![engineering stress vs true stress vs ultimate stress engineering stress vs true stress vs ultimate stress](https://media.cheggcdn.com/media/7d4/7d4c9de4-ed18-48d2-a24f-dd1cf0818c39/php4CThxg.png)
True stress values are more accurate measures of stress than engineering values for large deformations. True normal stress, σ, is based on the instantaneous cross-sectional area, A i: The stress and strain definitions in Equations 1 and 2 are based on the original cross-sectional area of the tensile specimen, so they are known as engineering stress and strain. However, the uniform elongation, e u, better represents the material's ductility or formability in uniaxial deformation, because after necking (when uniform elongation is exhausted), the material can be considered to have The total elongation, e t, at fracture and the total area of reduction at fracture, A r, are considered to be indications of material ductility. Area reduction, A r, is the percentage of reduction in the area, calculated by cross-sectional area at fracture, Af, and initial cross-sectional area, A 0:.Total elongation, e t (also known as elongation at break), is the elongation of the original gauge length of a tensile specimen at fracture, including both uniform (e u) and postuniform elongations.Uniform elongation, e u, is the elongation at the maximum load.Elastic modulus (E) (also known as Young's modulus) is the slope of the elastic part of an engineering stress-strain curve.Ultimate tensile strength (UTS) is the maximum engineering stress in a tensile test and signifies the end of uniform elongation and the start of localized necking.The intersection of this parallel line with the flow stress curve gives the value of Y.
![engineering stress vs true stress vs ultimate stress engineering stress vs true stress vs ultimate stress](https://i.ytimg.com/vi/q7g4MTSzESc/maxresdefault.jpg)
Y is determined approximately by drawing a parallel line to the linear elastic region of the flow stress curve from 0.2 percent engineering strain.
![engineering stress vs true stress vs ultimate stress engineering stress vs true stress vs ultimate stress](http://www.varmintal.net/engineering-stress-vs-strain-curve.png)
An extensometer is attached to the specimen to measure the elongation over an original gauge length, l0. 1 In a tensile test, a standard-size specimen is cut out from the sheet metal and pulled slowly until it breaks. The flow stress curve, which reflects the sheet material's elastic and plastic properties, typically is obtained from a tensile test or bulge test. This curve is one of the most important variables for calculating input data for the finite element (FE) and analytical methods used to predict metal flow and defects. They don't, however, always provide the true stress-true strain (flow stress) curve. Material suppliers often provide the yield stress and ultimate tensile strength for sheet materials. Editor's Note: Part II of this article will appear in the July/August 2011 issue.