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Thin stainless tube is always hard, anneal it should help, but can you just switch to mild steel, or a chrome moly tube? 316 is a bitch when it comes to work hardening, it happens fast and is a seriously common problem. Boat tanks made out of stainless frequently fatique crack do to the liquid sloshing, alu or steel holds up far far better in thoes applications, like your magnitudes better.
Other option may be ditch metal, use a composite, they can resist fatigue loads far far better.
You say thicker walled tubes held up, but not why you need/want to go to a thinner wall. How does the fusing of the felt influence wall thickness? Faster with a thinner wall?
Here's one more option. Take the lightest accelerometers you can afford to buy, and a storage oscilloscope. Instrument the tube along its length (concentrating on areas it's already cracked in), then record the vibrations/stress that section of the tube sees during operation. You may need to do at least two readings per section, at right angles to each other.
Then you can try tuning (damping) the greatest areas of movement (vibration) with clamp on weights, which could be as simple as split collars. If you can even out the stress along the length of the tube you'll greatly extend its work life.
I think others mentioned shot blasting. That will also help with crack resistance. Even a cryogenic treatment might help when using a ferrous tube material.
Work hardening is probably (bit) different issue. Stainless steel (or aluminium) is prone to fail at some point even if you don't stress it nowhere near yield limit if you just put enough load cycles at it. At 20khz you get 1 million load cycles in just 50 hours!
Most non-austenitic steel grades have nearly infinite load cycle life IF you manage stay below some certain stress level. IE:
https://www.mw-ind.com/pdfs/GoodmanFatigueLifeEstimates.pdf
Actually, much quicker than that. Each weld cycle is about 2 seconds. Approximately 400 parts per hour so 8 million cycles per hour.
Yeah, I meant to write that you get 1 million load cycles in 50 seconds of operation, not hours (stupid me)
For acoustic resonators (horns) the best materials are those with very high strength to mass ratios, such as titanium or Scandium alloys (very rare use). Fine grain tool steels such as D-2 and particle steels (M4, CPM-15V) are good also but take more energy for the same amount of face amplitude. Some designs cut against the horn face so it then has to be very hard.Neighbor toolmaker was making some ultrasonic horns. Worked thru several
failures, IIRC they settled
on d-2 heat treated right up to the upper hardness limit.
For acoustic resonators (horns) the best materials are those with very high strength to mass ratios, such as titanium or Scandium alloys (very rare use). Fine grain tool steels such as D-2 and particle steels (M4, CPM-15V) are good also but take more energy for the same amount of face amplitude. Some designs cut against the horn face so it then has to be very hard.
For the anvil, the part that supports the part being welded, the mass is not so important as it does not transfer acoustic vibrations to the workpiece but the strength should still be high as the vibration still conducts through it.
Uhm I thought you were asking for advice with this here thread ?
No need to lecture me sir.
Uhm I thought you were asking for advice with this here thread ?
No need to lecture me sir.
hm, but the lower stiffness material will have lower strengh and therefore lower fatigue life, no?
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