Critical shear stress

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The critical shear stress is a value of the shear stress , which usually indicates when a mass starts to move or when the plastic deformation of a malleable body begins.

Erosion in rivers

In connection with fluvial erosion , the critical shear stress characterizes the critical state in a flowing water at which the erosion of the soil material begins at the bottom of the water . In this state, the flow force of the flowing water begins to exceed the resistance of the sole material, so that the ground material is set in motion. If, for example, sand forms the bed of the river, the critical shear stress depends primarily on the grain size, but also on the shape of the sand grains and how much water is stored in the sand mass. Typical values ​​are approx. 2 N / m² for fine quartz sand (grain size 0.4 mm) and approx. 6 N / m² for coarse sand.

Flow behavior of viscous materials

Wall slide

When transporting certain highly viscous or viscoelastic materials in pipelines, it is of interest at which force relationships the material no longer adheres to the pipe (so-called wall sticking), but also begins to flow on the wall (so-called wall sliding). In the case of polymer melts , the critical shear stress at which the wall sliding begins depends, among other things, on the surface density of the attached molecules, the length of a monomer unit and the number of monomers involved in the temporarily formed entanglements in the molecular chains.

Change of flow behavior

According to a more general understanding, the critical shear stress indicates when the flow behavior of a highly viscous or viscoelastic substance changes in a characteristic way. The beginning of sliding on the wall is only one possibility. The critical shear stress can, for example, also characterize the transition between linear viscoelastic and nonlinear viscoelastic flow behavior.

Pressure oscillation

The specification of a critical shear stress as a limit value does not have to refer to when the material begins to flow or begins to flow in another way. The “critical shear stress” can also be defined as the value of the shear stress from which the pressure in the flowing mass begins to fluctuate (“oscillating pressure profile”). The critical shear stress for the start of the pressure oscillation is not only investigated during the extrusion of technical polymers , it was also determined, for example, for mozzarella cheese in a temperature range of 55–65 ° C. As the fat content increases, the critical shear stress that marks the beginning of the pressure oscillation in the flowing cheese mass decreases.

Deformation of metals

In crystal mechanics , the critical shear stress is the shear stress at which the transition from elastic to plastic deformation of metals takes place. If the critical shear stress in a lattice structure is exceeded, the atoms of a slip system consisting of slip plane and slip direction get into the field of action of the neighboring lattice plane and thus move abruptly (so-called dislocation movement ) to the next lattice location. They keep this place even after the tension is removed - the deformation is thus plastic.

Individual evidence

  1. ↑ Critical shear stress in river erosion geo.fu-berlin.de
  2. ↑ Critical shear stress bauformeln.de (typical values ​​for solids transport in water)
  3. ↑ Armin Merten: Investigations on flow instabilities in the extrusion of polymers with laser Doppler anemometry (PDF), dissertation at the University of Erlangen-Nuremberg, 2005, p. 22 f.
  4. ↑ See capillary rheometer for plastics processing: Simulation of rheological processes (PDF), KGK Kautschuk Gummi Kunststoffe, No. 9/2000, pp. 512–517.
  5. ↑ Bernhard Hochstein: Rheology of ball and fiber suspensions with viscoelastic matrix liquids (PDF), dissertation at the Technical University of Karlsruhe 1997, Chapter 9, p. 126 ff.
  6. ↑ Armin Merten: Investigations on flow instabilities in the extrusion of polymers with laser Doppler anemometry (PDF), dissertation at the University of Erlangen-Nuremberg, 2005, p. 2.
  7. ↑ Rheology of cheese: Thermo-rheological characterization of viscoelastic foods git-labor.de, February 13, 2015.
  8. ↑ Physical material properties ifw-dresden.de, lecture notes TU Dresden, see in particular Chapters 12 and 13.