The von Mises equation is a measure of the shear strain in the material and does not account for the hydrostatic straining component. For example, strain values of ε_x = ε_y = ε_z = 0.001 yield an equivalent strain ε_eq = 0.0.

The equivalent elastic strain is related to the equivalent stress when ν' = ν (input as PRXY or NUXY on MP command) by:

(17–128)

where:

σeq = equivalent stress (output using SEQV)

= equivalent elastic strain (output using EPEL, EQV)

E = Young's modulus

Note that when ν' = 0 then the equivalent elastic strain is related via

(17–129)

where:

For plasticity, the accumulated effective plastic strain is defined by (see Equation 4–25 and Equation 4–42):

(17–130)

where:

= accumulated effective plastic strain (output using NL, EPEQ)

This can be related to (output using EPPL, EQV) only under proportional loading situations during the initial loading phase and only when ν' is set to 0.5.

As with the plastic strains, to calculate the equivalent creep strain (EPCR, EQV), use ν' = 0.5.

The equivalent total strains in an analysis with plasticity, creep and thermal strain are:

(17–131)

(17–132)

where:

= equivalent total strain (output using EPTT, EQV)

= equivalent total mechanical strain (output using EPTO, EQV)

= equivalent thermal strain

For line elements, use an appropriate value of ν'. If > > , use ν' = 0.5. For other values, use an effective Poisson's ratio between n and 0.5. One method of estimating this is through:

(17–133)

This computation of equivalent total strain is only valid for proportional loading, and is approximately valid for monotonic loading.