Mechanism of Check Formation

Experiments with wood and plastic models have shown the type of second-order stress distribution which can be expected in the rays and prosenchyma in the surface layers of a flatsawn board during drying. During drying, tangential tensile stresses in the rays and tangential compressive stresses in the prosenchyma occur. In the early stages of drying tangential tensile stresses also exist du? to moisture gradients. Experiments with wood and plastic models showed stress distributions which would occur in isotropic rays and in wood free of moisture gradients. In the all-wood models where rays were anistropic and moisture gradients occurred, checks developed almost exclusively in the rays. Under certain conditions, the location of checking failures would depend on the relative magnitude of second-order drying stresses and thermal stresses. Since the tangential stresses at the surface tend to be highest at the juncture of the two tissues, checks would be associated with rays in either case but would occur within the rays when second-order drying stresses predominate, or are the only factor, and would occur just outside the rays in the prosenchyma when thermal stresses predominate.

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