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Need help analysing a failure
2

Need help analysing a failure

Need help analysing a failure

(OP)
Hi, I'm new here.  I just found this site and it looks like a great place to kick some ideas around.

I have a project I've been working on for some time now and I thought I had the solution worked out but ther was a failure recently and I'm having a hard time figuring out why.  Hopefully, if I can figure out why it failed, I'll also be able to figure out how to modify it to prevent future failures.

Ok, heres the situation.  I have a part that I designed for my race car.  It's a wheel hub.  The OE parts were not up to the task of road racing with big sticky tires.  The part I designed is forged out of 4340 steel, machined, through hardened to ~RC52 and then the critical dimensions are finish ground.  The part has the wheel flange and shaft forged from a single piece of material.  By my calculations, the maximum stress this part will see is 190,000 psi.  The stress peaks at the radius between the shaft and the wheel flange as you might expect.  According to the material data sheet I reviewed at my fabricators shop, the material used at the heat treated condition is supposed to be 250,000 psi.  The failure appears to be a sudden break.  Although there is some discoloration and what appears to be bits of red rust and patches of black oxidation in the break.  This might mean that the part cracked earlier and eventually completely failed.  But the hammered "ring" around the perimeter of the break that I would expect to see if that were the case is not there.

Questions:

1.  Any ideas why this part broke?

2.  Would induction hardening instead of through hardening be tougher?

3.  Is the material just not strong enough, either due to  an inferior  piece of 4340, or is 4340 the wrong material?

Thanks in advance for any insight you may be able to provide.  I'm sure there is more info you might need to help, so feel free to ask for any info I left out.

RE: Need help analysing a failure

Uh, 190ksi sounds like a pretty high design stress.  Can you put a bigger radius there?

RC52 is harder than an axe or a chisel.  You removed all the ductility with through hardening.

A few years ago, tiny wheels with large offsets became a fashion on automobiles.  The fashion went away quickly because of wheel bearing life measured in days.  
Are you running large wheel offsets with your wide sticky tires?  That stresses hub flanges in addition to bearings.

A detail drawing of the hub, and a section through the hub/wheel/tire/suspension assembly, might help our actual experts figure out what's going on.
 

Mike Halloran
Pembroke Pines, FL, USA

RE: Need help analysing a failure

Your design stress relative the ultimate strength of the material is high. You are at 75% of ultimate, so some yielding or minor cracking are possible with initial cyclic loading.  Once a small crack occurs, you have too little safety margin to allow fatigue cycling, thus you get a quick failure.

Since your part is subject to repetitive loading, check the endurance limit of the steel and design for that.

RE: Need help analysing a failure

(OP)
Where might I find the endurance limit?  Also, 250,000 psi is supposed to be the yeild strength, not ultimate.  Although I don't know what the ultimate strength is, it should be higher than the yeild, right?

The radius at the base of the shaft is 0.250" on a 1.5" shaft.

Wheel offset and suspension geometry were all factored when calculating the stress.  Stress was calculated at 1.6G.  The most I've ever measured on my car was 1.45 G with a single spike to 1.5 G.

The thing that has me puzzled is that the part that I'm breaking does not fail in the stock part.  In the stock part, the bearings fail.  Additionally, my part is a larger diameter than stock.

RE: Need help analysing a failure

Ah. We appear to  be talking about a spindle, not a hub.

You probably can't afford to duplicate the tightly controlled induction hardening that OEMs use in that area.

A drawing would help to prevent further misunderstandings.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Need help analysing a failure

(OP)
Well, in this case they are the same thing.  Yes a picture would help.  Unfortunately, I don't have have any handy.

Picture this:

The flange and shaft look like a very short axle shaft from a solid rear axle.  The bearing housing is bolted to the suspension upright.

What I did was machine out the stock housing to accept tapered roller bearings races(stock uses balls and has integral races in the housing and shaft).  

In a typical spindle set up, the shaft is fixed and the housing rotates.  In this case, the housing is fixed and the shaft rotates.

The pieces I'm having made cost me over $500 each now, would precise induction hardening increase the cost that much more?  More importantly, if it would improve the durability, it might be worth it.

RE: Need help analysing a failure

(OP)
Here's a link to some photos of the part.

RE: Need help analysing a failure

Okay, I think it's better to call that part a stub axle.

The radius at the thrust face of the larger bearing appears to be not more than .05".  Is that where the part broke?

How about some pictures of the broken part?

