Microscopic Characterization of Slip Lines and Compression Failures in Wood Cell Walls

Polarized light microscopy is used to observe slip lines or “slip planes” and compression failures in Douglas-fir, Japanese larch, and white pine when cut with a microtome knife. Various positions of knife to wood were used. No orientation of the knife provided sections entirely free of. slip lines. Fewest slip lines were observed when the fiber direction of the block was approximately parallel to the knife edge, where a relatively small cutting angle is employed. Boiling of the wood had a slight tendency to increase the number of slip lines. The slip lines make about a 70? angle with respect to the fiber axis. The compression failures for white pine were examined with the electron microscope using carbon replicas. This showed the deformations as folds or wrinkles. Then very small radial-longitudinal sections embedded in methacrylate were cut with a Porter-Blum ultramicrotome, the methacrylate removed and the sections shadowed with chromium. This showed that zones of displacement of the microfibrils developed principally in the S layer where the normal orientation is parallel to the cell axis and parallel to the compression stress acting along the grain. This generally agrees with results of optical studies. A good discussion and excellent illustrations are shown.

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