Help with the Three-Point Bend Test (mechanic experiment)

[thebookthief]

Hello,
For my science fair experiment, I'm 3D-printing a scalpel handle and testing to see whether it works as well as the gold-standard device. In order to measure the yield and stiffness of both the 3D-printed and the gold-standard against each other, I was thinking of using the 3-point bend test, but I'm not fully sure with how to actually apply all the equations to my experiment. One of the variables in the equation asks for the depth of the beam-like device (aka the scalpel) but I'm not sure how to do that.
Do you have any insight on how the Three-Point Bend test works, or if not, are there any other experiments in mechanics that I could use to test the strength of the scalpel?
Thanks.

[norman40]

Hi thebookthief,

The 3-point bend test seems to be appropriate for the comparison you want to make. In this test a force is applied to the midpoint of an object supported at each end. The amount of deflection due to the applied force is measured.

I think that you want the equation for “flexural strain” shown in the following link:

https://en.wikipedia.org/wiki/Three-point_flexural_test

This equation compensates the deflection (D) for the length (L) and thickness (d) of the beam-like object. The thickness (d) is the depth of the object in the vertical direction (same direction that the force is applied).

I hope this helps. Please ask again if you have more questions.

A. Norman

[michaelsosnowski]

Another approach would be to measure the hardness of the material or its ability to yield or resist to deformation under mechanical indentation compressive pressure. The modulus of the material may be also determined by the loading and unloading of the load displacement curve.

[thebookthief]

Hi Norman,

Is there any way to measure the deflection of the object without using a universal testing machine? The Wikipedia article and most of the other articles I've read on this topic require heavy industrial machines to apply the force at the midpoint of the object. Do you have any ideas for how I could measure the deflection and do the three-point bend test with cheaper materials?
My own idea was that to use some sort of derivation of Torricelli's experiment, where a fluid applies the force onto the midpoint of the object, and then I could measure the deflection of the object from its normal position that way, but I don't know whether this would be an accurate test, or how I would construct the test rig.

Is there any way to do the three point flexural test without a universal testing machine?
Thanks!

[michaelsosnowski]

A possibility is go to Home Depot where different springs are available and select one that had a diameter slightly smaller than a round marble diameter. If a U shaped object with enough depth to compress the spring backed marble into the 3D printed material to make a measurable impression was applied, a comparative value between gold standard and your material could be made.

[norman40]

Hi thebookthief,

You might be able to rig up a homemade bending test. You'll need a way to support the object, a way to apply the force and a method for measuring the distance the object bends. Take a look at the video at the following link:

https://www.youtube.com/watch?v=eRc-xlX9b9Q

This set up may give you some ideas about how to make your own test fixture. A crude way to go would be to clamp the test object to a workbench and hang a weight from the object's midpoint. Then use a ruler to measure the deflection.

I hope this helps. Please ask again if you have more questions.

A. Norman

[thebookthief]

So the 3D printed object that I have is a scalpel handle, which is around 14cm or 5.5 inches.

I could use a clipper to clip the ends of the scalpel to 2 benches (one end to each bench) and then hold a weight with a string from the midpoint of the object. I was thinking of hanging water inside a measuring cup from the midpoint, so that I could periodically increase the volume of water-- and thereby increase the force that the scalpel is able to handle. Then, the amount of water at which the scalpel fractures or breaks off from the benches would be the fracture point of my object.

However, for this procedure, I was wondering whether the shape and weight of the clippers I use would come into play in my calculations of the force the scalpel is able to carry. Also, would the position at which I clip the scalpel also impact my measurements?

Also, I don't really understand the description about using a U-shaped object to compress the spring-backed marble into the 3D-printed scalpel. It would be great if you could re-explain that process again.

Thanks!

[norman40]

Hi thebookthief,

The span of the object's supports does make a difference. As you can imagine the flex will be reduced if the supports are closer together. In the standard test the support span is specified as 16 times the depth of the object.

The test object is not clamped to the supports in the standard test. And clamping may influence the results. If you need to clamp the object to the supports to make your test work, be sure to use the same clamps and position them in the same way for all objects that you test.

You may want to review the standard test procedures here:

http://classes.engr.oregonstate.edu/mim ... sites).pdf

Perhaps Snowman will provide some additional explanation of the spring-backed marble test idea.

I hope this helps. Please ask again if you have more questions.

A. Norman

[thebookthief]

I can't seem to be able to view the document with the standard test procedures!

Is there any other document that has the details of the test you're talking about?

Thanks!

[norman40]

Hi thebookthief,

Sorry, I think something went wrong with the link I posted earlier. Try this one:

http://mahshahr.aut.ac.ir/lib/exe/fetch ... m_d790.pdf

I hope this helps. Please ask again if you have more questions.

A. Norman