Mechanics-guided exploration for the material design strategies from black carp teeth

Abstract

The black carp (Mylopharyngodon piceus), a species of fresh water fish, possesses a pair of powerful masticatory apparatus called pharyngeal teeth, which enable it to feed on snails and mussels equipped with hard shells. Such unique diet inspired us to investigate the teeth of black carp in an attempt to reveal the ingenious strategies for the design of engineering materials. To begin with, structural and mechanical characterizations were conducted on the pharyngeal teeth and the shells, showing that the enameloid (the outermost layer) of black carp teeth has similar elastic modulus and hardness in comparison to those of the pond snail shells. To shed light on the mechanics of black carp teeth crushing mollusk shell, parametric studies on the geometry of shells were conducted by using finite element analysis (FEA). It was found that whether a mollusk shell is crushable or not depends on the radius (R) and thickness (t) of the shell. A t-R map (predation map) was developed, giving rise to a region corresponding to crushable shells. Subsequent examination indicated that the geometries (R and t) of many fresh water species fall in the crushable region while those of the sea shells are beyond the crushing capability of black carp. These results agree well with the predation habit of black carp as a species of freshwater fish and are speculated as the consequence of natural selection. Preliminary studies indicated that the enameloid of black carp tends to fail, under indentation load, through ring cracking while some artificial brittle coatings (e.g. epoxy coated with glass) tend to fail through radial cracking. Therefore, the determinants of fracture modes (ring cracking & radial cracking) of brittle coatings under indentation are investigated. Rigorous theoretical analysis and numerical simulation indicated that fracture modes of brittle coating are determined synergistically by two dimensionless parameters (Modulus mismatch Ec/Es and mormalized fracture strength δf), which are functions of the mechanical properties and some geometric parameters. A map of fracture modes was developed, whereby the fracture mode should be directly predicated based on the two dimensionless parameters. Experimental verification of the fracture-mode map was also carried out by examining the fracture modes of fused quartz/cement bilayer materials under indentation. The experimental observation exhibited good agreement with the prediction by the fracture-mode map. The fact that black carp teeth still function well upon frequent mechanical interactions (e.g.squeezing and grinding) with the hard mollusk shells implies that they have prominent wear resistance. Thus, the subsequent attention was focused on the anisotropic wear resistance of black carp teeth. Nanoscratch tests showed the occlusal surface exhibits better scratch resistance compared to the other sections of the enameloid. To explain this anisotropic wear resistance, a theoretical scratching model was built up based on the classic wear theory. Results showed that for the given pyramidal scratching tool, solid surfaces with higher toughness/shear strength ratio (e.g. occlusal surface) tend to have better wear resistance. Therefore, the prominent wear resistance of occlusal surface can be attributed to the higher toughness/shear strength ratio of occlusal surface. In summary, above investigations not only elucidated the mechanics accounting for the superior mechanical properties of the black carp teeth but provided practical strategies for the design of synthetic materials and structures with better mechanical properties.