tension engineering stress vs true stress

As an alternative a method for the conversion of engineering stresses to Cauchy stresses for tensile and compression tests is presented, that considers the actual deformations of the test specimens. For the determination of Cauchy stresses in material tests the technique of digital image correlation (DIC) is often employed. The aim is to identify an effective stress along the entire test specimen as input for finite element analyses with polymers. Photographs of specimens made of a PLLA based polymer blend in different present configurations: A) foil specimen during tensile testing, B) cylindrical specimen during compression testing. With this method “true” stress-strain curves as input for finite element material models can be identified for arbitrary materials. Thus a method for conversion of engineering to Cauchy stresses in tensile and compression tests could be established considering the non-isochoric deformation in plasticity. The numerical results show good agreement with the experiments for the tested polymers.

For validation finite element analyses of the tensile and compression tests are performed using the identified stress-strain curves. The engineering stresses at several time points are converted into Cauchy stresses using newly developed formulas in consideration of the actual specimen geometry. The exact geometry of the specimens in the respective present configuration is determined in photographs, which are taken continuously throughout the test. A method for the determination of Cauchy stresses from tensile and compression tests is presented, that considers the actual deformations of the test specimens. Thermoplastic polymers exhibit non-isochoric behaviour during tensile and compression testing as well as particular deformations like local necking (tension) or buckling (compression).

Tension engineering stress vs true stress