Structure and Mechanical Performance of Teleost Fish Scales
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Structure and Mechanical Performance of Teleost Fish Scales Deju Zhu1, Lawrence Szewciw1, Franck Vernerey2, and Francois Barthelat1 1 Department of Mechanical Engineering, McGill University, Montreal, QC, H3A 2K6, Canada 2 Department of Civil Engineering, University of Colorado, Boulder, CO, 80309, USA ABSTRACT In this work, we have studied the structure and mechanics of fish scales from striped bass (Morone saxatilis). This scale is about 200-300 μm thick and consists of a hard outer bony layer supported by a softer cross-ply of collagen fibrils. Puncture tests with a sharp needle indicated that a single fish scale provides a high resistance to penetration which is superior to polystyrene and polycarbonate, two engineering polymers that are typically used for light transparent packaging or protective equipment. Under puncture, the scale undergoes a sequence of two distinct failure events: First, the outer bony layer cracks following a well defined cross-like pattern which generates four “flaps” of bony material. The deflection of the flaps by the needle is resisted by the collagen layer, which in biaxial tension acts as a retaining membrane. Remarkably this second stage of the penetration process is highly stable, so that an additional 50% puncture force is required to eventually penetrate the collagen layer. The combination of a hard layer that can fail in a controlled fashion with a soft and extensible backing layer is the key to the resistance to penetration of individual scales. INTRODUCTION Nature increasingly serves as a model and inspiration to scientists and engineers, and biomimetics has the potential to lead to novel engineering materials and systems with new combinations of properties, multi-functionalities, adaptability and environmental sustainability. In this work, we have studied the structure and mechanics of modern teleost fish scales, which have received relatively little attention in the past [1-3]. This type of scale displays interesting combinations of flexibility, strength, resistance to penetration, light weight and transparency. Fish scales exhibit large variations in shape, size and arrangement. The general classification includes cosmoid, ganoid, placoid, and elasmoid (cycloid and ctenoid) found in the modern teleost class of fishes [4]. The “primitive” cosmoid and ganoid scales are bulky, bony scales which offer very effective protective properties, through a multilayered structure capable of a variety of dissipative mechanisms [2]. However, over the course of evolution the reduction of the integumental skeleton has improved swimming performance [2, 5], and the “ancient” cosmoid and ganoid scales have been replaced by the thinner, more flexible teleost scales [6]. Teleost scales have excellent hydrodynamic properties [7] and provide a protective layer resisting penetration [2, 6]. THE HIERARCHICAL STRUCTURE OF FISH SCALE In this study we have investigated the structure and mechanics of a single teleost (ctenoid) fish scale from striped bass Morone saxatilis. Like many other structural bio
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