Andreas Öchsner

Numerical investigation of composite materials reinforced with waved carbon nanotubes

This paper studies the extent to which the effective stiffness of composite materials can be impacted by the characteristic waviness of nanotubes embedded in polymers. A three-dimensional finite element model is used to investigate the effect of volume fraction and waviness on mechanical properties, i.e. Young’s modulus and Poisson’s ratio, of composites reinforced with waved carbon nanotubes. According to the obtained results, the nanotube waviness causes a decrease in the longitudinal and transverse Young’s modulus of composites, compared to the straight nanotube reinforcement, but the change in the value of transverse Young’s modulus (Eyy) is less remarkable than the longitudinal Young’s modulus (Exx). Furthermore, the effect of fiber curvature on Poisson’s ratio has also been studied. The results show that the curvature has not much effect on Poisson’s ratio and when a fiber curvature changes, Poisson’s ratio value almost remains unchanged. In addition, the effect of fiber volume fraction on the longitudinal Young’s modulus and major Poisson’s ratio has been studied. As the mesh density may have a significant role in evaluating the model, several different meshes have been generated in order to predict their effect on the mechanical properties of the composite.

Influence of the Size and Amount of Cork Particles on the Impact Toughness of a Structural Adhesive

The inclusion of particles (nano or micro) is a method to improve the mechanical properties, such as toughness, of structural adhesives. Structural adhesives are known for their high strength and stiffness but also for their low ductility and toughness. There are many processes described in the literature to increase the toughness, one of the most common being the use of rubber particles. In the present study, natural micro particles of cork were used with the objective to increase the impact resistance of a brittle epoxy adhesive. The idea is for the cork particles to act like crack stoppers and absorb impact leading to higher absorption of energy. The influence of the cork particle size and amount were studied. Particles of cork ranging from 38 to 250 µm were mixed in the epoxy adhesive Araldite® 2020 from Huntsman. The amount of cork in the adhesive was varied between 1 and 5% by weight. Surface treatment (low pressure plasma) was applied to the cork powder to assess the effect of the interaction adhesive-cork with several degrees of adhesion. This evaluation was made using impact tests and it was evident that impact absorption was related to the size and amount of cork particles in the resin, considering a uniform particle distribution.

Permeability studies of artificial and natural cancellous bone structures

In the development of artificial cancellous bones, two major factors need to be considered: the integrity of the overall structure and its permeability. Whilst there have been many studies analysing the mechanical properties of artificial and natural cancellous bones, permeability studies, especially those using numerical simulation, are scarce. In this study, idealised cancellous bones were simulated from the morphological indices of natural cancellous bone. Three different orientations were also simulated to compare the anisotropic nature of the structure. Computational fluid dynamics methods were used to analyse fluid flow through the cancellous structures. A constant mass flow rate was used to determine the intrinsic permeability of the virtual specimens. The results showed similar permeability of the prismatic plate-and-rod model to the natural cancellous bone. The tetrakaidecahedral rod model had the highest permeability under simulated blood flow conditions, but the plate counterpart had the lowest. Analyses on the anisotropy of the virtual specimens showed the highest permeability for the horizontal orientation. Linear relationships were found between permeability and the two physical properties, porosity and bone surface area.

Finite element analysis of idealised unit cell cancellous structure based on morphological indices of cancellous bone

Human bones can be categorised into one of two types—the compact cortical and the porous cancellous. Whilst the cortical is a solid structure macroscopically, the structure of cancellous bone is highly complex with plate-like and strut-like structures of various sizes and shapes depending on the anatomical site. Reconstructing the actual structure of cancellous bone for defect filling is highly unfeasible. However, the complex structure can be simplified into an idealised structure with similar properties. In this study, two idealised architectures were developed based on morphological indices of cancellous bone: the tetrakaidecahedral and the prismatic. The two architectures were further subdivided into two types of microstructure, the first consists of struts only and the second consists of a combination of plates and struts. The microstructures were transformed into finite element models and displacement boundary condition was applied to all four idealised cancellous models with periodic boundary conditions. Eight unit cells extracted from the actual cancellous bone obtained from micro-computed tomography were also analysed with the same boundary conditions. Young’s modulus values were calculated and comparison was made between the idealised and real cancellous structures. Results showed that all models with a combination of plates and struts have higher rigidity compared to the one with struts only. Values of Young’s modulus from eight unit cells of cancellous bone varied from 42 to 479xa0MPa with an average of 234xa0MPa. The prismatic architecture with plates and rods closely resemble the average stiffness of a unit cell of cancellous bone.

Influence of the Morphology of Joining on the Heat Transfer Properties of Periodic Metal Hollow Sphere Structures

Hollow sphere structures (HSS) constitute a group of innovative materials which are characterised by more constant material properties compared to classical cellular metals [1]. Their big potential lies within multifunctional applications where combinations of their proper- ties yield symbiotic advantages. In the scope of this paper their effective thermal conductivity is investigated. In addition to the analysis of the dependency of this material parameter on the conductivities of the base materials and the sphere wall thickness, special focus is given to the influence of the morphology of joining.

Mechanical and microarchitectural analyses of cancellous bone through experiment and computer simulation

The relationship between microarchitecture to the failure mechanism and mechanical properties can be assessed through experimental and computational methods. In this study, both methods were utilised using bovine cadavers. Twenty four samples of cancellous bone were extracted from fresh bovine and the samples were cleaned from excessive marrow. Uniaxial compression testing was performed with displacement control. After mechanical testing, each specimen was ashed in a furnace. Four of the samples were exemplarily scanned using micro-computed tomography (μCT) and three dimensional models of the cancellous bones were reconstructed for finite element simulation. The mechanical properties and the failure modes obtained from numerical simulations were then compared to the experiments. Correlations between microarchitectural parameters to the mechanical properties and failure modes were then made. The Young’s modulus correlates well with the bone volume fraction with R2xa0=xa00.615 and P value 0.013. Three different types of failure modes of cancellous bone were observed: oblique fracture (21.7%), perpendicular global fracture (47.8%), and scattered localised fracture (30.4%). However, no correlations were found between the failure modes to the morphological parameters. The percentage of error between computer predictions and the actual experimental test was from 6 to 12%. These mechanical properties and information on failure modes can be used for the development of synthetic cancellous bone.

Uniaxial elasto-plastic behaviour of adhesively bonded hollow sphere structures (HSS): Numerical simulations and experiments

This paper is on the investigation of adhesively bonded metallic hollow sphere structures. Two different approaches, namely experimental analysis and finite element cal- culations are applied and the findings of both attempts are compared. In the scope of the numerical approach the influence of the mechanical properties of the adhesive on the me- chanical response of the structure is analysed. Based on these results, suggestions for design parameters are derived.

Effect of the amount of cork particles on the strength and glass transition temperature of a structural adhesive

The inclusion of particles (micro or nano) is a method to improve the mechanical properties, such as toughness, of structural adhesives. Structural adhesives are known for their high strength and stiffness but also for their low ductility and toughness. There are many processes described in the literature to increase the toughness, the use of rubber particles being one of the most common processes. In the present study, natural micro particles of cork were used with the objective to increase the ductility of a brittle epoxy adhesive. The idea is for the cork particles to act like a crack stopper. The influence of the amount of cork particles was studied. Particles of cork ranging in size from 125 to 250 µm were mixed in the epoxy adhesive Araldite 2020 from Huntsman. The amount of cork in the adhesive was varied between 0.5% and 5% in weight. This evaluation was made using tensile tests and it was evident that the failure strain was related to the amount of cork particles in the resin. The results concerning the single lap joints and the glass transition temperature confirm the increased ductility obtained in the tensile tests.

Calculations of the effective thermal conductivity in a model of syntactic metallic hollow sphere structures using a lattice monte carlo method

In this paper, a Lattice Monte Carlo method is used to determine the effective thermal conductivity in two dimensional models of adhesively bonded metallic hollow sphere structures (MHSS). In contrast to earlier approaches, more realistic distributions of spheres without the simplification of cubic symmetric arrangements are considered in this study. For the Monte Carlo analyses, two-dimensional periodic lattices representing different cutting planes through MHSS are generated. Therefore, an algorithm is used which sequentially fills the lattice by adding cut spherical shells and inclusions in the matrix. Another focus of this work is the analysis of the influence of different geometric circle distributions on the effective thermal conductivity. The findings of the random arrangements are also compared to a regular primitive cubic arrangement and with a Maxwell-type approach.

Lattice Monte Carlo Analysis of Thermal Diffusion in Multi-Phase Materials

This Chapter addresses the numerical simulation of thermal diffusion in multi-phase materials. A Lattice Monte Carlo method is used in the analysis of two- and three-dimensional calculation models. The composites considered are assembled by two or three phases, each exhibiting different thermal conductivities. First, a random distribution of phases is considered and the dependence of the effective thermal conductivity on the phase composition is investigated. The second part of this analysis uses a random-growth algorithm that simulates the influence of surface energy on the formation of composite materials. The effective thermal conductivity of these structures is investigated and compared to random structures. The final part of the Chapter addresses percolation analyses. It is shown that the simulation of surface energy distinctly affects the percolation behavior and therefore the thermal properties of composite materials.