In materials science the flow stress, typically denoted as Yf (or
), is defined as the instantaneous value of stress required to continue plastically deforming a material - to keep it flowing.
The flow stress changes as deformation proceeds and usually increases as strain accumulates due to work hardening, although the flow stress could decrease due to any recovery process.
In continuum mechanics, the flow stress for a given material will vary with changes in temperature,
; therefore it can be written as some function of those properties:[1] The exact equation to represent flow stress depends on the particular material and plasticity model being used.
Hollomon's equation is commonly used to represent the behavior seen in a stress-strain plot during work hardening:[2] Where
Generally, raising the temperature of an alloy above 0.5 Tm results in the plastic deformation mechanisms being controlled by strain-rate sensitivity, whereas at room temperature metals are generally strain-dependent.
[3] Independent of test conditions, the flow stress is also affected by: chemical composition, purity, crystal structure, phase constitution, microstructure, grain size, and prior strain.
[4] The flow stress is an important parameter in the fatigue failure of ductile materials.
The rate of crack propagation is inversely proportional to the flow stress of the material.