4_2: CROSS LINKING - OAK Oak wood has a lightweight, porous structure of cellulose/lignine (plus a lot of tannines). As all woods, most of the fibers are aligned longitudinally, but in oak a significant proportion of the fibers form long bridges perpendicular to the main direction; furthermore the longitudinal fibers are not straight, following instead wavy patterns. When subject to longitudinal cracking, the cross fibers oppose direct resistance, while the waviness of the main longitudinal fibers helps spread the crack into several parallel cracks, distributing the stress into many small splitted beams that work efficiently under flexure. The bulk of the surrounding porous wood has very low stiffness and flexes easily, further reducing the load concentration. The thin plank pictured has been split open for ? of the length, but the remaining ? is holding unscathed. Most of the other wood types would have failed by complete, neat splitting. The compactness of Oak wood made it a fundamental constituent of European engineering, navies and militaries for millenia; until almost all of the natural oak forests had been depleted and its wood superseded by the mass production of steel. Subject: Oak plank, split wet. Likely from Quercus genera, unidentified specie. Plank 3mm thick, 35mm wide, 150mm long. Crack max open about 7mm. Lens: 2 pieces Sagem 569937 350mm at f/9 in symmetric configuration, 1:1. Image projected on tilt-shift screen, photographed with 90mm macro f/8. Theme 4: TOUGHNESS OF COMPOSITES Development of artificial composite materials is today in a golden age; the extreme strength of artificial fibers allows us to fabricate new materials with unprecedented performances. However, when it comes to impact resistance, toughness and flexibility, we still have to rely on bulk materials like metals, plastics and elastomers. Our composites are not yet mature, performing way below the potential of their constituents. The positioning of the reinforcement fibers, their interlocking, the adhesion with the matrix, the control of cracks growth, those are the critical factors that can and need to be improved. Biological tissues instead, being built up from the molecular level, show an high degree of refinement in their micro and mesoscopic structure, resulting in lightweight and extremely tough materials. The images presented try to convey in a visual manner some solutions hidden in natural materials on how they fail progressively while holding structural integrity.