Effects Of Environmental Temperature On Acoustic And Mechanical Properties Of American Red Pine Standing Trees And Logs

Due to the implementation of the new forestry development strategy, China's timber resource structure is undergoing a fundamental change. The primary supply of wood raw materials has been shifted from natural forests to plantation-based forests. Because of the fast growing and short rotation cycle, plantation trees contain high proportion of juvenile wood. Compared to natural forests, plantation wood generally shows declined quality and increased property variability. The characteristics of this new resource pose a great challenge to the development and management of plantation forests and hinder the efficient utilization of the resources. One of the important research topics that need to be undertaken by the research community and wood industry is how to accurately measure and evaluate the quality traits of the plantation trees as well as the logs harvested from the plantations. The use of acoustic wave propagation method for plantation tree quality assessment has been a new research focus internationally. The key concept of this technology is based on the fundamental wave propagation theory, and utilizing the propagation speed of the acoustic waves to predict the elastic modulus of the wood under evaluation. Therefore, understanding the behavior of wave propagation and accurately measuring the acoustic wave velocity in living trees and logs is a critical step.Depending on the geographic locations and the timing of forest operations, trees and logs can be acoustically tested and evaluated in different climates or different seasons. The primary goal of this study was to investigate the effects of environmental temperature on acoustic wave measurements in standing trees and freshly cut logs. The specific objectives included the following:(1) Effects of wood temperature and moisture content on acoustic and mechanical properties of wood;(2) Theoretical modeling of the relationship between acoustic velocity and wood temperature;(3) Effects of seasonal temperature change on acoustic velocities in logs and in plantation trees.In this study, a comprehensive approach combining theoretical analysis, laboratory experiments, and field testing was used to explore the impact of environmental temperature on acoustic wave and mechanical properties of wood in standing trees and logs. The theoretical analysis was based on the assumption that frozen and unfrozen wood in trees and logs were physically constituted with wooden body (dry wood), ice/water, or ice&water. The relationships between acoustic velocity in wood and the temperature and moisture content of wood were modeled according to the principle of sound wave propagation in mixture. The experimental study was conducted on a single species-American red pine (Pinus resinosa). The impact of ambient temperature change on acoustic measurements on plantation trees and logs was examined through both laboratory experiments on small clear specimens and field testing on standing tree samples and freshly cut log samples.The research results summarized indicated that wood temperature had a significant effect on both acoustic and mechanical properties of frozen wood. Below the freezing point, acoustic velocity, peak energy, and MOE of wood continuously increased as wood temperature decreased. When wood temperature was well above freezing, velocity, peak energy, and MOE of wood decreased slightly as wood temperature increased; This changing trend is more apparent in dry wood (<FSP) than in green wood. But in general, the acoustic and mechanical properties of wood are not very sensitive to the temperature changes when it's above freezing. When wood moisture content is above the fiber saturation point, both acoustic and mechanical properties of wood changed abruptly around the freezing point (0℃). As wood temperature decreased from above freezing to below freezing, the propagation velocity and energy peak of the acoustic waves and the MOE of wood showed a sudden increase. The impact of temperature on acoustic wave velocity increased as the moisture content increased. When the timber changed from a high moisture content (such as living trees and freshly harvested logs) to0%moisture content (oven-dry), the linear relationship between the velocity and wood temperature gradually leveled off, that is, the effect of temperature on wave velocity gradually decreased.Based on the theory of acoustic wave propagation in the mixture and the propagation characteristics in different media (ice, water and pure wood substance), theoretical models of the relationships between acoustic wave propagation velocity and wood temperature were developed. The theoretical calculation indicated that when wood temperature was below the freezing point, the wave velocity predicted under different moisture levels were generally in agreement with the experiment results. The average prediction error was1.66%. The predicted acoustic velocity had a sudden change near the freezing step, which is consistent with the experimental result. But when wood temperature was above the freezing point, the prediction error was relatively big.Field acoustic measurements on the red pine plantation trees and logs indicated that the impact of environmental temperature on acoustic velocity of living trees and logs were most significant in winter seasons or when temperature is bellow freezing. In winter, longitudinal acoustic wave velocity of the red pine logs were averagely23%higher than those measured in other seasons; the longitudinal and transverse acoustic wave velocities of the red pine trees were respectively16%and24%higher than those in other seasons. When the ambient temperature was above freezing point, with the increase in temperature, the wave velocity of the red pine logs remained relatively constant, but the wave velocity of the red pine trees declined slightly. Statistical analysis indicated that the tree velocity at different temperatures above freezing showed no significant differences. Therefore, from the perspective of practical application, wave velocities of living trees and logs measured above freezing can be regarded as constant which does not need for a correction. Under the field test conditions, the sudden changes in velocity of the red pine trees and logs occurred in the temperature range of—2.5℃and0℃.