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High Stress On Fillets Analysis - Static Linear Analysis
2

High Stress On Fillets Analysis - Static Linear Analysis

High Stress On Fillets Analysis - Static Linear Analysis

(OP)
Hi all,

I am posting in hope of someone helping me analyze a high stress that I am seeing on a fillet in my FEA simulation.

For reference I am using Fusion 360 and running a static linear analysis. Everything else in my simulation is perfect except for this one fillet.

I am simulating with Aluminum 7075-T6 with a 455MPA Yield and 525MPA Ultimate.

As a side note, my simulation does converge as seen below, so I know this isn't a singularity / hot spot / calculation error.



The stress on the fillet is pictured below. It is a 5mm fillet. Making the fillet small makes the stress area longer. Removing the fillet makes the stress area disappear.




I am confused about this because I see many mixed opinions on how to handle this:

1. Oh just ignore it, its artificial, stresses like that on fillets don't make sense.
2. You need to fix it and make it disappear or except a failure.
3. Since the effected areas volume is so small you'll see a local yield which wont make the whole part fail, but it will fatigue over time. Compared the max stress to a S-N curve to determine how many cycles you'll be able to get out of it.
4. If you are really worried and don't trust any of the above, run a nonlinear analysis.

My personal analysis is leaning more towards number 1 & 3. Such a high stress on a fillet really doesn't logically make sense (I could totally be wrong), especially when the fillets actually bring out the stress in that area. Removing the fillet, the stress in that area goes away too. Since the effected volume is so small, its just a localized yield and those are OK and don't mean complete part failure instantly, just eventual fatigue failure over time.

Hoping someone can provide some insight on my personal analysis and tell me if I'm thinking correctly or not. I'm fairly new to FEA.

Many Thanks!

RE: High Stress On Fillets Analysis - Static Linear Analysis

remember you're running linear elastic FEA (I think, apologies if wrong).

3 seems about right … some local plasticity, an issue if this is a fatigue load.

another day in paradise, or is paradise one day closer ?

RE: High Stress On Fillets Analysis - Static Linear Analysis

(OP)
Correct. A static linear analysis. Completely forgot to state that and have updated the title and description.

Appreciate your response. 3 makes a lot of sense to me as well and I'm glad I could get another opinion.

I do want to make a note that my FEA simulation is using loads seen at ABSOLUTE MAX conditions, that might even be slightly unrealistic or never seen that many times (cycle wise). So the amount of time this load is going to be hit isn't a lot. In this case its not going ton be constantly cycling.

I like that term "local plasticity," I will need to do some googling with that key word and see what other threads and info I can find.

Many thanks,

Matt

RE: High Stress On Fillets Analysis - Static Linear Analysis

It's important to understand the source of suspicious stress concentrations. Since it's not a singularity, there must be a physical reason for such high stress. Maybe an error in boundary conditions or loads. Can you say more about them and show some pictures ?

Plasticity definitely should be included in this analysis. It may have large impact on results.

RE: High Stress On Fillets Analysis - Static Linear Analysis

First, I'm confused. RB1957 said "remember you're running linear elastic FEA (I think, apologies if wrong)" You said "Correct. A static nonlinear analysis." Is it linear or nonlinear? With von Mises stress 35% over Sy, it sounds like linear elastic.

Second, if you truly have plastic deformation, an SN curve is no good. You have to use strain life data and analyze for low cycle fatigue.

Third, if its not a singularity, you cant just ignore it. Either examine your model and if you can convince yourself your model is right and its a real stress, then you have to deal with it. I agree with FEAway. Looks like a BC or load problem. Hard to tell because you dont show them. Looks like the bolt holes are constrained and the back face.


Rick Fischer
Principal Engineer
Argonne National Laboratory

RE: High Stress On Fillets Analysis - Static Linear Analysis

(OP)
Sorry it is a linear static stress.

Sure.

I am simulating a redesigned transfer case for an AWD vehicle. Its contain a ring and pinon gear set. Its a two piece design, but I am simulating that case plate as bonded. I have done the bolt calcs by hand and also have tried sims constraining the plate to the case to make sure it doesn't negatively effect the case and it doesn't.



The case itself bolts to a transmission that looks like such :



I have my constraints setup as follows... basically the 5 bolt holes are constrained as well as a split face that the bolt head touches and a split faces that touches those offset bolt bosses on the transmission. I dont have the entire back of the flange constrained because only small faces make contact with the transmission.



