Diab W. Abueidda

Thermo-Electro-Mechanical Properties of Interpenetrating Phase Composites with Periodic Architectured Reinforcements

In this study, the multifunctional properties (thermal, electric, and mechanical properties) of a new type of three-dimensional (3D) periodic architectured interpenetrating phase composites (IPCs) are investigated computationally. These new IPCs are created using two interconnected, bicontinuous, and intertwined material phases. The inner reinforcing phase takes the shape of the 3D morphology (architecture) of the mathematically-known triply periodic minimal surfaces (TPMS). The TPMS reinforcements are 3D solid sheet networks with a certain volume fraction and architecture. The interconnectivity of the proposed TPMS-based IPCs provide a novel way of creating multifunctional composites with superior properties. In this study, the effect of six well-known TPMS architectures of various volume fractions on the thermal/electrical conductivity and Young’s modulus of the IPCs is investigated using the finite element analysis of a unit cell with periodic boundary conditions. The contrast effect (high and low) between the conductivities and Young’s modulus of the two phases is also investigated. The calculated effective properties are compared with some analytical bounds. The proposed TPMS-IPCs possess effective properties close to the upper Hashin-Shtrikman bounds. It is also shown that the effect of TPMS architecture decreases as the contrast decreases. Finally, the manufacturability of these new TPMS-IPCs is demonstrated through using 3D printing technology.

The ultrastructure of bone and its relevance to mechanical properties

Bone is a biologically generated composite material comprised of two major structural components: crystals of apatite and collagen fibrils. Computational analysis of the mechanical properties of bone must make assumptions about the geometric and topological relationships between these components. Recent transmission electron microscope (TEM) studies of samples of bone prepared using ion milling methods have revealed important previously unrecognized features in the ultrastructure of bone. These studies show that most of the mineral in bone lies outside the fibrils and is organized into elongated plates 5 nanometers (nm) thick, ~ 80 nm wide and hundreds of nm long. These so-called mineral lamellae (MLs) are mosaics of single 5 nm-thick, 20 – 50 nm wide crystals bonded at their edges. MLs occur either stacked around the 50 nm-diameter collagen fibrils, or in parallel stacks of 5 or more MLs situated between fibrils. About 20% of mineral is in gap zones within the fibrils. MLs are apparently glued together into mechanically coherent stacks which break across the stack rather than delaminating. ML stacks should behave as cohesive units during bone deformation. Finite element computations of mechanical properties of bone show that the model including such features generates greater stiffness and strength than are obtained using conventional models in which most of the mineral, in the form of isolated crystals, is situated inside collagen fibrils.

Modeling of Stiffness and Strength of Bone at Nanoscale

Two distinct geometrical models of bone at the nanoscale (collagen fibril and mineral platelets) are analyzed computationally. In the first model (model I), minerals are periodically distributed in a staggered manner in a collagen matrix while in the second model (model II), minerals form continuous layers outside the collagen fibril. Elastic modulus and strength of bone at the nanoscale, represented by these two models under longitudinal tensile loading, are studied using a finite element (FE) software abaqus. The analysis employs a traction-separation law (cohesive surface modeling) at various interfaces in the models to account for interfacial delaminations. Plane stress, plane strain, and axisymmetric versions of the two models are considered. Model II is found to have a higher stiffness than model I for all cases. For strength, the two models alternate the superiority of performance depending on the inputs and assumptions used. For model II, the axisymmetric case gives higher results than the plane stress and plane strain cases while an opposite trend is observed for model I. For axisymmetric case, model II shows greater strength and stiffness compared to model I. The collagen-mineral arrangement of bone at nanoscale forms a basic building block of bone. Thus, knowledge of its mechanical properties is of high scientific and clinical interests.