Testing Theories And Experiments Of Log Inner Decay Based On Stress-wave Technology

Wood inner decay is one of the main defects of timber. The extent of inner decay will has important impacts to the quality of bucking and will influence the log grading classification. Therefore, it is important to select a reasonable method and technique of testing the inner decay for improving the bucking quality and increasing the economical benefits of timbers.On the basis of expounding the research progresses of stress wave techniques in testing wood properties in abroad and home, the paper study systematically on the propagation theories of stress wave in timber, founds the stress wave propagation equations along the axis-direction, diameter-direction and string-direction of log, including the displacement equation, velocity equation, stress equation and strain equation. According to the reflection and transmission principles of stress wave propagation in wood, the testing theory of testing wood inner decay by using stress wave method is analyzed. By simplified analysis of the theories, the simulations of propagation tracks of stress wave in log are done by using the simulation software and the simulation results can indicate the factual tracks of stress wave propagation in log.After this, the testing experiments to Korean pine board samplings by using stress wave testing apparatus are done. The paper studies on the impacts of board length, the sizes and quantities of holes on propagation time and velocity of stress wave and wood dynamic elastic modules. The regressive analysis of the relations between the sizes of hole diameter, boards lengths and stress wave propagation parameters is done. The analysis results indicate that the propagation time and velocity of stress wave and wood dynamic elastic modules are positive relevance with board length respectively, and that propagation time is increased with the increasing of hole diameter and the propagation velocity of stress wave and wood dynamic elastic modules are lessened with the increasing of hole diameter. In addition, the sizes of hole in the end of board have no significant impacts on stress wave propagation parameters and wood elastic modules.The testing experiments of testing the inner decays in 13 main species in northeast forest areas in China are done by the stress wave testing apparatus. The comparison of tested dimensions and factual dimensions of inner decay are analyzed. The results indicate that the inner decays are figured out precisely by the measurement of stress wave apparatus. Not only this, the measurement can conclude the two-dimension images of the shapes of inner decay in logs approximately.The theories analysis, simulation and experiment results prove that the stress wave methods can testing the inner decay of log. The research results of this paper will has higher theoretical significance and application values to test the inner defaults of wood and wooden structure by using stress wave techniques.