Investigation On The Mechanical Properties Of Saxidomus Purpuratus Shells

Through a long process of natural selection, the structures and properties of natural biological materials could be greatly optimized, and their abilities to adapt the nature have tended to be perfect. In contrast, the artificial engineering materials normally cannot possess those superior properties owned by biological materials. Therefore, the investigations on the structure and mechanical properties of biological materials are significantly important and introductive for the improvement of the structures and properties of engineering materials. In the present work, the structure and mechanical properties (e.g., hardness, compression, bending and fatigue properties) of a bivalve clam, Saxidomus purpuratus shells, which were obtained from Dalian sea area, have been investigated systematically, focusing on the distribution of various mechanical properties at different positions in the shell. The relationship between the fatigue strength and the fracture strength of the shell is also discussed.The Saxidomus purpuratus shell has a hierarchical structure comprising three layers in thickness, i.e., inner, middle and outer layers, and X-ray diffraction data show that all the layers consist of aragonite. According to the structure of each layer, an overall structure profile of the shell was established. The shell shows a porous blocky structure without organic matters in the outer layer and a cross-lamellar structure in the inner and middle layers with little organic matters. The cross-lamellar structure of this shell is composed of numerous domains, each of which comprises parallel tiles with approximate thickness of 200-500 nm. The width of these domains ranges from 20μm to 80μm, and the tiles in any two neighboring domains make a certain angle mutually.Depending strongly upon the structural characteristics of each layer of the shell, the hardness of outer layer is obviously lower than those of middle and inner layers, for which the hardness is almost comparable. The characterization of the indentation using a newly-designed method shows that the damages induced by indentation relate not only to the magnitude of the indentation load, but also to the orientation between the load direction and the lamellae in the structure of the shell. When the indentation load is perpendicular to the surface of the crossed lamellae, the induced damage is more serious. Besides, the fracture toughness was roughly evaluated by hardness tests to be around 2.21 MPa·m1/2.Compression tests were performed with dry/wet shell specimens in different orientations. The results show that, as compared to the dry specimens, the wet have higher moduli and lower strengths. When the load is applied perpendicular to the surface of the shell, the compression strength is higher than that obtained with the load parallel to the surface of the wet shell. Analyses of compression stress-strain curves and the cracking paths demonstrate that the occurrence of steps in the curve should correspond to the formation of cracks. The continuous steps in the stress-strain curve imply that the crack propagation resistance becomes increased during the process of compressive deformation, causing an increase in the compression strength. Weibull statistics show that the compression strength (72 MPa) of wet specimens is quite close to that (73 MPa) of dry specimens, and that the compression strength (87 MPa) with the load perpendicular to the surface of the shell is higher than that (61 MPa) with the load parallel to the surface of the shell.The three-point bending mechanical behaviors of wet and dry specimens were investigated. It is found that the bending strength on the edge of the shell is higher; this is closely associated with the growth mechanism of the shell. The dry specimens near the mantle position have lower bending strengths than the wet, and the dry specimens in the other positions of the shell have higher strengths than the wet. Concerning the shell specimens of the same condition, the distributions of the fracture energy per area and the strength in the shell are identical. As compared to the case of fracture energy and strength, the distribution of the moduli is not of regularity, but the moduli of the wet specimens are a little higher than those of the dry specimens. The cracking paths are mainly affected by the weak lamellae and weak interfaces between lamellae and organic matters. Additionally, the tissues like adductor muscle etc. can also affect the cracking paths of the specimens. Weibull analysis indicates that the bending strength is 98 MPa for dry and 91 MPa for wet at a fracture probability of 50% [P(V)=0.5]. A prediction method was suggested to evaluate the fatigue strength in advance, so that the successful cases of high-cycle bending fatigue tests are obviously improved. Generally, the fatigue strengths of the shell specimens are lower than the relevant bending strengths, and the fatigue life is normally less than 106 cycles. The apparent "zig-zag" cracking path mode and the micro-step morphology on the fracture surface demonstrate that the shell has indeed undergone a process of fatigue failure. Weibull statistics show that at a fracture probability of 50%[P(V)=0.5], the fatigue strength of dry specimens is 85 MPa, a little lower than the bending strength (98 MPa).