Fatigue in suspension components

[Silente]

Hi,

i am wondering how you guys consider fatigue in your suspension members stress calculation.

I had some analysis on static load cases to understand when and which members are more stressed and to have a ballpark on how big they should be to have a certain safety factor.

But all this work doesn't analyze fatigue, since it is hard for me to think to a cycle with which every member could be stressed.

So my question is:
how do you consider fatigue when you do your calculations? you only use a bigger safety factor to be on the safe zone or you do different analysis to have a better idea?

On which members you would more concerned for fatigue failure?

Thank you

[fixitmattman]

You could turn to the "Fatigue Failure Theories" section in your textbook and go from there.

[Silente]

What kind of problem do you have?

If i asked here is probably because i know the theory but i need an advice on how to use it and maybe want to compare my ideas, don't you think?

A forum should be a place to discuss, not only to judge, like you are doing.

I would like to read your next question and see your face when somebody is going to answer in this stupid way.

Anybody else?

[Grant Mahler]

Actually, fixitmattman is right. The fatigue calculations aren't difficult, and are straightforward out of the book. Pick your component. Is it loaded in bending, compression, tension or some combination. Pick a number of cycles it must last. Run the numbers.

That said, most of the cars for this competition don't even get running until the week of comp, and run for a couple hours at comp (and poorly at that) and are not stressed anywhere near their fatigue limits. I doubt most teams have a clue what the true fatigue limit is.

The teams running carbon suspension components do an excellent job testing fatigue limits. If you absolutely must run this calculation, and you continue to be at a loss, ask someone whose continued success at competition depends on their having calculated the fatigue limits correctly.

[Sisyphus]

I have to disagree with Grant--fatigue calculations are generally difficult. I say this as having worked professionally in this area for several decades.

I am very happy to see that you recognize this is an important issue on a race car where the fatigue safety factors are low because of performance requirements. When we pulled the engine out of my friend's Formula Ford once, the back end of the car completely collapsed because all the welds in the space frame had fatigued. The car also broke during a race several times due to fatigue--only an idiot's design will fail due to tensile overload which is the failure mode typically taught in Machine Design.

I don't think fatigue gets nearly enough focus in the university (certainly was true at my school) but it is very important in industry (so you need to know about it).

One complication is that there are as many theories on fatigue as there are researchers in the field--and a theory that fits one situation well will not give reasonable results in another situation. One of the most prominent researchers in the field is Dr. Socie who has a free fatigue calculator on the web (www.fatiguecalculator.com). This site will give you some good insight but be sure you understand the physics of the models and don't use them blindly.

The most critical issue by far in making fatigue calculations is knowing the duty cycle. You need to think about the where the loads are coming from and figure out how many cycles of life you need and at what reliability--is a 50/50 chance of failure during Endurance OK? You will have to make many assumptions--when in doubt, be conservative. For example, if the loads are coming mainly from cornering, how many total number of corners will you have to negotiate during the event, including practice?

You also need accurate material data but if you are using mild steel or 4130 there is a lot of data available and these materials are well behaved. Check out Carroll Smith's books on materials and fatigue.

Good luck!

[BenB]

Silente, I would say it would be best to do the fatigue calculations. In FSAE I did the calculations for the suspension components I designed and I think they were used for several years in a row (which was my intention because we were trying hard at the time to build off previous designs so I thought it might go in the next car).

I think Grant is correct though. I was really ambitious with how much testing I thought we were going to have, and I would imagine now that those components are retired they only have used about 25% of their design life.

Once you figure out how many cycles you expect the car to do in a lifetime it is pretty easy to figure out the fatigue stress limit (and even easier to replicate those calculations for other components) so I would say do it!

[fixitmattman]

Originally posted by Silente:
What kind of problem do you have?

If i asked here is probably because i know the theory but i need an advice on how to use it and maybe want to compare my ideas, don't you think?

A forum should be a place to discuss, not only to judge, like you are doing.

I would like to read your next question and see your face when somebody is going to answer in this stupid way.

