# Formula for bending a rod in the elastic range

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1. May 27, 2017

### pistorinoj

Is there a formula to calculate the radius that a rod of a given radius can be bent around while staying in the elastic range?

For example, if I had a rod that was 1/4" in diameter and made of HDPE (which I think has a modulus of elasticity of 0.8 GPa), how would I calculate the minimum radius the rod could be bent around while still staying in the elastic range so that it could be unwound without deformation?

2. May 27, 2017

### Dr.D

The short answer is no. This depends entirely on the elastic limit, and that was not in your assumed input. The modulus of elasticity does not give you the necessary information.

3. May 27, 2017

### pistorinoj

Sorry, but I am not following you.

Are you saying that even knowing all the characteristics of HDPE, there is no formula for calculating the minimum bending radius?
Or are saying that I did not provide enough information in my question?

I do see places providing the following data for HDPE:
Film Tensile Strength at Yield, MD
21.0 MPa 3050 psi

Film Tensile Strength at Yield, TD
23.0 MPa 3340 psi

4. May 27, 2017

### Dr.D

I saw no mention of yield values in your original post, only the modulus of elasticity.

The initials HDPE mean nothing to me, but I presume that you are talking about one of the many plastic type materials in use today. There is some question as to just how well classical failure theories apply to such materials. I suspect you need something in the way of very new information, but I do not have such.

5. May 27, 2017

### Tom.G

HDPE is High Density PolyEthylene. Here in the States it is most commonly seen as gallon jugs for milk.

6. May 27, 2017

### pistorinoj

I think milk jugs are usually made from Low Density PolyEthylene (LDPE) but I will take a formula that works for either one.
HDPE is also commonly used in making robot parts.

7. May 28, 2017

### Nidum

There are simple and more complicated ways of getting an answer to your problem depending on the accuracy required .

Easiest way to get a ball park answer is to consider the bending of a cantilever made from the chosen material .

Work out the local radius of curvature at the fixed end when the maximum fibre stress is just at yield .

To be sure that the rod will unbend properly you would need to make the radius of curvature to be used in practice a little larger than the one as calculated above . This is particularly important for plastic materials like your HDPE .

Last edited: May 28, 2017
8. May 28, 2017

### Nidum

Beam bending theory is based on the relationship between local curvature and local bending moment .

Using this relationship it would be relatively easy to derive a formula relating local radius of curvature to maximum fibre stress for any given cross section geometry .

Please let us know if you want to look at this more analytic method for solving your problem .

Euler Bernoulli beam theory

9. May 28, 2017

### Nidum

Just did a sample problem to see what sort of radius of curvature is involved . Rod is 6,35 mm dia 75 mm long HDPE

Last edited: May 28, 2017
10. May 28, 2017

### pistorinoj

Yes, that would be great. Ideally, I could review a list of materials and find one that can be wound in the smallest space balancing stiffness and the overall diameter of the material. For example, I could use acetal (delrin), PTFE, composites, etc. at smaller diameters if those materials could be wound elastically in a smaller space yet be stiff enough for my purposes.

Could you show me how you calculated R156 for the HDPE size you used?

Finally, I see various references to engineering handbooks possibly having tables of this information. Is there one that you know of?

Thanks so much, this is really helpful.