Time-Temperature Superposition And Time-Moisture Superposition On Viscoelastic Mechanical Behavior Of Birch

Static viscoelasticity is one of the important mechanical performance indicators for evaluating the long-term performance and service life of wood.Although there are many studies on the static viscoelasticity of wood,there is lack of long-term viscoelastic model.Through time-temperature superposition and time-moisture superposition,the long-term viscoelastic behavior of materials can be effectively predicted.There have been studies on the applicability of time-temperature superposition to the viscoelasticity of wood,but most of them are limited to a single constant moisture content level.However,only a few studies have reported on the applicability of the time-moisture superposition to the viscoelasticity of wood.This study was aimed at exploring the applicability of time-temperature superposition and time-moisture superposition to the creep and stress relaxation of birch(Betula pendula)in a wide moisture content range and a wide temperature range,in order to promote the study of the long-term viscoelastic mechanical behavior of wood.The main research conclusions of this paper could be summarized as follows.(1)The test equipment for bending modulus of elasticity under the dual cantilever mode was designed independently.By measuring the oven-dry density and the bending modulus of elasticity in the dual cantilever clamp,specimens with similar physical and mechanical properties were selected for short-term static viscoelasticity tests and long-term viscoelasticity tests.(2)Through the dynamic thermomechanical analyzer and humidity accessory,the short-term creep and stress relaxation tests were conducted on the specimens parallel to the grain and perpendicular to the grain under different temperature and moisture content conditions,and the applicability analysis of time-temperature superposition and time-moisture superposition of the test results was conducted.The results shows that in the temperature range of 5 to 950C and the moisture content range of 0 to 24%,regardless of the grain direction of the specimens,theoretically,temperature and time are equivalent to the creep and stress relaxation of wood,and master curve and shift factor are affected by the moisture content and the reference temperature;similarly,moisture content and time are equivalent to the creep and stress relaxation of wood,and master curve and shift factor are affected by the temperature and the reference moisture content.(3)In order to verify the effectiveness of time-temperature superposition and time-moisture superposition,long-term creep tests were carried out on the specimens parallel to the grain and perpendicular to the grain with a moisture content of 12%at 250C.In order to ensure that the long-term creep test could be carried out continuously and stably,the creep measurement system and constant temperature and humidity system were independently designed.Then time-temperature superposition master curve and time-moisture superposition master curve under the same conditions were compared with the long-term creep test curves.The results shows that at the temperature of 25? and the moisture content of 12%,for the specimens parallel to the grain,the compliance predicted by the time-temperature superposition and time-moisture superposition is much greater than the compliance of long-term creep test;for the specimens perpendicular to the grain,the compliance predicted by the time-moisture superposition is comparable to the compliance of long-term creep test,whereas the compliance predicted by the time-temperature superposition is much greater than the compliance of long-term creep test.