Analysis of failed engine mount of an aircraft

A cracked engine mount of an aircraft was sent to the laboratory to analyze the cause of failure. Dye penetrant test indicated a crack at one of the weld joints. Investigation revealed this crack to be a fatigue crack. The fatigue crack had initiated at the weld – parent metal interface and propagated circumferentially about 12 mm. Microstructural study showed martensitic structure in the heat affected zone (HAZ). This was further corroborated by high hardness in this region. A section of the tube close to the HAZ was found to have low hardness (170-210 HV). This indicates improper heat treatment of the weld joint. It appears most probable that the abnormal microstructure in the HAZ and the low hardness of the tube adjacent to HAZ were responsible for the fatigue crack initiation. As per the specification, the tube should have hardness of about 270 HV