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Is this failure by shear or fatigue?
6

Is this failure by shear or fatigue?

Is this failure by shear or fatigue?

(OP)
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.

RE: Is this failure by shear or fatigue?

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

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

RE: Is this failure by shear or fatigue?

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?
 

RE: Is this failure by shear or fatigue?

Hi Naruwan

Looks like a fatigue fracture to me.

Desertfox

RE: Is this failure by shear or fatigue?

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?  

RE: Is this failure by shear or fatigue?

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?

RE: Is this failure by shear or fatigue?

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.

RE: Is this failure by shear or fatigue?

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

RE: Is this failure by shear or fatigue?

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  

RE: Is this failure by shear or fatigue?

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.
 

RE: Is this failure by shear or fatigue?

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

RE: Is this failure by shear or fatigue?

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.    

RE: Is this failure by shear or fatigue?

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
 

RE: Is this failure by shear or fatigue?

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
 

RE: Is this failure by shear or fatigue?

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.

RE: Is this failure by shear or fatigue?

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
 

RE: Is this failure by shear or fatigue?

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

RE: Is this failure by shear or fatigue?

(OP)
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!
 

RE: Is this failure by shear or fatigue?

2
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.

RE: Is this failure by shear or fatigue?

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?

 

RE: Is this failure by shear or fatigue?

(OP)
unclesyd, I don't have sufficient info regarding to the state of the rudder when it failed.  They just mentioned that the rudder is at the mid-ship position.  Thanks for the tip, I think I need to ask them more regarding that!

I had added in another close-up picture of a portion of the fractured surface.  I am quite doubtful that that was caused by flame cutting.  Ofcourse, I know to confirm it, I need to look at it's microstructure.

RE: Is this failure by shear or fatigue?

I think you are correct in you assumption. It is still weird as it is 180 degrees from the ratchet marks in the first picture.
A note on the rubber failing in the amidships position. A well designed rudder will go to this position in any failure that frees the rudder.  The degree of motion, 90.120.etc, of this rubber might help explain some of the features

Is the line starting from about 11:00 position in the last picture a greasw streak or is it grease emanating from a crack?

Another interesting features the series of parallel lines in the middle of your last picture. I've seen this on some lower strength steel that later turned out to features due to very high stress in will become crack origins.  During MT inspection these are normally ignored as they are considered ghost lines.
 

RE: Is this failure by shear or fatigue?

Naruwan,

I believe you are correct, there is no evidence of flame cutting.  What appeared to have been a flame-cut region in the orginal photo is clearly ratchet marks from a crack that initiated approximately 180 degrees away from the major crack.  This was probably a result of the increased bending the shaft saw in this area because of the reduced cross sectional area.

It would be nice to see some pictures after the grease has been cleaned.  I think beach marks would be clear, then.  Also, if you have the chance, a wet magnetic particle inspection of the OD would also be illustrative because I suspect it would reveal many more cracks.  The ratchet marks indicate multiple crack initiations that join as the crack fronts propagate towards the center.  THe large number of ratchet marks suggests there could be many other cracks that were initiated, but did not join the main group of cracks that eventually joined and propagated towards the center.  The number and location of these secondary cracks could reveal more information to the nature of the loading conditions that led to the cracking.

rp

RE: Is this failure by shear or fatigue?

This is a typical example of a rotating shaft fatigue failure. Form just this picture a failure analyst can tell the origin of the fracture (at one corner of the key hole ?), overload region (diametrically opposite end), and even the direction of the shaft rotation. Very similar examples are available elsewhere in text books and failure analysis courses. The beach marks are clearly visible and the rotation of the shaft makes them appear the way they are. Good post.

RE: Is this failure by shear or fatigue?

(OP)
Hi Guys! I'm back with new photos regarding to this case that I had started earlier on.  I had cleaned up the fractured surface with acetone and hey presto! I found beach marks! As you can see, there are ratchet marks laying on both sides of the shaft and they are exactly opposite to each other, just that one is larger than the other one.  Is it true that the formation of ratchet marks normally will result in beach marks?  In this case, the smaller ratchet marks were not able to form beach marks because the final rupture was to fast for the beach marks to be form?  I remembered Redpicker mentioned that this shaft had been subjected to bending and rotational stress.  The beachmarks formed like 30% of the whole shaft, I reckon that this shaft had been in high stress with low frequency type of fatigue.  I was still quite unsure of the crack initiating point.  Was it started at the 6 o'clock position (based on the photo) or was it started where the big & long ratchet marks are pointing (which is roughly at the 8 to 9 o'clock position)?

