Duration of Constant and Ramp Loading on Strength of Wood

Abstract

We present an analysis of time-dependent strength of cellulosic and polymeric materials under ramp loading. The analysis is based on the Reiner-Weissenberg thermodynamic theory of strength in conjunction with a nonlinear mechanical model. The strength model system has been applied with satisfactory results to predict rupture behavior of polymers and wood polymer composites under constant loading. It has the peculiar features of predicting (1) The upper stress limit at which the material ruptures immediately upon application of load; and (2) the lower stress limit at which the material can sustain the load indefinitely without rupture. Procedures to evaluate the model parameters based on existing constant and ramp-loading test data of Douglas-fir beams are demonstrated. Although the model parameters are defined physical quantities, their accuracy can be improved because of the complex material creep characteristics and mechanical properties.