conrod stress during each stroke and failure 2

[tomaschan]

Hi, as far as I know conrod is stressed by tensile, compression and bending stress.
I would be interested what stress is imposed on conrod at each particular stroke of 4-stroke engine and when is it most critical.
And at which place of shank (closer to big end or small end) is higher probability for conrod to break and why.

 

[SomptingGuy]

Scaling things massively up or down (away from passenger car sizes) is always a lot of fun. When I first started working we were given a top-to-bottom tour of the work that goes on. I recall a failure analysis guy pointing out the obvious cause for failure on a piston. I couldn't see it. I stepped over this huge hunk of metal that was in the way, looking for a mug-sized thing.

- Steve

 

[bentwings2]

This summer produced some interesting results along the rod failure line in our top fuel funny cars.

One motor severe detonation caused the aluminum rod to shatter like glass but remained lightly stuck together so that the failure was missed....ie the piston still went up and down the bore. However on disassembly the rod simply fell apart into several pieces. No damage to the crank but the bearing was squished severely. Several other pistons were badly burned but nothing was kicked out of the motor. All nine lives used on this one. haha


Our car suffered many burned pistons. The last run of the year severe detonation badly burned holes in all 8 pistons. The worst one burned thru the piston and scarfed about 3/8 inch of the wrist pin away and nearly burned thru the top of rod. Detonation hit this one so hard the top of the rod under the pin showed signs of getting ready to shear or slip from compression. The bottom end got hit so hard it ovaled the big end bad enough that we couldn't get the bolts out as they had been bent severely. We had to sawzall the rod off in several pieces. Ironically it only blackened the crank slightly and a weld repair at the local crankshaft company was all that was required. We were just plain lucky with this one. It still had 150 psi oil pressure as it was shutting down. Good oil pump I guess.

 

[140Airpower]

bentwings2, I assume that the shattered rod was not discovered on initial inspection, but only after disasembly, not that the engine was run with it in place? Anyway, this is an interesting occurrence. It sounds like it actually shattered in the process of disasembly and suggests some extreme embrittlement. I don't suppose it had been welded?

 

[TurboPutter]

There is a condition where tensile stress is higher than the exhaust stroke on a gasoline engine.

The transition between the exhaust stroke and intake stroke at full engine speed throttle closed operation can produce an even higher stress. A good number of the failed connecting rods I've seen have occurred when the driver closes the throttle at the maximum engine speed, whether to shift gears or slow down, as intake manifold pressure drops, the force required to bring the piston back down again can increase quite a bit.

 

[patprimmer]

I agree, broken rod bolts or caps most often happens at the end of the main straight at the start of the braking zone.

Regards
Pat
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[Tmoose]

Although the final failure may occur under a particulary high stress circumstance, I'd expect those busted rods were cracked rods for many minutes before actually breaking.
It would be interesting (to me) how long it had been since the rods had last passed a wet method mag particle inspection, and what preparation such as smoothing and shot peening may have been done prior to their racing career.

 

[140Airpower]

Yes, except for something catastrophic like breakage elsewhere, loss of oil film, detonation or the like, a rod will probably only break after fatigue. Then, flaws are the inevitable sites where the failure began.

 

[patprimmer]

Sure fatigue builds up over a period including periods of less than maximum stress, however the fatigue weakens the rod, it will let go when the stress required to break it is exceeded. That is most likely to be when at maximum stress unless a lot of fatigue occurs between maximum stress cycles.

Regards
Pat
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[Lou Scannon]

Pat, agreed, odds are in favour of ultimate failure during a moment of high stress.
Yet, I experienced failure of a cast crankshaft during cruise operation at about 2000 rpm. Examination of the failure location revealed that a fatigue crack had extended over more that 2/3 of the failed cross-section, before the final catastrophic fracture occured!


"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

I think fatigue is a process that can weaken the rod to the point where normal or even light operation can lead to failure. Used high-stress race engine parts should never be put in a street engine. If they were well engineered and stressed to their max while reliable in racing, they will most likely be subject to continued fatigue even with low stress operation. I don't recall the term used for this. A part can last an unlimited number of cycles when kept below a critical stress, but will progressively fail once the critical stress is exceeded, even at a low stress level.

 

[Lou Scannon]

Fatigue limit

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz