Flow stress is defined as the instantaneous value of stress required to continue plastically 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 = Kεn
Yf = Flow stress, MPa
ε = True strain
K = Strength Coefficient, MPa
n = Strain hardening 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.
The following properties have an effect on flow stress: chemical composition, purity, crystal structure, phase constitution, exit microstructure, grain size, and heat treatment.
The flow stress is an important parameter in the fatigue failure of ductile materials. Fatigue failure is caused by crack propagation in materials under a varying load, typically a cyclically varying load. The rate of crack propagation is inversely proportional to the flow stress of the material.