Ultrastructural Characteristics of Wood Fracture Surfaces
Keywords:Scanning electron microscopy, compression parallel to the grain, tension parallel to the grain, radial shear, tangential shear, intrawall failure, transwall failure, intercell failure, fracture paths, fracture surface
This study concentrated on the ultrastructural characteristics of hardwood fracture surfaces, but it included southern yellow pine as a representative softwood for comparison. Very small specimens were made, tested for compression parallel to the grain, tension parallel to the grain, shear in the radial plane and shear in the tangential plane, and were then prepared for scanning electron microscopy. Secondary electron micrographs of the fracture zones were recorded singly or in stereo pairs, and a number are used to illustrate the major findings.
Thick-wall cells tend to fail in an intrawall pattern at the S1/S2 interface, while thin-walled cells are more likely to fail with transwall fracture. In tangential shear tests of ring-porous woods, the plane of fracture follows the earlywood vessels which are thin-walled and have wide lumens. Large oak-type rays affect the fracture path in all of the test modes. Certain characteristic types of failure can be related to each of the testing modes utilized.
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