Summary
A yield criterion for elastic pure-plastic polycrystalline materials is generated under simplified conditions by assuming that for yielding a certain fraction Q c of the total number of slip planes in the material has to be active. This fraction Q c is called the critical active quantity. We suppose Q c to be independent of the state of stress. The yield criterion is mathematically expressed as an integral, which is a function of Q c. This criterion can also be used for anisotropic materials.
For isotropic materials the ratio (r) of the yield stress in torsion to that in tension is calculated as a function of Q c. We find 0.5≤r≤0.61.
The value r=0.5 (Tresca's criterion) is obtained for Q c=0 and Q c=1. The value r=0.577 (von Mises criterion) is obtained for Q c=0.34 and Q c=0.79. The difference between two criteria with the same r is the magnitude of the yield stress. We think the value Q c=0.79 corresponds to the experiments for f.c.c. materials, since a rough estimation gives Q c>0.75 for yielding.
The independence of Q c on the state of stress brings on that r>0.5 is more probable. This is caused by the slower increase to Q c in torsion compared with the case of tension.
From the theory follows that in the general case (Q c≠0) the middle principal stress has influence on yielding.
In this paper we don't determine Q c, but adapt its value to the experimental results. However, a rough estimation of Q c is given for isotropic materials.
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Crans, W. A generating function for the yield criterion of isotropic and anisotropic polycrystalline materials. Appl. Sci. Res. 16, 256–272 (1966). https://doi.org/10.1007/BF00384072
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DOI: https://doi.org/10.1007/BF00384072