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07-15-2008, 11:54 AM #1
Failed fan blade - metal fatigue?
We had a high speed fan that failed. It appears that the fan ingested something, which caused damage to the leading edge of all of the fan blades.
One of the fan blades broke off near the root. The thing that is surprising to me is the fracture patern in the broken fan blade. There appear to be two distinct mechanisms that cause the failure - does this suggest that cracking was occuring before the fan ingested something? A photo is attached.
Thank you in advance for your expertise and comments.
07-15-2008, 12:35 PM #2
another pix would be nice...
1. It looks as though the damage from ingestion happened awhile ago,
leading me to believe balance was a problem.
2. A top view showing how far out the blade would help, It looks like fatigue
based on the large radius frosting marks from both the upper surface
as well as the lower surface, like it was flapping up and down. The break
appears to be just past this radius.
3. This appears to be a casting, I don't like those marks on the root radius,
they look like shrinkage marks, indicating high tension from cooling.
4. The leading edge doesn't seem to have the same fillet at the junction
to the hub, it doesn't follow around the tip, it "drops out" right there,
looks like a good start for a crack, and the fracture marks sort of
make it look like an initiation point.
07-15-2008, 01:23 PM #3
maybe the blade broke off first then bounced around the housing causing the other damage. wild idea
07-15-2008, 03:50 PM #4
Digger Doug, thanks for your comments. The two hemispheres do indeed look different than the rest of the failure. The material is forged aluminum - it is forged as a billet and then machined. Here is a profile photo:
Surplusjohn, I think you may be on to something. Is there a way to determine the length of time a crack took to develop?
A related question would be if anyone knows of any reference books that would discuss this. I have ordered "Metal Fatigue in Engineering" by Stephens, but if anyone could suggest another reference, that would be greatly appreciated.
Thank you again for your expertise.
07-15-2008, 05:13 PM #5
the first picture you show with the two shiny and relatively flat radius areas is classic fatigue. The rest of the fracture would be over stress exemplified by a shear lip and the more irregular surface, but I can't see that from the orientation of the first photo. Look for the origins of the fatigue areas, likely you will find some damage or corrosion in this area. The origins can be found by following the chevrons in the fatigue regions. This may require a microscope and some side lighting.
Your fan ingested something before the final failure, damaging the blade and initiating fatigue. Alternatively it could have initiated by corrosion or a manufacturing defect. This failure was building up over time and did not occur in an instant.
A nice reference for this is "how components fail", it is an excellent read.
07-15-2008, 08:12 PM #6
I would guess that the notched blades set up an out of balance condition (vibration) which could cause the blade to oscillate at a relatively high frequency which could cause the apparent fatigue in a relatively short time.
You might try wiggling the remaining blades to see if you can break any of them off and see if they are on the verge of failure. Dave
07-15-2008, 08:56 PM #7
If you look at the top of the break area next to the "ingestion damage", that dark color makes me think that it has been cracked somewhat for some time. Hard to see in the photo, but if that area shows dirt penetration I would think that the "ingestion damage" is the root cause of the failure.