According to a fascinating article on the Key to Steel website, the breakage of the input shaft shows some characteristics of a "brittle fracture":
"Brittle fracture is characterised by the very small amount of work absorbed and
by a crystalline appearance of the surfaces of fracture, often with a chevron
pattern pointing to the origin of fracture, due to the formation of
discontinuous cleavage cracks which join up. It can occur at a low stress of
75-120 MPa with great suddenness; the velocity of crack propagation is probably
not far from that of sound in the material. In this type of fracture plastic
deformation is very small, and the crack need not open up considerably in order
to propagate, as is necessary with a ductile failure."
I'll just repeat the bit that really struck me: This thing broke apart at close
to the speed of sound. No, the fragments didn't necessarily go that fast (and
neither did I), but that's how suddenly the shaft broke, and that explains why
it looks like a grenade went off inside it.
On very close inspection, I can see several cracks at seemingly random angles to the breakage. It does show some signs of "ductile failure," where metal stretches as it is pulled, bent, or twisted apart. Some of the splines are bent, stretched, or twisted, although the surfaces between the fragments show no such signs. That makes some sense. As part of the shaft broke, neighboring areas were stretching to accomodate the change of shape. This same thing happens when you break a glass -- the definition of brittle -- the glass actually deforms a little before it breaks, which is why the pieces never fit together quite right again.
The good news is that the car is nearly together again. The two big pieces of car were reunited last night, and all that remains is a quick alignment check, gear change, and brake and clutch bleeding.
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