On inherent bending in tension tests of bamboo

Bamboo materials are often characterized using standard tension coupon tests. However, due to the functionally graded nature of bamboo, the standard tension test geometry is inherently flawed and may result in additional non-uniform bending stresses being introduced across the breadth of the cross section during testing. An experimental programme highlighting issues with, and interferences present, in current methods and practices regarding the tensile strength of clear bamboo specimens cut from the culm wall is presented. The study investigates the influence of fibre gradation on the specimen behaviour using tensile specimens having different in situ orientation; these include ‘radial’ specimens engaging the full gradation of fibres through the culm wall thickness, and ‘tangential’ specimens taken from varying locations through the culm wall thickness perpendicular to the radial direction. The study also considered the end restraint condition provided by the testing machine—for which there is no guidance in applicable standards. Digital image correlation was used to measure variation in strain in the test specimens related to their native orientation in the bamboo culm. Fibre density across the culm wall thickness and the end restraint conditions of the specimens were shown to have an influence on bamboo tensile test results and therefore must be considered in the development of improved standard test methods. It is recommended that tangential specimens are used since they possess a more uniform fibre density throughout their cross section and taken together can outline the fibre density and strength in the radial direction through the culm wall. Fixed end restraints result in marginally lower measured tensile strengths having marginally improved variation.