Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - TORWIRE (Mechanical Behavior of Microscale Metallic Wires under Torsional and Tensile Loadings at Elevated Temperatures)

Teaser

\"The on-going miniaturisation in many modern technological areas (e.g. micro/nanoelectronics, integrated circuits, micro-electrical-mechanical systems) requires the knowledge of mechanical properties at small scales. Metals are strong because of their microstructure. At small...

Summary

\"The on-going miniaturisation in many modern technological areas (e.g. micro/nanoelectronics, integrated circuits, micro-electrical-mechanical systems) requires the knowledge of mechanical properties at small scales. Metals are strong because of their microstructure. At small scales, constraints increase the stress at which plastic deformation can occur; these constraints can include physical boundaries, free surfaces, grain boundaries, or strain gradients. Since the geometric and microstructural dimensions of the components can range from microns to nanometers, the constraints of the dimensions on dislocation activities and the effects of surfaces and interfaces in the small-scale components result in some anomalous plastic properties, including the size effects, with smaller being stronger, plastic recovery, plastic recovery, with reverse plastic flows already occurring even when the stress is still under unloading, etc.
The outputs of this programme have a positive interaction with the EU 3-year project \"\"StrengthABLE\"\" funded by European Metrology Programme for Innovation and Research. Here, we focus on determining the length-scale dependence of strength and designing rules for materials strengthened by ‘length-scale engineering’. This project finally produced a novel micro-torsion instrument, which has been used for characterizing the mechanical properties of various fibrous materials, for example, Cu wire, spider silk etc. The experimental technique therefore has potential commercial value in the fields of textile, fibrous composite and medical devices, etc. Through this project, we contributed to the development of torsion tester at small scales. A long term goal is to exploit the size effect in novel high-strength, lightweight materials. The experimental programme, together with the theoretical interpretations on interpreting the data, is expected to lead to major advances in metallurgical engineering, with potentially trillion-dollar impact on a public good.
The key aim of this project is to develop a physical understanding of plasticity mechanisms operating in polycrystalline and single-crystalline solids, when material dimensions are reduced to the micrometer scale (or to the size of their instrinsic microstructure) subjected to nonuniform deformation at elevated temperatures. Special focus will be on the influence of microstructural properties (e.g. grain size, specimen dimension and texture) and loading conditions (e.g. with and without strain gradient, monotonic and cyclic torsion) on the mechanical behaviours of thin wires at different temperature. Torsion experiments for different specimen sizes and different materials have been conducted. The experimental results are used to be compared with the theoretical predictions. The results will give a deeper understanding of micro-scale mechanics.
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Work performed

During the performance of the project, five main works have been done. They are:
(1) Two micro-torsion testers based on different principle have been developed.
(2) The torsional properties of single fibers/wires, e.g. copper wires, carbon fiber, human hair, spider silk, etc. have been studied by using the torsion testers.
(3) The physical basis and the magnitude of the material length scale in theories of strain gradient plasticity are elucidated based on the critical thickness theory.
(4) The critical thickness phenomenon occurred in single-crystal wires under torsion is predicted by the continuum dislocation theory quantitatively. This links the continuum dislocation theory to the underlying physical picture of Matthews’ critical thickness theory.
(5) The physical nature of the flow rule for strain gradient plasticity theory proposed by Nix and Gao is discussed based on the thermodynamics paradigm developed by Gurtin and Anand. It is shown that the Nix-Gao flow rule is a combination of constitutive laws for the microstresses, balance law and a constraint.
As a leading author or corresponding author, the fellow has published seven peer-reviewed papers in the esteemed journals including International Journal of Plasticity, Acta Materialia, APL, etc. There are also three other papers in preparation. Most of these papers are strongly related to small-scale plasticity, and to torsional properties of thin wires.
Especially, the work on the torsional properties of spider dragline silk published on Appl. Phys. Lett. (111, 013701) was featured as a front-cover paper. In addition, Prof. D. J. Dunstan and the fellow were invited by the Editor of “The Conversation”, a respectable website that encourages working scientists to write popular articles, to write an article for the general public about our work on spider silk. Finally, collaborating with Prof. Markus J. Buehler of MIT, we finished one paper entitled “Spider dragline silk as torsional motor driven by humidity” that has been submitted to Nature Communications.

Final results

Metals are strong because of their microstructure. At small scales, constraints increase the stress at which plastic deformation can occur; these constraints can include physical boundaries, free surfaces, grain boundaries, or gradients of strain. Plasticity is the irreversible deviation from Hooke’s Law, which describes elastic behaviour. Torsion of small specimens such as thin wire is the most sensitive way of studying the early stages of plasticity, crucial to understanding failure by creep and metal fatigue. We have taken torsion experiments to new levels of sensitivity and a wider range of applications. First, by increasing the specimen length up to fifty metres, the torsion of thin wires can be studied at nanostrain – strain of one part in a thousand million. Second, inspired by the idea of critical thickness theory, an extended theory of small-scale plasticity for accounting for size effect is developed. Thirdly, a physical understanding of plasticity mechanisms operating in polycrystalline and single-crystalline solids is achieved, when material dimensions are reduced to the micrometer scale subjected to nonuniform plastic deformation.
An automated torsion equipment has be built by combing the load/unload method and the image processing method, capable of carrying out both of the two principal kinds of experiments. This equipment gives a much larger body of much more detailed data suitable for testing between the various theories for metals. Studies have also been made of new materials such as spider silks. New phenomena have already reported in spider silk. The work on the torsional properties of spider dragline silk published on Appl. Phys. Lett. (111, 013701) was featured as a front-cover paper. It has aroused remarkable media interest, with interviews on the BBC World Service, and pieces in the Science, Daily Mail, Physics word, Physics Today, AIP, and many others around the world. In addition, Prof. D. J. Dunstan and the fellow were invited by the Editor of “The Conversation” to write an article for the general public about our work on spider silk. We finally finished a report entitled “Why abseiling spiders don’t spin out of control-new research”, which attracted a lot of attention from the public.

Website & more info

More info: https://dreamerdabiao.wixsite.com/mechanics/torwire.