Research On Stress Wave Propagation Velocity In Both Frozen And Unfrozen Wood

In order to study the change rules of stress wave velocity (SWV) in both frozen and unfrozen wood, and provide the theoretical basis and scientific instructions for mechanical properities assessment and internal defect diagnosis of standing trees, logs and lumbers, two typical wood in northeast of China, Korean pine (Pinus koraiensis) and Ussuri poplar (Populus ussuriensis), were selected as experimental objects to conduct a series of stress wave tests and wood quality tests. Then, the effects of temperature, moisture content (MC), grain and cavity defect on stress wave velocity and transmitting time isolines were systematically analyzed, the relations between SWV and the mechanical properities of frozen and unfrozen wood were also discussed, and the mechanism of subzero temperature effect on wood mechanical properities and SWV was preliminarily illustrated.Firstly, the differential scanning calorimeter (DSC) was used to make cooling (temperature from 20℃to-60℃) and heating (from -60℃to 20℃) scan to each green specimen for studying the effect of temperature on the water states and contents in green wood.Then, at air dry and wet conditions, the modulus of elasticity (MOE), modulus of rupture (MOR), compression strength and SWV of the heart wood and sapwood of two species were measured at -20℃,-5℃,5℃, and 20℃temperature, respectively. The change rule of each parameter with temperature was analyzed and the correlations between the SWV and MOE or MOR were also dicussed. Based on this, the SWV change rule in Korean pine wood was studied when the moisture content ranges from 0 to above 90% and the temperature ranges from 20℃to -30℃.Finally, some Ussuri poplar lumbers sawed along the pith from a log and discs were prepared and measured using a grid point's method, and the stress wave transmitting time matrixs on the radial and cross section of frozen and unfrozen intact and defective wood which contain different size cavity were obtained. Then, the stress wave transmitting time isolines were drawed by processing the matrixs data, the effects of temperature, grain and cavity on the stress wave transmitting time and velocity were analyzed and the propagation rules of stress wave on the two typical setions of wood were also studied.The research results show that:(1)At subzero temperature, the phase of free water and a part of bound water in green wood can transform in sequence, which will change the inner structure of wood, so it can affect the mechanical properities and SWV of wood.(2)Temperature and moisture content are two important factors to influence the MOE, MOR, compression strength and SWV of wood. With the decreasing of temperature, each mechanics parameter and SWV increases. However, the change trends in air dry and wet wood are different. Each parameter in air dry wood increase linearly as temperature decrease, but the change curves of each parameter are nonlinear which increase slow between 20℃and -5℃but increase fast between -5℃and -20℃. There are strong linear correlations between SWV and MOE or MOR at different temperature, with R between 0.60 and 0.83.(3)In air dry state, with the decreasing of temperature, the longitudinal SWV in wood increase linearly. The SWVs in the heart wood of Korean pine and Ussuri poplar at -20℃are 730 m/s and 693 m/s larger than that at 20℃, and the increasing rates are 16.02%and 18.07%; in sapwood, the corresponding values are733 m/s and 589 m/s and the increasing rates are 17.23% and 14.95%. In wet state, SWVs also increase as the temperature decrease. The SWVs in the heart wood at -20℃are 1043 m/s and 1177 m/s larger than that at 20℃, and the increasing rates are 33.49% and 38.62%; in sapwood, the corresponding values arel050 m/s and 1164 m/s and the increasing rates are 33.96% and 37.90%.(4)At different temperature, the SWVs in wood decrease as the MCs increase. With the increasing of MC, the SWVs decrease fast below fiber saturation point (FSP, MC=32%) and slow above FSP. Furthermore, When the MC is below 50%, SWVs increase linearly as temperature decrease, but SWVs have an abrupt jump near 0℃temperature when the MC is above 50%. The main reason for the jump is that the phase transformation of free water in wood just occurs at that point.(5)The longitudinal SWV in wood is larger than the radial SWV, which is more easily affected by temperature. With the increasing of grain angleθ(the angle between propagation direction and wood fibre direction), the SWV decreases. Whenθ<45°, the SWV decreases fast; whenθ>45°, the SWV decreases slow.(6)Stress wave gradually travels from near to far area on the radial and cross section of frozen and unfrozen wood, which travels faster in frozen wood than in unfrozen wood. Cavity defect significantly affect the stress wave time, especially the time from exciting point to the area behind the cavity. In this area, the stress wave time is larger than that in intact wood, which increases as the size of cavity increases.(7)Tangential angle significantly affect the SWV in the cross section of log. The SWV increases nonlinearly with tangential angle 9. Whenθ<36°, the SWV increases fast; whenθ>36°, the SWV increases slow.