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Many wood and wood-composite structural elements are exposed to fatigue loading situations in service. Because wood is composed of a complex cellulosic fiber-based structure at the microscopic level, fatigue cracking phenomena are not commonly observed. Fatigue-related failures can look just like static failures. Fatigue cracking does occur under certain types of loading, however, and some examples are presented.
The special properties of wood are briefly examined to show why fatigue fracture/cumulative damage is so difficult to interpret in wood and wood products. Some historical insights are covered. The development of special tests to gather fatigue data for wood is discussed to further point out the unique concerns of designers working with the material.
Two case studies involving parallel to the grain fatigue fractures are presented. The case of a failed shotgun stock represents a regular repeated load situation on a component that was redesigned. Broken necks in bowling pins are due to a more complex dynamic loading regime, but exhibit similar cracking patterns in a laminated composite product.
wood, cellular structure, orthotropic properties, cellulose, grain direction
Professor, SUNY College of Environmental Science and Forestry, Faculty of Wood Products Engineering, Syracuse, NY