Thanks!

RE: Is this failure by shear or fatigue?

(OP)
This is another side view of another side of the shaft.  Maybe you are able to notice that a clean and straight fractured end was running along the circumference of the shaft which ended the fracturing.  Was it due to the brittleness of the material? (I'm yet to obtained the shaft's mill certificate)

Thanks!

RE: Is this failure by shear or fatigue?

Hi Naruwan

Thanks for the photo's.

Well the pics show the Instaneous Zone to be quite large and therefore the shaft was heavily loaded.
The side view showing the crack running diagonal is an indication of principle tensile stresses present in the shaft whilst being loaded in torsion.
I am guessing but I think the crack started at 6 o'clock looking at the first of your latest post because the river lines at that  point tend to curve to the outer surface but others might no better, however its clear the crack started somewhere on the left of the shaft and grew across to the right as we look at the picture.
According to the reference I left in my earlier post it says if the dividing line doesn't clearly indicate the crack start point then rotational bending was involved.

desertfox

RE: Is this failure by shear or fatigue?

Naruwan,
Things are a lot clearer after cleaning.  There is no doubt about it being fatigue, now.

The mag particle test I mentioned would be using a longitudinal field to detect circumfrential cracks on the OD.  I would expect them to be short indications (about the length of a single ratchet mark, or about 1 - 4 mm {maybe 1/32"-3/16"}) so wet flourescent mag particle may be needed.  Dye Penetrant testing may reveal them, but it is very messy to deal with (that dye gets everywhere) and its sensitivity is a bit low, so it may not detect them.

I put the origin at around the 8 o'clock position in your first photo.  Note that directly opposite this is where the crack comming from the other side is the deepest (the initiation point from the other side).  This suggests bending of the shaft from side-to-side, with the stresses on one side of the bend being greater than the other (probably because the support points are at different locations).  This would lead me to suspect the bushings/bearings that are supporting the shaft are a bit worn so one side is supported better than the other.  That's just a WAG, though, since I don't know anything about how this shaft is supported.

Thanks, Naruwan, this has been interesting.

rp

RE: Is this failure by shear or fatigue?

Another point to consider as rubber problems were mentioned in the OP is to look at the rudder design. If a rudder doesn't have the proper leasing edge,the portion in front of the ruder shaft, will impose much higher stresses in the rudder shaft.  This condition will cause the rudder to seek and shaft problems on hard turns.
I would look at the rudder design.

Sorry for the lack of proper nomenclature, it been too long away from the boats.  

RE: Is this failure by shear or fatigue?

(OP)
unclesyd,
I had managed to get a copy of the schematic drawing of the stock rudder with the bearing only.  In this attachment, the first drawing shows the original design of the stock rudder.  They re-designed it and came up with a thicker stock rudder with a bigger bearing to accommodate the new stock rudder.

Hope this helps...

RE: Is this failure by shear or fatigue?

(OP)
Hi, dear all,
this is a photo micrograph (at 100X) of the microstructure of the broken rudder stock which we are discussing earlier. You might be able to see that there are sulphide stringers embedded in the microstructure. The stringers aligned themselves in a way that they are along the longitudinal direction as the rudder stock.  Apparently, what I found out that the material grade is 16Mn which happened to be a Chinese grade.  Looking up the net, I found the equivalent grade to it will be ASTM A36.  The stringers are everywhere, which meant that if I were to perform a ASTM E45 test on it, this piece of steel will sure fail!  I am not quite sure whether the sulphide stringers maybe another factor which contribute to the failure of the rudder stock.  The microstructure looked strange in the sense that the pearlite are very angular and sharp in shape.  The grain size also seemed to be quite large which having a ASTM grain size number 6.0.

Is anyone able to advise me on this?

Thanks again!

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