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Fig. 5. Microindentation hardness showing a discontinuity through the thickness of crab claws and walking legs (data point: average; scale bar: standard deviation). The SEM micrograph above the plot documents the location where indentations were taken.

Figure 5 Microindentation hardness showing a discontinuity through the thickness of crab claws and walking legs (data point: average; scale bar: standard deviation). The SEM micrograph above the plot documents the location where indentations were taken.

Related Figures (13)

Fig. 1. (a) SEM micrograph of a cross-sectional fracture surface showing three different layers in the exoskeleton: epicuticle, exocuticle, and endocuticle. (b) Hierarchical structure of the exoskeleton of sheep crab, Loxorhynchus grandis. Chitin fibrils (~3 nm in diameter) wrapped with proteins form a fiber of ~60 nm in diameter. Fibers further assemble into bundles, which form horizontal planes (x—y plane) superposed in a helicoid stacking, creating a twisted plywood structure (180° rotation). In the z-direction there are ribbon-like tubules, 1 jum wide and 0.2 um thick, running through the pore canals.
Fig. 1. (a) SEM micrograph of a cross-sectional fracture surface showing three different layers in the exoskeleton: epicuticle, exocuticle, and endocuticle. (b) Hierarchical structure of the exoskeleton of sheep crab, Loxorhynchus grandis. Chitin fibrils (~3 nm in diameter) wrapped with proteins form a fiber of ~60 nm in diameter. Fibers further assemble into bundles, which form horizontal planes (x—y plane) superposed in a helicoid stacking, creating a twisted plywood structure (180° rotation). In the z-direction there are ribbon-like tubules, 1 jum wide and 0.2 um thick, running through the pore canals.
The properties are the average including their standard deviation: /, gage length; w, gage width; ¢, thickness; E, Young’s modulus. of, stress to fracture; and é, strain to fracture. Mechanical properties of crab exoskeletons from tensile testing in the longitudinal direction Table 1
The properties are the average including their standard deviation: /, gage length; w, gage width; ¢, thickness; E, Young’s modulus. of, stress to fracture; and é, strain to fracture. Mechanical properties of crab exoskeletons from tensile testing in the longitudinal direction Table 1
Fig. 2. Schematic representation of mechanical tests: (a) microindentation hardness test; (b) tensile test in the longitudinal direction; (c) tensile test in the z direction; (d) compressive test in the z-direction.
Fig. 2. Schematic representation of mechanical tests: (a) microindentation hardness test; (b) tensile test in the longitudinal direction; (c) tensile test in the z direction; (d) compressive test in the z-direction.
Mechanical properties of crab exoskeletons from tensile testing in the z-direction The properties are the average values including their standard deviation: d, diameter; t, thickness; E, Young’s modulus. of, stress to fracture; and é, strain to fracture. Table 2
Mechanical properties of crab exoskeletons from tensile testing in the z-direction The properties are the average values including their standard deviation: d, diameter; t, thickness; E, Young’s modulus. of, stress to fracture; and é, strain to fracture. Table 2
Mechanical properties of crab exoskeletons from compressive testing in z-direction The properties are the average values including their standard deviation: d, diameter; t, thickness; E, Young’s modulus. of, stress to fracture; and é, strain to fracture. Table 3
Mechanical properties of crab exoskeletons from compressive testing in z-direction The properties are the average values including their standard deviation: d, diameter; t, thickness; E, Young’s modulus. of, stress to fracture; and é, strain to fracture. Table 3
Fig. 3. (a) SEM micrograph showing the layered structure in endocuticle; (b) SEM micrograph showing the Bouligand structure, pore canals and pore canal tubules.
Fig. 3. (a) SEM micrograph showing the layered structure in endocuticle; (b) SEM micrograph showing the Bouligand structure, pore canals and pore canal tubules.
Fig. 4. SEM micrographs showing (a) ribbon-shaped pore canal tubules (arrows); (b) necked configuration of tubules in tensile traction (arrows); (c) tc view of the pore canal tubules showing (A) pore canals; (B) pore canal tubules; (C) chitin fibers.
Fig. 4. SEM micrographs showing (a) ribbon-shaped pore canal tubules (arrows); (b) necked configuration of tubules in tensile traction (arrows); (c) tc view of the pore canal tubules showing (A) pore canals; (B) pore canal tubules; (C) chitin fibers.
Fig. 6. Typical tensile stress-strain response of sheep crab exoskeleton in both dry and wet states in the y-direction. The results are compared with the mud crab exoskeleton by Hepburn et al. [18]. iLypical stress-strain curves in the dry and wet condi- tions are shown in Fig. 6. They exhibit a slightly convex shape stress-strain response which corresponds to elastic deformation. Table 1 summarizes the mechanical proper- ties of 34 tests taken from three different crabs. The wet samples have an average ultimate tensile strength of 31.5+5.4MPa at an average strain to fracture of 6.4 + 1.0%. The dry samples break at an average ultimate tensile strength of 12.9 + 1.7 MPa at an average strain to fracture of 1.8+0.3%. The stress-strain curves for wet samples are not perfectly linear. This may be due to sample alignment at the initial stage. The Young’s modulus was measured by taking the data points after 2% of strain and linear fitting. The average value of Young’s modulus for the wet samples was 518 +72 MPa, whereas it was 764 + 83 MPa for the dry samples. The work-of-fracture or toughness, as measured by the area under the stress— strain curve, is significantly affected by gradual fracture. The toughness for wet samples is 1.02 + 0.25 MPa, which is almost 10 times higher than that for dry samples, which is 0.11 + 0.03 MPa. Hepburn and co-workers [17] investi- gated the mechanical properties of mud crab, S. serrata, in tension. The stress-strain curves for both wet and dry mud crab samples are plotted in Fig. 6 along with the cur- rent results. There is a load drop in stress-strain curves at low strain. Hepburn et al. [17] concluded that this may due to the failure of inorganic minerals at low strain, followed
