EN380 - NAVAL MATERIALS WOOD AS A MARINE MATERIAL “Why wood is good.”
Naval Uses of Wood Ships Platforms Builidings Piles Superstructure Damage Control Aesthetics
Wood’s Pro’s & Con’s 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 Structure Oriented cellulose fiber/lignin cells (a natural composite) Hardwood/softwood Best described in cylindrical coordinates –Longitudinal –Radial –Tangential
Wood Structure (3) Properties vary throughout tree Warping a factor Marine-grade lumber is cut to minimize distortions Quartersawn or vertical grain (vg) Plainsawn
Wood Structure (4) Quartersawn is preferred, but is most expensive (often 2-3 times the cost) Quartersawn or vertical grain (vg) Plainsawn
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%
Mechanical Properties (2) Note: “Flexural Strength” is often called the “Modulus of Rupture”
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.
Joining Wood Joints are either “bonded” or “mechanically fastened” Bonded means “glued” Mechanically fastened uses nails, screws, bolts, rivets, special types. Trade-offs include…?
Bonded Joints Try to transfer load through compression or shear (“scarfing” transfers through shear)
Bolted Joints Similar geometries, but fasteners are used. These cause stress concentrations. Some joints have both glue and fasteners.
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
Plywood “Engineered Wood” addresses directional strength issues Advantage is large, watertight, stable panels Disadvantage is shaping Shaped-in-place is called “cold-molding”
Homework A Damage Control Problem Water filled compartment This or This?