Shell hardness and compressive strength of the Eastern oyster, Crassostrea virginica, and the Asian oyster, Crassostrea ariakensis.

The valves of oysters act as a physical barrier between tissues and the external environment, thereby protecting the oyster from environmental stress and predation. To better understand differences in shell properties and predation susceptibilities of two physiologically and morphologically similar oysters, Crassostrea virginica and Crassostrea ariakensis, we quantified and compared two mechanical properties of shells: hardness (resistance to irreversible deformation; GPa) and compressive strength (force necessary to produce a crack; N). We found no differences in the hardness values between foliated layers (innermost and outermost foliated layers), age class (C. virginica: 1, 4, 6, 9 years; C. ariakensis: 4, 6 years), or species. This suggests that the foliated layers have similar properties and are likely composed of the same material. The compressive force required to break wet and dry shells was also not different. However, the shells of both six- and nine-year-old C. virginica withstood higher compressive force than C. virginica shells aged either one or four, and the shells of C. ariakensis at both ages studied (4- and 6-years-old). Differences in ability to withstand compressive force are likely explained by differences in thickness and density between age classes and species. Further, we compared the compressive strength of differing ages of these two species to the crushing force of common oyster predators in the Chesapeake Bay. By studying the physical properties of shells, this work may contribute to a better understanding of the mechanical defenses of oysters as well as of their predation vulnerabilities.

Predicting failure in mammalian enamel.

Dentition is a vital element of human and animal function, yet there is little fundamental knowledge about how tooth enamel endures under stringent oral conditions. This paper describes a novel approach to the issue. Model glass dome specimens fabricated from glass and back-filled with polymer resin are used as representative of the basic enamel/dentine shell structure. Contact loading is used to deform the dome structures to failure, in simulation of occlusal loading with opposing dentition or food bolus. To investigate the role of enamel microstructure, additional contact tests are conducted on two-phase materials that capture the essence of the mineralized-rod/organic-sheath structure of dental enamel. These materials include dental glass-ceramics and biomimicked composites fabricated from glass fibers infiltrated with epoxy. The tests indicate how enamel is likely to deform and fracture along easy sliding and fracture paths within the binding phase between the rods. Analytical relations describing the critical loads for each damage mode are presented in terms of material properties (hardness, modulus, toughness) and tooth geometry variables (enamel thickness, cusp radius). Implications in dentistry and evolutionary biology are discussed.