2000 年 49 巻 4 号 p. 390-394
The longitudinal free vibration method is one of the practical methods of evaluating dynamic Young's modulus of commercial lumber. Young's modulus measured by this method, however, is the apparent value throughout the full length of the lumber regardless of whether it is clear or it contains knots or other defects. This fact is important in improving the reliability of this method as a nondestructive estimation of the strength of lumber. Because the strength of lumber with knots is mostly weaker than the strength of clear lumber even though both of them have the same Young's modulus.
It is well known that damping characteristic of lumber is affected by knots. Then, if we can introduce some information of the damping characteristic of lumber with knots in addition to the dynamic Young's modulus, the estimation of the strength of lumber by the longitudinal free vibration method may be improved to some degree.
In this study, dynamic Young's moduli and damping constants were calculated from the wave data of Douglas-fir specimens with or without knots obtained by the longitudinal free vibration tests, and the bending strengths of them were determined by static bending tests.
Then an empirical equation for the bending strengths was derived using the dynamic Young's moduli and the damping constants in the following way. 1) An exponential regression curve was fitted to the relationship between the dynamic Young's moduli and the bending strengths. 2) A regression line was fitted to the relationship between the damping constants and the ratios of the actual bending strengths to the strengths estimated by the regression curve above. 3) The exponential regression curve was modified by introducing the damping constants. This modification gave slightly better estimation of the bending strengths by the longitudinal free vibration method.