Materials and structures with negative Poisson's ratio exhibit a counter-intuitive behaviour. Under uniaxial compression (tension), these materials and structures contract (expand) transversely. The materials and structures that possess this feature are also termed as 'auxetics'. Many desirable properties resulting from this uncommon behaviour are reported. These superior properties offer auxetics broad potential applications in the fields of smart filters, sensors, medical devices and protective equipment. However, there are still challenging problems which impede a wider application of auxetic materials. This review paper mainly focuses on the relationships among structures, materials, properties and applications of auxetic metamaterials and structures. The previous works of auxetics are extensively reviewed, including different auxetic cellular models, naturally observed auxetic behaviour, different desirable properties of auxetics, and potential applications. In particular, metallic auxetic materials and a methodology for generating 3D metallic auxetic materials are reviewed in details. Although most of the literature mentions that auxetic materials possess superior properties, very few types of auxetic materials have been fabricated and implemented for practical applications. Here, the challenges and future work on the topic of auxetics are also presented to inspire prospective research work. This review article covers the most recent progress of auxetic metamaterials and auxetic structures. More importantly, several drawbacks of auxetics are also presented to caution researchers in the future study.

Auxetic metamaterials and structures: a review

The review outlines modern trends in theoretical developments, novel designs and modern applications of sandwich structures. The most recent work published at the time of writing of this review is considered, older sources are listed only on as-needed basis. The review begins with the discussion on the analytical models and methods of analysis of sandwich structures as well as representative problems utilizing or comparing these models. Novel designs of sandwich structures is further elucidated concentrating on miscellaneous cores, introduction of nanotubes and smart materials in the elements of a sandwich structure as well as using functionally graded designs. Examples of problems experienced by developers and designers of sandwich structures, including typical damage, response under miscellaneous loads, environmental effects and fire are considered. Sample applications of sandwich structures included in the review concentrate on aerospace, civil and marine engineering, electronics and biomedical areas. Finally, the authors suggest a list of areas where they envision a pressing need in further research.

Review of current trends in research and applications of sandwich structures

Bending behavior of sandwich composite structures with tunable 3D-printed core materials

Different from conventional materials, materials with negative Poisson's ratios expand laterally when stretched longitudinally. Known as ‘auxetic’ materials, the effect means they possess particularly fascinating properties, which have recently attracted considerable attention in the literature. A range of auxetic materials has been discovered, theoretically designed and fabricated. Developments in additive manufacturing (AM) techniques enable fabrication of materials with intricate cellular architectures. This paper outlines recent progress in the development of auxetic materials and structures, and their mechanical properties under quasi-static and dynamic loading are analysed and summarised. Limited experimental studies on 3D printed auxetic materials and structures are given more attention, ahead of extensively finite element (FE) simulations. A special focus is dedicated to their large, plastic deformation behaviour and energy absorption performance, which should be stressed in their engineering applications; no review paper has yet been found regarding this. Finally, this paper provides an overview of current study limitations, and some future research is envisaged in terms of auxetic materials and structures, nano-auxetics and additive manufacturing.

Large deformation and energy absorption of additively manufactured auxetic materials and structures: A review

Exploiting negative Poisson's ratio to design 3D-printed composites with enhanced mechanical properties

How does negative Poisson’s ratio of foam filler affect crashworthiness?

Composite materials has recently been widely used in the automotive, aerospace and other fields due to its light weight and good performance. At the same time, biological materials are also being paid more attentions in industrial application fields because of its economic, environmental, renewable and other advantages. Coconut mesocarp was chosen to be the core of composite sandwich structures with sheets of glass fiber reinforced plastics and carbon fiber reinforced composite were studied. Energy absorption behaviors were analyzed and compared. The thickness of all composite sheets is 1mm and the thickness of coconut mesocarp core is 20mm. Static compression tests were made by using universal testing machine. Results show that the composite sandwich structure with glass fiber reinforced plastic sheets and coconut mesocarp core has 1.0J/g of per weight energy absorption, which is higher than 0.91J/g of the composite sandwich structures with carbon fiber reinforced composite sheets and coconut mesocarp core. However, the peak crushing force of the former is less than the latter. Therefore, composite sandwich structures with glass fiber reinforced plastic sheets were selected for three points bending tests to study mechanical bending properties. By contrast with metal sheets sandwich structure and aluminum sheets sandwich structure, specific energy absorption of glass fiber reinforced plastic sheets sandwich structure is 2 times as much as metal sheets sandwich structure and 3–7 times as much as aluminum sheets sandwich structure, which shows an excellent crashworthiness performance of sandwich structures with composite sheets and bio coconut core.

Energy absorption characteristics of sandwich structures with composite sheets and bio coconut core

A potential natural energy absorption material – Coconut mesocarp: Part A: Experimental investigations on mechanical properties

Damping properties of plant fiber composites have been quite widely investigated based on macroscale techniques. Considering their heterogeneous and hierarchical microstructure, it is also necessary to study their damping behavior at the microscale to provide better insight into the dissipation origins and mechanisms. This article proposes a characterization of the dynamic mechanical properties of flax/GreenPoxy composites at microscale using dynamic grid nanoindentation technique. Two loading protocols are used: the classical Continuous Stiffness Measurement method (CSM) and a new method called Constant Amplitude Measurement method (CAM). Results show experimental protocols (CAM or CSM) do not have significant effect on storage modulus while they significantly affect the loss factor. The grid nanoindentation technique also provides an interesting map of the viscoelastic properties at the microscale in the cross-section of the composites and the damping contribution of the various components are highlighted. 

/pdf/in-situ-damping-identification-of-plant-fiber-composites-4xjvsp7x.pdf

Taiqu Liu

Papers

How does negative Poisson’s ratio of foam filler affect crashworthiness?

Energy absorption characteristics of sandwich structures with composite sheets and bio coconut core

A potential natural energy absorption material – Coconut mesocarp: Part A: Experimental investigations on mechanical properties

In situ damping identification of plant fiber composites using dynamic grid nanoindentation