Xiaonan Hou

A composite material with Poisson’s ratio tunable from positive to negative values: an experimental and numerical study

The Poisson’s ratio describes an extent of transverse deformation of a material when an axial strain is applied. A change of the Poisson’s ratio from positive to negative can equip a material with a set of specific properties. In this paper, a study of a composite material with a tunable Poisson’s ratio is presented. Samples of such a composite were first fabricated with two different polymers based on a composite structure proposed in our previous work using a multi-material additive-manufacturing system. Using both experimental and numerical methods, deformation mechanisms and mechanical properties of this composite material were analyzed. The obtained results demonstrate that its Poisson’s ratio can be reduced by increasing the difference in stiffness of constituent materials and turned from positive to negative when this difference is sufficiently high. Additionally, the study also validates a possible method to fabricate composites with designed structures and multi-constituent materials using additive-manufacturing techniques.

A study of computational mechanics of 3D spacer fabric: factors affecting its compression deformation

Mechanical properties of a 3D spacer fabric are determined both by mechanical properties of its components and its unique structure. However, at present, there is no well-received method to describe the mechanical performance of a 3D spacer fabric. In this article, numerical methods are used to analyze a compression mechanism of a typical 3D spacer fabric, as an example, based on features of its structure. Micro X-ray computed tomography was used to determine a precise geometry model of the studied fabric. To give a deeper understanding of the effects of the fabric’s structure on its mechanical properties, the finite element (FE) method was used to simulate the compression process of the fabric; seven FE models were developed to simulate its compression behavior. Based on results obtained with these models, the compression mechanism of the 3D spacer fabric is summarized and the factors affecting it are determined. The same method of analysis can be extended to other studies of 3D fabrics.

Metamaterials with Negative Poisson’s Ratio: A Review of Mechanical Properties and Deformation Mechanisms

Compared to conventional materials, materials with a negative Poisson’s ratio are endowed with many specific mechanical features; consequently, there are many potential applications for them. For the last two decades, many efforts have been made on this sort of metamaterial both experimentally and theoretically. This paper provides a brief review of those studies with a focus on mechanical properties and deformation mechanisms of the metamaterials. The latter are explained using a structure of a multi-phase metamaterial system for a more comprehensive understanding and as an inspiration for future works. Additionally, respective manufacturing methods and applications are also summarised.

Tailoring structure of inclusion with strain-induced closure to reduce Poisson's ratio of composite materials

A novel 3D continuum shell structure is introduced as inclusion for composite materials with special mechanical properties in this paper. Its geometry is based on a hollow re-entrant tetrahedron. In a composite, such an inclusion can demonstrate a closure effect induced by external compression. Its specific deformation mechanism results in a special character of deformation and affects effective (global) mechanical properties of the composite. A finite-element method is used to explore quantitatively and qualitatively the deformation mechanism of the suggested inclusion and its effect on the overall mechanical performance of the composite. In this study, geometrical features of the inclusion are used as parameters. The obtained results demonstrate that this kind of inclusion could reduce the composites Poissons ratio; moreover, its magnitude is adjustable by changing geometrical parameters of the inclusion. Besides, an overall hardening effect is achieved for the composite, with the magnitude of global stiffness also significantly affected by geometrical features of the inclusion. Thus, the developed inclusion actually provides a potential to develop new composites with a tunable Poissons ratio and enhanced mechanical properties.

Numerical Modelling of Thermally Bonded Nonwovens: Continuous and Discontinuous Approaches

Nonwoven fabrics are web structures of randomly-oriented fibres, bonded by means of mechanical, thermal or chemical techniques. This paper focuses on nonwovens manufactured with polymer-based fibres and bonded thermally. During thermal bonding of such fibres, as a hot calender with an engraved pattern contacts the fibre web, bond spots are formed by melting of the polymer material. As a result of this bonding process, a pattern of bond points connected with randomly oriented polymer-based fibres form the nonwoven web. Due to their manufacturing-induced composite microstructure and random orientation of fibres, nonwovens demonstrate a complex mechanical behaviour. Two distinct modelling approaches were introduced to simulate the non-trivial mechanical response of thermally bonded nonwovens based on their planar density. The first modelling approach was developed to simulate the mechanical behaviour of high-density nonwovens, and the respective fabric was modelled with shell elements with thicknesses identical to those of the bond points and the fibre matrix having distinct anisotropic mechanical properties. Random orientation of individual fibres was introduced into the model in terms of the orientation distribution function in order to determine the material’s anisotropy. The second modelling approach was introduced to simulate low-density nonwovens, and it treated the nonwoven media as a structure composed of fibres acting as truss links between bond points.