## 3 Results and Discussion
### Mechanical Properties  
Figure 12 illustrates the stress condition of hitting the ground and the supporting state when the Chinese mitten crab is moving, ignoring the force of friction and considering only the effect of the support reaction. Figure 12a shows the state when the tip of dactylopodite (T) comes into contact with the ground, and Figure 12b shows the state of the footstep during the supporting phase. When the dactylopodites are in contact with the ground and supporting, a bending moment will form at the end of the dactylopodite O\({}^{\prime}\), due to the impulsive force and the support reaction \(F_{\rm N}\). If the bending  
Figure 10: Results of XRD analysis. strength of the dactylopodite is insufficient, a bending fracture will occur. To observe the mechanical properties of dactylopodites, three-point bending tests were conducted on dry and wet specimens. The specimens for the three-point bending test were the second and third pairs of dactylopodites, which were divided into dry and wet samples. Table 2 shows the parameters of the samples. The tests were carried out by an in situ tensile tester. Each sample was fixed on the holders, and the span was 15 mm. Then load on it with pressure head driven by a motor at a constant speed until the samples were broken. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{4}{c}{**Parameters**} \\ \cline{2-5}  & **Moisture Content** & **Max. Outer Diameter** & **Inner Diameter** & **Length** \\  & \(\Phi\) (\%) & \(D\) **(mm)** & **\(D\) (mm)** & **\(I\) (mm)** \\ \hline Dry & 0 & 2.71 \(\pm\) 0.28 & 1.39 \(\pm\) 0.046 & 32.62 \(\pm\) 1.95 \\ Wet & \(41.06\pm 4.64\) & 2.50 \(\pm\) 0.15 & 1.31 \(\pm\) 0.005 & 30.88 \(\pm\) 1.16 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters of dactylopodite samples used for three-point bending tests. Figure 11: FTIR results for the dactylopodite exoskeleton. Figure 12: Forces on a dactylopodite during locomotion. (**a**) hitting the ground; (**b**) the supporting state. The bending strength of a dactylopodite, which can be calculated by Equation (1), is affected by a number of factors, including the structure, composition, and size. The cross section of the Chinese mitten crab dactylopodite is irregularly oblong with curved grooves on its circumference and has a variable cross section arc structure. Its composition is nonhomogeneous. The section modulus of the bending section is difficult to calculate accurately. Therefore the cross section of dactylopodite was approximated as annular in calculation. Thus \(M\) is proportional to load \(F\) and span \(L\), and the module of bending section is anti-correlated with the third power of diameter of section \(D\) approximately. The bending stress can be calculated accordingly:  
\[\sigma_{b}=\frac{M}{W}=\frac{FL/4}{\pi D^{3}(1-\alpha^{4})/32}, \tag{1}\]  
where \(M\) is the bending moment, \(W\) is the module of bending section, \(F\) is the load applied, \(L\) is span, \(\alpha\) indicates \(d/D\), \(\pi(1-\alpha^{4})/32\approx 0.092\). Figure 13 shows a set of curves for bending stress-displacement. The bending stress of the dry specimens increases linearly with the displacement of indenter, reaches a maximum value of 56.781 MPa, then drops rapidly and decreases at about 0.553 mm. The bending load of the wet specimen increases with displacement and reaches a maximum of 85.525 MPa at 0.780 mm, then begins to decrease slowly and drops dramatically at 1.357 mm. The results show that the dry dactylopodites exhibited typical characteristics of brittle material, and the change of the bending stress curve was linear and tended to break within a small displacement. The wet specimens were distorted when loaded during the test, so the maximum bending stress occurred after a longer displacement.