According to the constitutive equation during tensile deformation, the strain-rate-sensitivity exponent m is related to the true stress and true strain. However, in the articles, people draw the curve of flow stress as a function of strain rate and get the slope of the line as the value of m. So what is the flow stress? Is it the same thing with true stress?
some People consider flow curve and true stress strain curve as same. or they interchange these words.
True stress as definition says ist load/instantaneous area. But flow curve actually refers to plastic deformation or plastic flow of materials. So flow curve is something whic deals with plastic flow and hence the plastic region of true stress-strain curve is flow curve. i.e by removing elastic part from true stress-strain curve we get flow curve. Usually this difference you can see in any theory of plasticity books or metal formic books. So there is slight difference between them. Even though true stress and flow stress are quite Close values they are not identical (the the curve is rotated slightly anticlockwise to make slant elastic line as vertical i.e. removing elastic effect for perfect plastic flow analysis). Normally flow curves should not have elastic Region and will start from y-axis.
I agree with Abhishek Ramakrishna Shetty, "Even though true stress and flow stress are quite close values they are not identical "
In this competitive world no two things will exist if these are absolutely same. Basing on this paradigm the true stress and flow stress resort to two discriminate able things.
Theoretically: 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 = Kε
whereas True stress is the stress determined by the instantaneous load acting on the. instantaneous cross-sectional area.
The stress considering only the plastic deformation or plastic flow of the material is called flow stress. It is the intermediate value between the yield strength and ultimate or tensile strength.
Dear Ju,
True stress as definition says load/instantaneous area. But flow curve actually refers to plastic deformation or plastic flow of materials. So flow curve is something which deals with plastic flow and hence the plastic region of true stress-strain curve is flow curve.
Flow stress can also be defined as the stress required to sustain plastic deformation at a particular strain. The stress considering only the plastic deformation or plastic flow of the material is called flow stress. It is the intermediate value between the yield strength and ultimate or tensile strength.
The flow stress is a function of plastic strain. Properties have an effect on flow stress are 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.
True stress and strain are often not required. When the yield strength is exceeded, the material deforms. The component has failed because it no longer has the original intended shape. True stresses are instantaneous and hence take the reduction in cross-section due to straining into consideration. It makes the true stress to be higher than engineering one. At lower strain, these effects are negligible.
Even true stress and flow stress are quite close values they are not identical. Generally true stress and flow stress both can occur in the fatigue phenomena.
The definition of the true tensile stress is known as the derivative of force divided by the derivative of area at failure, (dF/dA), while the engineering stress is defined as the applied force divided by the nominal specimen area. There is a difference between the two.
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