High fatigue strength material?

I have a problem with which I could use some help.
I am in the process of designing a device, the purpose of which I am not
at liberty to divulge but may describe the relevant parts.
Picture a coiled stainless tube of 1/32 ID and 1/16 OD, seamless. It must be connected at its ends to two parallel shafts such that the coil is bent into a 180* bow. The coil is about 2.5" long and the tube from which it is made about 30" in length. The shafts rotate counter to each other. I have been experimenting with this awhile and the coil breaks at the point where it connects to the shaft, which is not surprising. After pondering what was needed I have come to realize that the strain profile of this coil must be altered so that the high strain at the point of attachment will be distributed over a much longer span of the tube.
To that end I would like to experiment with butting the tubes such that there is very little strain at the points of attachment and that the stress is distributed in an appropriate manner along some length of the coil. So imagine a straight tube, ID constant and OD starting to thicken at some point from the end and progressively thickening as it approaches the ends.
Would anyone have any suggestions of what material could be used to augment the straight tube in this manner? This material would have to bond permanently to the outside of the tube and would have to have a pretty goof fatigue strength. I am not yet sure if I can keep the strain of this thing well enough below the fatigue limit so I am hoping someone may know of a material with high fatigue resistance that might be appropriate for this sort of application. I will investigate fusible metals, but I'm not confident they will possess the desired properties.
I wish to engineer it to last at least 300 million cycles (full revolutions), which means keeping things well below the fatigue limit.
The arrangement looks like this:
________ / \ / \ / \<---coiled tube | | | | | | ___ ___ | | | | | | | | | | | | | | | | | | | |<----shafts | | | | | | | | | | | | | | | | | | | | |___| |___| Rotation-----> <-----Rotation
Any help would be greatly appreciated.
Thanks.
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On 01/11/2010 03:50 PM, Demon Buddha wrote:

easy! look up the appropriate s/n data for your grade of stainless, then calculate curvature radius such that surface stress in your coil falls below that required to yield your loading count. something >10m should do it.
or you could just be specific about your freakin' application so you're not wasting peoples time.
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I dont think your problem is the material, I think you just need to design it properly.

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Demon Buddha wrote:

Generally speaking, it sounds like you want a material of the type normally used for making springs. See _Spring_Manufacturing_Handbook_ by Carlson. There's a companion volume, _Spring_Designer's_Handbook_. And when it comes to spring design, my highest praise is reserved for _Spring_Design_ by Berry.
You can hunt for these classic volumes here:
http://used.addall.com /
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