THE TOUGHNESS OF SECONDARY CELL-WALL AND WOODY TISSUE

The 'across grain' toughness of 51 woods has been determined on thin w et sections using scissors. The moisture content of sections and the v arying sharpness of the scissor blades had little effect on the result s. In thin sections (< 0.6 mm), toughness rose linearly with section t hickness. The intercept toughness at zero thickness, estimated from re gression analysis, was proportional to relative density, consistent wi th values reported for non-woody plant tissues. Extrapolation of the i ntercept toughness of these woods and other plant tissues/materials to a relative density of 1.0 predicted a toughness of 3.45 kJ m(-2), whi ch we identify with the intrinsic toughness of the cell wall. This qua ntity appears to predict published results from K-IC tests on woods an d is related to the propensity for crack deflection. The slope of the relationship between section thickness and toughness, describing the w ork of plastic buckling of cells, was not proportional to relative den sity, the lightest (balsa) and heaviest (lignum vitae) woods fracturin g 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, th e hypothetical overall toughness of a thick (> 1 min) block of solid c ell wall material was calculated as 39.35 kJ m(-2), due to both cell w all resistance (10%) and the plastic buckling of cells (90%). This val ue successfully predicts the roughness oi. most commercial woods (of r elative densities between 0.2 and 0.8) From work area' tests in tensio n and bending. Though density was the most important factor, both fibr e width/fibre length (in hardwoods) and lignin/cellulose ratios were n egatively correlated with the work of plastic buckling, after correcti ng for density. At low densities, the work of plastic buckling in the longitudinal radial (LR) direction exceeded that in longitudinal tange ntial (LT), but the reverse was true for relative densities above 0.25 . This could be attributed to the direction of rays. Density for densi ty, the toughness of temperate hardwoods tested was about 20% lower th an that of tropical hardwoods. This is probably due to the much greate r number of vessels in temperate hardwoods. Vessels appear either not to display buckling behaviour during fracture at all or to collapse ch eaply. These general results have applications to other plant tissues.