A Comparative Study On Velocities Of Stress Wave Propagating In Standing Trees Under Both Frozen And Normal Temperature States

Nondestructive testing (NDT) technology based on stress wave was widely applied in wood industry, which had high practical value for defects identification and mechanical performance of wood. The previous research on nondestructive testing technology of wood based on stress wave were performed at normal or higher temperature, primarily aiming at the detecting of defects, such as holes, cracks, or mechanical properties of wood, and most objects detected using stress wave were almost processed wood samples, such as veneer, dowels and lumber, but less for logs, especially for standing trees. Northeast forest area in china was the largest base of reserves and supply of wood. As for its particular climate, the conservation and maintaining of forest and gradually harvesting operations were conducted in cold winter, and the felled trees were directly processed and utilized in frozen state. Because of its lower temperature and drought in winter, when the trees being under the dormancy state, the density and moisture content (MC) of standing trees largely distinguished from those in normal temperature state. Therefore, it was necessary to analyze whether and how the dynamic modulus of elasticity(MOE) would be different in frozen state compared with normal temperature, which provided strategic information not only can lay a foundation for achievement of the sorting and grading of standing trees, help make economic and environmental management decisions on treatment for individual trees and forest stand, improve thinning and harvesting operations, but also can enhance the accuracy and reliability of stress wave NDT as well as defects judgment, so as to efficiently allocate timber resources for optimal utilization in winter.The main objective of this study was to employ experimental method to test the transmission speed of stress wave and moisture content of standing trees in frozen and non-frozen states. Ten major species of northeast forest area collected from Harbin Forest Experimental Station were selected for the study. The testing data were processed and analyzed with the aid of the SPSS statistical package. The results showed that the longitudinal and radial propagation velocities of stress wave in frozen standing trees were all obviously higher than those of trees in non-frozen state, and there was a high negative correlation relationship between longitudinal velocities in frozen and non-frozen states, and the correlation coefficient was about 0.8; the longitudinal MOE in frozen state was 46% higher than normal temperature on average, and the radial MOE in frozen state was 76% higher than normal temperature on average. Additionally, the longitudinal propagation velocities of stress wave under the frozen state were significantly affected by moisture content, whose effect was significant under the fiber saturation point. Yet, the effect of MC which was over the fiber saturation point on ones under normal state was not evident. And the radial velocities didn't change too much with the change of MC under frozen and non-frozen states.It can be found that, by comparison study, the rigidity of wood in frozen state was stronger than ones in non-frozen state, which got a stronger capability to resist the distortion of wood in application. This also demonstrated the testing methods used in the normal temperature were not suitable for the testing under the frozen state. Consequently, it was necessary to consider separately the estimation of mechanical performance and accuracy of defects judgment in winter, so as to maximize the utilization of wood in winter.