DURATION OF CONSTANT AND RAMP LOADING ON STRENGTH OF WOOD

We present an analysis of time-dependent strength of cellulosic and po lymeric materials under ramp loading. The analysis is based on the Rei ner-Weissenberg thermodynamic theory of strength in conjunction with a nonlinear mechanical model. The strength model system has been applie d with satisfactory results to predict rupture behavior of polymers an d wood polymer composites under constant loading. It has the peculiar features of predicting (1) The upper stress limit at which the materia l ruptures immediately upon application of load; and (2) the lower str ess limit at which the material can sustain the load indefinitely with out rupture. Procedures to evaluate the model parameters based on exis ting constant and ramp-loading test data of Douglas-fir beams are demo nstrated. Although the model parameters are defined physical quantitie s, their accuracy can be improved because of the complex material cree p characteristics and mechanical properties.