Chromium Vanadium S-N curve 1

[rafamaniac]

Hello Everyone,

Can somebody tell me where i can find an S-N curve for this material, specifically for the SAE 6195, or any grade of chromium vanadium?
The problem is that i'm trying to calculate analytically the correspondent stresses for infinite life (10^6 to 10^7) in order to make the curve, having already calculated the endurance limit (Shingley Mechanical). I want to put these values in Ansys, however analytical formulas don't reproduce the real life S-N curve, obviously, because the material in this case is tested in laboratory and in rigorous conditions.

Thanks a lot,
David Marques

 

[mp87]

I am a bit confused by your request, since according to Shigley the endurance limit already corresponds to a fatigue strength in the range between 10^6 to 10^7 cycles. So, based on the classical infinite-life approach, you should keep stresses well below this limit (and take into account all factors that influence it as explained in Shigley's book).

Anyway, if you really want to build the entire S-N curve, considering that your fatigue limit is already approximated, you can assume a fatigue strength equal to 0.9 UTS at 10^3 cycles and connect the two points with a straight line.

 

[tbuelna]

rafamaniac-

I believe SAE 6195 is an obsolete alloy designation, but you'll have to check SAE J1249 to be sure.

Out of curiosity, do you have some specific application in mind for this material? SAE 6195 is a high carbon Cr-V steel, and it was typically heat treated to very high through hardness for applications like roller/ball bearings. Given how this material was typically used, I don't think you'll have much luck finding any tensile fatigue data for it.

 

[mp87]

Ditto tbuelna, 6915 is a former SAE grade that should not be anymore available.

I have only one objection, full hardening steels are actually finding an increasing use in diesel fuel injection equipment, and in such application the knowledge of fatigue data is of great relevance due to the high pressures involved.

 

[rafamaniac]

Hello,

Thank you people or the responses. i had to calculate the stress to the correspondent cycles to make the curve i had no other choice because i could not find the S-N. however i find a bit weird why is it so difficult to find S-N curves for steels online because all the materials have to be teste before going to the market. I was making my final projet in mechancial engineering and the object of interest was a bearing puller. Everywhere i look online about the material for the pullers they all say that is chromium vanadium but i read some comments where they say very rarely pullers are made of this material and its all a bunch of lies. The producers that put their products on amazon for example say they are made of this material because it looks great for the consumer because it is a high quality steel. However in reality its not. However as long as my calculations are correct i am happy.

Cheers,
David Marques

 

[mp87]

Okay that's a cute answer and perfectly legitimate considering that you are a student, now let me regale you with some words of wisdom

[ol 1]
[li]All materials are tested before being commercialised: it's generally true, but the test may vary a lot depending on the intended use of the material. Stainless steels are typically selected for their corrosion resistance, so they are likely to undergo corrosion testing before any rotating bending machine comes into play. Same story for an abrasion-resistant plate, the focus will be primarily on hardness and wear testing.[/li]
[li]Brochures don't always say the truth: I know, it may sound astonishing but that's how things work The fact is, unless you have no other clues to start your analysis from, it is advisable tokeep far from advertising material. That is intended for selling, not to provide technical advices. The good thing is that there is a myriad of other sources (handbooks, databases, scientific papers, to name a few) that put at your disposal (more or less) accurate and pertinent data.[/li]
[li]As long as your calculations are correct your prediction leads to a reliable and cost effective product, you're happy: Keep it in mind, numbers are easy to "adapt" so to demonstrate that everything is compliant, but then reality will spoil the inaccuracies you painfully tried to cover.[/li]
[/ol]

 

[EdStainless]

As mp87 said, for many alloys fatigue is not an important factor at all.
Do you have access to the latest copy of the MMPDS?
These are alloys for aerospace use and they have well defined strength and fatigue data. One thing that you will notice is how few alloys are listed.
That is how it works. The more complete the characterization the fewer grades you will find.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[rafamaniac]

Hello,

Thanks a lot for the feedback. I already finnished my projet. I calculated the endurance limit in "Shingley" and made the S-N curve out of it, because it is a more realistic approach of what this material would suffer from a real life fatigue (because it counts factors like temperature, surface finishing, size factor , load factor etc). Any S-N curve is data that is extracted from laboratory from very specific conditions, which isn't the case, just like this puller or basically any other tool.
Is this logic that applied correctly made?

Cheers,
David Marques

 

[mp87]

Hi,

Just to make sure we understand each other:
[ul]
[li]S-N curve -> used for finite fatigue life[/li]
[li]Fatigue limit -> used for infinite fatigue life[/li]
[/ul]

All modifying factors you mention usually apply to the fatigue limit only and are therefore used in the second approach.

The S-N curve, although primarily derived from standardised specimens in strictly controlled laboratory conditions, is a more versatile tool that can be applied directly to real components (for example, curves were developed for gears and welds). Needless to say, the more boundary conditions you introduce, the less general is the outcome of your test and therefore its applicability to different scenarios. Since in many practical cases an approximate S-N curve is sufficient, one can build it using basic material properties (typically tensile strength) under certain assumptions.

 

[metman]

Quote
"I was making my final projet in mechancial engineering..."

Design for RELIABILITY, manufacturability, and maintainability

 

[tbuelna]

Published S-N curves are usually based on results from testing a statistically relevant number of identical specimens under the same conditions. The analytical approach described that involves many adjustment factors, will naturally give a very conservative result. Since this application is for a puller tool where weight would not seem to be a concern, I imagine designing for very modest stress levels to ensure desired fatigue life capability should not present an issue.