


Example SF Adjustment

  • 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


Basic Loads to Consider

  • 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!

  • Ask an expert!

  • All of the above!

Cogito – Two time winner of the Little America’s Cup

No breakdowns so far!

IBEX 2004 - Session 102


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