Fatigue of Wood and Glued Joints Used in Construction

Procedures for determining fatigue behavior of wood and glued wood constructions using axial-loading fatigue machines are outlined. Douglas-fir and white oak were tested with resorcinol resin glues. Tests were made in tension parallel to the grain of solid wood, tension parallel to the grain of wood with 1:8 glued scarf joints, laminated glue-shear (which occurs when there is a discontinuity in one lamination and stress must be transferred by shear to the two laminations adjacent to it) and tension perpendicular to the grain (which occurs in edge-gluing boards). Glued laminated construction can be used without danger of failure in fatigue in any application where solid-wood construction has worked well. A scarf joint with a 1:8 or flatter slope in individual laminations will withstand repetitions of stress as well as a continuous lamination, provided that reductions in design stresses are made to allow for the reduction in static strength. Fatigue strength for straight-grained defect free wood in tension parallel to the grain at 30 million cycles of stress is about 50 percent of its static strength. The fatigue strength of a glue joint in shear is 40 to 45 percent of the static strength when a stress-raiser similar to the re-entrant angle used in these tests is present, and the condition of loading (stress ratio of 0.10) is the same.

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