Meeting Abstract
S7.12 Tuesday, Jan. 6 Micro-mechanics and material properties of the tessellated skeleton of cartilaginous fishes DEAN, MN*; YOUSSEFPOUR, H; EARTHMAN, J; GORB, S; SUMMERS, AP; UC Irvine; UCI; UCI; Max Planck Inst; UCI mdean@uci.edu
The natural history of sharks seems paradoxical: their long lives and swimming styles demand cyclic loading cycles on cartilaginous skeletons that cannot repair. Fatigue damage is proportional to loading cycle number and strain energy per cycle: shark skeletons should be irreparably fatigue damaged. To avoid this damage, structures can be either overbuilt (the excess safety factor decreases strain energy) or have properties to help dissipate strain energy. We have no evidence that cartilaginous skeletal elements have a larger safety factor than bony ones. We show, however, that elasmobranch skeletons are inherently fatigue-resistant; this is a function of the type of calcification of the tissue. The uncalcified hyaline-like cartilage core of each element is tessellated by a bark of abutting mineralized tiles (tesserae), adjoined by a fibrous phase. Indentation tests show that the mineralized tissue behaves nearly elastically and is more than two orders of magnitude stiffer than the uncalcified layer, which is highly viscoelastic. Using percussion testing, we show that tessellated cartilage is comparatively high in damping capacity and stiffness, a combination of cartilage- and bone-like mechanical behaviors. The damping capacity of tessellated cartilage (damping coefficient = 0.085) equals that of uncalcified cartilage, 50% greater than spongy bone (0.0552) and an order of magnitude greater than compact bone (0.085). However, the stiffness of tessellated cartilage approaches that of trabecular bone. A Reuss isostress model of a composite beam in bending shows tiling the surface of a gel shifts the strain energy into a less damaging loading regime by disproportionately loading the tissues compressive side. In this way, the tiled and calcified design of elasmobranch cartilage inherently imparts fatigue-resistance in a skeleton that cannot remodel.
