4_3: CHOPPED STRAND MAT - LEATHER The skin of mammals is made of a seemingly random network of short, curly collagen fibers. Under tension and flexure this fabric can deform considerably; it is very pliable for small deformations while it stiffens progressively when reaching its strength limits. The already elastic collagen fibers are interwoven in a loose and redundant manner, and when the skin is pulled a large number of fibers get tensioned and aligned against the stress. Only the most tensioned strands fail, while the rest, having worked mostly elastically, can return in their original position once the stress is removed. Animal hide has been in use by humans since prehistory and is still considered today the best available material for several applications. Artificial materials such as felt and non-woven tissue have similar geometry, while laminated Kevlar bulletproof vests use a similar load distribution mechanism. Subject: Cow skin, processed as suede, from an old shoe. Thickness about 0,8mm. Lens: Macro 90mm f/11. 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.