Determination of the fracture cause in an aircraft motor cylinder

One of the six cylinders of an aircraft four-cycle motor was fractured in two pieces between the 13th-14th cooling fins. To determine the root cause of the fracture, it was necessary a preliminary observation of the whole motor to verify the state of valves, spark plugs, seats, and their disposition in order to determine a possible motor malfunction previous to the failure. Afterwards, a chemical and mechanical material characterization and a fracture surface fractographic study were carried out determining the heat treatment state using metallography and microscopy techniques. Observations of the external and inner cylinder surface were made to detect the possible presence of cracks or pitting. Some pitting were observed in the cylinder external wall. It was determined that material was a quenched and tempered steel, type AISI 4140, and the fracture was brittle presenting fractographic signs of a progressive fracture, concluding that the fracture was originated by a corrosion pit localized in the cylinder external wall that progressed by a fatigue mechanism.