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Auxetic materials are those which exhibit negative Poisson’s ratio, i.e. these solids expand transversely when stretched longitudinally. In recent years the concept of semi-auxetics has been examined for cellular solids based on combination of re-entrant and hexagonal microstructures. In this paper we identify a type of rotating unit that gives positive Poisson’s ratio so that a study can be made on rotating sub-structures that exhibit both positive and negative Poisson’s ratio characteristics. A second type of rotating geometry, whose Poisson’s ratio shifts from negative to positive as stretching increases, has also been identified. Based on kinematical studies we explore the relationship between the on-axis Poisson’s ratios in terms of novel lattice geometry and the magnitude of deformation. [ABSTRACT FROM AUTHOR]

In 2003 Nature Materials article, Keckes et al. presented deformation properties of a variety of woods in relation to deformation of their individual wood cells. Their point is “The remarkable mechanical properties of biological materials reside in their complex hierarchical structure...”. This holds for mineral-based biological materials such as bone as well as for wood. Indeed, one of us (J.L.K.) introduced the concept that to explain the material properties of cortical bone, it was necessary to treat it as a complex material/structural hierarchical composite. Calculations to determine anisotropic properties of bone measured using ultrasonic wave propagation techniques, were extended to similar measurements on both soft and hard woods. These anisotropic properties calculations have been extended to include data based on mechanical measurements of orthotropic elastic constants of both soft and hard woods for comparison with both earlier ultrasonic measurements and mechanical testing on other woods. This work illustrates the fact that understanding and modeling the properties of wood is a complex task as the symmetry changes with scale. For example, lignin is isotropic, hemicellulose and cellulose are transversely isotropic, while the cells and microstructure have orthotropic symmetry. [ABSTRACT FROM AUTHOR]

Texture resulting from micromechanical processing plays an important role in the anisotropy of materials. The determination of texture components is a useful way to present texture data. The types of components present in the texture of a material can be related to other material aspects including predictions of yield loci using polycrystal methods. Al-Cu-Li 2195 thick plate was cold rolled to produce various reductions in thickness. Texture analysis was performed on the various rolled materials at different positions through the thickness of the plates. The texture components are consistent among the various rolled specimens at equivalent thickness positions. A texture gradient is observed to exist through the plate thickness that may indicate other microstructural information about the alloy. This texture gradient evidences the effects of increasing amounts of deformation on texture intensity, texture components observed, and changing modes of deformation. [ABSTRACT FROM AUTHOR]

The effect of stacking fault energy (SFE) on the evolution of microstructures during room temperature tensile testing has been investigated at two strain rates of 8.3×10−4 and 1.7×10−1/s in pure copper, Cu-2.2%Al, and Cu-4.5%Al alloys with SFE values of, approximately, 78, 20 and 4 mJ/m2, respectively. The mechanism of deformation changes from simple slip leading to cell formation in the high SFE metal, Cu, to overlapping and/or intersecting deformation twins in low SFE alloy, Cu-4.5%Al. The effect of strain rate is such that it results in rather poorly defined cell boundaries in copper, with a smaller cell size at higher strain rates for similar grain sizes and strain values. The alloys deform by twinning and the propensity of deformation twins increases with both a decrease in SFE value and increase in strain rates. [ABSTRACT FROM AUTHOR]