Failure Mechanisms?

Question

A crankshaft in a diesel engine fails. examination of the crankahaft reveals no plastic deformation. the fracture surface is smoth. In addition. several other cracks appear at other locations in the crankshaft.

i. from the above information, what type of failure mechanism would you expect?

ii. explain analysis

Answer 1

The crankshaft was too brittle(hardened too hard). The fractures   could have just easily been caused by the high compression of the engine alone being as brittle as it is.

Answer 2

No plastic deformation on fracture surface, thus, brittle fracture occurred. Likely indicative of very brittle material through (cranks are normally fairly tough). Smooth surface means brittle fracture, and all at once. Normally, you have a defect at which crack initiation occurs. Then, with each large stress, a small increase in the crack during the growth phase. This gives a striated (rough, with oriented lines) surface. Since this broke smoothly, looks like failed all at once.

So, I would say that too brittle material maybe at least partly due to excess surface hardening, possible overstress.