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I was looking at the elongation of various steel alloys, and the numbers were in the 10-20% range. Is this the amount of elongation at the point of failure? If so, what is the percentage of elongation that we would observe before reaching plasticity? Did I use the term plasticity correctly? I mean the point at which the material will no longer return to its original state.

I would assume there are probably two numbers here. The elongation just before yield stress, and then the recommended maximum elongation that would actually be safe to use in practice.

Thanks.

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  • $\begingroup$ Is that elongation due to load or temperature change? will it be different? $\endgroup$ – Solar Mike Oct 23 at 7:36
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The point where steel stops behaving in an elastic manner (i.e. return to its original stress) is essentially the yield point. The yield starts for most steels at approximately 0.2% elongation.

You might find some authors, that distinguish between the yield point and the Elasticity point, however IMHO for all intents and purposes they might just as well coincide. In any case, you will find yield strain/stress values a lot more often that the corresponding for elasticity point.

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For some high strength steels, there is no visible yield point, however in that case the failure strain is significantly less.

enter image description here

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  • $\begingroup$ does 0.2% refer to the dashed line in the lower graph? then it should be 2% $\endgroup$ – mart Oct 23 at 8:29
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    $\begingroup$ No, its 0.2% (it corresponds to the initial bump in the mild steel curve). The 2% line is probably showing some hysteresis, or the effect of work hardening. Probably I should change the graph to avoid confusion $\endgroup$ – NMech Oct 23 at 8:34
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It depends on how much distortion is acceptable for the application . Generally steel elongation to 0.5 % strain is acceptable . That is the elastic strain corresponding to 0.2% permanent strain defined as yield strength. However , high strength steels may reach over 0.7 % elastic strain before reaching 0.2 % permanent strain . This is a yield strength of about 125,000 psi , not exceptionally high in my opinion. I have seen pressure vessels with a few % strain that are acceptable for continued service. And furnace tubes with many % strain that are suitable for continued service ( relatively routine). So it depends on the application or, what do you want the answer to be ?

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