Anybody else?

I have a multitude of problems, don't think you want to be bored by the list of them though.

You question is right up the alley of "basic mechanics that should have been covered in class." Steel has well defined performance curves. If you're using an alternative material you will need to research the endurance characteristics of that material. Determine the forces, pick a loading case (fully reversed, repeated, etc.), design around the selected failure theorem. Do you want x cycles or infinite life? Do you want to safeguard against ultimate failure or yielding? Are you going to use a more conservative or more modern theory? These are things only you can answer, and they're in the fatigue failure theories section of any mechanical design textbook worth having.

FWIW I asked a question once, got a similar response, then felt bad because I asked an obvious question I should have researched. Felt like a jackhole after I found the answer.

[Adambomb]

Well, I can say that we have experienced fatigue failure before on suspension uprights after about 50 hours of run time, but those things were also so flimsy you could visually see like 5 degrees of flexure in photos of the car cornering. Of course the offending part was 4130, which is a good bit less forgiving in fatigue than mild steel. I've also witnessed a certain top-level car that will remain unnamed that experienced a fatigue failure on their 4130 chassis near where the shocks mounted.

I surely wouldn't discount it as something straightforward to calculate, either. Perhaps if you never hit any bumps, the load case is pretty easy, but I know there are a lot of us that drive on some pretty crappy lots for testing (or autocrossing for that matter). From there you pretty much are forced to either do some dynamic testing with strain gages or make some guesses. And that's just to get the loading figured out.

I've seen a lot more fatigue failures in the final drive. The shock loading can be pretty bad when a noob is behind the wheel. Also seen several engine vibration related fatigue failures once we switched to a single.

As for whether or not you will ever find the fatigue limit, it is pretty simple, 2 factors must be present: 1.) The part must be light enough to be just barely strong enough to not break under normal use, and 2.) The car gets a lot of run time. I would consider both of these items to be things that a GOOD team has to deal with. Some of the best teams get 50 hours on the car before they even pack for competition...

[Grant Mahler]

Originally posted by Sisyphus:
I have to disagree with Grant--fatigue calculations are generally difficult. I say this as having worked professionally in this area for several decades.

I am very happy to see that you recognize this is an important issue on a race car where the fatigue safety factors are low because of performance requirements. When we pulled the engine out of my friend's Formula Ford once, the back end of the car completely collapsed because all the welds in the space frame had fatigued. The car also broke during a race several times due to fatigue--only an idiot's design will fail due to tensile overload which is the failure mode typically taught in Machine Design.

I don't think fatigue gets nearly enough focus in the university (certainly was true at my school) but it is very important in industry (so you need to know about it).

One complication is that there are as many theories on fatigue as there are researchers in the field--and a theory that fits one situation well will not give reasonable results in another situation. One of the most prominent researchers in the field is Dr. Socie who has a free fatigue calculator on the web (www.fatiguecalculator.com). This site will give you some good insight but be sure you understand the physics of the models and don't use them blindly.

The most critical issue by far in making fatigue calculations is knowing the duty cycle. You need to think about the where the loads are coming from and figure out how many cycles of life you need and at what reliability--is a 50/50 chance of failure during Endurance OK? You will have to make many assumptions--when in doubt, be conservative. For example, if the loads are coming mainly from cornering, how many total number of corners will you have to negotiate during the event, including practice?

You also need accurate material data but if you are using mild steel or 4130 there is a lot of data available and these materials are well behaved. Check out Carroll Smith's books on materials and fatigue.

Good luck!

For the purposes of this competition, I respectfully disagree. I don't think that running to the absolute edge in terms of utilizing materials to their limit is the best way to earn points in this competition. I think, instead of running a fatigue simulation yet again, almost any team would be better off finishing the car a day earlier. An elementary calculation to ensure you are within the correct order of magnitude is more than close enough, and for that, a sophomore level mechanics book, or Carroll Smith's books will suffice.

