UC San Diego

This thesis investigates the effects of wood anatomy on the low velocity dynamic impact response of wood species in radial-loading drop-tower testing. The distribution of vessel elements was found to significantly alter the normalized impact energy of failure: diffuse porous species (uniform vessel distribution) saw improved impact resistance with increasing density, while ring porous species (tangential bands of vessels in the growth stage of growth rings) did not. Unlike in quasi-static conditions density alone was not an accurate predictor of failure energy in impact. Examination of quasi-statically and dynamically damaged ring porous white ash revealed that while vessels collapsed and absorbed energy under slow loading, they became stress concentrators in impact. Radial dynamic loading caused crack propagation in the tangential direction, which in the ring porous distribution of vessels in tangential bands allowed successive breaking of vessels. In diffuse porous species the uniform distribution of vessels did not provide any crack path and therefore saw improvements in dynamic work to fracture with increased density.

The dynamic load-time responses revealed higher amplitude oscillations in ring porous wood species near the initial moment of impact due to a combination of sequential vessel breaking and high fiber adhesion. Species able to maintain low peak forces exhibited failure mechanisms like progressive delamination observed in fiber-reinforced composites. Hierarchical deformation mechanisms were also observed such as tracheid unwinding in African mahogany and red alder. These findings may be used to inform bio-inspired impact resistant materials that incorporate porosity and hierarchical damage absorption structures.