The case has 4 combined radial bearing loads (represented as bearing loads) and 2 axial loads (represented as regular force loads).

Here are the pinon loads:



Here are the ring gear loads:



Here you can see we have high stress in two areas on the same fillet. One near the bottom and one near the top:

Bottom:



Top:



Looking at an extremely amplified view of displacement you can see the structure buckling near that bottom one, which is maybe causing that high stress area on the fillet?



The top is basically seeing the opposite and getting pulled apart (kinda hard to see without stepping through the scales).



What i just really dont understand is why the stresses in that area go away when the fillet is removed. I would think a finely meshed fillet would help alleviate them.

Hard to tell but the stresses are also really only on the surface. They dont extend deep into the piece.


RE: High Stress On Fillets Analysis - Static Linear Analysis

I too am confused on whether or not you are running a linear or non-linear plastic analysis? If you are running a linear analysis (which agrees with your title and verbiage but conflicts with the fact that you showed screenshots of your convergence method) then I would say I agree with number 3 and 4. If this is a nonlinear analysis, then it's hard to say what could be causing it. Since you say that removal of the fillet makes the plasticity go away, it would seem that there is something about how it is modeled that is causing it. Boundary conditions are a common culprit. Can you simplify your boundary conditions and load case and run a linear analysis.... check your displacements and stresses and see if the model is behaving correctly? Then introduce the correct boundary conditions, load case, and non-linear analysis one-by-one to see if you can spot the issue?

RE: High Stress On Fillets Analysis - Static Linear Analysis

(OP)
To clear things up it is a linear static stress. I made a typo and have updated my posts.

RE: High Stress On Fillets Analysis - Static Linear Analysis

Are these first or second order tetrahedrons ? Consider refining mesh in other areas as well (especially for the flange). I would also focus on boundary conditions since they are really close to these stress concentrations regions. There are various approaches to bolt modeling, try some of them and check their influence on the results.

RE: High Stress On Fillets Analysis - Static Linear Analysis

(OP)
F360 by default is second order tetrahedrons.

Continuing to play around with it tonight... I decided to try all of the different permutations of simplifying the BC's and loads.

I am using adaptive mesh refinement and making sure that the results from all of my simulations converge as to make sure there are no singularity's.

1. Full Load & Constrained Bolt Holes + Bolt Head Faces - Result can be found in previous posts. Very high stress on fillet. 860 MPA on lower fillet (0.52 Safety Factor).
2. 1/2 Load & Constrained Bolt Holes + Bolt Head Faces - High stress point in the exact same position as 1 but at exactly 1/2 the stress at 340MPA (1.05 Safety Factor).
3. Full Load & Constrained Flange Face (full back face) - High stress point in the exact same position as 1 & 2 but 486 MPA (0.93 Safety Factor).
4. 1/2 Load & Constrained Flange Face (full back face) - High stress point in the exact same position as 1 & 2 & 3 about exactly 1/2 the stress at 233 MPA (1.94 Safety Factor).

So the stress location is staying constant based on varying BC's and loads. Its hard to tell how and why that stress is originating. There is no displacement around or on it, it doesn't clearly stem from anything else, just kinda appears very thinly out of nowhere.

I guess I will have to try connecting more of that flange to the case. Specifically in the regions near those high stresses on the fillet.

RE: High Stress On Fillets Analysis - Static Linear Analysis

Since the OP quotes the applied loading to be "ABSOLUTE MAX conditions" in one of his previous posts, you could try to 'neuber-correct' the stress at this local area that is above Ftu.

There was a very good discussion on the neuber method on this very form at the below link:

https://www.eng-tips.com/viewthread.cfm?qid=210076

RE: High Stress On Fillets Analysis - Static Linear Analysis

I also vote for variant 3 - in fatigue you can not ignore stress at fillets and corners.

RE: High Stress On Fillets Analysis - Static Linear Analysis

is this an FEA package inside a CAD s/ware ? I'm looking at your constraints and it looks to me like you're constraining the surface (something I see in CAD FEA). I think you need to be careful with this type of constraint, it is easy to over-constrain the model which will create problems in the FEM results. Now I doubt this is causing your stress peak. I think you've a mild stress concentration, exaggerated by the linear FEA. If this is a fatigue load case and you're seeing plasticity then that's probably a bad sign. I suspect the only practical solution is to increase the fillet rad. There are other treatments but not really suitable for mass production. Possbily a material change ?

