rotating beam fatigue testing problems 2

[frank72]

I'm currently testing Ti6Al4V on the rotating beam fatigue test setup. the problem is that quiet some samples do break just at the end of their fixation. Actually all specimens were produced according to the relevant standards. We have been testing some other materials like TiCP, 316L Stainless, and so on without having problems. So far we tried to put a foil in between to avoid stress concentration but that doesn't change anything.
Has anyone experience with this system?

Regards
Frank

 

[redpicker]

I have seen "fretting" damage on Titanium alloys when attached to steel in a manner similar to the collet arrangement I believe you are using for "fixation". Examine the OD of the samples in the location of the fractures with a SEM and see if you don't have the same problem.

Solution? I don't know. In the application I was dealing with, we couldn't find one other than reducing the bending load (which in your case, would defeat the purpose). Maybe you could use a titanium collet?

rp

 

[EdStainless]

Ti alloys are not very damage tollerant. I have seen soft collet liners used, such as copper. You may need to look at alternate sample shape to reduce stress at the collet.

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[frank72]

thank'x for the advice. first of all - I'm sure we do have some fretting and or corrosion on the samples. the problem is that we do test up to 480 MPa load on the test specimens. I think there is some mircromotion between the samples and the collets. there is no way to fix it more tightly. as a result I've been thinking of making bushings of titanium or copper - atually what I would prefer would be some mateiral for the bushings with a young's modulus between titanium and steel. has anybody a good idea?

Regards
Frank

 

[CoryPad]

Gray iron is the only inexpensive material with elastic modulus between titanium and steel, the others being germanium, manganese, platinum and vanadium.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Metalguy]

If you're testing at room temp., you might try using a Loctite or similar "Bearing Mount" compound. Something like their no. 620 or 635 (as I remember) may prevent the fretting.

"When the Eagles Fall Silent
The Parrots Start Jabbering".

W. Churchill

 

[sreid]

If fretting is the problem you could try a couple of layers of Teflon Pipe Thread Tape. It acts as a lubricant.

 

[NickE]

I use a relatively thick teflon tape, 0.020", between my cantilever bend test specimens and the tool steel mounting block.

(This is for flat flex beams of 1095 steel HRc54 against A2 HRc58-62.)

I too noticed early failure and non-stress level related results due to fretting damage.


Nick
I love materials science!

 

[frank72]

Again thanks for all contributions!

@metalguy
I've tried several ways to avoid the friction by lubrication. actually non of them worked so far. We do have a special high moly grease and that doesn't change anything. I've also tried to work compleately "degreased" but that didn't help as well.

@NickE
I didn't think of that up to now. As we only had 0.11mm (.0043") I tried this. the advantage is that even as we had very vew vibrations the vibrations are even more reduced - BUT now i have the new problem that it is very hard to fix it to avoid a slip out of the test specimen - that happend now twice to me but I will try it again and I'll get some thicker teflon tape.

of course I'm still open for other inputs and I'll keep you infromed!

Regards
Frank

 

[EdStainless]

I didn't want to say this earlier, but you really need to repair your collets. They should fit very well with no sharp edges and full contact. You may need to be far more exacting on machining you samples in alloys with these low toughness issues.
In the past I have used very thin metal foil liners. The other thing is to run samples in under light load for a short period and then inspect to make sure that you have a good fit.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[frank72]

@EdStainless: We have been using brand new high precission collets with a polished edges. the samples have a roughness of 0.3micormeters - which corresponds to former N5. as mentioned above I've already tried to use a thin foil... but again it tells me that we already were in the right direction with our thoughts.
Regards
Frank