Studien über Wachstumsspannungen des Holzes—Dritte Mitteilung: Längenänderungen bei der Wärmebehandlung frischen Holzes

Summary

Green wood shrinks in heating up to 70°C and swells from 70° to 100°C. In cooling, reverse movements were observed. Besides these phenomena, plastic deformations running linear to the logarithm of time occur by heat treatment. Along the grain near the pith, green wood swells in hot water, because the high compression stresses in trees cause compression sets which are removed by boiling. In circumferential direction, a large elongation, up to 1.8 percent, takes place by boiling. This occurs in all parts of the stem but is smaller near the heart, where trees are tension stressed. Evidently growing wooden cells get a quasi compression set tangentially, which is diminished by the heart's transverse tension stresses. Along the rays, boiled wood shows some shrinkage near the stem's surface where tangential swelling is high. There is a small shrinkage in the inner parts which swell less tangentially. Probably this radial movement is a secondary effect of the tangential swelling, similar to the contraction of a tension stressed body according to Poisson's ratio. After long boiling times, wood shrinks in all directions. Plastic movements of woods have important consequences. Different elongations in the fiber direction at different distances from the pith cause warping and end cracks in logs and pith-containing boards. The circumferential expansion is the origin of heart cracks in steamed and boiled logs. In timber drying, the quasi compression set helps to prevent surface checks.