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.