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I was going through this website :https://www.tf.uni-kiel.de/matwis/amat/iss/index.html where it answered a question: Why is the maximum of the (nominal) stress-strain curve and the beginning of necking related? So for explaining the relation it stated that curves taken at different strain rates may not be very different, you always need somewhat larger stresses at higher strain rates to achieve the same strain. Why do you need somewhat larger stresses at higher strain rates to achieve the same strain achieved in lower strain rate?. Can someone give an explanation for such behaviour?

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  • $\begingroup$ Can you give some more details about exactly what sort of system you are considering. Both viscous and inertial forces could be the cause, but as it is you haven't given enough information for us to be able to answer. $\endgroup$ – John Rennie Nov 22 '18 at 6:58
  • $\begingroup$ I'm talking about stress strain curve for a specimen(steel) generated using Universal testing machine. $\endgroup$ – sb1 Nov 22 '18 at 7:55
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Dislocations have limited velocities. At slow rates of strain, materials can flow and creep and achieve large deformations without great force. Think of geology, glaciers, salt crystals, etcetera.

And it may be better to think of the forces and stresses as the independent variable, as the cause of the deformation. The convention of putting the strain on the horizontal axis may cause misunderstandings.

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