Failure analysis of an intake valve in a gasoline engine

This paper presents a failure analysis on a failed intake valve in a gasoline engine. During an endurance test، an intake valve was broken and caused some damages on the piston. Material examinations (by a light microscope and also a scanning electron microscope) were conducted on the fracture surface of the intake valve shaft. Results showed that no pores or defects could be observed and the microstructure and the hardness of the material were conformed to the specification (the standard map extracted by the manufacture). But a mechanical factor caused the damage which consisted of a reversal bending load due to the misalignment of the valve shaft. The misalignment was caused by the thermal deformation in the valve shaft. The engine was working in a hot condition. Also، it was a reversal condition (with zero mean stress) due to the rotation of the valve shaft during engine working. Also، the fracture surface demonstrated beach marks due to cyclic loadings. In this case study، it was found that cracks initiated from the outer surface of the valve shaft and propagated through the inside of the fracture surface.