The biomechanics of bamboo : investigating the role of the nodes

The stem of the bamboo plant consists of a hollow, tubular culm with periodic nodes, characterised by an internal diaphragm and an external ridge. Bamboo is a highly anisotropic material having a strong fibre orientation, but in the vicinity of the nodes, the fibres diverge from their longitudinal orientations. Previous researchers have claimed that the nodes have a biomechanical function, preventing failure by making the tube stiffer and stronger. To investigate this claim, tensile and bending tests were carried out on material samples and intact culms, both with and without nodes, to investigate culm stiffness and strength. Crack propagation tests were also carried out to determine the effect of nodes on fracture toughness. The results suggest that far from being a point of strength, the node maybe a point of weakness when loaded in tension. Material in the node has a significantly lower tensile strength; in bending tests on intact culm lengths, failure occurs when the stress on the tensile side is exactly equal to the node’s tensile strength. Failure occurs by longitudinal splitting: it is proposed that this may be initiated by cracks forming in the nodes. The spacing of nodes is too large to affect the stiffness and strength of the tube as a whole and also greater than the critical crack length for brittle fracture. Thus, the diaphragm and ridge structure of the node can be explained as an attempt to reinforce a biologically essential feature which would otherwise be a point of weakness.