Wood

Wood is not ideally elastic as it takes time for it to recover. However, residential deformations are recoverable. Wood strength varied depending on species, loading condition, load duration, number if assorted material and environmental factors. Property value in longitudinal axis (parallel to grain) is higher than tangential and radial axes.

When load applies parallel to grain, it produces stress that short grain cell longitudinally. Microfribils begin to fold where weakness spot formed. As stress increase, wood cell will fold ‘S’ and wrinkles is visible. Next if stress continues, the crushing of internal complex cellular structure occurs.

Compression test on wood is usually done on 50x50x200 or 50x50x100 parallel to grain and 50x50x150 perpendicular to grain. Failure symptoms are crushing, wedge split, shearing, splitting, brooming and end rolling.

Tension of parallel grain is better. Failure symptoms are cell-to-cell slippage (adjacent sliding of cells) and cell wall failure (rupture of cell wall). However, initial failure of wood occurs.
Bending test are used for flooring or rafter system beam wood (50x50x760 / 25x25x410) which the strength is less than tension and more that compression due to tension parallel grain is better. In all, fatigue strength of wood is better than metal.