Part failure - need help figuring out why - Stress Concentration

[DWLetourneau]

Hello everyone I have a problem that I need some help with.

At my place of work as a co-op we had a part fail. This part is a shaft thats dia. 6" and steps down to a dia. 4.125". There is a 3/8" fillet to help reduce stress. The material is AISI C-1015HF, I believe. This part snapped in half at the step down between the two diameters.

I have two questions...

1)I was under the impression that for ductile materials, stress concentration factors should can be ignored. This part, I beleive, is considered a ductile material, but it has been heat treated for hardness. So at what point does a part go from ductile to brittle, and when should stress concentrations become a factor again (In the case of considering the stress at the fillet of these to diameter features)??

2)Also I have another question about static loading. If you have a shaft on the ground and lift it up by the ends and held it say 3' off the ground.When you pick the shaft up a moment created, correct?. Is this still considered a static load even though there is a moment created in lifting the object?

If you have any questions let me know. I can draw a quick sketch.

Thanks.

 

[TVP]

1) Your impression is incorrect.

2) Does this shaft rotate due to the application of a torque somewhere? If so, then it is not static. Likely this failed due to fatigue not overload, but that is a complete WAG that I am making...

 

[Twoballcane]

Hmmm...off the top of my head...
1, Ductile or brittle every material has a yield and utimate strength. If you do a dynamic analysis (assuming this is rotating), you can figure out the torque and then the stress at the point on the shaft. If this is more than yield, your starting to deform. Once you are over ultimate, you will fail.

2, There will be two moments. One is when you pickit up and the other at static. When you pick it up you are accelerating the part in the upward direction. Due to this you have a G case. Once it comes to rest the moment is due to the mass.

Tobalcane
"If you avoid failure, you also avoid success."

 

[TheTick]

Hardened = no longer ductile --> cracks can propagate more readily.

As far as additional loads due to lifting, F=m*a for additional load. This assuming your constraints are constant. How fast did you pick this up, anyhow?

 

[DWLetourneau]

Well this shaft was not in use (not rotating). It was being lifted and snapped off at the end. Sorry I should have stated that in the orignal post.

I'm not sure on the speed the lifted the part at. I was just told they lifted it and it snapped. I would assume that it was somewhat slow, but again that is an assumption.

Another thing we were worried about was that our machinest had welded a small block on the end of these right near the step/fillet. I thought maybe the since the material was already hardened the heat from welding could have weakened the "joint" significantly.

I will try to get pictures of the broken part in the next couple days and let you all see.

 

[desertfox]

Hi DWLetourneau

A sketch giving some dimensions of the shaft and the lifting points would be useful including any pictures of the broken part.
Have you made these shafts before? if so have you had any problems like this with them, its good to look back at history of a component if you can.


regards

desertfox

 

[Twoballcane]

hmmm...this smells fishy. Unless this shaft is so long that its own mass can deform to the point of failure does not sound right. It does not sound like a design problem, but a handeling probem as in sombody droped it, broke, and now trying to cover it up by blaming the design. Just my thoughts.

Tobalcane
"If you avoid failure, you also avoid success."

 

[jistre]

I'd also like to know a couple of things that might help narrow down the search. A lot of times, you can zero in on a cause for a failure simply by first ignoring the particulars of the part and looking at the circumstances surrounding the failure.

1) Was this a brand new part or had it come out of a machine for overhaul/repair?

2) Was this a proven design with handling procedures that had been used successfully before, or was it also a new design that had never been handled by the shop floor before?

 

[mikek10]

I agree with the last poster to a certain extent.

Blaming of operators / maintenance is over the top sometimes and an easy way out for the designer - but this sounds too much, it sounds unlikely for a piece take a workload but not its own weight under handling??

It could be it was already damaged - why were the welded blocks put in place? - had it already fractured once?

More info would be good!

 

[mikek10]

I meant Twoballcane by last poster - jistre posted while I was typing

 

[rb1957]

yeah, things just don't break in two for no reason ... i don't care how fast it was being lifted, that wouldn't do it. it clearly nothing to do with fatigue or notches. possibly something to do with the heat treat ?? or maybe something happened whilst it was being lifted ... picture a parent, a child and a broken window "i didn't do anything, i was just sitting here and the window broke !"

 

[hydtools]

If the shaft is AISI 1015, not much happens in heattreat unless it was quenched rapidly to force it to harden. That could create stress cracks. Was the heattreat a case hardening process? Done improperly would cause case cracking.

Ted

 

[jet1749]

Fracture face analysis would surely explain the mode of failure and its origin

regards
John

 

[Artisi]

a photo of the failure area would help.

 

[NickE]

"Hardened = no longer ductile --> cracks can propagate more readily."

I would not say that is a general rule. Often hardened and tempered steels are so incredibly strong that the forces required for cracks to grow are also quite enormous.

1015 wont even harden if you quench it in cold salt brine, not enough carbon.

so if this is actually hardened it is as a previous poster stated and cased of some type, nitride, carburized, etc...

Also as others have said, failure analysis would be really helpful in this case.

Nick
I love materials science!

 

[TheTick]

I humbly accept NickE's correction.

I thought 1015 didn't sound like a hardenable grade. Perhaps hardening did more harm than good.

If this company is misapplying processes and materials like this, who knows what else is wrong. First step is to get the material and processes in order before jumping to conclusions about design.

 

[Gymmeh]

NickE, I thought you could carburize low carbon (C<0.30%) steels to increase surface hardness.???

 

[patdaly]

Gymmeh,

Certainly you can Carb and Harden low carbon steels, but that should be specified in the OP question.

 

[NickE]

"so if this is <b>actually hardened</b> it is as a previous poster stated and cased of some type, nitride, carburized, etc..."

Sorry hard to read grammar.. Yes low carbon steels can be carburized, nitrided, nitro-carburized, etc... as a means to produce a hard strong case surrounding a tough core.

 

[bcd]

One other root cause to consider is Hydrogen embrittlement.

Heat treated, welding near the stress concentration, possibly a rapid fracture when lifting....Would have been the first thing I looked into.

Could also be a defect in the base material. The fracture surface will lead you in the right direction. Have a metallurgist look at it.

bcd