You are correct that there is a time and a place for a complex and consequently more accurate model. In my opinion, this is not it.

[Adambomb]

Originally posted by Grant Mahler:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Sisyphus:
I have to disagree with Grant--fatigue calculations are generally difficult. I say this as having worked professionally in this area for several decades.

I am very happy to see that you recognize this is an important issue on a race car where the fatigue safety factors are low because of performance requirements. When we pulled the engine out of my friend's Formula Ford once, the back end of the car completely collapsed because all the welds in the space frame had fatigued. The car also broke during a race several times due to fatigue--only an idiot's design will fail due to tensile overload which is the failure mode typically taught in Machine Design.

I don't think fatigue gets nearly enough focus in the university (certainly was true at my school) but it is very important in industry (so you need to know about it).

One complication is that there are as many theories on fatigue as there are researchers in the field--and a theory that fits one situation well will not give reasonable results in another situation. One of the most prominent researchers in the field is Dr. Socie who has a free fatigue calculator on the web (www.fatiguecalculator.com). This site will give you some good insight but be sure you understand the physics of the models and don't use them blindly.

The most critical issue by far in making fatigue calculations is knowing the duty cycle. You need to think about the where the loads are coming from and figure out how many cycles of life you need and at what reliability--is a 50/50 chance of failure during Endurance OK? You will have to make many assumptions--when in doubt, be conservative. For example, if the loads are coming mainly from cornering, how many total number of corners will you have to negotiate during the event, including practice?

You also need accurate material data but if you are using mild steel or 4130 there is a lot of data available and these materials are well behaved. Check out Carroll Smith's books on materials and fatigue.

Good luck!

For the purposes of this competition, I respectfully disagree. I don't think that running to the absolute edge in terms of utilizing materials to their limit is the best way to earn points in this competition. I think, instead of running a fatigue simulation yet again, almost any team would be better off finishing the car a day earlier. An elementary calculation to ensure you are within the correct order of magnitude is more than close enough, and for that, a sophomore level mechanics book, or Carroll Smith's books will suffice.

You are correct that there is a time and a place for a complex and consequently more accurate model. In my opinion, this is not it. </div></BLOCKQUOTE>

I won't necessarily disagree, instead I would say it depends on the team. Some teams are ready to take it to this level, although honestly most aren't. Personally, I've considered it, and even spent that one day on it, and let's just say that the one day I did spend on the calculations gave me a bit of a baseline and some insight that I could use to develop a faster, simpler method for future validation. Lets us run closer to the edge, without spending a lot of time trying to find the edge! If I had to take a random guess, only being within an order of magnitude will net you a 600 lb car. In the end though, it all depends on where your team is at. Bear in mind, I remember in the '07 Optimum G seminar, it was mentioned that if you want into design semi-finals, your car needs to be under 400 lbs. I doubt there are too many reliable, sub-400 lb cars out there that haven't had at least a little bit of fatigue analysis done on them...

[vandit]

Originally posted by Silente:

I had some analysis on static load cases to understand when and which members are more stressed and to have a ballpark on how big they should be to have a certain safety factor.

But all this work doesn't analyze fatigue, since it is hard for me to think to a cycle with which every member could be stressed.

So my question is:
how do you consider fatigue when you do your calculations? you only use a bigger safety factor to be on the safe zone or you do different analysis to have a better idea?

On which members you would more concerned for fatigue failure?


Using static safety factors based on FE analyses for Fatigue analysis can be misleading as the FE calculation will not take into account the plasticity nature of material (unless the material Tensile test data is specifically used during FE analysis). Keeping the stress values at critical regions of components within the elastic limit of material is a safe practice (for fatigue analysis also ) which theoretically will ensure that you are not beginning the Neuber's behavior in your component and hence no damage with each loading cycle (atleast theoretically).

Lets assume for simplicity that Fatigue failure will happen in your component at maximum stress regions (which can be not so in some cases). So to determine either safety against Fatigue loading or determine number of cycles you will have to generate local S-N curves at nodes (nodes in critical regions of components).

