Flow stress is defined as the instantaneous value of stress required to continue deforming the material - to keep the metal flowing. It is the yield strength of the metal as a function of strain, which can be expressed:

Yf = Keⁿ

• Yf = Flow stress, MPa
• e = True strain
  Strain (materials science)
  In continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...
  
  
• K = Strength Coefficient, MPa
• n = Strain hardening
  Work hardening
  Work hardening, also known as strain hardening or cold working, is the strengthening of a metal by plastic deformation. This strengthening occurs because of dislocation movements within the crystal structure of the material. Any material with a reasonably high melting point such as metals and...
  
   exponent

Hence, Flow stress can also be defined as the stress required to sustain plastic deformation at a particular strain.

The flow stress is a function of plastic strain.

Flow stresses occur when a mass of flowing fluid induces a dynamic pressure on a conduit wall. The force of the fluid striking the wall acts as the load. This type of stress may be applied in an unsteady fashion when flow rates fluctuate. Water hammer is an example of a transient flow stress.

The following properties that have an effect on flow stress: chemical composition, purity, crystal structure

Crystal structure

In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...

, phase constitution, exit microstructure, grain size, and heat treatment

Heat treatment

Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...

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