Cast Iron Metal Fatigue Failure

California failure analysis expert performs a failure analysis on a failed cast iron "latch lever" from a wood stove. This failure initiated as a fatigue crack at a casting defect. It progressed as a fatigue crack through most of the thickness until the final overload failure. Upper left is a photograph of the backside of the lever, and it shows the casting defect. Upper right is a 10X photo-micro-graph of the fracture surface. Cast iron fracture surfaces are "grainy". Fortunately, oblique lighting reveals the fatigue stria on the fracture surface. When latched, the surface with the casting defect is in compression; it is believed that this is why the crack progressed mostly as a fatigue failure.