Shaft failed by torsion? 2

[Naruwan]

Hi guys,
I just got this one and only photo from my client regarding a broken shaft. Based on this picture, I reckon it was failed by torsion. Of course I will be accessing this shaft tomorrow and hopefully will be getting more photos. Any comments?

Thanks!

 

[desertfox]

hi Naruwan

Not the best pic I have seen (grin) looks like a failure caused be principle tensile stress acting on principle planes ie at 45 degrees to shaft axis and yes those tensile stresses could be caused by torsion.
Would like to see other half of shaft and the failed faces.

desertfox

 

[Guest102023]

A failure analysis isn't done by looking at a photograph. Find a reputable metallurgical lab and have a complete (root cause) investigation performed.

 

[desertfox]

It might need a lab at the end of the day but from my experience looking at the failure and knowing typical signs and features of different failures can tell you an awful lot.

desertfox

 

[desertfox]

hi

This article might help:-

http://www.plant-maintenance.com/articles/rcfa.shtml

 

[Guest102023]

Useful article, but looking at the fracture will only tell you the fracture mechanism. It won't tell you what other fracture mechanism or small defect might have led to fracture, it won't tell you material characteristics, and it certainly won't tell you the root cause. If this failure looks just like the previous 5+ identical-looking failures, then go ahead and draw conclusions.

I recently completed an investigation on a tube that had at least three distinct fracture mechanisms, and a root cause that happened years earlier. It required metallography, optical and SEM fractography, chemical analysis, and extensive microhardness testing, as well as understanding of the alloy characteristics, fabrication methods, and service conditions.

 

[Naruwan]

Hi guys, I'm back with some more better photos. Apparently this hollow shaft was a part of a drilling equipment in a marine environment. The shaft had been in operation since the year 2005 under a seawater condition.

The fracture surface is covered with brown rust and only a small portion is still with the colour of fresh fractured surface. Close to the crack initiation point is dark brown rust. The fracture happened at an angle, which is about 45degrees to the axis of the shaft, however it ended flat and perpendicular to the longitudinal direction of the shaft. It was noticed that a thin but sharp edged layer was encircling the fractured surface’s rim.

Multiple marks, straight aligned and evenly spaced, are seen throughout the OD’s surface. The marks are believed to be caused by clamping devices. ID’s surface at the fractured end of the shaft is observed to be covered with rust. A screw thread is seen running internally at the other end of the shaft. At this end, grease is covering the threaded portion and not must rust-ing was observed.

I reckoned that this is a torsion failure however I was curious why the fracture had turned from a 45 degree rupture to a flat one (perpendicular to the longitudinal direction of the shaft)?

Thanks again!

 

[Naruwan]

This is another side view of the broken shaft.

 

[Naruwan]

A 'top' view of the broken shaft showing the fractured surface.

 

[Naruwan]

End view of the shaft showing the fractured surface from another perspective.

 

[Naruwan]

A close up view of fractured surface

 

[desertfox]

Hi Naruwan

Thanks for the pic's.

I agree it looks like a torsional failure due to principle tensile stresses as I mentioned in my previous post.
The reason I think the fracture went flat at the end is that at any other orientation other than at roughly 45 degrees there are both tensile and shear stresses acting, so its my guess that as the crack progressed there was insuffient material towards the end to resist the tensile and shear stresses on the other planes and hence the final straw that broke the camels back so to speak.

Regards

desertfox

 

[Naruwan]

Desertfox,
so eventually, the shaft was failed by overloading. I was going to take some more photos (fractography) using SEM, checking it's chemical composition (They claimed that it's of AISI 4140), check on it's mechanical properties with impact test and tensile test.

However I think there won't be any surprises in those results that I will be getting. Anyway,
thanks guys.. probably I will upload some SEM photos next week for you guys viewing..

Regards

 

[desertfox]

Hi Naruwan

Yes I think near the end it overloaded.
If you can let us know the outcome of the findings.

Regards

desertfox

 

[Naruwan]

Hi guys,
I had did some SEM analysis on the failed hollow shaft especially on the torsion part. However, I am quite bewildered that they don't seemed to be microvoids which have been elongated in the direction of shear.

 

[Naruwan]

This is the same location taken at 500X.

 

[redpicker]

It's difficult to tell, but did this fail at a threaded connection? What is the OD? (I'm guessing around 6 inches) Did it fail while it was in service?

If it is a threaded connection, you are looking at a tensile overload failure, failing in the last engaged thread. Tensile overload probably occuring from bending or a combination of bending and applied torsion (with the threads applying tensile stresses to the connection). The 45 degree angled portion is mostly irrelevant, as the fracture initiated in the flat (transverse) portion at the 12 O'clock position in your photo 1990.jpg. There may be evidnece of a small fatigue crack at the thread root in that location. The remainder of the fracture is a rapid fracture cause by the low toughness of the material.

If the material is 4140, you will find the yield strength to be below 70,000 PSI (it should be better than 100,000 PSI) and the Charpy impact to be less than 25 Ft-Lbs (it should be better than 30 Ft-Lbs). If you do get higher than 30 Ft-Lbs with a room temperature test, find out what the temperature was when it failed and test again at that temperature. If the material has been heat treated, you will find the hardness at the OD surface to be around 30-35 HRC but fall to less than 25 HRC at the depth the fractutre initiated (very shallow hardening). It's a toss-up whether or not you are looking at as-rolled/annealed 4140, poorly heat treated 4140, or just a plain carbon steel. Let us know the chemical analysis and mechanical property results.

Oh, you won't see any microvoids because this is a low-ductility (brittle) fracture. If you can get a clean enough surface, you will find a combination of cleavage and quasi-cleavage.

Yeah, I've seen a few of these, why do you ask?

rp

 

[Naruwan]

Hi guys,
I am back with some chemical and mechanical results.

The chemical results are as follows: C:0.39%, P:0.007%, Sulphur:0.017%, Si: 0.26%, Cr: 0.95%, Mo: 0.18%.

Mechanical results:
UTS: 944 N/mm2
Yield stress: 563 N/mm2
Hardness (HRC): 287 averaged of three readings.

It seemed that this material is within the AISI 4140 specifications, however there are inclusions seen everywhere. I did an EDX of the tensile fractured surface and found that its Sulphur Manganese.

Eventually, we found out that the shaft (which was part of a very long drilling component for sub-marine usage) buckled and bent. There should be some torsion or atleast twisting when the component broke off, that's why we are seeing some signs of torsion. We had deduced that the component failed by overloading.

 

[desertfox]

Hi Naruwan

Thanks for the update, just to clarify when you say "we deduced failure by overload" do you mean after getting the SEM results?
Also what you're saying is there was no fatigue just purely overload?

desertfox