Crankshaft Failure Mode Analysis

[pbiebach]

I have a broken crankshaft where I am trying to determine the cause of failure.

here are some pics of the cracked surface.

what is known:
1) the crank shaft was under very light load at time of failure

What do you guys think?

 

[brewnog]

Fatigue. Crack propagation originating from the oil drilling shown.

 

[pbiebach]

Thanks for the reply, could you explain how you are able to tell this?

 

[brewnog]

1) Beach marks clearly visible showing nucleation site is likely somewhere around the fracture face on the drilling.

2) 90% of the time they fail like that.