Biomechanical Characterization of Scallop Shells Exposed to Ocean Acidification and Warming

Abstract

Increased carbon dioxide levels (CO₂) in the atmosphere triggered a cascade of physical and chemical changes in the ocean surface. Marine organisms producing carbonate shells are regarded as vulnerable to these physical (warming), and chemical (acidification) changes occurring in the oceans. In the last decade, the aquaculture production of the bivalve scallop Argopecten purpuratus (AP) showed declined trends along the Chilean coast. These negative trends have been ascribed to ecophysiological and biomineralization constraints in shell carbonate production. This work experimentally characterizes the biomechanical response of AP scallop shells subjected to climate change scenarios (acidification and warming) via quasi-static tensile and bending tests. The experimental results indicate the adaptation of mechanical properties to hostile growth scenarios in terms of temperature and water acidification. In addition, the mechanical response of the AP subjected to control climate conditions was analyzed with finite element simulations including an anisotropic elastic constitutive model for a two-fold purpose: Firstly, to calibrate the material model parameters using the tensile test curves in two mutually perpendicular directions (representative of the mechanical behavior of the material). Secondly, to validate this characterization procedure in predicting the material's behavior in two mechanical tests.

Keywords: FEA; biomechanics; bivalves; elastic anisotropy; mechanical properties.

FIGURE 1

Specification for grips and samples for the tensile tests: (A) shows the sample design for the tensile test, the principal’s directions, and orientations of the samples. (B) shows the cross-sectional area of the samples and how it is estimated. (C) shows the tensile test assembly. All dimensions are shown in mm.

FIGURE 2

Flexocompression test configuration: The base for the flexo-compression test and the test configuration are shown, in which the punch is located at the cusp of the shell (outer surface).

FIGURE 3

Parametric surfaces adjusted to the barycentres of meshes for 45° tensile and whole shell specimens: (A) shows the 45° tensile test specimen surface fitting, and (B) shows the entire shell parametric surface fitting.

FIGURE 4

Element directions fitting for meshes used in FEM simulations: The boxes are shown the orientations of the particular coordinated system concerning a global orthotropic behavior. (A) shows the element directions for the 45° tensile test specimen, and (B) shows the element directions for the entire shell specimen.

FIGURE 5

Flowchart of the fitting procedure of elastic properties through numerical simulations.

FIGURE 6

Results of the tensile test apparent behavior in engineering stress against engineering strain: (A–C) shows the tensile results for 90°, 0°, and 45° orientations, respectively. (D–F) compare the mechanical behavior on the maximum strain, maximum engineering stress, and the elastic modulus. (●): extreme values of the distribution.

FIGURE 7

Flexocompression test results: (A) shows the behavior of complete shells in a flexo-compression test. It is shown at the application of force on the valve and displacement of the punch (vertical) at the cusp of the shell. (B) shows the comparison of the behavior of shells subjected to flexo-compression tests. The top graph shows the maximum force applied to the shell (N), and the bottom graph shows the displacement of the punch (mm) before the fracture. (●): extreme values of the distribution.

FIGURE 8

Results of simulations and comparison with experimental tests: (A) shows the simulation results of the tensile test, both for the elastic properties fitting stage (longitudinal and transversal direction) and for the validation stage (diagonal). (B) shows the validation of the orthotropic model adopted with the adjusted properties obtaining an error of approximately 23%.

FIGURE 9

Principal stresses magnitude and directions (in the boxes) in the shell-punch contact area: (A–C) show the principal σ _I, σ _II, and σ _III on the inner (left) and outer (right) surface, respectively.