Development of a hollow cylinder test for the elastic modulus distribution and the ultimate strength of bamboo

Bamboo, which has become increasingly popular as a renewable structural material in past decades, is a natural functionally graded material whose elastic modulus gradually increases from the inner to the outer surfaces. A hollow cylinder test has been developed to characterize the strength and stiffness of bamboo specimens in the cross sectional plane. A hydraulic jack is used to inflate the rubber hose, which is inserted into a bamboo specimen to apply an inner pressure. The strains on the inner and outer surfaces of the bamboo are measured under an increasing inner pressure until that the bamboo specimen splits. The effective elastic modulus of bamboo varies continuously in the radial direction that is tentatively approximated by a power, exponential or linear function, respectively, each of which includes two parameters to be determined experimentally. Given an inner pressure, the stress and strain distributions along the radial direction can be analytically derived for the three forms of elastic modulus distribution functions, respectively. Fitting the test data with the formulation, we can obtain the two parameters for each elastic modulus distribution function. Based on the cellular microstructure and comparison of the elastic modulus distribution, the exponential function of elastic modulus provides the most favorable results and is thus recommended for a future standard test method, which can be used in bamboo material characterization and quality control.