a strange failure of an unalloyed Tantalum bolt 1

[elinah34]

I have a 8-33 Tantalum bolt which we manufactured according to ANSI Standard.
When tightening the bolt we got a strage and unfamiliar (at least to me) failure of the bolt's thread (I attached photos).
I thought that when designing according to the standard the shank should fail before the threads.
In addition, even more strange is the thread didn't really tear off the shank but it looke like it plastically deformed.

What can you say about it ?

 

[3DDave]

I can say - how much engagement was there?

 

[elinah34]

5 pitches

 

[hydtools]

The screw failed in torsion and wrinkled the thread in the area of failure. Presummably the threads not engaged in mating threads.

Ted

 

[elinah34]

hydtools - very interesting!
I read a little about common failures and it wasn't mentiined.

 

[Artisi]

A cross section sketch of the bolting arrangement might put a bit more light on the problem, and would confirm hydtools observation / comment that certainly looks like the answer.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[TugboatEng]

To me it looks like the bolt yielded in torsion while tightening and the yielded again during loosening to give that back and forth wrinkle. It seem like the threads seized or the bolt bottomed out in the hole.

 

[dik]

Did you try an anti-seize treatment? What are the tolerances of the threads?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[3DDave]

Pure tantalum has a 50% elongation - per one bolt source. And still has a non-zero yield strength. That's an uncommon material specification, so it's not a surprise about the uncommon failure.

I agree that the screw must have either bottomed out or snagged something in the hole as it isn't a tensile failure. I've seen those in stainless steel and the main ductile change was a large increase in thread pitch.

 

[mfgenggear]

so I tried to find a data sheet on this material and all I found was this white paper.
depending on the alloy tensile properties and hardness value are all over the spectrum.
please advise alloy of this material. material certification would give important data.
my opinion is the tensile properties (hardness) is in adequate for this application.
the material seized while installing and failed during assembling and then on removal.
as hydratools posted. or the the threads were incorrect on the mating part or this bolt.
was the threads on both parts verified with thread gages.

https://apps.dtic.mil/sti/pdfs/AD0266415.pdf

 

[elinah34]

Hey, nice to see a lot of answers.
First of all here is a link to the manufacturer material in which you can find some data regarding Tantalum:

https://www.plansee.com/en/materials/tantalum.html

The bolt couldn't bottom out since the mating thread is a through one, so the bolt might protrude out.
I also suspected the bolt seized (cold welded) but I didn't feel it stuck while tightening it, and there was no typical sound I know when such thing happens.
About the tolerance - in the drawing it was written to stick to the standard definition.
Can you explain what you meant by a "a large increase in thread pitch"?

 

[Compositepro]

I agree that this appears to be torsional yielding of the bolt where the threads were not engaged with other threads. The waviness is due to compression buckling of the thread tips.

 

[3DDave]

As tension is developed by tightening the bolt elongates and the thread pitch increases with it, much like a Slinky toy the pitch increases with tension. If the bolt has significant elongation ability the un-engaged threads in tension will stretch. In this case the pitch appears unaffected, just angular displacement, so the waviness wasn't a result of tension.

The material is soft, so there would be no noise to its galling.

 

[saplanti]

How did you take it out might direct us to answer correctly. Did you find any failure on the inner thread? What exact materials and heat treatment were used on both sides? Note that first four threads did not fail like the others.

I am with Compositepro, 3DDave and Hydtools for the cause of failure until I see further information.

 

[elinah34]

saplanti, what do you mean by how did I take it out?
If I understand the question, so I unscrewed it without any excessive force.

 

[saplanti]

Yes, you understood corrrectly.
You say you unscrewed without any excessive force. In that case, when you screwed it first time the first four threads engaged without any problem and fifth and sixth were damaged as given on the photos. This seem to be unusual isn't it?
In that case, if possible, I would check the threads on the bolts, there might be a manufactural mistake between pitches on the same bolt.

Or there might be something on the fift and or sixth threads during the tightening that caused this kind of failure. Is this happening on all the threaded connections? If not this might be the cause. If it is happening I would question the pitches of the threads on the same bolt and/or between threads on the oppsite connected members.

 

[Artisi]

Is this the first time you have used this bolt/material in this application?
Could be worthwhile repeating the assembly / bolting while carefully observing what is taking place, ie is the bolt free in the tapped hole, any signs of binding, what is the surface finish on the female thread -rough /smooth any sign of galling, try lightly torquing the bolt, remove and check thread surfaces for damage, was the failed bolt over torqued?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[mfgenggear]

elinah34

can you please explain the the machining process? were these single pointed on a lathe. please specify the thread type and size.
on the link you provided it states this material does not machine very well, because it is dead soft it will have a poor finish if machined.
were there burrs present, were these deburred properly, if there can not be a material change then rolling these threads may be a better
application instead of single pointing.
one other question why was this material chosen?
Edit: also can you provide a screen shot of the thread requirements, I am also puzzled by the question you asked about the enlarged thread pitch.
and I really recommend to purchase thread gages, what this will provide that the thread bolt will assemble to the mating part. it incorporates, all errors in the thread well as
to verify the correct geometry, and pitch diameter. and plug gages for the mating part. since this material is soft it will be prone to galling and seize.
a coating of either dry film lube or anti seize as mention earlier.

 

[elinah34]

Hey mfgenggear
I can't give details why using this specific material, though I am sure it's interesting. I can just say it's a requirement/constraint.
You are definitely right - this material isn't ideal for machining, and the right way of producing bolts made of tantalum is rolling.
But when contacting manufacturers about rolling tantalum bolts we got an enormous minimum ordering amount since this product is a custom one for us, and as for now we can't afford it.
So, we machined it by turning (lathe). The quality wasn't perfect, and indeed we could recognize the surface wasn't smooth but quite rough. I am attaching the drawing that was used to manufacture this bolt.
About thread gauge - I was sure it's the manufacturer role and duty to check with this instrument:/

 

[elinah34]

Here is my drawing