Fatigue on Super Duplex Welded Tube 3

[Golum]

Can anyone please advise,

I have a requirement to perform fatigue testing on some Super duplex tubes which contain butt welds in the centre.

Strain has to applied in the weld area.

Resonant frequency testing has been specified which is fine but very expensive.

An alternative suggestion for a pulsating tensile test is being considered. My concern is how do I ensure the strain is applied at the weld if we only apply direct tension.

 

[MintJulep]

I don't know what a super duplex tube might be, but generally if you apply an axial load to a tube-like thing you get strain everywhere in the tube like thing.

How will the thing actually be loaded in its application? The loading obtained by tension will be very different than the loading obtained during a resonant dwell test.

 

[Golum]

The application shall subject the tube to both tension and bending over a sustained period with bending stress being by far the dominant feature.

My question I guess is if the test is only tension will the strain applied be uniform in the sample including at the welds.

 

[stanier]

Tke a sample of the tube containing the weld. Install the tube in a lathe. Arrange for a weight to be attached to the end of the tube. Run the lathe.

The tube weld will be subjected to cyclic bending strain.

Although this does not imapct a tensile strain it can achieve somethnig that you seek.

 

[metengr]

Golum;
You need to think about this a little more before going down the path of testing and finding out the results are worthless.

You have a tube and now you introduce a butt weld. The first question that needs to be asked is this butt weld a full penetration butt weld?

If so, the service application needs to be considered. If the tube contains internal pressure and is exposed to service temperatures above ambient service temperature, one has to consider thermal and mechanical stresses on this butt weld – not uniaxial tension.

So, I would think you would need to have a test set-up where a three point beam fatigue test is required to obtain any meaningful results. You are trying to simulate possible bending stresses along the OD surface of the tube at the location of the butt weld.

Last, introducing a butt weld in thin wall heat exchanger tubing or thin wall tubing is not good engineering practice.

 

[Golum]

metengr,

Thanks for the advice, The butt welds are indeed full penetration and the tubes shall be subjected to internal pressure and temperatures above ambient (this is the reason for the fatigue tests, we are validating an SN curve)

Your points on uniaxial tension are well received and back up my concerns on testing with the welds.

Resonant Frequency testing is normal for testing of line pipe, we have just not used it on something smaller.