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Why is it, that - if displayed in a strain/stress-diagram - the curve will at first be linear, and then change into a "curved shape", where it will first decrease again, and only later increase again? Is there any tendency, for which materials this holds true and for which it doesn't? Can I identify these by certain characteristics.

As an example graph, please consider this graph:

sample stress-strain curve

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closed as not a real question by Manishearth Jun 7 '13 at 5:44

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

    
Possibly related: physics.stackexchange.com/q/11724/2451 –  Qmechanic Jul 7 '11 at 19:22
    
Where did You copy this diagram? Somewhere about plastics? Did You find the words "hardening/softening"? –  Georg Jul 22 '11 at 13:47
    
This is quite broad unless the type of material is specified. –  Manishearth Jun 7 '13 at 5:45