Theories for Anisotropic Shrinkage

The cell wall of wood can be considered to consist of a non-crystalline matrix of lignin and hemicelluloses in which strong, stiff cellulosic microfibrils are embedded. The crystalline microfibrils exhibit no tendency either to adsorb moisture or to change in length or cross-section. On the other hand the non-crystalline isotropic matrix can lose and gain water and shows a considerable tendency to shrink and swell. In isolation one would expect the matrix to shrink or swell equally in all directions, that is Ox = Oy = = tto and Ovoi = 3ao i.e. ao is the isotropic shrinkage in 

Shrinkage ratios will vary according to the stiffness (E) of the microfibriis and the shear modulus S) ol the matrix. 

In this theory the difference in radial and tangential shrinkage of many species grown in the temperate zone is attributed to differences in density of early and latewood. The shrinkage of dense latewood cells is greater than that of earlywood. 

In the tangential direetion the earlywood and latewood must move together (they aet in parallel). The earlywood is foreed to shrink more than it would wish by the latewood, while the latewood is slightly restrained by the weaker earlywood and does not shrink quite as mueh as it would wish. Where latewood is eonsiderably denser and stronger than earlywood the bands of latewood foree the weak bands of earlywood to shrink tangentially by almost as mueh as an isolated band of latewood 

Ray-free wood Wood with fine Wood with all Isolated ray 

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