Bending overload and thermal fatigue fractures in a cast exhaust manifold

The left exhaust manifold of a Ford Y-Block engine failed at two different locations after nearly fifty years of service. A study was conducted in order to determine if the main causes and the sequence of failure of this component were related to its operation. A detailed analysis comprising materials characterization, fractography and a thorough finite element modeling was carried out. The results showed that the normal operation of the engine induced several thermal fatigue cracks, which were primarily located in the outermost runners of the component. One of these cracks propagated unstably during the operation, producing one of the fractures. The other one was generated during an overhaul, when a bending load and an improper assembly condition propagated a preexisting crack. In addition, the presence of a mixed graphite distribution comprising A, B and D flake graphite compromised the mechanical properties of the exhaust manifold, and aided in the process of crack propagation.