Mechanics-based analysis of selected features of the exoskeletal microstructure of Popillia japonica
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We explore key mechanical responses of the layered microstructure found in selected parts of the exoskeletons (pronotum, leg and elytron) of Popillia japonica (Japanese beetle). Image analyses of exoskeleton cross-sections reveal four distinct layered regions. The load-bearing inner three regions (exocuticle, mesocuticle, and endocuticle) consist of multiple chitin-protein layers, in which chitin fibers align in parallel. The exocuticle and mesocuticle have a helicoidal structure, where the stacking sequence is characterized by a gradual rotation of the fiber orientation. The endocuticle has a pseudo-orthogonal structure, where two orthogonal layers are joined by a thin helicoidal region. The mechanics-based analyses suggest that, compared with the conventional cross-ply structure, the pseudo-orthogonal configuration reduces the maximum tensile stress over the exoskeleton cross-section and increases the interfacial fracture resistance. The coexistence of the pseudo-orthogonal and helicoidal structures reveals a competition between the in-plane isotropy and the interfacial strength in nature’s design of the biocomposite.
I. INTRODUCTION
The hard exoskeleton of a beetle is an important part of the arthropod’s body. It has outstanding structural properties with multifunctional capabilities, such as supporting the body weight, filtering chemicals, and resisting external loads (e.g., enemy attack). The multifunctional properties are obtained by constructing an intricate, hierarchical structure, using primarily the biopolymers chitin along with associated proteins.1,2 Thus, the exoskeleton is a multifunctional biological composite. Unlike the mineralized exoskeletons of crustaceans such as Homarus americanus (American lobster) and Callinectes sapidus (Atlantic blue crab),3 the majority of insects do not have a significant amount of minerals (e.g., calcite) in their exoskeletons. Minerals would make insects too heavy to fly.1,2 The exoskeleton of a beetle is typically divided into four regions: the epicuticle, exocuticle, mesocuticle, and endocuticle.1,4 The outermost region, the epicuticle, is the thinnest region (typically less than 1 mm) and consists of lipids and proteins. It acts as a diffusion barrier and is not load-bearing.1,2 The remaining exoskeleton (exocuticle, mesocuticle, and endocuticle) provides the load-bearing capacity. Here, the major building materials are chitin fibers, and the associated proteins are supplemented with lipids, pigments, inorganic materials, and water.1,2,5 Chitin has an elastic modulus up to 100 GPa,6,7 and even higher a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0409 J. Mater. Res., Vol. 24, No. 11, Nov 2009
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values have been reported (the order of 100–200 GPa8,9), and forms microfibrils with a diameter of approximately 2 to 3 nm.10 The microfibrils and the surrounding protein matrix (elastic modulus less than 1 GPa6) form a composite structure.1,2,11 Furthermore, bundles of mi
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