The shear-induced solid-liquid transition in yield stress materials with chemically different structures

The shear-induced solid-liquid transition of viscoplastic materials has been studied extensively through various steady-shearing experiments including steady stress sweep, stress ramp, creep and strain recovery. The results are consistent in showing a clear change in behaviour of the materials from solid-like to liquid-like. A transition stress plateau separates the regions of solid behaviour and liquid behaviour. The slope of the plateau reflects the uniformity of the structure, and hence the distribution of bonding strength within this structure. Depending on the structure of the material, the yielding process of viscoplastic materials can occur over a wide or a narrow range of stress, which represents “ductile-type” or “brittle-type” failure. Altering the concentration and extent of particle flocculation (for suspensions) or polymer chain entanglements (for polymer gels) can vary the bonding strength and strength distribution, and therefore change the slope of the stress plateau. The continuous solid structure exhibits creep at stresses well below the yield stress and fails at a critical strain. The yielding of the material seems to be characterised by a critical strain rather than by a critical stress or a critical shear rate. The deformation of viscoplastic materials can be recovered fully or partially in the solid-like region once the stress is removed. This is a significant difference from the behaviour of purely viscoelastic materials. Strain recovery tests result in two characteristic strain values that can be used to define the two commonly used “yield stress” values, the higher one of which is in good agreement with the traditional value of yield stress as measured by vane torsion, for example.