Elastic wave reflection in beams with acoustic black hole termination under axial excitations

The wave speed in structures can be controlled by altering their stiffness through variations of the geometry and dimensions of the cross-section along its length. Theoretically, slowing down the elastic waves can lead to a zero-reflection effect, which is referred to as acoustic black hole (ABH). ABH has been demonstrated in metallic structures with isotropic-like material properties; while the effects of material anisotropy such as wood mechanical properties on the ABH effect are yet to be explored. We demonstrate numerically and experimentally the ABH effect in wood beams (Pinus radiata) of different lengths by applying an exponential reduction of the circular cross-section. The beams are knot-free and made of cuts of 40mm, 80mm, 100mm, 120mm and 160mm length along the grain, which could lead to a small Poisson ratio by about one order of magnitude. An increase in moisture content of the ABH wood beams by about 8-10% of the dry weights(N=5 samples) and a higher volume fraction of fibrous grains have been shown to reduce the wave reflection, as the drier and lower volume fraction the fibre, the higher the reflection becomes. This work can lead to design of novel elastic wave-guides for timber structures as used in buildings and wooden infrastructure or construction processes