# 11.5: Isotropic and Kinematic Hardening


It should be noted that in the case of uniaxial stress, $$\sigma_2 = \sigma_3 = 0$$ and Equation (11.3.10) reduces to $$\bar{\sigma} = \sigma_1$$. Likewise, for uniaxial stress $$\dot{\epsilon}_2 = −0.5\epsilon_1$$ and $$\dot{\epsilon}_3 = −0.5\epsilon_1$$ and the equivalent strain rate becomes equal to $$\bar{\dot{\epsilon}} = \dot{\epsilon}_1$$. Then, according to Equation (??), $$\bar{\epsilon}_1 = \epsilon_1$$. The hypothesis of the isotropic hardening is that the size of the instantaneous yield condition, represented by the radius of the cylinder (Figure (11.4.1)) is a function of the intensity of the plastic strain defined by the equivalent plastic strain $$\bar{\epsilon}$$. Thus

$\bar{\sigma} = \sigma_y(\bar{\epsilon})$

The hardening function $$\sigma_y(\bar{\epsilon})$$ is determined from a single test, such as a uniaxial tension. In this case

$\bar{\sigma} = \sigma_1 = \sigma_y(\bar{\epsilon}) = \sigma_y(\epsilon_1)$

Thus the form of the function $$\sigma_y(\bar{\epsilon})$$ is identical to the hardening curve obtained from the tensile experiment. If the tensile test is fit by the power hardening law, the equivalent stress is a power function of the equivalent strain

$\bar{\sigma} = A\bar{\epsilon}^n$

The above function often serves as an input to many general purpose finite element codes. A graphical representation of the 3-D hardening rule is a uniform growth of the initial yield ellipse with equivalent strain $$\bar{\epsilon}$$, Figure ($$\PageIndex{1}$$).

In the case of kinematic hardening the size of the initial yield surface remains the same, but the center of the ellipse is shifted, see Figure ($$\PageIndex{1}$$). The coordinates of the center of the ellipse is called the back stress. The concept of the kinematic hardening is important for reverse and cyclic loading. It will not be further pursued in the present lecture notes.

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