Investigations On Microstructures And Fracture Behavior Of Layer-structured Biological Shells

Various biological shells have received considerable attention, especially in the aspect of fundamental investigations and relevant bionic applications, owing to their unique mechanical properties, e.g., high fracture strength and superior fracture toughness. The shells named as Saxidomus Purpuratus (called Zishifangge in China) were adopted as the target materials in the present work, and the microstructures, general mechanical properties and fatigue fracture behavior of the shell were systematically investigated and analyzed. It is expected that this work could provide some valuable experimental data and theoretic fundaments for the further design of artificial high-performance composite materials, and also deepen understanding of the specific fracture behavior of such natural bio-shells.Firstly, microstructures of the shell were carefully observed with optical microscopy viewing from different layers and its cross section. It is indicated that the microstructure exhibits distinctive features at different parts of the shell. For example, the microstructures corresponding to the outer or/and central layers of the shell are mainly featured by typical layered structures, while a crossed lamellar structure presents in the inner layer of the shell. In addition, observations on the cross-section showed that, from outer layer to inner layer, the layers do not align strictly in parallel towards only one direction, but present a gradually-increasing deflection angle.Micro-hardness tests at different locations of the shell demonstrated that there exits an evident anisotropy of the micro-hardness, which is closely related with the distinctive microstructures. The micro-hardness of inner layer is significantly higher than that of outer layer, and the micro-hardness on cross-section is obviously higher than that on the top surface of the shell.Three-point bending tests showed that there are different values of bending strength at various locations of the shell, which is closely related with the distribution of calcium carbonate depositions in shells. According to the special characteristics of self-secretion and natural deposition of calcium carbonate, the calcium carbonate more easily deposits on the edge of shells, causing that the anti-bending strength of the edge of shells is significantly larger than that of the middle. Taking samples from different locations in the shell, the fracture modes and cracking paths of these samples were found diversified. Generally, the fracture strength is smaller, provided the transition between layers is more abrupt in the process of cracking. SEM observations of fracture surfaces revealed that the fracture features can be primarily embodied by a layer-by-layer cracking phenomenon. A "weak interface" may exist between some layers, which could lead to a rapid cracking of the shell.Finally, the three-point bending fatigue life of the shell, which is a typical kind of brittle materials, was tentatively measured experimentally. Here, a loading frequency f of 30 Hz, a stress ratio R of 0.1, and a maximum applied load of 14 N were adopted for the dynamic bending tests. A fatigue fracture took place after the shell was cyclically deformed to 5.37×106 (i.e. fatigue life). SEM observations of fatigue fracture surfaces indicated that the fatigue fracture process is comparatively complicated, e.g., obvious cleavage step-like patterns, many fracture debrises and etc. This part of work has laid a fundament for the further study of fatigue deformation behavior of such shell materials.