Safety factor against fatigue loading can be estimated using

j = local endurance strength / local equivalent stress

where 'local equivalent stress' can be used as maximum stress value from FE analyses but to determine the 'local endurance strength' demands lot of consideration of various factors such as UTS, notch effect, Surface finish, statistical influence etc. which will modify your material S-N curve.

From FE analyses , this notch effect is taken into account using stress gradient in the critical regions. The effect of this gradient dominates the modification of your material S-N curve and YES, in a not so simple manner. Bringing the effect of mean stress correction and the correct choice of mathematical model for multiaxiality further increases the complexity of determining safety factor against loading.

To get the number of cycles of operation is easy once you can generate local S-N curves.

Personally, i would love to Study Fatigue strength of uprights and the welded mounts on chassis.

[Drew Price]

I don't think the suggestion was whether or not it's necessary to consider fatigue failure criteria in designing the car - it was whether the OP (or other designers who have to ask here a very general question about design) should really consider fatigue in his analysis.

Yes, to get into finals you would absolutely need to show some justification and fatigue criteria for quite a portion of the car - but someone asking about how to get started on that is probably not in that situation.

When I was just starting out with steel arms, I designed mine with buckling failure in mind, namely, I didn't want to be in danger of tearing out a mounting lug or shearing a mounting bolt if the car got curbed (so the wheel at least stayed attached), so if it's at least that strong, it's plenty, and that was a 'good-enough-to-move-on-to-something-else' with a quick back of the envelope run at a fatigue loading condition.

If I were designing carbon arms with bonded mounting lugs, that method wouldn't get me a car that I'd be comfortable driving. (Or maybe I would be comfortable, because I'd be ignorant to the fact, which I sometimes wish I could go back to....)

If you have to ask a general fatigue failure question here, then you need to just make your arms from 0.75"x0.049" and be done with it - move on to the next thing.

Unless your car does weigh 600lb. Then please use 0.75"x0.065".

Best,
Drew

[vandit]

yes, designing a component in FSAE from Fatigue point of view is not something you must do but rather you can do if you have time.

Frankly no one worries too much about Fatigue failure in FSAE unless you do stupid design mistakes like we did in past

http://fsae.com/eve/forums/a/t...10371641#96810371641 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/51210030541?r=96810371641#96810371641)

[flavorPacket]

Originally posted by Drew Price:
Yes, to get into finals you would absolutely need to show some justification and fatigue criteria for quite a portion of the car

No, you don't.

-suspension designer who made finals and has never done any fatigue analysis because it's pointless in FSAE

[Mike Cook]

Originally posted by flavorPacket:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Drew Price:
Yes, to get into finals you would absolutely need to show some justification and fatigue criteria for quite a portion of the car

No, you don't.

-suspension designer who made finals and has never done any fatigue analysis because it's pointless in FSAE </div></BLOCKQUOTE>


Agreed.

The only time I really consider fatigue is in spinning aluminum applications (spindles, wheels). For 6061, I try to keep my components peak stress at under 15ksi and it will pretty much last longer than the car will.

If you are going to be building right at the ragged edge in terms of stress/fatigue, you better know your loads exactly, which requires strain gauging, calculations will not be sufficient. Even if you measure them, there is the possibility that you will drive on a different track with different load conditions. FSAE cars shouldn't be operating this close to the limit in my opinion. And if you don't believe me then look at why half to 2/3s of the cars never finish endurance.

[exFSAE]

With only 1 exception I can think of... the failure mode for every broken FSAE part I've seen is pure 'static' overload rather than fatigue.

Limited-to-no true knowledge of load cases.. blatant manufacturing errors.. completely bogus FEA constraints.. outrageously large mesh sizes.. limited experience with weld metallurgy.. etc.

[flavorPacket]

couldn't have said it better myself, Mike and exFSAE.

