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14 Fundamentals of Wood/Epoxy Composite Boatbuilding Some boat designers set their priorities and trade off between stiffness, one-time strength, weight, and resist- ance to fatigue. We use wood, so we don’t have to compromise. It is the material of choice when the primary goal is to gain maximum stiffness and excellent strength and at least weight, and wood will retain these qualities for many years of hard sailing. Wood Strength In The New Science of Strong Materials, J. E. Gordon suggests that a simple way to understand the strength of wood is to think of a bundle of drinking straws which have been glued together. These represent tubular cellu- lose fibers bonded with lignin and clearly illustrate what happens to a piece of wood under load. When equal tension and compression loads are applied to opposing faces of this bundle, effects differ. The bundle is anisotropic: longitudinal strength, with the grain of the straws, is much greater than horizontal or cross-grain strength. When too much compression is applied to any face, the straws buckle and then crush. The bundle is generally stronger in tension than in compression, but in spite of all this pushing and pulling, the straws are very hard to actually break. Strength is a measurement of the amount of force needed to break something. In wood, strength values vary significantly according to the orientation and type of load. Mechanical properties are about 20 times higher in the direction of grain than across it, either parallel to or at right angles to the growth rings. Tensile strength of most wood is quite high and compressive strength somewhat lower. When it is overloaded, wood will buckle and crush, with cells deforming up to 20% before initial failure. It will then gradually yield before finally breaking. The strength of any particular piece of wood is greatly dependent on its quality, moisture content, and density. Grain irregularities, knots, and other defects may cause local weakness. Variation in moisture content may increase or reduce the ultimate strength of an entire board. Figure 3-10 illustrates how changes in levels of moisture affect wood’s ultimate mechanical properties, especially in compression. Wood density varies from species to species. See Appendix B for a listing of the ultimate static mechanical properties of selected boatbuilding woods. Wood has unusually good resistance to fatigue. Although its one-time load capability may not be as high as that of other materials, a significantly high percentage of it is available for long-term fatigue life. This is not surprising when we consider that nature spent millions of years evolving trees in a competitive environment. But while this capability was empirically proven for years—anyone who watches a tall old tree survive repeated winds knows that wood is strong over time—little specific design data was available to support engineering efforts for repetitively loaded wooden structures. Laminated veneer testing In 1978, with the sponsorship of NASA, we began a program to determine the fatigue characteristics of wood in laminate form. Earlier fatigue testing, performed in 1943, showed that Sitka spruce exhibits excellent fatigue behavior, but failed to provide enough specific information for us to develop design allowables Figure3-2 ThisDouglasfirlaminateaugmentedwithcarbon fiber failed in compression at about 20,000 psi.PDF Image | Gougeon Brothers on Boat Construction
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