A preliminary explanation for subzero temperature effect on stress wave velocity in green Korean pine wood

The objective of this study is to preliminarily illustrate the reason of subzero temperature effect on stress wave velocity in green wood. The stress wave velocities in Korean pine standing trees at normal and subzero temperature were respectively measured and compared. Then, the effects of temperature and moisture content on stress wave velocity were studied by measuring the small clear wood samples obtained from the standing tree specimens. An unusual phenomena was found the stress wave velocity does not increase linearly as temperature decreases when the moisture content of wood is above 50%, but has an abrupt jump near 0°C temperature. For explaining this phenomena, the phase and content of different water fractions in green wood samples at subzero temperature were determined using the differential scanning calorimeter. The intrinsic relation between the water states and stress wave velocities in wood was also analyzed. The research results show that all free water and a small portion of bound water in green wood transform from water to ice as temperature decreases from above zero to subzero. Because the stress wave velocity in ice is higher than that in water, the main reason for the jump of stress wave velocity near 0°C temperature in wood when the moisture content is above 50% is that the phase transformation of free water in wood just occurs at that point.