Probably you want a fatigue test. You may want to consider something we know (in aircraft structures) … an overload really helps the fatigue performance. if you load to 120% the fatigue load you'll set up a plastic zone in this area and that'll create compressive stresses to retard the fatigue of the fillet.

another day in paradise, or is paradise one day closer ?

RE: High Stress On Fillets Analysis - Static Linear Analysis

(OP)
@nlgyro

Thanks for the response. I am going to do some further research on neuber correction. It seems to me this is an alternative to doing a full blown non-linear analysis.

@rb1957

Correct this is a FEA package inside of my CAD environment. Autodesk Fusion 360 has an integrated simulation environment. Let me better show how I have my constraints setup. Basically I am constraining the bolt holes as well as the area the bolt head touches and the area the flange touches on the transmission bosses.

Are you saying that constraining just those surfaces could cause issues? Attached are 3 images of the areas I am constraining.

1.Hole
2.Bolt Head Face
3.Tranmission boss face






Maybe it will be worth my time to run a non-linear analysis as well. I do have access to that in my simulation package.

I assume if I see the same stress concentration occurring in a nonlinear analysis then its not a mild stress concentration being exaggerated by linear analysis and is a very big problem.

I unfortunately don't have access to any fatigue loading simulations. This item is going to be used for drag racing and its really only going to be seeing these crazy forces in 1st gear, so it is ok if the cycles are limited. Maybe its time to switch simulation software.

On a side note, I really appreciate everyones time. This forum really does have a great group of people, so cheers everyone!

RE: High Stress On Fillets Analysis - Static Linear Analysis

In this thread you will find a list of references about Neuber's rule: https://www.eng-tips.com/viewthread.cfm?qid=456136

Nonlinear simulation (with plasticity) is the first thing I would try in this case. Simple bilinear model with hardening should be enough. But I also suggest modeling bolted connection in more realistic/detailed way.

RE: High Stress On Fillets Analysis - Static Linear Analysis

yes, that's what I thought. I suspect that locking surfaces in this way is overconstraining the model and not, IMHO, good practice.

But I also doubt that it is affecting significantly your stress peak.

The key thing to understand is the loading … is it an extreme static case or something typical, representative of fatigue loading ?

An interesting sidebar would be to look at the current transmission piece (or something similar) … if it shoes a similar stress peak and has acceptable service experience then you have your answer (it looks acceptable).

another day in paradise, or is paradise one day closer ?

RE: High Stress On Fillets Analysis - Static Linear Analysis

(OP)
OK... So I ran a non-linear analysis and the results are pretty interesting.

I still have stresses in the same spot, but the magnitude is quite smaller. I am now seeing around the 480-530 MPA range (which is barely above yield), where before they were 860 MPA... surprise

Here is how I setup my material:



And here are the results...



I think its safe to say now that the stress concentrations were definitely being exaggerated by the linear analysis. From my understanding non-linear takes into account not only non-linear area of the stress strain curve of the material but also the change in stiffness in the high stress areas.

Let me also give a little more background on the problem...

So basically the setup for this is as follows... the engine is connected to the transmission and this AWD transfer case is connected to the final drive of the transmission and it sends torque to the rear.

Since its connected to the final drive the input torque it sees is pretty massive. In this gear the ratios are as follows...

1st gear - 3.07
2nd gear - 1.77
etc etc
Final Drive - 4.00

So I want my case to be able to handle 1000nm engine torque in all gears with the possibility of 100% torque being sent to the rear wheels through this case. This would happen if the front lifts or the front loses traction.

So if we multiply things out our input into this gear set becomes...

1000nm * 3.07 * 4.00 = 12280nm
1000nm * 1.77 * 4.00 = 70008nm

Then basically since its hypoid spiral bevel gear set I can calcualte the gear tooth forces and then calculate the combined radial and axial bearings loads which I then use in the my FEA...

I am reusing the stock gearset, just creating a stronger case and explained more of my gear tooth / bearing load calcs here : https://www.eng-tips.com/viewthread.cfm?qid=463926

So basically I am simulating the absolute max condition it would see in 1st gear with 1000nm engine tq and 100% to rear. This is slightly unrealistic as 1000nm engine torque in 1st gear is massive, usually the engine can't build enough load to even produce that and only 100% torque would go to the rear if the front of the car completely lifted off the ground.