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Need help analysing a failure

Wait, there's more.
The only .25" radius on the parts in the pictures is at the root of the wheel pilot.
A radius nearly as large as that _should_ be at the thrust face of the larger tapered roller bearing. ... but it's not there.

The obvious next question is whether your stress model even looks like the part.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Need help analysing a failure

(OP)
I think what you don't see and what I forgot to tell you is that the thrust surface for the outer bearing is a separate piece.  The .25" radius is underneath it.  The inside of that ring is machined to match the radius.

It is possible that the stress model is off, but I think it's pretty good.

I'll see if I can dig up a couple pics of the broken part and post them.

RE: Need help analysing a failure

Okay, now draw a section through the assembly.
You need to include the bearing, the stub axle, and spacer.
The spacer, adjacent its internal diameter, has one radiused edge and one sharp edge.
The sharp edge of the spacer abuts a radiused edge inside the bearing.
The sharp edge of the spacer, where it contacts the shaft, constitutes, IMHO, a stress raiser.
Not by coincidence, that's where the shaft of the stub axle broke.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Need help analysing a failure

To me, it looks like you have a classic bending fatigue failure. I would take the piece to a metallurgical lab to verify this. The stresses on the part can be much higher than what the models show, due to residual stresses from manufacturing processes, such as grinding.

RE: Need help analysing a failure

Hi MJMinear

Having seen the values for the endurance limit for the steel you are using on the previous link I posted and combining it with this site which gives typical examples of different shaft failures I would agree with swall its a classic rotational bending fatigue failure.

http://www.asminternational.org/pdf/spotlights/jfap0502p011.pdf

see page 14 of the above link

desertfox

RE: Need help analysing a failure

(OP)
Mike,

The fracture is actually 100% on the raduis all the way around.  Meaning it's completely under the spacer.  No part of the fracture is at or outside the edge of the spacer.

Looking at the articles linked above, this appears to best fit the low stress riser, high bending stress example.

RE: Need help analysing a failure

You were right in your suspicion, that thing had cracked all around and the gray patch in the middle was all that was holding it when it let go.  Classic rotational bending (cyclic) stress failure.  I agree with Mike Halloran, the through hardening removed the ductility.  Also, whoever said you don't have enough of a safety factor is right too.

Without spending any mental effort nor time on it, I will posit that this might be one of those cases where you're better off with un-heat-treated material.  One time, the younger, fresher engineer who had preceded me at a job had made the tailshafts on some 78" dia screw classifiers out of 4340HT.  The same thing happened, they broke.  I figured out the issue and changed it, but they were no where near the 190ksi you're talking about, so that might not be applicable.  Perhaps you will have to revisit the design geometry.

RE: Need help analysing a failure

I must disagree with tr1ntx. Higher fatigue strength comes with higher tensile, higher yield. This drives you to a fully hardened part. A non hardened part is the wrong way to go.

RE: Need help analysing a failure

Refering your questions:
1. Part broke, even tho the applied stress in the bulk section was less than yield, because either there was an undetected crack, or the fracture toughness was inadequate (and allowed a crack to nucleate underthose conditions).

2. Don't know.  4340 steel is pretty 'deep-hardening' so you may still get thru hardness even with induction hardening and an air quench.  Maybe something like 4140, 4130, would give a softer and tougher core.  Check the literature, see what spindles are typically made of.

3. Is 4340 not strong, inferior or wrong? No, no, and maybe.

I think you need to do any combination of:
[a] inspect for pre existing surface flaws, esp. in the high stress areas (e.g., mag particle or dye penetrant).  From the photos, looks like there may have been a crack that grew by high cycle fatigue.
[b] eliminate high stress conditions thru redesign with large fillet radii, if possible.
[c] use material with a higher, plane strain fracture toughness.  4340 at 52 HRC will have very low K_Ic because of the inverse relationship.  You really want toughness in this application.  Do [b] then drop the UTS and hardness.  Otherwise you may have to go with another alloy like maraging steel ($).
 

RE: Need help analysing a failure

(OP)
Thanks for all the great replies.  I've been without a PC for a few days so I just got a chance to read all of them.  

The radius I'm using is pretty well maxed out within my space constraints.  Besides, modeling shows that even doubling the radius results in only about 5% reduction is stress.

From the input above, I think the best thing to try next would be induction hardening instead of through hardening.  Does anyone have any reference material on how to calculate the resulting strength and how to calculate the desired depth of hardening?

