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The cohesive failure of wood studied with the scanning electron microscope
Author(s) -
Borgin Karl
Publication year - 1971
Publication title -
journal of microscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.569
H-Index - 111
eISSN - 1365-2818
pISSN - 0022-2720
DOI - 10.1111/j.1365-2818.1971.tb02356.x
Subject(s) - composite material , materials science , perpendicular , brittleness , slippage , scanning electron microscope , shear (geology) , tension (geology) , lamella (surface anatomy) , compression (physics) , geometry , mathematics
Summary The purpose of the present investigation was to establish the value and suitability of the scanning electron microscope for research on the mechanism of fracture and cohesive failure of the structure of wood. Wood samples, mainly consisting of various pines, were stressed to the point of failure in tension and shear parallel or perpendicular to the main fibre direction. The rate of loading and the temperature were varied between wide limits. The fractured surfaces were studied and gave the following results: (1) In tension parallel to the grain, fissures developed between the microfibrils in the cell walls causing the wood to rupture perpendicular to the longitudinal axis of the fibres. At both high and low temperatures the matrix, in which the microfibrils are embedded, and to a much lesser extent the middle lamella between the fibres failed by plastic flow when the external load was applied very slowly. Under such conditions strands or bundles of microfibrils were pulled out and fractured in an irregular pattern. Cohesive failure in tension caused by sudden impact was characterized by limited slippage of microfibrils and a near brittle failure of the embedding matrix. A remarkable clean cleavage approaching a true brittle failure was observed when the wood was stressed rapidly perpendicular to the grain. (2) Cohesive failure when the wood was exposed to shear along the grain and tension across the grain, showed clearly that the weakest part of the ultrastructure of wood under such conditions was the bonds between parallel strands of microfibrils which were pulled out with little resistance from the matrix. However, the adhesion between the middle lamella and the cell wall or the middle lamella itself failed when the wood was exposed to cleavage parallel to the grain. If the process of cleaving was carried out extremely fast, as in the machining of wood, the rheological properties of the middle lamella did not allow for plastic deformation with subsequent failure, and the bulk structure of the wood tissue behaved almost as a homogeneous material. The use of the scanning electron microscope for the study of the structure of wood under stress and the mechanism of its cohesive failure can supply information which seems to be unobtainable by any other method.