stress-strain

An interesting feature in plasticity is the increase /decrease of axial strain with shear stress (for finite deformation). The increase/ decrease is more (even for small deformation) in case of non- proportional loading i.e, when axial stress is not zero.?
I am seeking the explanation for this unusual behavior in terms of micro-mechanics? and also if anybody has noticed this behavior. kindly illustrate?
Thanks A.

Amit Pandey wrote:
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I'm not sure I understand your question as stated (IIRC) - I saw some modeling done using FEM on floc structures to predict elastic constants I'll have to dig up some old references. The connectivity - Fractal dimension and inter particle attraction.... will determine when the material starts to flow under applied stress. As shear rate increases- so does shear stress - the shear stress will break up more flocs and decrease the apparent viscosity. You may want to look through the literature on gels
Having stated the above - that model does not even take into account the deformation of the double layer (for aqueous systems of particles) or more involved scenarios involving polymer chains.
Clay is plastic because it's crystal structure enables it to bind water to the surface. This layer of water keeps the particles separated enough so they can be sheared apart. Clay is also shaped like flat hexagonal plates with the edges and surfaces having different charges. Under most pH conditions, the edges are attracted to an adjacent particle face - this produces an open floc structure which is easily sheared apart. giving plastic behavior.
Gregg