Mechanical APDL provides a capability to model nonlinear (multilinear) elastic materials (input using TB,MELAS). Unlike plasticity, no energy is lost. (The process is conservative.)

Figure 4.23: Stress-Strain Behavior for Nonlinear Elasticity represents the stress-strain behavior of this option. Note that the material unloads along the same curve, so that no permanent inelastic strains are induced.

Figure 4.23: Stress-Strain Behavior for Nonlinear Elasticity

The total strain components {ε_n} are used to compute an equivalent total strain measure:

(4–129)

is used with the input stress-strain curve to get an equivalent value of stress σ_e .

The elastic (linear) component of strain can then be calculated:

(4–130)

and the "plastic" or nonlinear portion is therefore:

(4–131)

In order to avoid an unsymmetric matrix, only the symmetric portion of the tangent stress-strain matrix is used:

(4–132)

which is the secant stress-strain matrix.