My ideology is that if I can design a case strong enough to withstand that absolute maximum even if its slightly unrealistic it should be indestructible at more realistic conditions such as lower torque and less torque transfer to the rear. And this also gives me some wiggle room for possible error in my gear math. As you go up to 2nd gear you can see the forces basically 1/2 and at that point all of the stresses are gone and this is still assuming 100% tq transfer and its usually 50%.

Ive never seen the stock case fail at the flange area. Usually it always fails at the pinon area, but because the pinon area was never strong enough maybe it wasn't possible to put enough torque through the thing to see failure at the flange area.

RE: High Stress On Fillets Analysis - Static Linear Analysis

(OP)
Here you can also see my constraints causing some issues. This IMO is totally safe to ignore. Thats clearly being caused by the split face that is constrained (to represent the bolt head).

Any suggestions on how to better constrain this? Presumably without modeling bolting this piece to a plate (transmission).

RE: High Stress On Fillets Analysis - Static Linear Analysis

If you are applying extreme loads then mild stress concentration is acceptable, because fatigue loads are much lower.

From your NL run you're predicting a peak stress of 550MPa (quite a bit above the yield of 450 MPa). What's the endurance limit of your material ?
from your linear run the peak stress was 850 MPa, so you need to say fatigue loads are 50% of the extreme load before you don't get yielding under fatigue loads (which you want to avoid). At the endurance limit (ok, there isn't one for Al) is much lower than that.

Your later post is a stress concentration at a spot face. This is also near one of your constraints. Which is causing it ? both are ??

another day in paradise, or is paradise one day closer ?

RE: High Stress On Fillets Analysis - Static Linear Analysis

(OP)
OK. A lot of this is starting to make a lot more sense... so let me recap in my own words to make sure I am on the right page.

When analyzing local stress concentrations we must do the following:

1. Perform a linear analysis on the piece
2. Make sure that our simulation is converging so that we know for a fact this is real stress and not a singularity due to poor meshing.
3. If the concentration isn't yielding and we want to determine fatigue cycles we can use a S-N curve. All is good.
4. If the concentration is yielding and its a very high stress spike, we should either use a nueber correction or run a non-linear analysis to get a better idea of whats going on.
5. If the non-linear analysis returns lower stresses we know the stresses in the linear analysis were exaggerated.
6. If we are in the elastic deformation range we can use a S-N curve to determine fatigue. If we are in the plastic deformation range we can now use a Stress Life curve to determine low cycle fatigue.

In this case since my stress concentration is small and local I can use a stress life curve to see how many cycles I can get out the piece at this extreme load.

RE: High Stress On Fillets Analysis - Static Linear Analysis

5 is given if linear stresses exceed yield.

one way you can infer a "real" stress is to assume the same strain would develop …

FE gives you a stress, strain = stress/E
read this strain onto the real stress/strain curve to see the "real" stress.

In determining fatigue a couple of options open up …
1) do you use the peak (element averaged) nodal stress with Kt = 1; or
2) do you infer a stress concentration, Kt, based on the peak stress and some (guess) as to the nominal stress.
3) maybe repeat 2 with several Kts

another day in paradise, or is paradise one day closer ?

RE: High Stress On Fillets Analysis - Static Linear Analysis

(OP)
Continuing to play around with things...

Now that I have a better understanding on how to analyze these stress concentrations I wanted to do some experimenting.

I wanted to be able to make the fillet where I am having those stress concentrations larger. Unfortunately due to the size of the neck (that connects the case to the flange) it made this difficult because anything bigger than a 5mm fillet would cause it to interfere with the bolt holes.

I decided to try and make the neck smaller so that I could make the fillet bigger. This basically just ended up making those stress concentrations on the fillet spread out over a much larger area. I suspect this is because in order to make the fillets larger I had to reduce the size of the neck thus reducing the strength of the neck.

Below are some results from my non-linear analysis.

We can see it didn't really make the stress higher but the area of the stress is way larger. Its no longer a small localized stress concentration.




It also made the displacement more than double (before 1.8-2mm)!!



