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.
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
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
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
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
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
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
RE: Need help analysing a failure
http://www.mjmracing.shutterfly.com/
RE: Need help analysing a failure
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
I found this site and reading your posts I would say that your 250,000psi is the UTS in which case your endurance limit according to this site is about 85000 psi ie 30% of UTS.
http
desertfox
RE: Need help analysing a failure
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
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
RE: Need help analysing a failure
RE: Need help analysing a failure
RE: Need help analysing a failure
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
RE: Need help analysing a failure
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.
htt
see page 14 of the above link
desertfox
RE: Need help analysing a failure
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
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
RE: Need help analysing a failure
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
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
RE: Need help analysing a 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
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
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
This phenomenon is why well funded race teams throw way "perfectly good" used parts.
RE: Need help analysing a failure
RE: Need help analysing a failure
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
http://www.dynamicmetals.net/
RE: Need help analysing a failure
http:
Like%20Scratch%20on%20the%20Fatigue%20Life%20of%204340%20Steel.pdf
http://www.shotpeener.com/library/pdf/2002006.pdf
ht
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
RE: Need help analysing a failure
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