Unloading after Plastic Deformation 2

[Sparweb]

When a structure (say, a cylindrical tensile test coupon, for simplicity) is loaded beyond Fty, but not broken, and then unloaded, what does the unloading curve really look like? It's drawn often enough in the textbooks: they always show a straight line back down with slope E to the x-axis, and then mention that the strain at this point is the permanent deformation of the structure. Assuming that the material in question has a well defined elastic zone, and the peak stress is in the plastic range, why is the unloading line straight? Can the moving dislocations and grain boundaries in the material (when it is being stressed plastically) go a little bit back to their initial state when relieved?


| __________Ftu
| __----/
| _/ /
| ^Fty_/ /
| / / <--- Is the unloading line really straight?
| / /
| / /
|/ /
--------------------------
0 e_permanent

(Hope the drawing works out)

I expect the answer is no, but I'm trying to make sense of some test data...


STF

 

[EnglishMuffin]

Even the loading portion of the elastic deformation curve isn't exactly straight, so the unloading curve won't be either (the interatomic forces follow a non-linear relationship). But I can't see how you can get any appreciable plastic deformation during the unloading portion.

 

[CoryPad]

Assuming that the material is metallic, the elastic response is approximated as linear, but there can be non-linearity. This is dependent on second and third order elastic constants, and processes such as dislocation bowing. For most metals, the elastic response is > 95% linear. Thus, the elastic unloading should be essentially linear. The reason elastic unloading is linear is that the crystalline structure has been changed to accomodate the external work and now has a new, expanded elastic response regime. The dislocations and grain boundaries don't really move during unloading - they moved during the loading phase until the peak stress, then they essentially are stable until move work is applied to the system.

Regards,

Cory

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