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Toward a further understanding of size effects in the torsion of thin metal wires: An experimental and theoretical assessment

dabiao liu's picture

Recently published in International Journal of Plasticity:

http://www.sciencedirect.com/science/article/pii/S0749641912001234?v=s5

 http://dx.doi.org/10.1016/j.ijplas.2012.08.007

As you know, there is a great deal of interest
and excitement recently in understanding the mechanism(s) of size effects in
the plastic deformation of crystalline materials. In this paper, we have
developed a new torsion balance technique to obtain the torque-twist data of
micron-scale metal wires straightforwardly. Especially, an in-situ torsional
vibration method for calibrating the torque meter with precision is addressed. Our
paper provides a direct experimental observation that a significant size
effect in both the initial yielding and the plastic flow is present in torsion,
whereas only a minor effect in tension. The data here are abundant and
reproducible, and they are the direct complements to the
measurements of Fleck et al. (Fleck, N.A., Muller, G.M.,
Ashby, M.F., Hutchinson, J.W., 1994. Strain gradient plasticity: theory and
experiment. Acta Met. Mater. 42, 475-487.) In particular, the physical
basis of the size effects in wire torsion is elucidated in the light of the
geometrically necessary dislocation argument and the critical thickness effect.
Moreover, three phenomenological theories of strain gradient plasticity are
assessed within the context of wire torsion, and the corresponding rigid-plastic
solutions are derived. Distinctions between the theories are highlighted
through comparison with experiment, emphasizing the difference in predicted
trends in the size dependence of initial yielding and of hardening rate. The systematic experimental and theoretical assessment suggests that the size effect in the initial yielding is mainly due to the constraints that the external geometrical size put on a finite strained volume, while the size dependence in the plastic flow is principally owing to the geometrically necessary dislocations associated with the plastic strain gradients. These
findings may have implications in the basic mechanism for the size effects of
crystalline materials under un-uniform plastic deformation. We believe that it
is a comprehensive study on the size effects in the torsion of thin metal wires
both experimentally and theoretically.

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v.tagarielli's picture

Vito Tagarielli, PhD

Department of Aeronautics

Imperial College London

v.tagarielli's picture

Vito Tagarielli, PhD

Department of Aeronautics

Imperial College London

dabiao liu's picture

Thanks very much for your comments. I hope this work can bring you several inspirations. Thanks.

Dabiao Liu (Ph.D. Candidate)

Department of Mechanics

Huazhong University of Science &
Technology

Wuhan P.R.China, 430074

dreamer.dabiao@163.com

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