In this paper, we consider the fatigue strength of wood structural mem
bers. That is, we develop a mathematical model for time-dependent stre
ngth under sinusoidal load. This work extends the model for time-depen
dent strength under constant load and ramp load derived previously by
two of the authors. It is based on the statistical theory of the absol
ute reaction rate in a version favorably reviewed in the literature. U
nder the isothermal condition, the model predicts that the time at fra
cture is independent of stress frequency. The need to evaluate experim
entally some of the model parameters that may depend on stress frequen
cy indirectly through temperature changes is discussed.