07 Jun 2019 Engineering Stress vs True Stress
The data obtained from a Tensile Test (https://www.fea-solutions.co.uk/tensile-tests/) are forces and displacements.
In order for this to be useful to an engineer, the applied force has to be converted into a stress. (https://www.fea-solutions.co.uk/stress-strain-curves/)
There are two types of stress which can be calculated, Engineering Stress and True Stress.
Engineering Stress is defined as the Force applied divided by the original, undeformed cross sectional area A0, or as an equation σe=F/A0.
True Stress on the other hand is the Force applied divided by the actual cross sectional area, which changes noticeably once the specimen is deforming plastically (https://www.fea-solutions.co.uk/elasticity-and-plasticity/).
As a mathematical equation, True Stress is defined as σ=F/A.
As it is extra effort and actually quite complicated to measure the cross sectional dimensions of a specimen during a Tensile Test, most stress-strain curves and also most Ultimate Strength values published represent Engineering Stress.
Engineering Stress is appropriate for the most common FEA application, which is linear-elastic stress analysis. For a FEA that includes plasticity it is however required to use True Stress data, which are often difficult to obtain.
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