The cohesive failure of wood studied with the scanning electron microscope

Abstract

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.

Investigations into the photodegradation of wood using microtensile testing

“…Zimmermann et al (1994) also noticed this effect, determined by the speed of failure. If the external load is applied slowly, plastic flow would result in the pulling out of strands and bundles of microfibrils (see Borgin 1971, Derbyshire and Miller 1981, Zimmermann et al 1994. Sudden impact and a fast failure propagation results in brittle-like failure of the embedding matrix.…”

Cutting distance as factor to evaluate the quality of wood machined surfaces: A preliminary study

“…According to Borgin (1971), this type of fracture expose the S-1/S-2 region, these surfaces are highly polar with cellulose-and hemicellulose-rich of hydroxyl units available for wetting. Finally, River et al (1991), indicates that once the cutting tool has accumulated more wear, the cell walls are compressed forming a smoother and even less porous surface than that formed with a sharp tool.…”

Fracture Initiation and Progress in Wood Specimens Stressed in Tension. Part III. Clear Wood Specimens with Various Slopes of Grain

“…Cleavage produces smooth fracture surfaces with no or little unwinding of tracheids or cell wall distorsions (Debaise etal 1966;Borgin 1971;Tan etal. This is indicated by the similarity in fracture surface morphology of the 30°-, 20°-and 10°-specimens and of wood specimens fractured in cleavage tests parallel to the grain.…”

“…1995). By contrast, shear failures are ragged and fuzzy due to large-scale cell unwinding and tearing of cell walls (Borgin 1971;Cote and Hanna 1983;Zink et al 1994;Tan et al 1995). This difference in fracture behaviour appears to be determined by cell wall thickness (Cote and Hanna 1983).…”