Broken drive shaft...is it Torsional fracture?

[vandit]

hey , couple of days back while testing our old car in college campus, we had driveshaft break down....this thing happened in first gear(guessing due to max torque).....also this drive shaft was little modified from stock one.....it was cut, turned and was again splined at the end to fit into our track width

the fracture happened at the point where the splines start , and the two broken pieces are very neatly separated , like being cut under cutter ....

after zooming in , i can see whirlpool sort of pattern being generated before breakoff tip.....does this suggest torsional fracture?

secondly, can these drive shafts actually break like this?....or i am missing out vital points...please explain...eager to know about this.....

also i would like to upload pics ...how can i do that?

[Drew Price]

Shanky,

I think (from what I understand of the conditions you describe) you are correct in assuming that the failure was torsional in nature. The 'whirlpool' pattern shows where the two pieces tore away from each other.

You can upload photos to a hosting site, like www.photobucket.com, (http://www.photobucket.com,) and then link to the URL's of the images to post them on the board. You can link to images in a Facebook or MySpace account if you already have one of those set up as well.

As for shaft failures, the failure you are describing sounds like a ductile failure, your shaft was too soft. It probably looked something like (photo from BEAR Metallurgy (http://www.bearinc.com/metallurgy.html):
http://www.bearinc.com/mechanical%20engineering-4.jpg

Remember that ductile and brittle materials fail under different conditions, ductile materials usually fail in shear, and brittle materials usually fail in tension. A shaft loaded in torsion has the max shear plane perpendicular to the axis of rotation, so you see the clean cleaving of the two shaft halves.

In a brittle, or brittle-surfaced material, like a properly treated drive shaft we might see a failure like below, where the max tensile loading plane is at 45 degrees to the axis of rotation (similar to how the max shear stress in a tensile specimen is at 45 degrees to the direction of loading). Photos below from Smith's 'Engineer to Win':
http://i289.photobucket.com/albums/ll207/Drew-Price/Smith-ShaftFailures001.jpg

A shaft which has failed under fatigue loading will have varying section patterns, sort of like the photo below, where the 'whirlpool' or 'beach mark' pattern us where a crack or imperfection has propagated smoothly spreading through the loaded section, and when the remaining material could no longer carry the full load, the remaining section tore away from the rest, leaving the rough texture on the left of the shaft. The starting point of the fatigue fracture (point 'A' in the photo below) is a surface flaw, like a scratch, a nick, or a rock chip in the material. Shafts in torsion carry very high stresses on the outer surface of the shaft, so a very good surface finish is a necessity. If your shaft looks rough or has grooves from the turning operation then this could be a source of the starting point for a fatigue failure. The roots of the splines are another excellent place for a fatigue failure to start.
http://i289.photobucket.com/albums/ll207/Drew-Price/Smith-ShaftFailures003.jpg

As for the fracture happening at the root of your splines, I am completely unsurprised. Review or expand your knowledge of stress concentrations, or stress raisers, and look at what cutting splines into your shafts does to their strength if you do not change the geometry to accomodate their presence. If the splines were cut into the outer surface of the shaft, as in the middle drawing below, then the roots of the splines have reduced the cross-sectional area of the shaft, and have reduced it's strength in the process. The splines have pretty sharp roots as well, and act to concentrate stress flow right where you have weakened the shaft by reducing it's area, thus the shaft will always break at the root of the splines in a design like this.
http://i289.photobucket.com/albums/ll207/Drew-Price/Smith-ShaftFailures002.jpg

I hope that explains some things, let us see photos of your shaft though, always fun to see what happens when things break.

Best,
Drew

[vandit]

ya, understood.....i guess it was our mistake only....as we cut them, we provided a notch or gap sort of thing at the end of splines(or the beigining of splines)...this notch is the point where a clean fracture has taken place....

after looking at 2D drawings of shaft in you rreply, i got the flaw....there is suppose to be a smooth uphill joint to the main shaft from the splines and not like a gap/notch like us....

man, this fracture mechanics is interesting

http://i296.photobucket.com/albums/mm190/vanditgoyal/6.jpg
http://i296.photobucket.com/albums/mm190/vanditgoyal/4.jpg
http://i296.photobucket.com/albums/mm190/vanditgoyal/2.jpg
http://i296.photobucket.com/albums/mm190/vanditgoyal/1.jpg

[Drew Price]

Yup! You pretty accurately reproduced a #1 above, when you actually want a #3.

Make sure your 'uphill' slope increases in diameter as you go from the shaft center out to the splines. The circle around the roots of the splines should be larger diameter than the diameter of the middle of the shaft to keep the stress concentration to a min.

Best,
Drew

[vandit]

yeah understood, now when i look at the stock shaft ....it has this uphill smooth continuation and also the diameter of the shaft after uphill is reduced for some 50mm length and again a uphill and then smooth uniform diameter diameter.....

and also i would like to mention this point, this drive shafts we bought comes with induction hardening.....but we turned this shaft and reduced it's top later by 2 mm...do you think that could have further lessen the strength of the shaft ......frankly i dont have much knowledge on induction hardening....

[Mike Sadie]

shanky, you are correct about that too. Induction hardening is basically a typical heat and quench process. The entire shaft (through thickness) is heated via induction rather than an oven. The part is then quenched in some liquid, resulting in a very hard shell, with decreasing hardness towards the center. This agrees with the ductile shear failure

If you saw the bulletin from Taylor racing a couple months back, the surface of their shafts measure Rc 53 treated vs Rc 15 not treated.

