Effectn of Wave Form on Dissipated Energy During Fatigue of Wood.

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Failure mechanism of wood was investigated from the time dependency of wood failure.This research revealed that the mechanism of fatigue failure was similar to that of static failure but the process differs. The fatigue failure deals with not only time factor but also the factor of load repetition number.The dissipated energy generated during the repetitive loading provides important information on the fatigue failure. Amount of the dissipated energy which is measured during the fatigue process consists of two parts, the part caused by internal friction of rheological property of wood and another part is that by microscopical damage which induces the failure of a testing specimen. The fatigue limit stress can be estimated by the dissipated energy obtained by the stepwise repeat test. Namely the fatigue limit is given as the stress at the point where the dissipated energy begins to surpass the amount which is calculated from the linear relation between the dissipated energy and the stepwise stress. On the other hand the fatigue failure time can be estimated from an empirical relation ( an exponential function) between the dissipated energy related to the damage and the stepwise stress. More over understanding physical meaning of the empirical function will help to explain the mechanism of fatigue failure.The time- and the repetition factor dependently cause damages to the specimen. It needs a non-linear damage theory consisting of a mutual interference of time and repetition factors, to clearly show the mechanism fatigue failure.