Evaluation of loggerhead sea turtle carapace properties and prototype biomimetic carapace fabrication

Abstract

The research presented in this study has been conducted in an effort to aid in the creation of a biomimetic shell that may be employed in full-scale field experiments to determine the efficacy of mitigation options to limit loggerhead mortality in boat strike incidents. The objectives of this research include the development of experimental testing procedures for the material characterization of the loggerhead carapace, and the design, fabrication, and evaluation of an artificial prototype carapace.

A photographic database of wounded sea turtles in Georgia was evaluated in order to determine the primary sources of loggerhead collision injuries and the most common regions of the carapace damaged in boat strike incidents. Skeg impact was found to be the most common source of injury, with a frequency of 44%. In addition, 74% of the sea turtles reviewed sustained injuries to the center third of their carapace length, indicating this region as the most probable impact location.

Material testing procedures were developed for evaluating the material properties of the loggerhead carapace. This was followed by the material testing of three loggerhead shells for the purpose of determining localized mechanical properties. Samples were harvested from the shells in a manner designed to identify potential variations in properties due the location and orientation of the coupons within the carapace. Each coupon was subjected to axial tension or three-point bending. Specialized tabs were designed for tension testing in order to accommodate the coupon's irregular geometry and minimize curvature-induced moments. The tensile test results indicated that the longitudinal and transverse properties of the loggerhead carapace were similar. The tensile strength, elongation at failure, and modulus of elasticity were determined to have percent variations of 12.2%, 10.7%, and 10.1% respectively. In contrast, the three-point bending test results indicated that the modulus of rupture and flexural modulus for the transverse samples were approximately four times greater than those of the longitudinal samples. This variation may be attributed to regions of weak tissue running transversely through the carapace.

The results of the material testing were utilized in the design of two prototype composite shells. The prototypes were successful in simulating the strain at failure and force per unit width to within 10% of the loggerhead carapace. The resulting procedure may be used to create artificial shells suitable for prototype scale tests in natural environments. In addition, the material testing methods developed for this investigation may offer insight into procedures for evaluating alternate forms of rigid or curved biological specimens.