s-n curve 301 spring temper
s-n curve 301 spring temper
(OP)
Can someone point me to a source for fatigue limit curves for 301 stainless strip? I'm working on a part .01 inch thick, .25 inch wide and about 1 inch long. It's a spring detent function. Loading is in one direction only, lifetime cycles only a few hundred, normal operating position approximately half deflection, static load. I calculate bending stress at max deflection of 160ksi. Would a material with 200ksi proof strength and 0 to 120ksi fluctuating fatigue limit (Sandvik 12r11 218ksi for example)work? I have a tight space constrait to work in, hence my dilemma. Thanks in advance for any advice.





RE: s-n curve 301 spring temper
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RE: s-n curve 301 spring temper
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First ASTM A666 only takes you to 185ksi UTS at full hard.
For this material A-L give an endurace limit of 80ksi.
You are either going to need an actual fatigue curve, and/or a stronger material.
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RE: s-n curve 301 spring temper
RE: s-n curve 301 spring temper
RE: s-n curve 301 spring temper
If this part is statically loaded at ~ 80 ksi, and then periodically dynamically loaded to 160 ksi, then I would be more concerned with force loss/compression set then fatigue life. Probably Sandvik 12R11 would be ok for fatigue life based on several hundred cycles to 160 ksi.
RE: s-n curve 301 spring temper
Any feel for how much compression set might be expected? Say 20%? More or less? I'm just after a gut feel for now so I can get the design close. Then we'll test it.
I was thinking that a slight yield during initial operation might be advantageous in that it would redistribute stresses and accomodate tolerance stack-ups. Is this a reasonable thought or am I asking for trouble?
RE: s-n curve 301 spring temper
There is no substitute for calculations for the minimum properties of the strip. When your calculations for minimum material properties show that it will do the job, then repeat the calculations for the actual material properties and compare it to the tests. If you have a match then you can assume that your calculation for minimum properties are OK.
RE: s-n curve 301 spring temper
RE: s-n curve 301 spring temper
It also gives the S-N curve for differing tensile strength levels.
(reverse bending stress at 20C Mean Stress=0)
CODE
Prob -- Stress Level
50% -- +/-550N/mm^2
10% -- +/-485N/mm^2
0.4mm / 1700N/mm^2
50% -- +/-575N/mm^2
10% -- +/-525N/mm^2
0.4mm / 1900N/mm^2
50% -- +/-595N/mm^2
10% -- +/-530N/mm^2
IF you find that your design is robust enough as far as max stress and relaxation you may be able to use a better steel such as 11R51 (17-7PH) or even 7C27Mo2 (MOD420) both of which have higher tensile strengths and higher endurance limits. However they are way way more expensive. One thing to remember when designing with Sandvik grades of steel is that some grades (11R51 & 7C27Mo2) are extra clean for fatigue applications. Also you may be able to use a carbon steel if the environment is dry and clean, or oil immersed.
Sandvik steel along with Hitachi and Uddeholm are some of the better steels for fatigue applications. They seem to have better control on properties adn gauge.
Nick
I love materials science!
RE: s-n curve 301 spring temper
11R51 is not similar to 17-7, the sandvik name for that is 9RU10. 11R51 is roughly the same chemistry as 12R11 just with a 0.7 nominal Moly addition. This "...has better tempering properties. The Mo additive also gives it a slightly superior corrosion resistance.
RE: s-n curve 301 spring temper
Good comments toward consideration of minimum properties. I hadn't considered that issue.
NickE,
Those figures are close to what Sandvik currently posts on their website. I headed directly to 301-type material because we already use it in other (less demanding) applications in the same outdoor agricultural machinery environment. Plated carbon steel could perhaps be an option. This spring will be inserted in a die cast aluminum housing so I do need to consider galvanic action. The other end of the spring will be rubbing against acetal. Does the S_N curve you have go down to 100s of cycles or does it start at 10,000 cycles? Any chance you could post an image or pick off some points for me?
Thanks.
RE: s-n curve 301 spring temper
Only down to 10,000cyc.
Nonetheless here are the starting points:
CODE
Prob--Cycles--Stress
50% -- 10^4 -- +/-1025N/mm^2
10% -- 10^4 -- +/-950N/mm^2
1700N/mm^2
50% -- 10^4 -- +/-1075N/mm^2
10% -- 10^4 -- +/-1012N/mm^2
1900N/mm^2
50% -- 10^4 -- +/-1125N/mm^2
10% -- 10^4 -- +/-1075N/mm^2
The curves all have the basic S-N Curve shape, the knee starts around 1x10^5 and the curve goes flat again around 6x10^5.
You will have a glavanic problem if the aluminium and the steel take any baths together. And ambient moisture will cause trouble after time. I tend to believe in the "design for roubustness" theroy, so I would use SS of some sort, or maybe another spring alloy if the corrosion issue is severe. Even these austenitic hard rolled grades will corrode if the environment is right.
Be careful of crevices! also surface condition is going to be important. Some sort of finishing process should be applied after stamping.
(oh and I should have ended the quote in my correction post above, Thats directly from the sandvik book. They do have a heat treatment for 11R51!)
Nick
I love materials science!