Microstructural Characterizations And Mechanical Behavior Of Veined Rapa Whelk Shells

For many natural biological materials, especially for biological mineralized material, such as bones, shells, teeth and etc, their material microstructures and the corresponding properties have tended to be perfectly optimized through a long-term evolution process together with organisms, Although the basic unit of biomineralization materials is the most common materials like calcium carbonate or calcium phosphate, they often possess a assembly of complex structures for adapting to environmental and functional needs. Therefore, biological materials can exhibit the outstanding material properties, and the traditional synthetic materials cannot compare these properties. Shells, as a kind of natural biological materials, have been concerned by material designers and investigators because of its unique structure and excellent mechanical properties. In the present work, a kind of natural biological material called veined rapa whelk shell was selected as the target material, and its microstructure, crystal compositions, vickers hardness, compressive mechanical behavior were studied and discussed by using a variety of analysis and testing techniques, with the purpose of providing some valuable reference data for the in-depth investigation on the structure and mechanical behavior of natural biological materials.The microstructures of the veined rapa whelk shell were observed on different layer surfaces and differently oriented cross sections, it was found that the shell exhibits a hierarchical structure in its entirety, The inner layer and the middle layer of the shell are a staggered structure of sheet nacre, and outer layer is a prism-like structure. A very clear interface can be observed between the outer and middle layers. The microstructures observed along the cross sections with different orientations (e.g., with vertical orientation or parallel orientation) of veined rapa whelk shells also exhibit some differences, indicating an anisotropic characteristic.XRD analysis of the different layers of veined rapa whelk shells demonstrates that the components and content of different layers are different. The main ingredients of the inner layer and the middle layer are made of crystalline CaC03(Vaterite) and its decomposers, and the composition type and content are different in these two layers. The outer layer contains a kind of calcite calcium carbonate, which was detected to be different from the calcium carbonate crystals found in the middle layer. This type of calcite crystals is the major component of the outer layer of shells.Microhardness has been measured on different layers and cross-sections with different orientations of shells, and it was found that microhardness decreases gradually from the inner layer to the outer layer. For instance, hardness changes from312Hv to155Hv on the different locations of cross section under a load of50g. Observations and measurements of indentation on different layers of the shell indicate that the indentation crack path, crack growth distance and the indentation-induced damaged area are different, indicating that the fracture toughness decreases in steps from the inner layer to the outer layer, and that the fracture mechanism and toughening mechanism of each layer are different.A series of compressive tests were performed on the differently-oriented veined rapa whelk shell samples, for which the compressive loading direction makes different angles with the shell conchoids. The elastic modulus, the first fracture stress, the maximum fracture stress and fracture energy density have been measured on these oriented samples, and it was found that the veined rapa whelk shell samples with various orientations have different measurements. Generally, all the measurements are highest for the sample having0f°-orientation but lowest for the sample with60°-orientation. This indicates that mechanical properties of veined rapa whelk shells is dependent upon the sampling orientation. This phenomenon is believed to result from the existance of different stress states for the coarse and thin shell textures during compression. Moreover, investigations on the compressive fracture behavior, fracture paths and frature morphologies of the shell reveal that the fracture behavior of the veined rapa whelk shell under compression is generally different from that of conventional brittle materials. The fracture process of the shells takes place gradually with a relatively high ductility, if compared to the case of general brittle materials.