Thanks,

RE: Need help analysing a failure

You have to consider the section size of the material that will be hardened (the case) as well as the section size of the unhardened core. If you were to choose 1045, induction hardened, you would have a case that would run approximately Rc 53, which converts to 269,000 UTS. For 1045 hot rolled, which would be the core, an old handbook from Republic shows 45,000 psi tensile (typ) for this material.

RE: Need help analysing a failure

52 HRC is just too high for 4340 in a rotational bending application.  At that hardness, the toughness is so low that even small imperfections will propagate and lead to failure.

Try dropping to hardness to 38/42 HRC and shot peening the radius afterwards.  Look up Metal Improvement, Inc., for a shot peening vendor near you.  This is much less expensive than induction hardening and avoids problems induction hardening could cause.


rp

RE: Need help analysing a failure

(OP)
The only problem with the HRC ~40 is that the listed tensile strength is right at my predicted peak stress (about 190 ksi), ie no safety factor at all.  This level of stress is only reached momentarily, but may be reached many times per lap.

I studying the section, I think a 0.05" increase in the radius diameter can be achieved if I can find the appropriate seal.  That would help a little.  That coupled with the increased toughness at a lower HRC might be enough.

HRC 45 has a tensile strength of 228 ksi, Do you think that might be a better compromise between toughness and strength that will still maintaining enough UTS?

RE: Need help analysing a failure

MJMinear;
If you absolutely need strength greater than 190 Ksi based on current design constraints, and you can't reduce stress concentration effectively, you may need to investigate using a maraging alloy, like Maraging 300. The UTS for this alloy is 290 Ksi min, with a yield of 285 Ksi, and it is tough material.

RE: Need help analysing a failure

I looked at this briefly and am currently at the opinion of saying you're not going to be able to make a part that won't break.  You're going to be able to increase the time (load cycles) until it breaks using suggestions above, but at 190 ksi, it's going to break.  I don't know about the Maraging Alloys though, maybe that will work.

This phenomenon is why well funded race teams throw way "perfectly good" used parts.

RE: Need help analysing a failure

(OP)
Are the specs on 300M you list for the base material or are they heat treated numbers?  If 300M is required, that's what I'll do.  Per my fabricator, it will add about $100 per part.  They are already pretty expensive, so if it takes an extra $100 to make it right, that's right thing to do.

RE: Need help analysing a failure

MJMinear;
What I mentioned is not 300M (a modified 4340), it is a different alloy containing much higher nickel and cobalt. It is more expensive than you think, but it will work under your conditions. It is strengthed by aging and not by conventional heat treatment for alloy steels.

You should look at 300M, but I am not sure that you will have the necessary through thickness mechanical properties and toughness that is needed for this service duty.

http://www.steelforge.com/alloys/?alloy=Maraging%20300

http://www.dynamicmetals.net/

RE: Need help analysing a failure

These papers discuss the effects of aforementioned shot peening on high strength 4340 steel.

http://www.metalimprovement.com/PDF_DOCS/The%20Effects%20of%20a%20Machining-

Like%20Scratch%20on%20the%20Fatigue%20Life%20of%204340%20Steel.pdf

http://www.shotpeener.com/library/pdf/2002006.pdf

http://www.gurteq.com/RAYSTRESS%20DOWNLOAD/RAYSTRESS%20005.pdf

Paper on variable amplitude loading

http://www.eurojournals.com/ejsr_29_2_02.pdf

Here is an off the shelf Astralloy V bar material that I've used to overcome numerous problems with fatigue, especially in very high loads and load reversals.  I have in-service highly loaded shafts that are over 25 years old.  

The people at Astralloy should produce the numbers for the  comparison data for Astralloy V versus other alloys.
 
http://www.astralloy.com/pdf/astralloy_v_plate.pdf

RE: Need help analysing a failure

To me, all these measures as discussed will have influence on fatigue life. Considering the function of the part, it WILL fail through fatigue at some point.  

RE: Need help analysing a failure

Have you done an analysis with the mass of the spacer integrated with the mass of the stub axle?  

I.e., I don't think the spacer buys you anything.  At the thrust face of the bearing, put a radius just a tick larger than the bearing edge radius, and a shoulder big enough to pick up the thrust.  You can still pick up whatever remains of your big radius, so on a macro scale, the shoulder becomes a bump on the radius.

 

Mike Halloran
Pembroke Pines, FL, USA

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