Halfshaft failure

[Denis Sweeney]

Bob FYI the 90% 10% ratio is courtesy of Professor Maddox of the Welding Institute, Cranfield, a long time ago, I certainly would not argue with a guy understood to be one of the then leading authorities on fatigue

[Chris KC]

It's a rotating - bending fatigue failure with multiple points of origin. Alternating compressive and tensile stresses in the surface of a driveshaft are normal; the shaft encounters bending loads if only from supporting its own weight. Fatigue is about the cumulative impact of smaller loads. As others have observed the hardness test mark may have been the point of crack initiation, but any tool mark would do.

[Dave Mann]

The initiation point was from a number stamping adjacent to the hardness test.

[Steve kay]

The only time I have ever had a broken half shaft was in the Ulster, after a full day’s merciless trialing. We then stopped for much needed therapeutic refreshments, Bathams, and the half shaft broke post pub about a hundred yards from home. One of those occasions when you pat it on the nose, or bonnet, and say well done little car.

[Bob Culver]

In front stub axles cracks advance vertically from below and/or above until just a thin horizontal slice remains.. With rotation tend to advance less directly leading to the final more circular core.
Of two trailers  I have been associated with both had/have axle cracks! And two left front hubs in the Seven. I have a collection of failed railway fishplate bolts I used to pick up walking alongside the tracks locally. Very many bolts were missing! Walked past a parked 4 wheel trailer recently with one wheel missing. The stub had failed mainly from the top down.
Shot peening, rolling etc can greatly increase resistance to fatigue. The old counter was polishing but it was discovered that the mechanical working of the surface had the effect and not the shine. Some reckon the original Austin axles were case hardened, which also provides protection.
The first photo seems to have two opposite origin points.

[Colin Morgan]

Interesting subject - metal fatigue.  Plenty of reading out there for those who want to know more.

Apparently. once a crack gets itself together and starts to propagate is might not be long before failure - however, it does depend on what loads the item sees in service.  If stresses are above a general threshold , fatigue failure can start from any minor defect that produces a stress concentration, but the initial damage is likely to be down at the microscopic level.  However, once there is a sharp crack, the stress can be 100x background at the tip, and so the more it spreads, and the the load it sees is greater - hence a one-way road to failure.  Fortunately, steel is a tough material - from its structural imperfections, for one thing - and items can survive a for long time despite their inevitable (sudden) demise in the end.  

The following photos show what failures initiated from surface defects and internal defects tend to look like - similar to the photos above?
  Fatigue failure - surface v internal defect.jpg (Size: 128.36 KB / Downloads: 284)

[Bob Culver]

Thanks Colin

Failure from internal inclusions is rare esp with clean modern steels. The example is likely a lab straight tension situation. Except  in very ht steels the final failure  metal is usually quite evident. 

In the original second photo the rotation has caused the crack to extend full circle much as for an inclusion in straight tension, although progressing inward instead of outward.

I dont know if the key postion differs in detail between the early thin .875 axles and the 1930s thicker .890 ones but I have encountered after market later shafts thin at the outer end. The taper contact then ends very close to the keyway with inevitable early cracks 

The first posted photo seems to have a turning groove around it. perhaps it was a reclaimed shaft.

I have observed Hillman Hunter axles. These have been heavily surface rolled all over to place the surface in protective compression

There is a stress limit below which steel lasts forever. But there is probably no part of the car, bodywork included, where the level is not exceeded occasionally, even if only in exceptional circumsatnces. So if you drove to infinity everything would fail eventually.

I like the photos of workshops. Which is oldedest? The vice or lino/carpet?

[Dave Mann]

That brings back memories Colin, I'd just started a new job in 1971 and was sent to investigate a piston seizure on a 6000 BHP engine. When I returned and told the boss the 6" diameter gudgeon pin had broken he went ballistic and didn't believe me until I dumped the bits on his desk. The cause was a slag inclusion, we then had the job of tracing all the pins from that batch.
Good question Bob the vice is at least 70 years old whilst the carpet was of a similar age but has now gone to be recycled. When we get a new carpet the old one gravitates to my workshop.

[Stuart Giles]

Failure of a factory Halfshaft at the Brinell test indentation was mentioned earlier in this thread. I had an object lesson in stress raisers caused by a sudden change in section some years ago with my Special.



You can see part of the Brinell indentation on the broken end in the photo. What's not  so obvious is where I've blinded it out (the shiny area just behind the new keyway) in the short Halfshaft  that I created from a knackered long one above the broken end.

[Bob Culver]

Hi Stuart

Gratifying that all those old axles can be utilised. Unfortunatel I do not have  a swb car to utilise my acquired collection. Presumably you weld up the old keyway. Regent axles in particular seem very hard but nothing carbide tools cannot handle.

On many axles a rough light pass has been made over the indent with a coarse grinder, suggesting the edge of the crater was a recognised  problem. Would expect to worsen!

If I had known of the propensity to break there the cornering antics of my youth would have been very curtailed.

Unfortunate that the break  too far in the be inspected. With a tested crank and key way I had great faith in my car and explored many out of the way places very far from home. And no AA membership....!

Coincidentally last night on TV "Engineering Failures" a tremendous US rail smash when axle on an oil wagon failed. It had been cast or forged with a huge defect hollow centre and fatigue eventually doomed it.