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.
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?
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Plymouth Tube
RE: Is this failure by shear or fatigue?
Can you provide more details as to material and service conditions?
RE: Is this failure by shear or fatigue?
Looks like a fatigue fracture to me.
Desertfox
RE: Is this failure by shear or fatigue?
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?
Any chance this is a very long shaft subjected to "whipping" in the middle?
RE: Is this failure by shear or fatigue?
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?
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?
_____________________________________
"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?
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?
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?
RE: Is this failure by shear or fatigue?
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?
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?
RE: Is this failure by shear or fatigue?
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?
RE: Is this failure by shear or fatigue?
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?
RE: Is this failure by shear or fatigue?
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?
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?
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?
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?
Look at this site it might help:-
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desertfox
RE: Is this failure by shear or fatigue?
RE: Is this failure by shear or fatigue?
Thanks!
RE: Is this failure by shear or fatigue?
RE: Is this failure by shear or fatigue?
Thanks!
RE: Is this failure by shear or fatigue?
Thanks again!
RE: Is this failure by shear or fatigue?
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?
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?
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?
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?
RE: Is this failure by shear or fatigue?
Thanks!
RE: Is this failure by shear or fatigue?
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!