Cast Alum Fatigue Crack , But No Fractographic Evidence 1

[Metalguy]

Have a baffling problem involving the sudden fracture of a motorcycle fork tube. The fork is cast alum. alloy, composition unknown. The fracture surface was not damaged during the event, but there is no "classical" evidence of any fatigue cracking, visually, under a stereo microscope, and under a good SEM.

I am of the opinion the fork HAD to have some amount of fatigue cracking present before the sudden failure. This fork was on a "high quality" on-road/off-road type motorcycle, but the final failure happened suddenly while riding on a smooth road.

There are at least a few other similar events, and the manufacturer has redesigned and strengthened the fork in the failure area (where the front axle is attached) while denying the existence of "any known problems".

Can anyone shed some light on this apparent paradox? The few alum. failure analyses I've performed where fatigue cracking was involved had clear evidence of fatigue cracking you could see across the room, including a large cast turbocharger impeller. Why is this fork so different?

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling

 

[metengr]

Metalguy;
Low cycle fatigue crack propagation. Did the failure initiate at a stress riser or pre-existing flaw?

 

[TCAir]

Was this on a Honda Dual Sport? I know there was a recall a few years ago due to a poor batch of forks.

Not a very engineering answer but the point is that the issue was known.

Engineering has always been my love, but it ended up being my second career...

 

[swall]

Metalguy--you may be missing the obvious. It could just be a static overload fracture. Unusual, but it can happen. Cast aluminum (except for die castings)generally has a sufficiently coarse structure so you can see fracture directionality similar to chevron markings you would find in high strength steel and then find the initiation point. One thing I would look for (and this is just a WAG) would be pre-existing cracks due to thermal deburr. I have looked at hundreds of service fractures in cast aluminum and you will always see at least macroscopic evidence of fatigue if it is truly a fatigue failure.

BTW, is this the failure that happened near Destruction Bay?

 

[Metalguy]

Thanks for the replies. So far, I haven't been able to spot the initiation point on either fork. I suspect the right side failed first, which then caused the left side to fail via gross overload.

The castings appear to be of high quality, with no obvious casting defects. The bike in question (not a Honda) had been in an accident several months prior to the fork failure, so I thought it would be easy to find any evidence from that accident, but no such luck.

I am looking at this failure gratis; also no charge from the SEM lab I used. And yes, it is the "Destruction Bay" bike, and the photos of that girl lying in the middle of the road with her broken arm kind of got to me (ADV forum--Adventure forum). My name there is "Benesesso", a neat word I coined from my experiences in Italy.

I would prefer not to name the manufacturer at this point, or the rider.

If anyone who has a lot of experience with alum. casting failures wants to help out gratis, I can send you the broken pieces. The vast majority of my work has been with steels, stainless and "Alloy 600", heh heh.

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling

 

[swall]

Metalguy--I am aware of the particular crash and I would be willing to take a look at the pieces. I have done some gratis work with failures on the brand of motorcycle in question several times over the last 20 years.

 

[Metalguy]

Swall,

Great! Hopefully you'll see something I've missed. BTW, I just received a met. failure report on another fork failure (same kind of bike), and there was a typical fatigue crack that went most of the way thru the wall before it reached critical size and broke via brittle fracture.


Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling