V.H. Carneiro

Auxetic materials — A review

Auxetic materials are endowed with a behavior that contradicts common sense, when subjected to an axial tensile load they increase their transverse dimension. In case of a compression load, they reduce their transverse dimension. Consequently, these materials have a negative Poisson’s ratio in such direction. This paper reviews research related to these materials. It presents the theories that explain their deformation behavior and reveals the important role represented by the internal structure. Their mechanical properties are explored and some potential applications for these materials are shown.

Effect of grain and secondary phase morphologies in the mechanical and damping behavior of Al7075 alloys

The present study evaluates the role of the microstructure in the static and dynamic mechanical behavior of as-cast Al7075 alloy promoted by ultrasonic treatment (US) during solidification. The characterization of samples revealed that US treatment promoted grain and intermetallics refinement, changed the shape of the intermetallic phases (equilibrium phases of soluble M and/or T (Al, Cu, Mg, Zn) and their insoluble Al-Cu-Fe compounds) and lead to their uniform distribution along the grain boundaries. Consequently, the mechanical properties and damping capacity above critical strain values were enhanced by comparison with values obtained for castings produced without US vibration. This results suggest that the grain and secondary phases refinement by US can be a promising solution to process materials to obtain high damping and high strength characteristics.

Modeling and elastic simulation of auxetic magnesium stents

Auxetic materials are characterized by getting thiner/ larger in tension/compression. This counterintuitive behavior is advantageous in specific applications such as self-expandable stents. There are currently some stents that make use of this behavior, nevertheless there are still auxetic geometries that are not explored in this field. Additionally, Pure Magnesium is a promising material to manufacture bioabsorbable stents. This study presents the modelation of novel auxetic self-expanding stents based in Reentrant and Chiral geometries. They are simulated using Finite Element analysis to determine the presence of negative Poissons ratios and if they are a possible solution for further stent development. It is concluded that such modelations show low values of Poissons ratio and may be a viable possibility to obtain a new generation of self-expanding stents.

Comparison of the Poisson's Ratio of Simulated Rigid and Elastic Auxetic Models Using Kinematic and Finite Element Analysis

Materials that possess a negative Poissons ratio are called Auxetics. They are characterized by the counterintuitive behavior of expanding in tension and contracting in compression. To justify this deformation behavior, there have been developed theoretical modelations like the reentrant and bowtie models. However, the most generalized models are based on geometries that possess rigid trusses and hinging nodes. This does not portrait the reality of the application of these models. In this study, there have been performed simulations, using kinematic analysis to characterize the auxetic behavior of the theoretical rigid/hinging models and finite element analysis to characterize elastic models that represent real bodies. There were determined and compared the Poissons ratios of the theoretical and elastic reentrant and bowtie. Additionally it was shown that there is a significant difference between the results. In conclusion the theoretical models predict lower values of Poissons ratio, while the elastic models that simulate a real body show less auxetic behavior.

Magnesium alloy biodegradable scaffolds: Simulation of casting and manufacturing

Metallic scaffolds of magnesium alloys combine the properties of biodegradability and biocompatibility of the base material with the best of physical and mechanical properties of these structures, such as high energy absorption, controlled degradation, high permeability for gas and liquid and high strength-to-weight ratio. All these characteristics allow a wide application of these metallic structures in biomedical fields, for instance prostheses and even for the control of release, diagnosis and detection of drugs in the organism. Furthermore, once fabricated in open cells scaffolds, these structures allow the growth of new bone tissue and ease the transport of body fluids. A large number of processes are commonly used to manufacture these metallic structures, such as castings process, powder metallurgy, additive techniques, among others. The purpose of this work is the manufacturing of metallic scaffolds in magnesium alloys (AZ91D Eco) by vacuum investment casting process. Open cell models were obtained with additive manufacturing techniques which were used to produce the plaster moulds. The molten alloy was then poured, in a vacuum chamber (0–1 bar), to obtain the final scaffolds. Different experimental values of vacuum pressure and geometries were analyzed to study the filling capacity and compared with numerical casting simulation performed in NovaFlow&Solid software package. The results allowed to conclude that the increase of vacuum pressure promotes a better filling of the mould cavities and a better sanity of the final samples. In addition, the variation of the geometry had large influence in the filling behavior.

Mechanical behavior of honeycomb lattices manufactured by investment casting for scaffolding applications

Lightweight metallic lattices in the form of honeycombs are long known to exhibit a good mechanical strength/weight relation, given their geometry and relative density, in comparison with bulk materials. Due to the current developments in additive manufacturing techniques, the production of honeycombs by investment casting is now easier and may be a competitive route when compared to welding and gluing of sheet metal. This study explores the importance of the manufacturing design when producing honeycombs by investment casting. It is shown by numerical simulation and experimental procedures that mold filling in directions where horizontal ribs are present may induce defects such as interdendritic porosities. These defects have a relevant role in the elastic domain of the lattices, decreasing the apparent Young’s modulus and the plastic collapse stress. In terms of energy absorption, it is shown that these porosities have no significant effect due to the fragile fracture of both casting directions.