Flexural anisotropy of rift-sawn softwood boards induced by the end-grain orientation

Sung-Wook Hwang,Junji Sugiyama,Hiroshi Isoda,Takafumi Nakagawa

doi:10.1186/s10086-021-01946-y

Sung-Wook Hwang, Junji Sugiyama + Show 2 more

Open Access

https://doi.org/10.1186/s10086-021-01946-y

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Abstract

Inspired by the use of rift-sawn softwood board for covering curved surfaces in Kokerabuki, a traditional Japanese roofing method, we investigated the flexural anisotropy of wood caused by its end-grain orientation. We measured the flexural displacement of softwood species, Chamaecyparis obtusa and Cryptomeria japonica, and hardwood species, Populus suaveolens and Cerasus serrulata. For the softwood species, this was approximately five times longer for the rift-sawn specimens than for the other grain patterns. Using the replica method to measure the softwood tracheid deformation with different flexural displacements, we confirmed the different deformation mechanisms of the tracheid in the flat- and quarter-sawn specimens, and rift-sawn specimens. In the flat- and quarter-sawn woods, on-axis loading was generated, in which the stress was concentrated on the radial and tangential cell walls parallel to the direction of tension or compression. By contrast, in the rift-sawn wood, off-axis loading was generated, in which the stress was evenly distributed throughout the corner cell walls without the wall directly resisting the tensile and compressive forces. We also concluded that the tapered shape of the tracheid walls contributes to the excellent flexibility of rift-sawn softwood.

Highlights

Kokerabuki is a unique Japanese roofing method of stacking thin wood boards
If physical and mechanical anisotropy is induced by endgrain orientation, it should be considered when designing products based on thin boards
The differences in the modulus of elasticity (MOE) and modulus of rupture (MOR) of the species tested were consistent with their density differences

Summary

Introduction

Kokerabuki is a unique Japanese roofing method of stacking thin wood boards. This method is applied to many of the traditional Japanese architectures designated as important cultural properties [1]. Traditional Japanese roofing methods are categorized into several types, such as Kokerabuki, Tokusabuki, and Tochibuki, depending on the thickness of the boards used. Flat-sawn boards are Studies of wood’s grain orientation generally concern axial grains, which represent the longitudinal arrangement of wood cells. This is because the mechanical performance of wood is considered to be best in the longitudinal direction [3,4,5,6,7,8]. In addition to Kokerabuki, these products include acoustic boards for musical instruments and veneers for wood composites

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