Effect of cyclic long-term temperature exposure on the bending strength of lumber

This research evaluated the historical assumption that repeated exposure to elevated temperatures has a cumulative effect on wood properties. This recommendation was given in a paper by J. D. MacLean in 1951 and is a critical assumption when estimating the permanent effect of temperature on wood properties. No experimental results to support the recommendation were presented by MacLean. Approximately 670 southern pine and Douglas-fir solid-sawn 2x4's of two mechanical grades and one visual grade were subjected to cyclic and continuous exposure at 82 degrees C and 30% RH for periods up to 30 mo. They were then tested after equilibration to room temperature and 20% RH. The cyclic exposure specimens alternated between 1 mo at 82 degrees C and 1 mo at room temperature. The results show that there is no significant difference between the residual modulus of rupture (MOR) of the cyclic and continuously-exposed specimens for equivalent exposure periods. Trends in residual arabinose also supported this conclusion. Plotting the residual MOR of the cyclic specimens as the summation of the time they were exposed to the higher temperature provided a conservative estimate of the permanent effect of temperature. The results discussed in this paper are a small subset of a larger study and are not intended for use in general engineering design.