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Where it all began...

Where it all began. EN380 - NAVAL MATERIALS. WOOD AS A MARINE MATERIAL “Why wood is good.”. Naval Uses of Wood. Ships Platforms Builidings Piles Superstructure Damage Control Aesthetics. Light (floats!) Easy to work Abundant Inexpensive High specific properties

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Where it all began...

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  1. Where it all began...

  2. EN380 - NAVAL MATERIALS WOOD AS A MARINE MATERIAL “Why wood is good.”

  3. Naval Uses of Wood • Ships • Platforms • Builidings • Piles • Superstructure • Damage Control • Aesthetics

  4. Light (floats!) Easy to work Abundant Inexpensive High specific properties Good fatigue resistance Good insulation Flammable Rots Gets eaten by insects and borers Not strong for the volume Wood’s Pro’s & Con’s

  5. Wood Structure • Oriented cellulose fiber/lignin cells (a natural composite) • Hardwood/softwood • Best described in cylindrical coordinates • Longitudinal • Radial • Tangential

  6. Wood Structure (2) • Stiffness and Strength Ratio’s • Longitudinal =“100” • Radial = “5-10” • Tangential = “2-10”

  7. Wood Structure (3) Plainsawn • Properties vary throughout tree • Warping a factor • Marine-grade lumber is cut to minimize distortions Quartersawn or vertical grain (vg)

  8. Wood Structure (4) Plainsawn • Quartersawn is preferred, but is most expensive (often 2-3 times the cost) Quartersawn or vertical grain (vg)

  9. Mechanical Properties • A function of moisture content • Wet wood is as low as half as strong as dry • “Wet” (or “green”) means moisture content >18% • “Dry” means ~12%

  10. Mechanical Properties (2) Note: “Flexural Strength” is often called the “Modulus of Rupture”

  11. Mechanical Properties (3) • Wood is an orthotropic material, but • Analysis usually uses either: • Isotropic beam theory for lumber, or • Transversely isotropic plate theory for plywood • (See the notes for typical equations) • Wood “creeps”, so a FOS of 2 (for creep) is added for long term use.

  12. Joining Wood • Joints are either “bonded” or “mechanically fastened” • Bonded means “glued” • Mechanically fastened uses nails, screws, bolts, rivets, special types. • Trade-offs include…?

  13. Bonded Joints • Try to transfer load through compression or shear (“scarfing” transfers through shear)

  14. Bolted Joints • Similar geometries, but fasteners are used. These cause stress concentrations. • Some joints have both glue and fasteners.

  15. Biodegradation • Wood wants to degrade, but given the right conditions, it can carry loads for centuries. • Basic problems include: • “Rot”, which is fungal attack • Insect attack • Marine borer attack • Prevention includes: • Paint, preservatives, ventilation

  16. Plywood • “Engineered Wood” addresses directional strength issues • Advantage is large, watertight, stable panels • Disadvantage is shaping • Shaped-in-place is called “cold-molding”

  17. Homework • A Damage Control Problem Water filled compartment This or This?

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