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Good day, I am a mechanical engineering student, we conducted three-point bending test for a metal specimen and I am wondering why the temperature of the specimen did not increase as the UTM certainly did work on the specimen, therefore some thermal energy increase should be present.

This is in comparison when we conducted tensile test to another specimen in which we realized that the metal became appreciably hot.

I think it was because bending involved tension and compression, reasoning that because they are opposite of the other, while tension releases thermal energy because of the pulling away (rupture) of interatomic bonds, compression accepts thermal energy because it needs for the compression of the bonds? Therefore no (or minimal) thermal energy increase could be observed after bending.

I can't research anything related to this phenomena on the internet. (Also we had never conducted yet a compressive test, so I don't know if compressing the metal reduces its temperature). I also know the physics of thermal expansion (metal expands with increase in temperature), I am not just sure if it applies the other way around.

I just want to know if you think my hypothesis is correct, or if not could you provide a link or explanation? Thank You

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    $\begingroup$ How much temperature rise did you expect to see, based on the amount of work done and the mass + specific heat of the sample? $\endgroup$
    – Dave Tweed
    May 22, 2016 at 11:17
  • $\begingroup$ I doubt the compression accepted enough (if any) thermal energy to negate the energy from the bending. It is not something I have ever heard of! Can you give some figures? Total mass, dimensions, bending energy input, bending force & distance, thermal conductivity, accuracy of temperature measurement, duration of vending, time between bending and temperature measurement? $\endgroup$
    – Jodes
    May 22, 2016 at 11:25
  • $\begingroup$ Length of specimen: 18.2 cm <br/> Weight of specimen: 0.369 kg <br/> Nominal Cross Section Area: 262.16 mm2 <br/> Degree of bend of neutral axis: (from 180) to 9 degrees <br/> Distance between support: 130mm <br/> Punch diameter: 30 mm <br/> Max Bending Load: 28.2016 kN <br/> Bending Stress: 1 530 880 kPa $\endgroup$
    – MathForPrecision
    May 22, 2016 at 12:14
  • $\begingroup$ The specimen is reinforcing steel bar $\endgroup$
    – MathForPrecision
    May 22, 2016 at 12:20
  • $\begingroup$ I expect to observe a temperature rise like in our experiment in tensile test, as it is the same kind of specimen and they both experience plastic deformation. So I am confused why there is no temperature increase. $\endgroup$
    – MathForPrecision
    May 22, 2016 at 12:26

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First of all, I don't think likely that your hypothesis is true. I don't think the heat generated in tension would be "absorbed" in compression. That seems unlikely and I've never heard of such.

Also, I don't think termal expansion is what is going on when you run a tensile test and the metal gets hot. The heat that is liberated if you 'stretch' a specimen is not working through the same mechanism that a metal raises volume when heated. Those are two different effects.

What I think is going on is something that someone already suggested in the comments: when you ran the tensile test, you had a big part of your specimen getting stretched thus generating significant heat. When you did the bending test only some localized areas of your specimen suffered significant traction, and it probably was not as much traction as it was subject on the pure tensile test.

Probably the small areas of your specimen that did suffer traction did also generate heat, but it was less heat than you were expecting. If you should ran the same test and measure the heat on the outside of the area that is being directly being bent (suffering traction) I think you would detect a temperature change.

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