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Engineering Strain Vs. True Strain

For many engineering applications, the use of 'engineering stress and strain' values for material stress-strain curves will be sufficient for obtaining correct answers in a plasticity analysis. Engineering stress and strain are commonly represented in 2D by these equations:

e = DL / Lo            s = F / Ao

However, engineering strain is a small strain measure which is invalid once the strain in your model is no longer 'small' (approximately 5%). True strain, which is a nonlinear strain measure that is dependent upon the final length of the model, is used for large strain simulations. True stress and strain are commonly represented in 2D by these equations:

e = ln(L / Lo)            t = F / A

As engineers are often supplied with engineering stress and strain test data, a conversion to true stress and strain (also called log strain) is needed before inputing these material properties into ANSYS. For uniaxial stress-strain data, engineering stress and strain can be converted to true stress - log strain by:

el = ln (1+e)            t = s (1+e)

For your convenience, DRD has supplied the following spreadsheet that has stored these formulas and can be used to convert from engineering stress and strain to true stress - log strain. Simply put your test values in the left columns and the right columns will be filled with updated values for use in a large strain analysis.


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