S. Anup

Influence of platelet aspect ratio on the mechanical behaviour of bio-inspired nanocomposites using molecular dynamics

Superior mechanical properties of biocomposites such as nacre and bone are attributed to their basic building blocks. These basic building blocks have nanoscale features and play a major role in achieving combined stiffening, strengthening and toughening mechanisms. Bioinspired nanocomposites based on these basic building blocks, regularly and stairwise staggered arrangements of hard platelets in soft matrix, have huge potential for developing advanced materials. The study of applicability of mechanical principles of biological materials to engineered materials will guide designing advanced materials. To probe the generic mechanical characteristics of these bioinspired nanocomposites, the model material concept in molecular dynamics (MD) is used. In this paper, the effect of platelets aspect ratio (AR) on the mechanical behaviour of bioinspired nanocomposites is investigated. The obtained Young׳s moduli of both the models and the strengths of the regularly staggered models agree with the available theories. However, the strengths of the stairwise staggered models show significant difference. For the stairwise staggered model, we demonstrate the existence of two critical ARs, a smaller critical AR above which platelet fracture occurs and a higher critical AR above which composite strength remains constant. Our MD study also shows the existence of mechanisms of platelet pull-out and breakage for lower and higher ARs. Pullout mechanism acts as a major source of plasticity. Further, we find that the regularly staggered model can achieve an optimal combination of high Young׳s modulus, flow strength and toughness, and the stairwise staggered model is efficient in obtaining high Young׳s modulus and tensile strength.

Influence of viscoelasticity of protein on the toughness of bone

Bone is an ultrafine composite of protein (collagen) and mineral (hydroxyapatite). An analysis to determine the influence of the viscoelasticity of protein on the toughness of bone at the ultrafine scale is conducted by developing a discrete lattice model appropriate for the ultrafine scale called the incremental continuous damage random fuse model (ICDRFM). Collagen viscoelasticity at ultrafine scale is shown to contribute significantly to the toughness of bone. The results obtained are important in the design of biomimetic ultratough artificial composites.

INFLUENCE OF RELATIVE STRENGTH OF CONSTITUENTS ON THE OVERALL STRENGTH AND TOUGHNESS OF BONE

Biocomposites such as bone exhibit synergistic superior mechanical properties compared to its constituents, protein (collagen) and mineral (hydroxyapatite). The importance of properties of constituents at the submicron scale with regard to the toughness and strength of bone is investigated employing a discrete lattice model. The results show that matrix failure as opposed to platelet breakage provides better toughness to the bone. There is a fairly sudden increase in the toughness of bone when the strength of mineral platelet to that of protein crosses a particular critical value. These could provide clues to the preparation of ultra-tough artificial composites and the treatment of diseases related to fragility of bone.

Influence of initial flaws on the mechanical properties of nacre

Nacre is a bio-composite made up of hard mineral and soft protein, and has excellent mechanical properties. This paper examines the effect of naturally occurring defects (initial flaws) in nacre on its mechanical properties such as toughness and strength. A random fuse model is developed incorporating initial flaws. Numerical simulations show that initial flaws affect different mechanical properties at different rates. The variation in the experimentally obtained mechanical properties of nacre reported in the literature is shown to be due to initial flaws. The stress in the mineral and protein increases due to initial flaws, but by different amounts. The results obtained in this study are useful for gaining insight into the failure of nacre and development of nacre-inspired composites.

Mechanical behaviour of bio-inspired brittle-matrix nanocomposites under different strain rates using molecular dynamics

We study the generic mechanical behaviour of ceramic–ceramic nanocomposites inspired from biological materials. The nanocomposite models considered in our study are the regularly and stairwise staggered arrangements of stiff brittle platelets embedded in compliant brittle matrix. Molecular dynamics simulations are carried out to investigate the effect of strain rate on these nanocomposites. The variation in stress–strain behaviour and mechanical properties are analysed. The evolution of deformation processes is also investigated. Our results show the existence of different strain rate regimes separated by critical strain rate. Deformation mechanisms such as matrix cracking, crack bridging, interfacial debonding and hence platelet pullout are observed at lower strain rates. Amorphous deformation and direct debonding without matrix cracking are observed at higher strain rates.

Bio-inspired composites with functionally graded platelets exhibit enhanced stiffness

Unidirectional composites inspired from biological materials such as nacre are composed of stiff platelets arranged in a staggered manner within a soft matrix. Elaborate analyses have been conducted on the aforementioned composites and they are found to have excellent mechanical properties like stiffness, strength and fracture toughness. The superior properties exhibited by these composites have been proved to be the result of its unique structure. An emerging development in the field of composite structures is functionally graded composites, whose properties vary spatially and possess enhanced thermo-mechanical properties. In this paper, the platelets are functionally graded with its Youngs modulus varying parabolically along the length. Two different models-namely, tension shear chain model and minimisation of complementary energy model have been employed to obtain the stiffness of the overall composite analytically. The effect of various parameters that define the composite model such as overlapping length between any two neighbouring platelets, different gradation parameters and platelet aspect ratio on the overall mechanical properties have been studied. Composites with functionally graded platelets are found to possess enhanced stiffness (upto [Formula: see text] higher) for certain values of these parameters. The obtained solutions have been validated using finite element analysis. Bio-inspired composites with functionally graded platelets can be engineered for structural applications, such as in automobile, aerospace and aircraft industries, where stiffness plays a crucial role.

Natural Optima in Human Skull: A Low-Velocity Impact Study

Abstract Human skull bone is a three-layered structure with a compliant viscoelastic cancellous bone layer sandwiched between elastic compact layers. The objective of the work was to check whether the skull bone is innately optimized with respect to the various geometrical and material parameters and to find out the parameters with which they are optimized. The effect of varying the thickness of layers and short time modulus of the interlayer are studied for optimality using finite element analysis of the skull subjected to low-velocity impact. Results show that they are optimized to meet different objectives.