Fig. 6. Typical tensile stress-strain response of sheep crab exoskeleton in both dry and wet states in the y-direction. The results are compared with the mud crab exoskeleton by Hepburn et al. [18]. iLypical stress-strain curves in the dry and wet condi- tions are shown in Fig. 6. They exhibit a slightly convex shape stress-strain response which corresponds to elastic deformation. Table 1 summarizes the mechanical proper- ties of 34 tests taken from three different crabs. The wet samples have an average ultimate tensile strength of 31.5+5.4MPa at an average strain to fracture of 6.4 + 1.0%. The dry samples break at an average ultimate tensile strength of 12.9 + 1.7 MPa at an average strain to fracture of 1.8+0.3%. The stress-strain curves for wet samples are not perfectly linear. This may be due to sample alignment at the initial stage. The Young’s modulus was measured by taking the data points after 2% of strain and linear fitting. The average value of Young’s modulus for the wet samples was 518 +72 MPa, whereas it was 764 + 83 MPa for the dry samples. The work-of-fracture or toughness, as measured by the area under the stress— strain curve, is significantly affected by gradual fracture. The toughness for wet samples is 1.02 + 0.25 MPa, which is almost 10 times higher than that for dry samples, which is 0.11 + 0.03 MPa. Hepburn and co-workers [17] investi- gated the mechanical properties of mud crab, S. serrata, in tension. The stress-strain curves for both wet and dry mud crab samples are plotted in Fig. 6 along with the cur- rent results. There is a load drop in stress-strain curves at low strain. Hepburn et al. [17] concluded that this may due to the failure of inorganic minerals at low strain, followed
Fig. 7. Typical tensile response of sheep crab exoskeleton in the z- direction (dry condition).
Fig. 7. Typical tensile response of sheep crab exoskeleton in the z- direction (dry condition).
Fig. 8. Typical compressive stress-strain response of sheep crab exoskel- eton in the z-direction (dry and wet conditions).
Fig. 8. Typical compressive stress-strain response of sheep crab exoskel- eton in the z-direction (dry and wet conditions).
‘ig. 9. Fracture surfaces of the exoskeleton in (a) dry and (b) wet conditions; A indicates chitin/mineral bundle fracture and B shows bundle separatior Fig. 11 shows a schematic representation of the fracture processes. The breaking of chitin bundles when they are parallel to the loading direction (regions A in Fig. 9a) and their separation when they are parallel (regions B in Fig. 9a) are the main fracture modes. Intermediate orienta- tions in the Bouligand will fail by either normal bundle fracture or bundle separation. Fracture in the x—y tensile specimens will preferentially travel through the orifices which provide the stress concentration sites. This is shown in Fig. lla. For failure when loading is parallel to the z- direction (Fig. 11b), separation of the chitin bundles is fol- Fig. 9 shows the surface of a tensile specimen fractured in the longitudinal direction (dog-bone-shaped sample). The fracture surface of a dry sample is shown in Fig. 9a.
‘ig. 9. Fracture surfaces of the exoskeleton in (a) dry and (b) wet conditions; A indicates chitin/mineral bundle fracture and B shows bundle separatior Fig. 11 shows a schematic representation of the fracture processes. The breaking of chitin bundles when they are parallel to the loading direction (regions A in Fig. 9a) and their separation when they are parallel (regions B in Fig. 9a) are the main fracture modes. Intermediate orienta- tions in the Bouligand will fail by either normal bundle fracture or bundle separation. Fracture in the x—y tensile specimens will preferentially travel through the orifices which provide the stress concentration sites. This is shown in Fig. lla. For failure when loading is parallel to the z- direction (Fig. 11b), separation of the chitin bundles is fol- Fig. 9 shows the surface of a tensile specimen fractured in the longitudinal direction (dog-bone-shaped sample). The fracture surface of a dry sample is shown in Fig. 9a.
Fig. 11. Schematic drawings showing tensile failure in the (a) y-direction and (b) z-direction.
Fig. 11. Schematic drawings showing tensile failure in the (a) y-direction and (b) z-direction.
Fig. 10. Fracture surfaces of the exoskeleton after tensile test in the z-direction: (a) fracture between exocuticle and endocuticle; (b) fracture withi endocuticle; (c) schematic drawing showing the arc pattern on oblique surface.
Fig. 10. Fracture surfaces of the exoskeleton after tensile test in the z-direction: (a) fracture between exocuticle and endocuticle; (b) fracture withi endocuticle; (c) schematic drawing showing the arc pattern on oblique surface.
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Fracture Mechanic
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