Surface treatment for compressive pre-stress 1

[moon161]

A tubing spreader crossed my desk, engineer asked me to make a drawing to source from a different shop.

Tool is 2 halves of conical spike used to spread apart tubing so a barb can be put into it. Stainless is preferred because it's medical mfg. Current tool is hardened stainless, failed in fatigue from the bending stress 3-5000 cycles.

I remember hearing shot/bead blasting put a compressive prestress into the surface. Anyone work with this to delay or reduce fatigue problems?

 

[zeusfaber]

If you use "shot peening" as your search term, you'll find some information.

A.

 

[CoryPad]

Shot peening is a very common process used to impart compressive residual stress at the surface of parts to improve fatigue life. Other techniques include roller burnishing, deep rolling, and laser peening.

 

[iainuts]

Only 3000 to 5000 cycles? The part is has to be over stressed to begin with. Shot peening isn't likely to help. Look for stress concentrations first. Make the part beefier if at all possible. Lastly, look for a material with a higher fatigue strength. Shot peening will help a bit for very high cyclic fatigue failures, but I doubt it will do anything for such a low cycle failure.

 

[unclesyd]

The Metal Improvement site has a lot of information on shot penning.
If your part can be black you could use the Kolene QPQ or Nitromet FNC process on material like 17/4 PH SS.

http://metalimprovement.com/joinnow.php

http://www.nitromet.com/

 

[tbuelna]

moon161,

Processes like laser or shot peening improve fatigue life by inducing a compressive pre-stress on metal surfaces that are subject to high localized tensile stresses in service.
As others noted, if your tool failed structurally after less than 5K load cycles, then you have problems that shot peening won't help.

A more effective approach might be to change the shape/profile of your expander tool such that any stress concentrations are minimized, and the bending stresses it experiences during operation are more evenly distributed within its structure. It's an old axiom that "stiffness draws stress". So making a part more flexible overall will usually help to reduce high localized stresses. A good FEA would probably give you some guidance on how to best address your problem.

Good luck.
Terry

 

[Ron]

Agree with iainuts...this part is overstressed and shot peening won't help much. Redesign.