Abstract
The development of high-performance, veneer-based wood composites is a topic of increasing importance due to the high design flexibility and the comparable mechanical performance to solid wood. Part of this improved mechanical performance can be contributed to the size effect present in wood. Based on previous findings in the literature, this size effect can be either strengthening or weakening. The presented study investigates the influence of thickness and load angle on the tensile strength and tensile stiffness of peeled veneers compared to thin sawn timber. Veneers with thicknesses of 0.5 ± 0.05 mm, 1.0 ± 0.05 mm and 1.5 ± 0.05 mm as well as sawn wood with thicknesses of 1.5 ± 0.1 mm, 3.0 ± 0.1 mm and 5.0 ± 0.1 mm were tested in tension under different load angles (0°, 45° and 90°). The results only partly confirm a size effect for strength parallel to the grain. The strength perpendicular to the grain increased significantly between 0.5 mm and 1.5 mm, with a significant decrease between 1.5 mm and 5.0 mm. The presence of lathe checks diminished the strength perpendicular to the grain of the veneers by about 70% compared to solid wood, partly overshadowing a possible strengthening effect. It was concluded that a transition from a strengthening to a weakening behaviour lies in the range of multiple millimetres, but further investigations are needed to quantify this zone more precisely. The presented results provide a useful basis for the development of veneer-based wood composites with a performance driven layer-thickness.
Highlights
IntroductionThe mechanical performance of wood and wood-based products is influenced by a multitude of inherent (e.g., density, grain angle and moisture content) as well as external (e.g., production technology, final dimensions and load case) factors [1]
The mechanical performance of wood and wood-based products is influenced by a multitude of inherent as well as external factors [1]
While the mechanical performance in loading parallel to the fibre direction is somewhat similar for veneers (σ0,mean = 126 ± 34 MPa and E0,mean = 15.6 ± 3.4 GPa) as well as solid wood (σ0,mean = 119 ± 37 MPa and E0,mean = 14.8 ± 2.7 GPa), there is a substantial difference when loaded perpendicular to the fibre
Summary
The mechanical performance of wood and wood-based products is influenced by a multitude of inherent (e.g., density, grain angle and moisture content) as well as external (e.g., production technology, final dimensions and load case) factors [1]. The general perception in literature states that, with increasing dimensions, the strength of a material decreases [3,4]. This relationship is commonly known as the size effect and is usually described applying Weibull’s [5] weakest link theory for brittle materials. The relationship between specimen size (length, width and height/thickness) and mechanical performance has been investigated in different directions (parallel and perpendicular to the grain), in different load cases (tension, compression and bending) and on different size levels (macroscopic and microscopic). The results presented in the literature (see Table 1) partly confirm a size effect for wood, depending on the load case and investigated size scale
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