After the sample is again uniformly deformed, the increase of stress with the progress of extension results from work strengthening, that is, dense dislocations induced by plastic deformation hampers the further motion of dislocations. Explicitly, heterogeneous plastic deformation forms bands at the upper yield strength and these bands carrying with deformation spread along the sample at the lower yield strength. The stress of the flat region is defined as the lower yield point (LYP) and results from the formation and propagation of Lüders bands. In this region, the stress mainly increases as the material elongates, except that for some materials such as steel, there is a nearly flat region at the beginning. This region starts as the stress goes beyond the yielding point, reaching a maximum at the ultimate strength point, which is the maximal stress that can be sustained and is called the ultimate tensile strength (UTS). The second stage is the strain hardening region. The stress component of this point is defined as yield strength (or upper yield point, UYP for short). The end of the stage is the initiation point of plastic deformation. In this region, the material undergoes only elastic deformation. The stress is proportional to the strain, that is, obeys the general Hooke's law, and the slope is Young's modulus. The first stage is the linear elastic region. To clarify, materials can miss one or more stages shown in figure 1, or have totally different stages. There are several stages showing different behaviors, which suggests different mechanical properties. Stages Ī schematic diagram for the stress-strain curve of low carbon steel at room temperature is shown in figure 1. If not mentioned otherwise, stress–strain curve refers to the relationship between axial normal stress and axial normal strain of materials measured in a tension test.Ĭonsider a bar of original cross sectional area A 0 ranging from 0.02 to 0.5.

The form of deformation can be compression, stretching, torsion, rotation, and so on. The stress and strain can be normal, shear, or mixture, also can be uniaxial, biaxial, or multiaxial, even change with time. Generally speaking, curves representing the relationship between stress and strain in any form of deformation can be regarded as stress-strain curves.

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