Measuring longitudinal elastic modulus of bamboo through free vibration approach

Bamboo as a natural fiber reinforced composite material possesses anisotropic mechanical properties. The highest strength and stiffness are found when bamboo was loaded along the longitudinal direction of fibers. Therefore, evaluating the elastic modulus of bamboo along its longitudinal direction is necessary for structural design. In this work, a nondestructive, dynamic testing method of free vibration was used to measure the elastic constant of bamboo in the longitudinal direction. Specimens with various lengths were tested under bending vibration conditions to determine the damped frequencies and damped periods through the Fast Fourier Transform (FFT). Then, the fundamental solution to the Euler-Bernoulli equation for free vibration was used to calculate the elastic moduli. It is found that the moduli of elasticity for specimens with various lengths were in the range from 14.1 to 16.7 GPa under two different loading configurations. The static tests by three-point bending produced elastic modulus results of as received bamboo: 14.24 ± 0.57 GPa for loading configuration 1 and 14.58 ± 0.08 GPa under loading configuration 2. The elastic moduli varied from as low as 11 to as high as 19 GPa depending on the moisture contents and testing methods. It is concluded that both static and dynamic tests delivered close elastic modulus values. The dynamic test results are slightly higher than the static ones due to the deformation rate effect. Moisture content in bamboo has significant effect on the stiffness of the composite. The higher the moisture content, the lower the stiffness of the bamboo. The overall deviation of the dynamic testing for determining the elastic modulus is about 5.74% considering the errors from geometry, density, and frequency measurements.