Quick question about threaded-rod tensile test 1

[mxracer17788]

I am going to perform a standard tensile test on a fully threaded 5/8" rod. I had two fixture blocks made that are threaded for this bolt pattern, and the tensile tester holds onto these steel blocks when it pulls. The rod is 6.5" in length, and has a thread engagement of 1.5" on each side leaving 3.5" of material between the two fixtures. My question is, where will this threaded rod tend to break? If it breaks at the surface of the block, it will be difficult to remove the small remaining piece that is still threaded into the fixture block.

 

[mxracer17788]

Is there a way to predict where the rod will break?

 

[EdStainless]

Not unless you machine a section. If your set up is not super straight it will break at one of the holders.
Before you test cut a slot across both ends of the rod. Then when it breaks you can use a screw driver to remove a short piece.

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Plymouth Tube

 

[mxracer17788]

Also, what material should I make these threaded fixtures out of? the rod is a36

 

[EdStainless]

I would make the end threaded holders out of the hardest material that I could thread.
Maybe a 0.30-0.35%C steel, heat treated with a temper high enough to just make it threadable.

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Plymouth Tube

 

[TVP]

I agree with Ed's suggestion, but would offer that 4140 alloy steel, with slightly higher carbon content, is more widely available and can be quenched and tempered to the necessary strength/hardness (> 40 HRC).

 

[racookpe1978]

The logic there being that you want to test (to stretch to failure) the threaded rod, not the test fixture itself nor the test clamp for the rod.

So, to exaggerate slightly, if your test clamps were threaded, and were made out of some material less rigid, and overall less strong than the threaded rod, the test fixtures would bend and distort before the rod would.

Now, what are you trying to test for actually?

The threads themselves as a stand-alone failure point? If so, then your assembly needs to remove as a failure point everything except the tips and roots of the threads of the "average" rod.

The final tensile strength at failure of the (average ? minimum? Maximum? Nominal? 5 sigma deviation weakest?) rod? How will you define average rod material strength and average thread and rod dimensions? How many rods will you use to develope your std deviations and statistics? How will your tests vary based on changes in rod threads and rod dimensions, compared to rod material deviations? And, how will you tell the difference when each rod has failed?

The tensile strength of an (average) threaded rod held in place by an (average) nut tightened to an (average) torque value? After all, if a nut is held in place by tension, doesn't that tension increase the local stress in the rod-and-threads that can induce failure of the assembly? So, shouldn't you be worried about how much the assembly can hold, not how much a rod itself will fail at?

 

[mxracer17788]

All I'm doing is performing a simple tensile test on these threaded rods (all identical) to get a baseline strength for another experiment. I expect that if the rods are all tested in a similar manner, that the results will also tend to be similar. I am not concerned with the strength of thread engagement, and I am trying to eliminate this variable (or at least control it for all rods) by picking a material for the fixture that wont deflect appreciably at the applied load of around 40,000 lbs. On that note, what is usually the threshold hardness for steel that can be threaded. (i.e. hardest steel that is threadable by any standard machining practices).

 

[mxracer17788]

I should also mention that the rods were manufacture at the same time from the same heat of steel.

 

[mfgenggear]

mxracer

A-2 tool steel, machine complete, then heat treat, 60 HRc, it's distortion is very minimal.
chase the threads after, & grind surfaces if needed.
Mfgenggear

 

[mxracer17788]

Is it absolutely necessary to use something like A2 or 4140 for this test? I do not want to spend a lot of money on the holding fixtures so I'd rather not purchase an expensive alloy and have it heat treated. If the threaded rod is 7/8"-9 and I have at least 2" of thread engagement, can't I just use a medium strength steel like A-36?

 

[Engdoitbetter]

Usually the stress distribution across the engaged thread is not uniform, so it'll probably break near the holders, but it depends also on the load direction.

 

[EdStainless]

You need something harder than A36 if you want to reuse them. Otherwise the treads will distort enough to make testing difficult and sample removal even harder.
Use 4135/4140. Machine the outside and drill the hole.
Q&T to some med hardness (1000F temper). Ask your heat-treater, I would aim for about 32RC.
Tap threads and use.

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Plymouth Tube

 

[mxracer17788]

Okay perfect. I think I am going to use A514 or A517 in order to keep costs down and avoid having to source out heat treatment. The hardness for this material as delivered should be around 20RC.

 

[Compositepro]

How about using off-the-shelf high strength nuts as part of your fixture?