RCS
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Registered: 26th Jan 05
Location: Lichfield/Dundee
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Couple of months ago we had a catastrophic rod failure in the 206 engine.
Did some investigation into the cause of the failure using an electron microscope. They have initially failed due to buckling from cylinder pressure which has caused major misalignment. As the rod has tried to restraighten itself this has introduced increased tension into the rod which has lead to crack propogation where the shank blends with the small end.
In the above picture you can see the grain particles of the powder forging. The indicated areas are voids which arise during the manufacturing process. The voids in the material posses a surface energy and as the rod has been put into increased tension due to the misalignment, the voids try to reduce this surface energy by merging with other voids - in other words they form small cracks.
As the engine cycled through a few revolutions after the buckling failure, these cracks have opened up and eventually led to the catastrophic failure of one of the rods.
I'm such a geek 
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flybikeslee
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Registered: 2nd Jan 07
Location: Liverpool
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geeky but cool to see things like that
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IvIarkgraham
Premium Member
Registered: 27th Mar 04
Location: Ellesmere Port, Cheshire
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one reason why rods should be shot peened
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*JonnyG*
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Registered: 2nd Jun 08
Location: Lincolnshire
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Wow.. so much detail
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RCS
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Registered: 26th Jan 05
Location: Lichfield/Dundee
User status: Offline
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quote:
Originally posted by IvIarkgraham
one reason why rods should be shot peened
The rods had been shot peened.
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IvIarkgraham
Premium Member
Registered: 27th Mar 04
Location: Ellesmere Port, Cheshire
User status: Offline
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quote:
Originally posted by RCS
quote:
Originally posted by IvIarkgraham
one reason why rods should be shot peened
The rods had been shot peened.
 obviously not properly
shot peening takes away fattigue from machining
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Ben-B
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Registered: 28th Jul 08
Location: the lovely Nottingham
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quote:
Originally posted by flybikeslee
geeky but cool to see things like that
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RCS
Member
Registered: 26th Jan 05
Location: Lichfield/Dundee
User status: Offline
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quote:
Originally posted by IvIarkgraham
quote:
Originally posted by RCS
quote:
Originally posted by IvIarkgraham
one reason why rods should be shot peened
The rods had been shot peened.
obviously not properly
shot peening takes away fattigue from machining
Not really.
It compresses the surface layer of the material producing compressive stresses which helps to prevent fatigue crack propogation. It's not done on the machined surfaces.
And shot peening wouldn't save a connecting rod from buckling.
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Ellis
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Registered: 11th Sep 07
Location: Aberdeenshire
User status: Offline
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Great thread 
Did a quality unit at Uni about crack propogation, fatigue, failure analysis etc... loved it
Not quite as cool as doing it on engines though, good work 
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crazybrightman
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Registered: 30th Mar 09
Location: leicestershire
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french engineering!
i take it some h section rods are going in now? you cant make up for poor metal quality what ever treatment you add.
i take it the 206 is in some sort of high revving high comp state of tune?
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RCS
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Registered: 26th Jan 05
Location: Lichfield/Dundee
User status: Offline
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There is a process which can remove the voids in the material but its too costly to be done.
Currently designing some h-section rods and performing static buckling analysis with correlated test data. If all goes ok hopefully Arrow will manufacture some for us!
Wasn't high revving, they buckled under gas pressure at 6.5krpm
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Ellis
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Registered: 11th Sep 07
Location: Aberdeenshire
User status: Offline
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Will you doing any finite element on the new design, if so - request for screen shots
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Nick-S
Member
Registered: 3rd Mar 04
Location: Leigh. Drives: RS Megane 230 F1 Team R26
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thats quite cool but i wouldent call that a catastrophic failure. You should off seen the old block, crank, rods and pistons that gaz'd brought to pvs  i only wish id have had a camara to take pics. now that was a catastrophic failure!!
[Edited on 09-06-2009 by GSi_16v]
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crazybrightman
Member
Registered: 30th Mar 09
Location: leicestershire
User status: Offline
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quote:
Originally posted by RCS
There is a process which can remove the voids in the material but its too costly to be done.
Currently designing some h-section rods and performing static buckling analysis with correlated test data. If all goes ok hopefully Arrow will manufacture some for us!
Wasn't high revving, they buckled under gas pressure at 6.5krpm
oooh good quality but wouldn't like to thing how much one offs are going to cost! no off the shelf options then? or the posability of using rods from another pug lump? spose not if your going the arrow route really.
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