The toughness of secondary cell wall and woody tissue

Abstract

SUMMARYThe ' across grain ' toughness of 51 woods has been determined on thin wet sections using scissors. The moisture content of sections and the varying sharpness of the scissor blades had little effect on the results. In thin sections ( 0.6 mm), toughness rose linearly with section thickness. The intercept toughness at zero thickness, estimated from regression analysis, was proportional to relative density, consistent with values reported for non-woody plant tissues. Extrapolation of the intercept toughness of these woods and other plant tissues\materials to a relative density of 1.0 predicted a toughness of 3.45 kJ m −# , which we identify with the intrinsic toughness of the cell wall. This quantity appears to predict published results from IC tests on woods and is related to the propensity for crack deflection. The slope of the relationship between section thickness and toughness, describing the work of plastic buckling of cells, was not proportional to relative density, the lightest (balsa) and heaviest (lignum vitae) woods fracturing with less plastic work than predicted. The size of the plastic zone around the crack tip was estimated to be 0.5 mm in size. From this, the hypothetical overall toughness of a thick ( 1 mm) block of solid cell wall material was calculated as 39.35 kJ m −# , due to both cell wall resistance (10 %) and the plastic buckling of cells (90 %). This value successfully predicts the toughness of most commercial woods (of relative densities between 0.2 and 0.8) from ' work area ' tests in tension and bending. Though density was the most important factor, both fibre width\fibre length (in hardwoods) and lignin\cellulose ratios were negatively correlated with the work of plastic buckling, after correcting for density. At low densities, the work of plastic buckling in the longitudinal radial (LR) direction exceeded that in longitudinal tangential (LT), but the reverse was true for relative densities above 0.25. This could be attributed to the direction of rays. Density for density, the toughness of temperate hardwoods tested was about 20 % lower than that of tropical hardwoods. This is probably due to the much greater number of vessels in temperate hardwoods. Vessels appear either not to display buckling behaviour during fracture at all or to collapse cheaply. These general results have applications to other plant tissues.

The fracture properties and mechanical design of human fingernails

“…However, the reason why the toughness of the complete nail is greater than the average for the three layers may instead be due to a volume effect. Unlike in hair, cell walls are very conspicuous in nail (Baden, 1970) and it is possible that cutting through nail could cause them to buckle plastically, just like the cell walls of wood (Lucas et al, 1997). Thicker samples of nail would consequently be tougher than thin ones because the size of the plastic zone would be greater, just as in wood (Lucas et al, 1997).…”

“…The scissor cutting tests were similar to those devised by Lucas and Pereira (1990), and widely used since to investigate the toughness of leaves and other plant tissues (Wright and Illius, 1995;Choong, 1996;Lucas et al, 1997Lucas et al, , 2000. A pair of precision scissors (Radio Spares model 539-609, Corby, UK) was mounted on the lower platform of a model 4301 universal testing machine (Instron, High Wycombe, UK), with its downward-pointing blade held rigid.…”

Tooth use and the physical properties of food

“…For example, Peters 23 found that woodland nuts are not as strong in compression as nut species found in tropical forests. Similarly, Lucas et al 30 found that tropical hardwoods are tougher than temperate species. Yamashita's study 26 suggested that shear strength of food is strongly seasonal for lemurs, with stronger foods being ingested during the dry season.…”

Leaf cellulose density as the key determinant of inter- and intra-specific variation in leaf fracture toughness in a species-rich tropical forest

“…Even though we considered only the bulk abundance of cellulose without considering structural organization, we could detect the overriding role of cellulose in explaining interspecific variations in leaf toughness. It was somewhat surprising that lignin and hemicellulose, which cross-link and bind cellulose microfibrils, had negative contributions to fracture toughness, similarly for leaves (table 2) and wood [38] analysed with the same cutting test. Detailed ultrastructual analyses might shed light on the reason why an increase of lignin, which enhances stiffness, somehow results in less work required for cracking.…”