SURFACE CRACKING OF QUENCHED AND TEMPERED STEEL BY SHEAR INSTABILITY-CAUSE OF PREMATURE FATIGUE CRACKS IN HELICOPTER MAIN ROTOR SPINDLE LUGS?

Cases of early fatigue cracking of main rotor spindle lugs made from a quenched and tempered steel have been studied. Experiments have shown that the material in question is prone to surface cracking by shear instability when moderately strained under a multiaxial stress state resembling that of a lug and pin joint. Fatigue testing has shown that the presence of shear instability cracks can lead to rapid, multiple fatigue initiation in a local area, as was observed in the real cases of lug cracking. It is considered that the first step in the initiation of the spindle lug fatigue cracks is some degree of highly local plastic deformation in the lug, triggering microcrack formation by shear instability. Arguments are presented to explain how deformation may have occurred in the critical position of the spindle lug.