DEVELOPMENT OF TENSION AND COMPRESSION CREEP MODELS FOR WOOD USING THE TIME-TEMPERATURE SUPERPOSITION PRINCIPLE

This paper describes the development of long-term creep models for woo d in tension and compression using the time-temperature superposition principle (TTSP). Shortterm accelerated creep tests were conducted in both tension and compression in a controlled environment. The tested s pecimens include southern pine (Pinus spp.), Douglas-fir (Pseudotsuga menziesii), and yellow-poplar (Lirodendron tulipfera). Test temperatur es ranged from 20 degrees to 80 degrees C. The moisture contents (MCs) for testing tension parallel to the grain were 6, 9, and 12 percent f or Douglas-fir and southern pine, and 6 and 9 percent for yellow-popla r. The MCs for testing in compression parallel to the grain were 9 and 12 percent for Douglas-fir and southern pine, and 6, 9, and 12 percen t for yellow-poplar. The strain was measured using bonded strain gauge s. The individual compliance curve for each temperature was plotted ag ainst the log-time axis to obtain a master curve. The temperature shif t-factor relationship was then plotted. A nonlinear regression analysi s was used to estimate the model parameters. The results show that the TTSP was successfully applied to all three species tested at the MCs just stated in both tension and compression. Success of the TTSP was s upported by smooth-fitting master curves, conformance to the Arrhenius equation, and the activation energies falling within published result s.