Vibratory Stress Relief for improving fatigue life

[kiwinjuneer]

Can anyone clarify the effectiveness of Vibratory Stress Relief for improving the fatigue life of carbon steel?

Can a component that may be subject to fatigue failure, be stress relieved on a regular basis using VSR and have its service life prolonged?

Does VSR actually convert Martensite back to Austenite either in part or in full and will it change the elongation properties of the metal?

 

[Pressed]

It is a un-clear topic to say the least. It has been reported to work best during the welding solidification process. I have heard about it being used for stress relief on a regular basis but haven't read any scientific reports to say much.

There aren't many places that have this equipment. Those that do, it seems 50/50 on opinions; but as it can't be seen or touched, I understand why that is.

The third paragraph about converting, I would say no.

I suggest doing a test, if possible. Maybe borrow the equipment or go with your part/s to where the VSR machine is.

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[kiwinjuneer]

Thanks Pressed

Interestingly, "something" happens to the crystaline structure of the metal with VSR as I have used it on numerous occasions to pre-camber crane and bridge beams. You simply set the beam down on numerous packers of appropriate thickness, plug in the VSR unit in the evening and bingo, by the morning you have a perfectly cambered beam. At the time, we never questioned what it was doing to the metal but I now have an application where it would be very nice if the same technique could release at least some service induced stresses and thereby enhance fatigue life.

I guess my best bet is to conduct some accelerated fatigue testing with/without VRS and see what happens. Unless anyone else out there has the answers...?

 

[metengr]

You might want to check these threads below for added information;

thread330-131445

thread330-92768

thread330-77148

 

[swall]

Setting aside the issue of vibratory stress relief, fatigue life of a carbon steel component will be improved only if residual tension stresses in the high stress area of the component are reduced. So to evaluate stress relief for any given situation, you need to know the areas of the component will see the highest stresses in service.Then you need to know what the residual stresses are in that area.

 

[kiwinjuneer]

Thanks metengr and swall and I do apologise to all for not using the right search box on the tool bar. The eng-tips forum search only seems to pick up the current threads while the google one gets those older ones referenced above.

After reading them I am still confused though and it seems that a lot more research is required. One post states that the effect is of a limited elastic nature while another says plastic. Not being a materials guy, I'm not qualified to comment. But I do know that the impact VSR has on pre-cambering 36" deep beams was not my imagination and certainly not a temporary thing. In fact so far as I know, the cranes, bridges, syncrolifts etc are all performing to spec and still in service.

 

[bklauba]

Most users of vibratory stress relief do so to enhance dimensional stability during machining, assembly, transport, and field use. In a paper presented at ASM International's Trends in Welding Research Symposium last year, it was reported that 30% saw an improvement in fatigue properties that they attributed to use of vibratory stress relief, but 95% used the process for dimensional stability purposes. (None reported a detriment to fatigue.)

There is no evidence that suggests that any metal's physical properties are altered by vibratory stress relief.

Current modeling of vibratory stress relief, chiefly presented in the work of Walker, Waddell and Johnston, from the U. of Strathclyde, Bill Hahn while he was at U. of Illinois and Alfred University, and Y.P. Yang of the Edison Welding Institute, would suggest that loading that would normally be only elastic in nature, causes plastic events due to combining with high-level stresses within the material, triggering or unleashing a form of potential energy.

Can you describe the workpieces involved in detail?

More info can be found at:

http://www.airmatic.com/content/view/50/70/

BK

 

[BTIGUY]

Kiwinjuneer,

Nice questions. As one who has witnessed this industry for over 30 years, a mfr of the Meta-Lax line of eq, and a planer-mill machine shop owner, I can answer your first 2 q's (and I don't know the third). But before I do let me point out that "vibration" is much like heat - there are lots of diffrent energy levels. Like heat, there is an optimum zone that will produce successful results. Also, like heat there is an energy zone that will adversely affect your parts including causing fatigue. What we have found from our own experiences in our machine shop is that SUB-harmonic energy is optimum when using vibratory stress relief. We call this process META-LAX. This was also mathematically verified by a PE from Lockheed. Successful results from using sub-harmonic vibrations will be CONSISTENT machine stability, reducing "premature" fatigue, and reducing delayed distortion. Now for the answers.

1. Clarify fatigue life improvement. By applying sub-harmonic vibrations "premature" fatigue can be reduced. The fatigue life of sub-harmonic treated parts will be extended compared to non-treated parts simply because sub-harmonic treated parts tend to last up to their designed strength whereas non-treated parts tend ot break well before. An example of this is from Corrosion Engineering who makes vibration screen decks for the brutal mining industry. They reported in an article that after using sub-harmonic vibrations on a very critical part of the screen deck called cross-tubes that they eliminated cracking from use in all 600 cross-tubes they mfrd strecthing over a 4-year period.

2. Reducing stress buildup by periodic SR. YES, here is an example. The aluminium die cast industry tears up its tooling routinely around 125,000 shots. Yet if sub-harmonic vibrations are used periodically (we suggest at 20,000 shot intervals) die casters report sometimes up to a 4-fold improvement. Similar examples can be found from high performance racing.

Caution, beware of putting any value on reports that were conducted by students which contain no hands-on application for proving their thesis, like the CEER report.

There are well over 140 application photos on

http://www.meta-lax.com

website, many of those indicating longer service life from treated components.

What is your application?

BTIGuy

 

[bklauba]

To dismiss Dr. Bill Hahn's work as "that . . conducted by students which contain no hands-on application for proving their thesis" seems ill-founded. A great deal of published research is made possible by the hard work of graduate students, published under the name of their supervisory professor. Knowing Dr. Hahn, I also doubt that he would allow his name to be associated with anything he did not approve. Both CEER and Alfred University have published this work.

But Hahn is not the only researcher who has compared the effects of resonant and non-resonant approaches to vibratory stress relief. Both Shankar when he was at the Oregon Graduate Center (now part of the Univeristy of Oregon) and a research team led by Dr. Y. P. Yang of the Edison Welding Institute have done in-depth studies of this issue.

Shankar's work can be seen at:

http://www.vsrtechnology.net/images/stories/pdfs/Shankar.pdf

The paper written by Yang et al can be seen at:

http://www.vsrtechnology.net/images/stories/pdfs/Yang_Jung_Yancey.pdf

Other research papers on vibratory stress relief can be accessed at:

http://www.vsrtechnology.net/index.php?option=com_content&task=view&id=22&Itemid=34

All of these researchers have reached the following conclusions:

1. Both resonant and non-resonant approaches to vibratory stress relief can relieve residual stresses.

2. Resonance was more effective in side-by-side comparisons.

BK

 

[EdStainless]

I have used these systems, and like many I was most interested in dimensional stability. However fatigue was also a concern. We decided that the net effect was to "average out" the residual stresses. Stresses overall didn't go away, or even get smaller, but it did relax locations of highest stress.
We didn't see improvements in fatigue life specifically, but shafts that had been VSRed were straighter after use and developed fewer alignment and wear problems.
If you are truely fatigue limited tehn you need to redesign the part. You cannot count on stress relief to make up for material property variations and machining issues.

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