Indented rings (hazel growth) of Norway spruce reduce anisotropy of mechanical properties

This study presents three-point bending test results of Norway spruce clear wood specimens loaded on the radial-longitudinal plane in two different load cases. The tested samples were graded as resonance wood for instrument making and were characterised by narrow annual rings and relatively low density. The modulus of elasticity (MOE) and the corresponding modulus of rupture (MOR) are illustrated separately for the samples with straight grain and the group showing the specific growth pattern of indented rings (‘hazel growth’). With the longitudinal wood anatomical direction parallel to span width, the fibre deviation caused by the indents reduces MOE and MOR values, whereas a ‘reinforcing’ effect of the indents could be observed for the load case with span width parallel to the radial direction. Both aspects lead to a reduction in anisotropy for hazel-growth Norway spruce (anisotropy MOE: indented rings 11.6, straight grain 14.7, anisotropy MOR: indented rings 6.9, straight grain 8.9), which partly explains the exceptional position of this growth pattern for the construction of high-class musical instruments with outstanding mechanical and acoustical performance.