Experimental and Analytical Analysis of Mechanical Response and Deformation Mode Selection in Balsa Wood

Abstract

This study investigates the influence of relative density and strain rate on the compressive response of balsa wood as a sandwich core material commonly used in naval structures. Compressive strength, plateau stress and densification strain of balsa wood along the grain direction is investigated over its entire density spectrum ranging from 55 to 380kg/m³ at both quasi-static (10⁻³ s⁻³) and dynamic (10³ s⁻³) strain rates using a modified Kolsky (split Hopkinson) bar. Scanning electron microscopy is used on recovered specimens subjected to controlled loading histories to identify the failure mode selection as a function of density and strain rate. The results indicate that compressive strength of balsa wood increases with relative density though the rate of increase is significantly larger at high strain rates. The failure of low-density specimens is governed by elastic and/or plastic buckling, while kink band formation and end-cap collapse dominate in higher density balsa specimens. Based on the experimental results and observations, several analytical models are proposed to predict the quasi-static compressive strength of balsa wood under uniaxial loading conditions. Results also show that the initial failure stress is very sensitive to the rate of loading, and the degree of dynamic strength enhancement is different for buckling and kink band modes. Kinematics of deformation of the observed failure modes and associated micro-inertial effects are modeled to explain this different behavior. Specific energy dissipation capacity of balsa wood was computed and is found to be comparable with those of fiber-reinforced polymer composites.