Wouldn’t the rust and lack of grease put extra side to side strain on the piston? It seems to me that if the joint doesn’t move freely, then the piston would take quite a load from the side, which it was not designed to do. A defect in the weld was probably gave it the greater propensity to break, and the rust in the joint was what set it off. Correct me if I’m wrong though. That’s just what it seems like to me.
If the pin is seized it would create a bending moment on the rod. The highest stress point would be where it broke. The back and forth movement of the blade would cause fatigue and it would eventually fail in the same location if that was the case.
That is true however, the fracture does not indicate a fatigue failure from bending. The dull grey surface that resembles granet crystals on the surface of the fracture is indicative of a rupture of the material. Instantaneous fracture from a load that is beyond the yield point of the material propert.
When looking at fracture due to fatigue there are two zones to look for. One is the fatigue zone and the other is the rupture zone.
In a situation where a small crack develops (from fatigue) and slowly spreads through the rod, the crack leaves striation marks which are like the material ripping apart little by little until the cross sectional area of the rod is too small to support the load then it ruptures.
The striated zone from crack propagation is the fatigue zone.
The surface where it ruptures looks identical to the surface in your one pic.
The striated surface from crack propagation will be dull from the surfaces moving back and forth and rubbing over each other. If the crack growth is slow the fatigue zone may be corroded also depending on it's environment.
The amount of overstresing can be determined by the size of the fatigue zone compared to the rupture zone. A large rubture zone and small fatigue zone indicate highly overstressed.
Remember, a fatigue faikure will have both zones. From your picture we can only see approximately 80-90% of the surface but, all of the visible surface I see indicates a rupture type failure.
It's possible there is a fatigue zone but without a better picture straight on I cannot tell. If there is no discernable fatigue zone it means it still failed due to highly stressed. I'm not an expert but my experience and education tells me this was certainly overstressed. A legitimate failure analysis would look a lot closer identify a manufacturing defect thst would cause a stress riser. If none is found then it could be concluded it failed because the load was greater than the materials ultimate strength. They would also test it's hardness to see if the welding changes it's mechanial poroperty and contributed to a brittle faikure.
