Crack initiation and growth in a special quasi-sandwich crossed-lamellar structure in Cymbiola nobilis seashell

Sandwich structure consisting of three crossed-lamellar layers (inner, middle and outer) is one of the most common structures found in mollusk shells, and is normally arranged in a 0 degrees/90 degrees/0 degrees or 90 degrees/0 degrees/90 degrees mode. However, the Cymbiola nobilis seashell in the present study is observed to exhibit a unique quasi-sandwich structure, where the inner and middle layers have an similar to 15 degrees rotation in comparison with those of typical sandwich structures, resulting in a 15 degrees/75 degrees/0 degrees or 75 degrees/15 degrees/90 degrees mode. This has been identified as the weak/tough/weak and tough/weak/tough modes, and the sample arranged in the 15 degrees/75 degrees/0 degrees mode with a tough layer in the middle has a higher strength than that arranged in the 75 degrees/15 degrees/90 degrees mode with a weak layer in the middle. The fracture resistances of these two types of structural arrangements depend mainly on crack propagation. The interfaces between the macrolayers can effectively arrest the crack propagation especially when the tough layer is positioned in the middle (15 degrees/75 degrees/0 degrees mode), hence increasing the strength and toughness of materials. Salient toughening mechanisms involving crack deflection together with zig-zag crack propagation paths as well as the fiber pull-out of second-order lamellae are identified. Moreover, triangular fracture paths with a convex morphology are observed to form due to the concurrent occurrence of fiber fracturing and channel cracking in single 1st-order lamellae. These findings shed light on the superb crack propagation resistance of the unique quasi sandwich structure in the C. nobilis shell, thus paving the way for the development of bio-inspired advanced structural materials.