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Composites- Engineering Basics

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

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  1. Composites- Engineering Basics Paul H. Miller, D.Eng., PE Naval Architecture Program United States Naval Academy IBEX 2004 - Session 102

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. FEA Example IBEX 2004 - Session 102

  9. 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

  10. 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

  11. 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

  12. D3039 Tensile C393 Flex Standard Tests IBEX 2004 - Session 102

  13. Impact – A hull panel IBEX 2004 - Session 102

  14. Prototype – a deck panel IBEX 2004 - Session 102

  15. 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

  16. 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

  17. “You've got to ask yourself one question: 'Do I feel lucky?‘” IBEX 2004 - Session 102

  18. 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

  19. 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

  20. 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

  21. 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

  22. Basic Loads to Consider • Tensile • Compression • Buckling • Shear • Bending IBEX 2004 - Session 102

  23. 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

  24. Guides and References • Everything you need to know about basic structural analysis? • My most-used reference! IBEX 2004 - Session 102

  25. Guides and References • The classic • Structural details • Basic properties • Carpet plots • OK if building to 60’s technology IBEX 2004 - Session 102

  26. 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

  27. 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

  28. 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

  29. Guides and References • A $35 program that uses CLT. • My most-used composites software! • Works great with a spreadsheet. IBEX 2004 - Session 102

  30. 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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