Is this failure by shear or fatigue? 6

[Naruwan]

Dear all,
I had just received a photo of the failed shaft's fractured surface and I was wondering if this fracture surface is caused by shear or fatigue. I had asked our mentor regarding this and he claimed that it was caused by fatigue due to the fact that there were ratchet marks at the left side of the shaft. Seriously, I am not convinced.

 

[EdStainless]

You need higher magnification to prove it, but that is the look of fatigue beach marks.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[metengr]

It looks like evidence of fatigue crack propagation under torsion, and not just torsional overload. It also appears as though there is a machined step or diameter change along a portion of the fracture surface.

Can you provide more details as to material and service conditions?

 

[desertfox]

Hi Naruwan

Looks like a fatigue fracture to me.

Desertfox

 

[unclesyd]

I have to make the same request as posted above, one needs a little more information and a picture. The red rust is one sign that the shaft took a while to break.

Is there any cracking in the larger cross section and also the smaller cross section?

Did someone get after the shat with a torch, evidenced by the apparent burn marks in the top quadrant?

 

[Ron]

Agree with metengr that those are fatigue beach marks. As you can see, the cross section was reduced by the fatigue, enough so that a fast torsional failure then occurred.

Any chance this is a very long shaft subjected to "whipping" in the middle?

 

[blacksmith37]

I suggest getting a book by Don Wulpi- "How Thing Fail" ,or Why. It is old, several versions. It has a lot of excellent photos of mechanical failures in steel equipment (He used to do failure analysis with International Harvester).
Red rust (Fe2 03) is evidence of fretting "corrosion" ;dry steel surfaces rubbing together with very limited movement.

 

[redpicker]

I agree with fatigue, but I don't necessarily agree with propagation under torsion. It looks like just bending to me. It is hard to tell from only a photograph, but it looks like the initiaiton point is around the 7:30 position. Ratchet marks radisting from either side suggest an high bending load (much higher than the fatigue limit, or only one crack would have formed, not multiple cracks as suggested by the ratchet marks). These ratchet marks diverge approximately equally on both sides. I would expect them to take on a directionality if there were torsional loads involved.

The last to fracture has rusted because grease covered the advancing fatigue crack, slowing down the corrosion. On final fracture, the crack progression was so rapid that the fracture surfaces did not get coated with grease.

rp

 

[arunmrao]

I am learning,good discussions,though I observed the gas cutting at the edges and rust region ,perhaps some more input from OP can help. If there is a rust does it mean that the shaft was used in wet environment and water seeped through the crack before failure happened.

_____________________________________
"The richer we have become materially, the poorer we have become morally and spiritually." Martin Luther King Jr

 

[metengr]

rp;
As usual gun slinger metallurgy based on only one picture. The only reason I thought that there was a torsional component to fatigue crack propagation is simply because the fracture surface does not appear to be simply transverse to the main axis of the shaft if this was pure bending. If there is a machined step, the fracture surface would be significantly out of plane or at an angle, which would explain why the OP mentioned shear versus fatigue.

 

[redpicker]

metengr,
Agreed. It is difficult to tell from only one picture. When I think of torsional fatigue in a shaft, I typically think of the shaft is rotating, which I don't believe is the case here. While I'm sure there is a torsional compoent to the loading (it is, after all, a shaft) I don't believe it is rotational; that is, the shaft does not rotate completely, and primarily, in one direction. It would help to know if that is a concentric step and if the cracked formed in the fillet at the edges of the step.



rp

 

[Guest102023]

There is evidence of fatigue, but that is about all you can say. Colours on a fracture surface are no indication of prior heat treatment or possible overheating. A side view would be helpful in establishing the fracture surface topography, as well as other features like geometric transitions or welds.

 

[gearcutter]

I'm only new at this so please forgive me; I can see plenty of ratchet marks but I can't see any beach marks. Can someone point out their position please?
The surface does not appear, to me, to be smooth and fatigued. It looks rough and faceted suggesting a fast brittle failure rather than a slower ductile one.
Looks to me like a high nominal stress, mild to sharp notch, rotating bending failure with minimal signs of fatigue.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[gearcutter]

Once again, I'm only new at this but I would have thought that these images are more representative of a fatigue failure. Beach marks are clearly decernable and the fatigue surface is smooth.
Please correct me if I'm wrong.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[metengr]

In the photo with the OP, look at the series of distinct radial ridges emanating from the shaft OD surface (left side of the photograph). As you follow the ridges toward the mid-section of the shaft look at around the 8 to 9 o'clock location and you can see a noticeable thumbnail-shaped region that exhibits a different surface texture (where the ridges seem to end). This region has the appearance of a beach mark or crack arrest line, looks like one of several. Different lighting and angle would be required to confirm.

 

[gearcutter]

Thanks metengr.
I can see the area you are referring to but, to me, it looks like an area where the grease is making the rough fracture surface look smooth.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[blacksmith37]

Classic low stress (small final fracture, single origin) bending (flat fatigue surface- torque stress fatigue is at 45') fatigue starting in the keyway .

 

[Naruwan]

Dear all, I manage to get some more photos ( however not able to upload all at once. Attached is a closeup picture of the ratchet marks. Edstainless, could you point out where is the beach marks? I am not able to identify them. Regarding there's some remarks about had the shaft been flame cut out (as it seemed that a part of it revealed flame cut marks), I really need to ask the owner of this vessel.

This is a part of the stock rudder that failed. When the steering gear was operating, the rudder was observed to be in the mid-ship position. It was discovered only when an Auto Pilot failure alarm was sounded. The ship was not able to steer even when it was changed back to manual steering. Apparently, these are the info that I have obtained from them.

Referring to back my first photo attachment, the fractured surface is quite flat, therefore it might not involve torsion. The initiation of the crack should be starting at the 6 o'clock position where it propagated to into almost a quarter of the shaft's diameter. Then it started to propagate further but slowly which also allowed lubricant or grease to seep into the crevice. The 3rd part the fracture is the rusted part. Based on the surface, it's a brittle fracture. Rust came in probably because lubricant can't go further into the crevice. The final fracture was at the 2 o'clock position where the material just tore off.(Correct me if I'm wrong)


Thanks again!

 

[swall]

This photo does not show beachmarks. If you look at the ratchet marks, you can see a smooth area as they are ending. This smooth area shows a crack arrest right at the rough texture a little further in. If you do SEM in the area where the crack arrest is, you should see some nice fatigue striations. You MIGHT see some beach marks in this area if you clean the oil off with some spray degreaser or contact cleaner.

 

[unclesyd]

I've seen this type failure before in rudder stock on work boats. The usual results for the reason is the rudder was flapping in the breeze. I've seen breaks quite similar to this and some with longitudinal 45 degree cracking.

What is the configuration of the shaft at the break?

What is the material?

Was there any problem with searching while steering the boat?