I have concluded that taking this approach is not going to make things better. I think my best bet is how I had originally with the large neck and small 5mm fillet... and I think that the small local stress concentrations I am seeing there are OK. At least when we are talking about absolute max loading and when there won't be many cycles at that load. I think aluminum will be OK for a prototype and my initial testing, but I don't think its going to take much more than a few hundred cycles at that absolute max load condition, if that. 4340 or 8620 would be a good option, but they're 3x heavy. But honestly, who knows If ill even be getting close to that max load condition.

I still need to do some more strain life chart anaylsis but I am having a hard time finding such charts for aluminum. Anyone have any good sources?

~Matt

RE: High Stress On Fillets Analysis - Static Linear Analysis

Honestly, with your issues being so close to the fasteners, I think a lot of your stress is due to having to deal with your fixed boundaries at the fasteners, especially since your fasteners are going to have some give axially due to how long and thin they are. Rather than considering them fixed, I would create a 1D spring element with the appropriate fastener stiffness (such as via the Huth formula), connecting one end to the part, and grounding the other end. In Nastran, I would create this element as a zero-length element right at the interface between your part and the transmission, and then connect this to a second 1D element that goes through the hole and spans the thickness of your part and connect it to both surfaces of your flange via RBE3s, and then make the stiffness of this part such that the fastener load is split up roughly equally to the two surfaces.

Though, it might be easier to just model the fasteners in 3D elements, bond it to the surface of your part, and let the other end be fixed...

RE: High Stress On Fillets Analysis - Static Linear Analysis

(OP)
Hey Countryman thanks for the reply.

Even though Fusion 360 Static Stress does use Nastran behind the scenes I unfortunately don't have access to all of those different element types you are talking about.

Your last sentence about modeling the fastener in the 3D got me curious. I decided to give it a try...

Basically I modeled a very simple bolt with a 22mm head and a 10.5mm shaft (hole is 10.5mm too). I then aligned up everything properly.

Here is how I setup my contacts.

Between the bolt head and the flange I set that up as bonded.



Between the bolt shaft and flange hole I set that up as sliding (not sure if this is correct maybe this should be bonded as well?).



Lastly I fixed the end of the bolt.



The results were pretty similar in terms of that fillet area vs used fixed constraints.



High stress concentrations on the bolts.


RE: High Stress On Fillets Analysis - Static Linear Analysis

Well, that's unfortunate. The only other thing that comes to mind is to see if you can design in some small gussets between the attach flange and neck to take some load away from that fillet. It might be tricky getting in them in such that you can still install the fasteners, though.

If it helps, I've attached some strain life constants I got when I took a class a while back. I don't have a proper reference for these, so use at your own risk!

RE: High Stress On Fillets Analysis - Static Linear Analysis

(OP)
Appreciate the strain life data countryman as well as the help you have given me.

I did try adding some gussets on the bottom most portion of the flange as well as one of the sides. Unfortunately it didn't help much :/ and clearance in the areas where I can add gussets is very tight.

One thing that one of my friends suggested to me was to cut the neck in half and basically machine the flange / half of the neck out of steel such as 4340 and then bolt that part to the 7075 main case.

Basically as follows:



It works really really well and completely fixes those stress concentrations due to the flange and those problematic fillets being made out of steel when I simulate the flange and the main body as bonded.

The issue is simulating the bolts...

The way the fusion 360 bolt connector constraint works is it uses beam elements by having one beam element represent the shaft, where the node ends of the beam element are located at the centers of the circular edges that are used for the head and thread locations. Then at each end of the beam element used for the shaft more beam elements are used to connect to the hole edge in a kind of spoke wheel circular pattern. Those spokes are connected to the 3D solid elements at the hole edge. Since the load is being transferred through an edge, a stress concentration will likely manifest. Like found below...



I could try 3D modeling the bolts and then manually setting all of the preloads & contacts but it becomes unmanageable.

Ill keep plugging away...

RE: High Stress On Fillets Analysis - Static Linear Analysis

I'd've thought making it a two piece ass'y would have increased the cost a lot ?

Is there no way to change the fillet geometry ? I'd've thought a simple change to the fillet contour would be cheap to implement ?

Could you shot peen ? probably too expensive ?

Can you benchmark with a successful design ? How do we know that this peak isn't on current designs (which are successful) and so is really a "model artifice" ?

another day in paradise, or is paradise one day closer ?

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