Whoops clang !!

[misterdimwiddy]

I am happy to be corrected by any metallurgists out there but here is my interpretation of the damaged rod.

Small end condition is unmarked and there is no discoloration. Teere is one small surface damage feature which resulted from the broken lower rod end entering the underside of the piston and whacking the small end.

Image 1 shows the full fracture face with high cycle fatigue initiating at the lhs towards the upper corner very close to the outer surface of the rod.

This is more visible in image 2 (better lighting) with a clear low point with curved 'beach marks' or 'shell lines' showing the gradual propagation of the crack under cyclic loading, these concentric lines radiating from the initiation point. These lines are an unambiguous characteristic of fatigue failure. Fatigue crack growth always propagates on a plane at right angles to the principle tensile stress.

The shell marks are surprisingly still clearly visible even though the faces of the advancing crack would have been hammered together under compressive load. Under high magnification you can see that the surfaces appear hammered.

Looks as though this crack growth has advanced to the point where the lh half of the cross section area has been compromised which eventually overloads the remaining section leading to a more rapid ductile failure (general yielding) and eventual very rapid fracture (dark grey very smooth narrow area) at the rhs. The more ductile failure gives a coarse surface appearance as seen in figure 5.

Before total failure, the loading on the rod has become asymmetrical resulting in cracks near the surface at 3 locations within the fatigue damaged area. There is a clear twist in the 'I' section shape of the rod that I believe has led to cracking on the lower side of the web that joins the right and left hand sides. This crack has started at the radius where the shape changes, cracking generally starting where stress raisers occur.

Image 3 and 4 show the initiation point. Note how coarse the surface texture is when magnified, each surface irregularity being a potential stress raiser.

The nucleation of cracks occurs either from flaws in the casting (voids, particulates etc) or from surface stress raisers. This point coincides with a profile (radius) change on the outer surface (figure 3) and what looks like a small dark void type feature on the rhs of figure 4.

Polishing rods or introducing a compressive surface stress condition (shot peening) reduces the probability of crack initiation and hence fatigue failure.

Overall it looks like very bad luck that this TRX motor had a flawed rod and I doubt you can reliably blame your lad for over revving the poor thing.

Fatigue crack growth is interesting in that it occurs at stress levels that can be just 40% of the tensile yield strength of a material. The phenomenon first became apparent when Liberty ships started breaking in half when they were in cold arctic waters during WW2. The Comet airliner also suffered from this because test aircraft were highly loaded statically before fatigue testing which gave a false indication of the fatigue resistance of a previously unstressed airframe.

[masterbill]

Hello,

Gosh

Thank you

[M.V.]

Bravo sir, well done.

[dandywarhol]

Far from dim misterdimwitty - impressive analysis - I even think I understand it when you relate it to the photos

[QuickDraw]

If Shakespeare had ever done a rod, this is how I imagine he'd have told it. Love it! Nice work MDW

[flyinghigh001]

Can't see the most important tool on the bench which is always required when such a meeting of components come
to pass..... A BIG MUG OF COFFEE! TC