What is the impact of Youngs Modulus on copper cable torqueing?

[MandyVol]

I hope someone can help. I am an Production Engineer. We have a situation where we have to torque a 16mm copper cable in a box terminal by 4Nm. The cable will be used to carry a high electrical current.

We find that after torqueing (About 5 hours or so) the terminal can be re-tightened back to 4Nm.

I am told this is to do with Youngs Modulus and the characteristics of copper.
I have taken a look at Youngs Modulus but it all seems centred around the stretching of copper and not the compression / "squashing" of copper.

Would anyone know how to work out:
1. What interval we should wait before re-torque,
2. How many times you would need to re-torque,
3. Is re-torque the best option or is there an alternative.
4. Am I even on the right track with regards to Youngs Modulus or are there any other suggestions?

Any assitance would be greatly appreciated.

 

[Q_Goest]

Hi Mandy,
The modulus of elasticity (Young's modulus) is the same in both tension and compression, but the reason for the relaxing of the torque isn't due to the modulus. If the stress on the copper doesn't exceed yield during operation, it shouldn't require retorquing, but I suspect because the yield strength for copper is so low that it is being deformed when torquing and again to a small degree during operation.

As you crush (or stretch) a material like copper, it work hardens, so the yield strength increases. There's a risk of fatigue if there are alternating stresses in the material, but I suspect the alternating stresses are low and can be ignored. You'd need to look at that separately. The more important issue is getting the material to stop yielding during operation. Perhaps by overtorquing the copper to well above 4 Nm (if it can handle it) then removing the load and reapplying to 4 Nm, it might not need periodic retightening. Otherwise, I suspect you will need to retorque a number of times, each time you'll find it being a bit tighter than previously and eventually it won't need to be retightened at all. But I think it's getting loose over time because the copper is yielding.

If you try this, let me know as I'd be interested in your results. I have a similar issue using annealed copper.

 

[MandyVol]

Thanks a lot! I will try this and let you know if it works, any other suggestions will still be appreciated.