Mechanical Performance of Turtle's Shell

Mechanical Performance of Turtle’s Shell

Biological composite shields have been increasingly investigated in recent years. In turtle shell, there is a unique arrangement of alternating rigid rib and flexible suture elements that give rise to superior mechanical performance. The rigid structure of turtle shell protect the internal organ from external damage while some degree of flexibility allow the respiration and locomotion. The top dorsal of the turtle shell (namely carapace) was reported in the scientific literature and it found that carapace contains unique macroscopic configuration of complex alternating strips of rigid boney ribs. The carapace is consists of both alternating rigid (rib) and flexible (suture) elements in an arrangement of zigzag tips of rib connects to the flexible suture sites. The image of turtle shell has been shown in Figure 1. 

Figure 1. Ventral view (inside-out) of turtle shell. The red arrow mark the individual suture adjoining to rib

Flexural high stress cyclic loads were applied to both rib, suture, and complex specimens obtained from the carapace.  Static bending test were tested for specimens cut from the carapace to measure the strength for each specimen. The average strength for different part of the turtle shell has been shown in Table 1.

Table 1. Average strength for different part of turtle shell measured by quasi-static bending stress

Type of specimen	Strength (MPa)
Suture	51.3
Complex (Whole shell)	71.2
Rib	121.6

Based on the results, it showed that the rib is the strongest as compared to suture and the complex structure of turtle shell. The ribs demonstrate better fatigue resistance than sutures due to layered sandwich micro-structure (two perpendicular parallel-fibered sub-layers). However, the complex specimen made of a sequence of rib-suture-rib-suture-rib elements are able to withstand repeated loads due to its fast unlocking mechanism.

            According to the CT scan, it showed a decreasing mineral concentration from the shell toward the suture. The un-mineralized suture integrate with the rib allow extra degree of flexibility underload. This interdigitating nature of the structure of the sutures allows them to move freely towards each other under small load. However, the shell becomes rigid when adjoining dermal bones meets under critical deformation threshold. The concept of the nature of the shell structure is depicted in Figure 2. The SEM images of the complex 3-dimensional structure of suture joining the rib of the turtle shell have been shown in Figure 3a, b, c.   

Figure 2. Schematic depiction of unloaded and loaded deformed beam. The parameter D, W, and α are denote as pitch of the zigzag, gap of the suture, and maximal bending angle. 

Figure 3. SEM images complex 3-dimensional structure of suture joining rib. (a) at the interdigitating suture, (b) at the cancellous region adjacent to suture. The thin dorsal cortex region shown below the dashed green line is gradually thicken away from the center from suture to rib , (c) Higher magnification of (b). The red elipse marks the fracture region.

References

1. B. Achrai and H. Daniel Wagner, "The red-eared slider turtle carapace under fatigue loading: The effect of rib–suture arrangement", Materials Science and Engineering: C, vol. 53, pp. 128-133, 2015.

2. R. Shahar, S. Kraus, E. Monsonego-Ornan and P. Fratzl, "Mechanical Function of a Complex Three-dimensional Suture Joining the Bony Elements in the Shell of the Red-eared Slider Turtle", MRS Proc., vol. 1187, 2009.