[Fyhr]

Two things, 1: stress concentration due to bad design as stated earlier.
2: Fatigue break due to bad material choice and/or selected heat treatment process.
edit: Also machining after the heat treatment process may be a very bad idea, what steel are you using?

[Drew Price]

Pontus,

I think I am going to call you out on the fatigue failure, it looks like a one time overload to me, like the driver dumped the clutch, or smacked the throttle coming out of a turn in a low gear and just sheared the shaft rather than a fatigue failure.

I don't see any smooth beach marks, or fracture progression coming from the edges or anything. Just twisted in two.

EDIT: Though it is certainly possible that once the shaft sheared, the two pieces rubbed together enough to wipe the marks away, but on a shaft that looks so soft it just ripped in two, could a fatigue crack have contributed significantly? I thought that was less of a problem with softer materials? No way to know the hardness of that shaft unless Shanky gets it tested and lets us know though.

http://www.msm.cam.ac.uk/phase-trans/2006/SI/9.jpg


http://www.meic.com/Newsletters/2000/summer00a.JPG



Rotating Shaft Failures (http://www.meic.com/Newsletters/2000/Summer00.htm)


Best,
Drew

[Fyhr]

you may very well be right, looking again the smooth surfaces on the outermost part that got me thinking fatigue was the cause may very well have been grinded down by the shaft rotating against the splined part.

[Drew Price]

My freshman year I competed with the Baja team, since we did not yet have an FSAE team at Northwestern. The driveline was set up such that the CVT drove the input shaft to a homebrew 2-stage gear reduction box the guys had made the previous year. The input drove a bevel, and the output pointed straight down into the input for the ATV diff housing the car used. During the big, muddy, incredibly fun race, the input shaft sheared right in two. We tried to jiggle the shifter forks, making sure that it wasn't just out of gear or anything, and in the process polished the surfaces of our new two piece input shaft nice and smooth, making any analysis pretty impossible. I think that failure was from material made too soft (I believe it was a low carbon shaft, with a honkey woodruff key groove all the way down the length of the input side), and I have that image stored away of the failure zone being obliterated by the parts being nice and close to each other.

And Shanky, I completely agree with your statement that 'fracture mechanics sure is interesting.'

Best,
Drew

[vandit]

@ all

sorry for being absent from important discussion for one day....

from above posts i figured out this, since we turned down our drive shaft - we actually removed the hard uniform coating from it and exposed the softer material inside making it more vulnerable. this could be explaination to absence of any crown projections on the circumference of broken shaft (which is present in very first pic of this thread)......

secondly, i would like to describe the exact conditions of the car from the date these shaft has been installed. the car(or basically the shaft) took part in germany'07 where it ran smoothly.....after coming back we had some only 8-10 km of testing spread over 3-4 days in different months....so i guess nothing significant enough to support fatigue....also on the day of breakdown, the car was running smoothly for couple of kms.....then driver complained of wheel wobbling(same side of the broken drive shaft)....but i guess it must have been loose toe rod.....then again he tried to take off the car from stall in first gear, and then it all happened.....

so i guess, sudden torque - stress concentration on notch - softer material - high twisting - may be a sudden bump under the wheel, led to quick snap off.

[Someones]

I agree that it would probably be an overload failure.

However as Drew said... the smearing of the fracture surfaces damages any evidence that would indicate a fatigue type failure.

Also: that it failed toward the centre of the shaft also suggests an overload failure as in torsional fatigue failures the final overload stage generally occurs more towards the outer edge of one side...

[JD232]

Fatigue failure... hmm a possibility simply for the fact that it didnt break off in initial runs.. What surprises me is the fact that the it broke off really clean, almost like an impact failure (remember charpy test?) which again really negates the probability of a fatigue failure.

The only other factor is the stress concentration due to that groove. The high torque of first gear can cause overload leading to this failure... do u use any cushioning effect on ur sprocket mount??

But hey shanky, why is there that groove there in the first place???

[drywater]

hey
i guess the groove is there for the circlip ,which is used to retain the tripod on the splines..i think i am right.
moreover i have anothere query shanky,i am also a member of indian team from delhi college of engineering,which material do you use for your driveshafts,because we do not get sae 4340 here,alternatively we use en 24 alloy steel..it is working well,,we have used that for the past 7 years.But still i was curious where do we get sae 4340 shafts or blanks.

[SNasello]

I think that EN24 is essentially the same alloy as SAE 4340, just a different name for it. They are at least very close in composition.

[Grant Mahler]

MatWeb for EN-24/4340 (http://www.matweb.com/search/DataSheet.aspx?MatGUID=b0f23fe4fbb844f5a3e71cfa0e3 1f513&ckck=1)

[vandit]

@silverback

Sorry for being absent from the forum. The notch where the fracture has taken place is just bad manufacturing. and no sprocket cushioning.

@Dry water
no this groove is not for circlip. it was just bad manufacturing finish. No we are not making our own drive shafts yet. We turned a stock maruti 800 drive shaft(which was the first mistake) and then shorten it and then resplined. What we learned, maruti drive shafts are decent enough to sustain if you dont fiddle with them much. Yes you are right, getting grades of steel you desire is very difficult anywhere in indian market. You only get couple of EN series because they are widely used in industrial sprocket and gear manufacturing.