Abstract

The Bauschinger effect (BE), i.e. asymmetric plastic flow, and partial recoverable plasticity were observed in both copper and gold polycrystalline wires under cyclic torsion tests. These experiments provide a direct evidence for a class of strain gradient plasticity theories involving both energetic and dissipative length scales. Systematic experimental observations imply that the geometrically necessary dislocations (GNDs) induced by the plastic strain gradients give rise not only to the size effect, but also to the Bauschinger effect and plastic strain recovery at small scales. The back stress generated from the long-range interaction of GNDs plays an important role in the asymmetrical plastic flow. The observed anomalous plasticity in cyclic torsion is interpreted in the light of a strain gradient plasticity involving both energetic and dissipative length scales. The corresponding papers can been seen in: http://www.sciencedirect.com/science/article/pii/S0921509315303117

Keywords

• Torsion test;
• Strain gradient plasticity;
• Geometrically necessary dislocations;
• Bauschinger effect;
• Plastic recovery

A detailed paper on the experimental aspects can be seen at: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.110.244301

Recently, I am reading several classic works on crystal plasticity. But, it seems to be difficult for me to obtain several very old papers in this area. For example, one of the classic papers on Hall-Petch relation,

Petch, N.J., The cleavage strength of polycrystals. The Iron and Steel Institute, 1953, 174: 25-28. I have tried many ways to get the original paper, but I finally failed. Could someone please send me a copy of this paper? Thanks a lot. Here is my email: dbliu2009@gmail.com

Our new paper entitled "Anomalous
plasticity in the cyclic torsion of micron scale metallic wires" has been
accepted by PRL

(see: http://link.aps.org/doi/10.1103/PhysRevLett.110.244301)

The plasticity of micron-scale Cu and Au wires
under cyclic torsion is investigated for the first time using a torsion balance
technique. In addition to a size effect, a distinct Bauschinger effect and an
anomalous plastic recovery, wherein reverse plasticity even occurs upon unloading,
are unambiguously revealed. The Bauschinger effect and plastic recovery have
been observed in molecular dynamics and discrete dislocation dynamics
simulations of ideal single-crystal wires; the results here are an excellent
confirmation that these effects also occur in experiment in non-ideal polycrystalline
wires. A physical model consistent with the simulations is described in which
the geometrically necessary dislocations induced by the non-uniform deformation
in torsion play the key role in these anomalous plastic behaviors.

Firstly, this study has revealed, for the
first time, a series of anomalous plastic phenomena in the cyclic torsion of
micron-scale Cu and Au wires, including the size effect, distinct Bauschinger
effect, recoverable plasticity, different hardening behaviors between two
metals, etc. Each phenomenon may attract many attention. Secondly, these new
experiments provide an excellent confirmation of the previous molecular
dynamics and discrete dislocation dynamics simulations. These findings may have
significant implications in the basic mechanism for the cyclic plasticity at
small scales. Thirdly, these observations are physically explained by using the
dislocation theory. Especially, a physical model for the plastic recovery is
proposed. Two important conclusions are implied: (i) the geometrically
necessary dislocations contribute to the size effect in non-uniform
deformation, (ii) the geometrically necessary dislocations are another physical
origin of the Bauschinger effect and plastic recovery at small scales.

Liu, D., He, Y., Zhang, B., 2013. Towards a further understanding of dislocation pileups in the presence of stress gradients. Doi: 10.1080/14786435.2013.774096

http://www.tandfonline.com/doi/abs/10.1080/14786435.2013.774096#preview

This study is an essential complement and extension to the stress-gradient concept recently proposed by Hirth. An analytic method is presented for studying the behaviour of double-ended dislocation pileup in the presence of various stress gradients by solving a singular integral equation based on the continuous approximation of dislocations. Four special cases of double-ended pileup in the presence of stress gradients are discussed in detail. The corresponding dislocation distribution, the length of pileup, the total number of dislocations within the pileup and the force on the leading dislocations at the pileup ends are derived, respectively. It is shown that both the number of dislocations and the force on the leading dislocation in a pileup are sensitive to the relative magnitude of stress near the dislocation source and both are less than that in constant stress case. Of particular importance, it is indicated that the small-scaled materials subjected to a stress involving a gradient would be stronger than that under a constant stress. Applied to wire torsion and foil bending, the stress gradient model predicts an increase in the initial yielding, which is in reasonable agreement with the recent experimental data. The proposed stress gradient concept may provide a new physical insight into the size-dependent plasticity phenomena at small length scale.

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.

Our new paper entitled "Size Effects in the Torsion of Microscale Copper Wires: Experiment and Analysis", which has been accepted for publication in Scripta Materialia. As you know, there is a great deal of interest and excitement recently in understanding the mechanisms of size-scale effects in the plastic deformation of crystalline materials. Our manuscript provides the direct experimental evidence that strain gradient is still considerable in the torsion of microscale polycrystalline wires, wiping off the influence of external geometrical size and grain size. In our tests, we find significant size effects in both the plastic flow stress and initial yielding in torsion, while only slight size effects present in tension. The experimental result here is the first for complement to the tests performed by Fleck et al. (N.A. Fleck et al., Acta Metall. Mater. 42, 475 (1994). Cited times: more than 1200 times) to date. Our micro-torsion tests have higher resolution in the low strain region. Moreover, we have developed, for the first time, an automated micro-torsion test apparatus based on the torsion balance principle for obtaining the torque-twist data of microscale metallic wires straightforward. The experimental results are abundant, accurate and reproducible, which are also in excellent agreement with a theory of strain gradient plasticity. These findings may have implications in the basic mechanism for size-scale effects of crystalline materials in plasticity under un-uniform deformation.

You can download the paper from the link below:

http://www.sciencedirect.com/science/article/pii/S135964621100741X

We are pleased to see your comments. And you also can contact with me through Email: l_dabiao@hotmail.com.

who can tell me more about the critical thickness theory ?I will be very excited if you can introduce some reference to me.I know that critical thickness theory is solved rigorously and ursed to validated a useful approximation which is combined with slip distance theory modified for a finite structure size . (physical life review letters :elastic limit and strain hardening of thin wires in torsion)