Microstructure and mechanical property of Ruditapes philippinarum shell.

In order to provide reference for the development of relevant dredging and processing machinery of the manila clam (Ruditapes philippinarum), the clam shell´s microstructure, phase composition, microhardness, nanoindentation hardness and elastic modulus were investigated, the quasi static compression and orthogonal compression tests of live clams were carried out as well. The shell consists of the outer, middle, and inner layer, which correspond with the composite prismatic structure, crossed-lamellar structure, and homogeneous structure, respectively. X-ray diffraction (XRD) results indicated that all the three layers are made up of pure aragonite phase, the outer layer of the shell displays greater intensity and more diffraction peaks than the middle and inner layer. The microhardness of the inner layer, ~ 3.00 GPa, is harder than the middle and outer layer. Both in the middle and outer layers, the microhardnesses of the vertical section perpendicular to the growth lines are obviously higher than that of the cross section parallel to the growth lines. The similar trend was observed in nanoindentation hardness (H) and elastic modulus (E), but the H/E ratios between three layers and two sections are close to 0.05. The quasi static compressive strengths of live clams with loading along the X (beak horizontal), Y (beak upward and umbo downward), and Z (beak vertical) orientations were tested, and the differences were identified. The lowest strength was found with loading orientation Z, the Weibull strength at 50% probabilities of fracture force is just 153 N. The results of the orthogonal compression tests demonstrated that the fracture force of clams varies from approximately 100 N to 400 N, the effect of the clams´ age on fracture force is more significant (p < 0.01) than loading orientation and loading speed. Therefore, the clams´ age and ultimate load of fracture should be taken into account during clam production mechanization, for the purpose of maintaining the clams´ integrity and survival prior to sale.