tension engineering stress vs true stress

Distortion energy failure theory is comparison between 2 kinds of energies, 1) Distortion energy in the actual case 2) Distortion energy in a simple tension case at the time of failure. The concept of Von mises stress arises from the distortion energy failure theory. We will explore what Von Mises stress is in the coming section.

The most preferred failure theory used in industry is ‘Von Mises stress’ based. Many years of engineering experience has shown that normal stress theory doesn’t work in most of the cases. If you use such an assumption, you would be using a failure theory called ‘normal stress theory’. Can we say here also, that the material fails when the maximum normal stress value induced in the material is more than the yield point value ?. Now consider the situation in second part of Fig.1, an actual engineering problem with a complex loading condition. Fig:1 A simple tension test and a real life loading condition The simple tension test is a unidirectional test, this is shown in the first part of Fig:1. When the material reaches the yield point (for ductile material) the material can be considered as failed. Here the material is pulled from both ends.

One of the most easy way to check when a material fails is a simple tension test. In the following sections we will have a logical understanding of Von Mises stress and why it is used. It works well for most cases, especially when the material is ductile in nature. Von Mises stress is considered to be a safe haven for design engineers.Using this information an engineer can say his design will fail, if the maximum value of Von Mises stress induced in the material is more than strength of the material. What is Von mises stress ? December 29, 2012

Tension engineering stress vs true stress