Abstract: Two bolts fractured in the process of installation.Based on macro-observation,micro-observation,microstructure testing and microhardness testing,the fracture mechanism was confirmed.And based on loading analysis,ring-draw examination,friction coefficient testing and finite element analysis,the fracture cause was found.The fracture was caused by overload.Under the co-effect of castor oil and TB06-9 lacquer,the friction coefficient reduced greatly.As a result,the lognitudinal stress of bolts increased remarkably under a certain moment of torsion.Then the bolts fractured.