[vandit]

Originally posted by exFSAE:
Limited-to-no true knowledge of load cases.. blatant manufacturing errors.. completely bogus FEA constraints.. outrageously large mesh sizes.. limited experience with weld metallurgy.. etc.

+1 ...very true

[Adambomb]

Well, allow me to go ahead and "buck the trend." I've seen several 4130 components in some of our old cars suffer fatigue failures, including drive hubs, halfshafts, and a front spindle. I've witnessed low-cycle fatigue in 1018 steel on our exhaust mounts after we switched to a single. It does happen, it has happened to us. All suffered brittle crack propagation under normal loading, wasn't like we hit a curb or anything. BUT...the cause for the fatigue failure was not because there wasn't enough material there. Just remember that when dealing with hardened 4130 stress concentrations should not be ignored.

But yes, beyond that, I'm on the same page as Mike Cook. Start out by doing a very simple fatigue analysis on an aluminum hub and it should give you an idea of what sort of fatigue life you have. I won't give any recommendations beyond that, other than to say that I found it instructive and enlightening. Also bear in mind whether or not the component is strength or compliance limited.

I've seen a few fatigue failures in FSAE, but they were almost all hardened 4130, and it almost always happened at a stress concentration.

[Drew Price]

Originally posted by flavorPacket:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Drew Price:
Yes, to get into finals you would absolutely need to show some justification and fatigue criteria for quite a portion of the car

No, you don't.

-suspension designer who made finals and has never done any fatigue analysis because it's pointless in FSAE </div></BLOCKQUOTE>


http://fsae.com/groupee_common/emoticons/icon_frown.gif http://fsae.com/groupee_common/emoticons/icon_frown.gif http://fsae.com/groupee_common/emoticons/icon_frown.gif

I guess I held higher oppinions of you guys than you deserved! http://fsae.com/groupee_common/emoticons/icon_wink.gif

Best,
Drew

[Adambomb]

Originally posted by Drew Price:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by flavorPacket:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Drew Price:
Yes, to get into finals you would absolutely need to show some justification and fatigue criteria for quite a portion of the car

No, you don't.

-suspension designer who made finals and has never done any fatigue analysis because it's pointless in FSAE </div></BLOCKQUOTE>


http://fsae.com/groupee_common/emoticons/icon_frown.gif http://fsae.com/groupee_common/emoticons/icon_frown.gif http://fsae.com/groupee_common/emoticons/icon_frown.gif

I guess I held higher oppinions of you guys than you deserved! http://fsae.com/groupee_common/emoticons/icon_wink.gif

Best,
Drew </div></BLOCKQUOTE>

I wouldn't necessarily go that far. It really depends on your design strategy. I'm guessing Penn State did a pretty fair amount of fatigue analysis to get their '09 car under 300 lbs, and they made it to design finals. Can't really speak for the others though, nor do I care to comment on how much we do! http://fsae.com/groupee_common/emoticons/icon_razz.gif

[flavorPacket]

Originally posted by Drew Price:

I guess I held higher oppinions [sic] of you guys than you deserved! http://fsae.com/groupee_common/emoticons/icon_wink.gif

Best,
Drew

If the design event were called the "fatigue analysis event", then maybe you'd have a point. But last time I checked, it was about designing an entire car and integrating several systems into a cohesive, fast, and reliable package.

Even the most talented and knowledgeable FSAE members know next to nothing when compared to professionals. So why try to be one? Just have fun while you still can. You'll have plenty of chances to waste your company's money on intellectual masturbation when you're older.

[Drew Price]

Originally posted by flavorPacket:
Even the most talented and knowledgeable FSAE members know next to nothing when compared to professionals. So why try to be one? Just have fun while you still can.


My team could have used this advice about 3-4 years ago - we'd probably be a little less sinister now.

Best,
Drew

[exFSAE]

Originally posted by flavorPacket:
Even the most talented and knowledgeable FSAE members know next to nothing when compared to professionals. So why try to be one? Just have fun while you still can. You'll have plenty of chances to waste your company's money on intellectual masturbation when you're older.

This x100.