Ultimate Tensile strength of ductile materials

[Abhishekdas]

In case of ductile materials we know that there is a substantial difference between the ultimate tensile strength and yield strength. As long as the specimen stress is below the yield strength we need not worry about engineering stress and true stress, as they as essentially same. But when necking starts and by the time the specimen actually fails there is a considerable difference between the engineering stress and true stress. The value at failure reported by UTM testing is supposed to reveal the Ultimate Tensile Strength of a material. So these reported values say 550-600 Mpa (normal values for steel) are the Engineering Stress values at failure or the true stress values at failure? Thank You

 

[billy_joule]

The value at failure reported by UTM testing is supposed to reveal the Ultimate Tensile Strength of a material. So these reported values say 550-600 Mpa (normal values for steel) are the Engineering Stress values at failure or the true stress values at failure? Thank You

I'm unaware of any UTM can measure true stress or strain. They do not have the means to measure instantaneous cross sectional area. It can be done from the UTM data after (IME of 3-4 digital UTM's they don't even output stress strain data - just load-extension, the rest is up to the user)