Local orientation analysis using X-ray microdiffraction reveals that the crystalline calcium carbonate fraction in lobster cuticle is associated with the chitin/protein fibers oriented perpendicular to the surface. The calcite crystallites show a fiber texture with the crystallographic c-axis pointing perpendicular to the cuticle surface, suggesting an orientation relationship between calcite and the organic chitin/protein fibers.
The exoskeletons of the American lobster Homarus americanus and of the edible crab Cancer pagurus were analysed with structural and chemical methods. The exoskeletons consist of crystalline magnesian-calcite in the form of nanocrystals (domain size about 20 nm), amorphous calcium phosphate (ACP), and a-chitin. The composition is different at different parts of the skeleton and also between these two species. This can be related to the mechanical requirements and also to the biological escape behaviour of these animals upon attack of predators. The finger and the claw are used for cutting and therefore strongly mineralized and very hard. The shell of the body (the carapace) is less mineralized and therefore more elastic. The lobster as mobile, fast-swimming animal typically escapes from a predator's attack whereas the crab clings to the ground and buries into the sand. Consequently, the shell of the lobster is less mineralized (and therefore lighter and less hard) than the shell of the crab.
In this project we model the elastic properties of bone at the level of mineralized collagen fibrils via step-by-step homogenization from the staggered arrangement of collagen molecules up to an array of parallel mineralized fibrils. A new model for extrafibrillar mineralization is proposed, assuming that the extrafibrillar minerals are mechanically equivalent to reinforcing rings coating each individual fibril. Our modeling suggests that no more than 30% of the total mineral content is extrafibrillar and the fraction of extrafibrillar minerals grows linearly with the degree of mineralization. It is shown that the extrafibrillar mineralization considerably reinforces the fibrils properties in the transverse directions and the fibrils shear moduli. The model predictions for the elastic moduli and constants are found to be in a good agreement with the experimental data reported in the literature.
This study presents a novel experimental approach to the characterization of the deformation of a mineralized biological composite using arthropod cuticle as a model material. By performing tensile tests combined with a detailed strain analysis via digital image correlation, the elastic-plastic deformation behavior of the endocuticle of the American lobster Homarus americanus is examined. The test specimens originate from the pincher and crusher claws. For evaluating the effect of moisture on the deformation behavior, the samples are tested both in dry and in wet state. Sample characterization using the digital image correlation method requires a stochastic spot pattern on the sample surface. Digital images are then taken at subsequent deformation stages during the mechanical test. These images are used to calculate the displacement, the displacement gradient, and the strain fields via pattern correlation. The method is applied both, at a global scale to measure with high precision the stress-strain behavior of the bulk cuticle and at a microscopic scale to reveal strain heterogeneity, strain patterning, and strain localization phenomena.
The effect of sterilization on the structural integrity of the thermoplastic matrix composite polyetheretherketone (PEEK) reinforced with carbon fibers (CF) is investigated by nanoindentation and nanoscratch tests. The use of the material as a medical implant grade requires a detailed understanding of its micromechanical properties which primarily define the in-vivo behavior. Sterilization is a mandatory process for such materials used in medical applications like bone-implants. The steam and gamma radiation sterilization processes employed in this study are at sufficient levels to affect the micromechanical properties of some polymer materials, particularly in the interphase region between the polymer matrix and the reinforcing fibers. Nanoindentation and nanoscratch tests are used in this work to reveal local gradients in the hardness and the elastic properties of the interphase regions. Both methods help to explore microscopic changes in the hardness, reduced stiffness, and scratch resistance in the interphase region and in the bulk polymer matrix due to the different sterilization processes employed. The results reveal that steam and gamma radiation sterilization do both not entail significant changes of the reduced elastic modulus, hardness, or coefficient of friction in the bulk polymer matrix. However, minor material changes of the PEEK matrix were observed in the interphase region. Of the two sterilization methods used, the steam treatment has a more significant influence on these small changes in this region and appears to increase slightly the thickness of the interphase zone.
