Restricted life of after burner manifold assemblies due to stress raisers

Rejection rate of A/B manifold assemblies of an aircraft got radically increased. Statistics indicated that maximum assemblies were rejected due to the presence of hair-line cracks on the face end of fuel inlet joint block.Detailed study ascertained that the cracks were initiating from coarse grinding marks and engravings which were present on the face end. It is known that residual stresses are induced in the material by grinding and engraving, consequently they act as stress raisers. In extreme service conditions (high temperature and stresses), cracks were initiated and propagated by the mechanism of thermal fatigue.