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Part failure - need help figuring out why - Stress Concentration

Part failure - need help figuring out why - Stress Concentration

Part failure - need help figuring out why - Stress Concentration

(OP)
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.

RE: Part failure - need help figuring out why - Stress Concentration

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...

RE: Part failure - need help figuring out why - Stress Concentration

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."

RE: Part failure - need help figuring out why - Stress Concentration

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?

RE: Part failure - need help figuring out why - Stress Concentration

(OP)
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.

RE: Part failure - need help figuring out why - Stress Concentration

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

RE: Part failure - need help figuring out why - Stress Concentration

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."

RE: Part failure - need help figuring out why - Stress Concentration

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?




RE: Part failure - need help figuring out why - Stress Concentration

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!

RE: Part failure - need help figuring out why - Stress Concentration

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

RE: Part failure - need help figuring out why - Stress Concentration

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 !"

RE: Part failure - need help figuring out why - Stress Concentration

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

RE: Part failure - need help figuring out why - Stress Concentration

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

regards
John

RE: Part failure - need help figuring out why - Stress Concentration

a photo of the failure area would help.

RE: Part failure - need help figuring out why - Stress Concentration

Quote (TheTick):

"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!

RE: Part failure - need help figuring out why - Stress Concentration

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.

RE: Part failure - need help figuring out why - Stress Concentration

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

RE: Part failure - need help figuring out why - Stress Concentration

Gymmeh,

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

RE: Part failure - need help figuring out why - Stress Concentration

Quote (me):

"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.

RE: Part failure - need help figuring out why - Stress Concentration

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

RE: Part failure - need help figuring out why - Stress Concentration

Thank you for clearifying.

RE: Part failure - need help figuring out why - Stress Concentration

(OP)
Hey fellas thanks for the replies.  Sorry I have been out of the office (I'm actually still in school and only work on Tue/Thur part time).  I have attached a sketch of the part that I have been reffering to.  I will try to get a picture of the actual failed part this afternoon.

Attached is the end of the roll.  It broke at the very end diameter to the next step (at the 3/8" fillet).  The overall dimension of the shaft is 137.125" and acutally steps up a few more times in diameter.  The part is symmetrical as well.

We have used these many many times.  The difference this time was there shafts were regrinded for smoothness.  The blocks that were welded on at the end where to mark which side of the roll for the dolly to pick the shaft up on.

The way they pick these shafts up are in two.  They use a coupling to attach to shafts by end and lift by the middle (both side balance out).  When they lifted this one it sheared right off.

Ill get picutres of the carnage today.  I'm not to the point yet in my education career to know failure analysis (i'm a co-op) so I'm hoping you might be able to learn me :)

Thanks again guys.

RE: Part failure - need help figuring out why - Stress Concentration

(OP)

RE: Part failure - need help figuring out why - Stress Concentration

only pic 6 came across ...
you say they couple two of these 138" ingots of steel (7" dia) together and lift ... does the coupling reach onto the full diameter of the bar, or only the reduced rod end ?

if it's just the rod end, then there's 130" x 50in2 of steel ... 6500in3 *.3 = 1 ton
moment something like 2200*65 = 143000 inlbs
sounds like s lot (but then its a big rod end).

if your place does this often, then something went wrong with this one.  maybe it was the grinding (unlikely IMHO), maybe it was the lift (sudden loading ?) ... maybe things were unbalanced ???

RE: Part failure - need help figuring out why - Stress Concentration

I only can see files 1 & 2... There are features on the fracture surface that appear to be common to ductile failure... however it doesnt appear that the part bent before breaking, at all.... Weird...

RE: Part failure - need help figuring out why - Stress Concentration

At first look, the fracture runs close to the end of one of the welds.  Nasty welding.  In picture 001, there appears to be a heat affected zone that is cresent shaped and may have been the start of the fracture.

I would not expect welding would be allowed on a heattreated part.  That would create a discontinuity or crack in the case.  Were the blocks welded on differently than had been done in the past?

Ted

RE: Part failure - need help figuring out why - Stress Concentration

(OP)
Yes these blocks were welded to let the dolly drivers know what side to pick the shafts up on.  There may have been other reasons that are unknown to me.

It looked like to me as well that part of the fracture happened right around the penetration of the weld.  I thought the heat from the weld might have caused a problem in the material (being that it was already heat treated).

RE: Part failure - need help figuring out why - Stress Concentration


If the 3/8" fillet is produced by grinding, it may have had grinding cracks produced in it, although this would be evident as an early fatigue failure.


Cheers


Harry

RE: Part failure - need help figuring out why - Stress Concentration

how does the coupling pick up the two ingots ?

RE: Part failure - need help figuring out why - Stress Concentration

If you examine the broken faces you can usually tell if a crack already existed. The "old" crack usually appears darker than the area that has finally broken. If the shaft was long and picked up with the original crack at the bottom it may have broken under its own weight.

RE: Part failure - need help figuring out why - Stress Concentration

i guess your load condition is the torque type.The normal failure in toque is due to section variation.anyways if you are using any kind of stress analysis software failure should be detectible.

RE: Part failure - need help figuring out why - Stress Concentration

Either the two pictures of the broken end piece are not the same piece, or there are two welded-on blocks on both sides of the broken end piece. I thought only one block was welded onto the end piece.

In any case, I think the end piece broke due to a hair line crack. The crack most likely occured during the heat treating process. Probably caused by thermo shock due to improper temperature control. I do not think the welding had anything to do with it. The heat from the weld is at most only local. Yes, the welding job looks terrible, but it's not an extensive weld.

Eugene Kim
www.eeksolutions.com

RE: Part failure - need help figuring out why - Stress Concentration

Looking at picture 6, which is the only picture that clearly shows the fracture face, it's clear that this is a fatigue failure.  The "beach" marks evident in picture 6 leave no doubt that this part failed in operation.  

The end of the shaft finally breaking off during handling is largely coincidence, not the root cause of failure.  I'd say it's a possibility that the weld in pic 1 may have been about the only thing holding the shaft together.

Without more good pictures of the parts it is difficult to provide much information.  It does appear that there is also a stress concentration issue, but from what I can see in the pictures provided it looks like rotational bending was a larger issue.

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