Due to difficulties in determining modulus of elasticity of wood, only strain or deflection profiles caused by relaxation of internal stress are normally evaluated for industrial kiln-dried lumber. To directly assess the level of internal stress within the lumber, a new technique of measuring the restoring force on a half-split specimen has been presented and the corresponding device has been designed and constructed. Kiln-dried rubberwood specimens with dimensions of 30 or 50 mm (thickness), 130 mm (width) and 50 mm (length) were used in the study. The measured restoring force appears to vary with half-split length and specimen thickness. A mathematical model based on an elastic cantilever beam theory has been successfully developed to describe the restoring force behavior in a flexural response regime. The magnitude of the maximum linearly averaged internal stress σ can be derived without prior knowledge of the modulus of elasticity of wood. For the 30-mm-thick lumber, the derived values of σ are in general agreement with the ones obtained from the conventional McMillen slice test. Very close agreement is observed when the internal stress is at a relatively low level and its profile is approximately linear. But for the 50-mm-thick lumber, the determination of σ is less appropriate because of its relatively short flexural range. A restoring force–internal stress chart has been proposed for practical use in the lumber industry. This assessment was performed to investigate the evolution of internal stress during the conditioning and storage stages of kiln-dried rubberwood lumber.
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