Modes of mechanical failure of hollow, septate stems

Abstract

Three general modes of mechanical failure were observed when hollow, septate stem segments of Arundinaria tecta (Poaceae) were axially compressed and caused to fail: (1) rupture of tissues at the opposing ends of the nodal transverse diaphragm parallel to the plane of stem flexure; (2) localized catastrophic transverse invagination of internodes (Brazier buckling); and (3) longitudinal rupture of internodal walls along the convex surface of stem flexure attended by nodal tissue shearing. The frequencies of occurrence of these three modes were not equivalent among the 100 stem segments examined; stem failure was dominated by nodal tissue shearing (i.e. 67% of the nodes failed by shearing; 52% of the internodes longitudinally invaginated; 27% of the internodes failed in Brazier buckling; 21% of the internodes failed by rupturing). Computer simulations of axially compressed stem segments consisting of one node and one internode composed of non-linearly elastic, anisotropic materials (tissues) successfully predicted observed strain patterns and revealed that the pathway to stem failure contains a bifurcation point below which deformation patterns coalesce on a single configuration, and above which stem failure by Brazier buckling or nodal tissue shearing are essentially mutually exclusive responses to excessive stem flexure. The patterns of actual and simulated stem flexure were consistent with the hypothesis that nodes store strain energy as stems flex, and release this energy to restore stem shape when bending forces abate (i.e. nodes operate as spring-like joints). However, strain energy 'sinks' identified by simulations were also located in bending internodal walls which can store elastic strain energy to do work. Nodal diaphragms and internodal walls likely comprise a complex and global rather than a simple and local elastic system for the recovery of stem shape.