Crack propagation and fracture toughness of solid balsa used for cores of sandwich composites

Abstract

To better understand the suitability of balsa wood as a core material for sandwich composites, this paper presents a detailed study of the fracture properties of balsa. The experiments looked at mode I, mode II, and mixed-mode fracture with crack growth both parallel and perpendicular to the wood grain direction. The experiments monitored toughness as a function of crack growth to record crack resistance or R curves. The mode I toughness was lower than most other wood species, but considering its low density, the toughness of balsa is higher than previously expected. The mode I toughness increased as a function of crack growth due to a fiber-bridging process zone at the crack tip. The mode II toughness was higher than mode I toughness, but was much less affected by fiber bridging. The mixed-mode failure envelope had an unusual shape, which could be explained by an influence of fiber bridging. Overall, the toughness of balsa is adequate for sandwich composite core materials and likely better than some alternative core materials.