Browsing by Author "Bargmann, Swantje"

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    Hierarchical Microstructure of Tooth Enameloid in Two Lamniform Shark Species, Carcharias taurus and Isurus oxyrinchus
    (2021-04-09) Wilmers, Jana; Waldron, Miranda; Bargmann, Swantje
    Shark tooth enameloid is a hard tissue made up of nanoscale fluorapatite crystallites arranged in a unique hierarchical pattern. This microstructural design results in a macroscopic material that is stiff, strong, and tough, despite consisting almost completely of brittle mineral. In this contribution, we characterize and compare the enameloid microstructure of two modern lamniform sharks, Isurus oxyrinchus (shortfin mako shark) and Carcharias taurus (spotted ragged-tooth shark), based on scanning electron microscopy images. The hierarchical microstructure of shark enameloid is discussed in comparison with amniote enamel. Striking similarities in the microstructures of the two hard tissues are found. Identical structural motifs have developed on different levels of the hierarchy in response to similar biomechanical requirements in enameloid and enamel. Analyzing these structural patterns allows the identification of general microstructural design principles and their biomechanical function, thus paving the way for the design of bioinspired composite materials with superior properties such as high strength combined with high fracture resistance.
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    Micromechanical modelling of advanced hierarchical composites
    (2020) Griffiths, Emma; Reddy, Batmanathan; Bargmann, Swantje
    Nanoporous metals are uniquely interesting materials. Their high ductility and impressive strength in compression make them a favourable candidate for use in structural applications. However, these materials under-perform when tested in tension. This issue may be addressed by impregnating the nanoporous metal with a polymer. In this work the behaviour of a polymer impregnated nanoporous gold (NPG) composite is explored using the finite element method in three different scenarios: linear elasticity, fracture and electrically stimulated actuation. Using representative volume elements (RVEs), previously unexplored relationships between the macroscopic material response and its microstructure as well as interesting mechanisms and deformation strategies are explored. Firstly the homogenization and micromechanical response under compression of a gold/epoxy nanocomposite is investigated. Investigation into the stress-strain response within the material reveals a complex interaction between the constituents resulting in both compressive and tensile strains. With specific focus on the loading modes of the individual ligaments, significant axial and bending loading as well as an unexpectedly large amount of shear stress is seen. Following this the improved ductility and resistance to fracture of a gold/polymer nanocomposite compared to the pure NPG material is revealed using computational compact-tension tests. It is observed that the polymer stabilizes the gold thus preventing ductile fracture. Several toughening mechanisms are also revealed. Previously unexplored effects of increasing the volume fraction on the ductility and strength of the composite are also explored. The functionality of the gold/polymer nanocomposite as an actuator material is then investigated. A coupled chemo-electro-mechanical material model is adopted to model the electrically stimulated deformation. This is carried out in Abaqus using a novel staggered explicit-implicit solution scheme. Simulation of several RVEs with different gold volume fractions show that while the gold provides strength and support, increasing its volume fraction hinders both the ion transport speed and the total deformation of the nanocomposite. A complex interaction between the stress response and the gold volume fraction is also observed.
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    Multiscale modelling of sutures in a high-performing biological protective structure: the turtle shell
    (2022) Alheit, Benjamin; Reddy, Batmanathan; Bargmann, Swantje
    Many natural protective structures, such as alligator armour, turtle shells, and the skulls of many animals including humans, contain networks of sutures; those are, soft tissue that bonds adjacent stiff plates typically made of bone. Such protective structures ought to withstand large loads associated with predator attacks. If one considers the optimization process of evolution and the ubiquity of suture networks in natural protective structures, it is reasonable to hypothesize that sutures improve the mechanical behaviour of protective structures during predator attacks. However, the effect of sutures in such loading scenarios is not well understood. We address this by using computational models of turtle shells where special attention is paid to the influence of the network of sutures. Additionally, we elucidate the structure-function relationship using parametric studies varying the suture geometry. Computational experiments are carried out at the suture scale to elucidate its mechanical behaviour and at the shell scale to elucidate the effect that sutures have on the shell. Among other insights, we show that: the compliance of the shell during small deformations can be increased by increasing the height of the interlocking bone protrusions and suture thickness; the bone plates interlock for sufficiently large deformations of sutures with sufficiently long protrusions; suture geometry can be used to tailor stress-wave propagation; and the presence of sutures can reduce the maximum strain energy density, a key indicator for a material failure, during a predator attack by 31 times. The work presented paves the way for the inclusion of sutures in biomimetic protective structures such as helmets and body armour. Computational solid mechanics aspects include multiscale modelling, model order reduction, and finite strain constitutive modelling aspects, such as viscoelasticity, hyperelasticity, and anisotropy.