Dave_Stohler said:
looks like ductile failure to me......
Hmmm... I don't see anything ductile about that failure (except for the bending apart of between the two sides of the crack, but by the time it starting bending apart, the crack was already far along the road to failure).
This looks like a typical fatigue failure to me. In a fatigue failure, a crack slow grows from an initiating site. (The initiating site could be a scratch, a pit, a void or defect in the material, or just a point of high-stress concentration due to geometry and loading). There is little ductility (plastic deformation) in the area around the crack during the first stages of crack growth - the part maintains its original size and shape, with the only obvious indication being the edge of the crack visible on the surface of the part. It is only when the crack has grown far enough to seriously weaken the part that any change gross mechanical property changes - that change in property being either a change in stiffness, a plastic deformation, or in many cases, simple a brittle failure of the remaining cross-section next to the crack.
Fatigue is caused not by a sudden, excessive force, but occur slowly with many cycles of lesser force. Cracks may take many months or even years to grew before the part finally fails. In many cases, supposedly "sudden" failures of parts were actually long in the making, as fatigue cracks slowly grew. Because there is not gross change in material properties of the component, fatigue is often not detected until ultimate failure - but that doesn't mean that the cracks weren't arlready there before the final failure. Fatigue cracks are frequently able to be detected by inspection before failure - but many parts are infrequently (or never) inspected, so the cracks were never noticed.
