In the case where the user elects to input only an initial yield stress SIGY and the tangent modulus Etan in lieu of a true stress vs. In any case, the first plastic strain value should be input as zero and the first stress value should be the initial yield stress. For plastics/polymers, you probably should consider the increase in recoverable strain as stresses increase (since the elastic component of strain may be quite large). For metals, E is very large compared to the yield stress so it's fairly common practice in the case of metals to just subtract off a constant value equal to the strain at initial yield from all subsequent strain values. Note that as the stress value increases, the recoverable strain ( true stress/E) increases as well. Įffective plastic strain (input value) = total true strain - true stress/E Using experimental data from a true stress vs. True stress is input directly for the stress values.

effective plastic strain curve in a LS-DYNA plasticity model should be the residual true strains after unloading elastically. The effective plastic strain values input in defining a stress vs. Full iterative plasticity can be invoked for shells, at greater expense, for material models 3, 18, 19, and 24 by setting MITER=2 in *CONTROL_SHELL. Input of noisy experimental data may cause spurious behavior, particularly in the case of the default, 3-iteration plane stress plasticity algorithm for shells. When using *MAT_24, one should input a smoothed stress-strain curve utilizing a minimal number of points. True stress = (engineering stress) * exp(true strain) = (engineering stress) * (1 + engineering strain) where exp(true strain) is 2.71 raised to the power of (true strain).īe aware that experimental data always includes some degree of error and thus tends to be somewhat noisy or erratic. True strain = ln(1 + engineering strain) where ln designates the natural log

true strain, not engineering stress or strain. First of all, you may check that your experimental data from a uniaxial tension test is expressed in terms of true stress vs.

Engineering stress vs true stress pdf