Mechanical response and failure of balsa wood under compression
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Balsa wood is a natural cellular material with excellent stiffness-to-weight and strength-to-weight ratios as well as superior energy absorption characteristics. These properties are derived from the microstructure, which consists of long slender cells (tracheids) with approximately hexagonal cross section that are arranged axially. Parenchyma are a second type of cells that are radially arranged in groups that periodically penetrate the tracheids (rays). Under compression in the axial direction the material exhibits a linearly elastic regime that terminates by the initiation of failure in the form of localized kinking. Subsequently, under displacement-controlled compression, a stress plateau is traced associated with the gradual spreading of crushing of the cells through the material. The material is less stiff and weaker in the tangential and radial directions. Compression in these directions crushes the tracheids laterally but results in a monotonically increasing response typical of lateral crushing of elastic honeycombs. The elastic and inelastic properties in the three directions have been established experimentally as a function of the wood density. The microstructure and its deformation modes under compression have been characterized using scanning electron microscopy. In the axial direction it was observed that in the majority of the tests, failure initiated by kinking in the axial-tangential plane. The local misalignment of tracheids in zones penetrated by rays ranged from 4° to 10° and axial compression results in shear in these zones. Measurement of the shear response and the shear strength in the planes of interest enabled estimation of the kinking stress using the Argon-Budiansky kinking model. The material strength predicted in this manner has been found to provide a bounding estimate of the axial strength for a broad range of wood densities. The energy absorption characteristics of the wood have also been measured and the specific energy absorption was found to be comparable to that of metallic honeycombs of the same relative density.