Failure of Locking Wires of an Aeroengine Component: Attributed Primarily to Over-twisting and Secondarily to Engine Vibration and Improper Material Selection

Abstract

Each of failed, working and unused (as-fabricated) locking wires used in fastening engine exhaust pipe of an aeroengine was analyzed during the course of failure investigation. Relevant background information is as follows: (1) Specification GOST 18143-72 was followed in manufacturing those wires, and (2) wires were for one-time use only. The failed and all other wire types were found to be made of as-specified AISI 321 austenitic stainless steel, based on compositional and microstructural analyses with high volume fraction of phases of Ti–C–N family. High volume fraction of Ti–C–N phases led to unacceptably higher strengths in each of the failed as well as non-failed wires (UTS value in excess of 580 MPa), while the as-specified material is supposed to have breakage strength of 520 MPa. The principle contributing factor to failure of one wire of the locking wire pair is over-twisting of the wire during fixing it to the main assembly. Effect of engine vibration in form of vibration fatigue was found to be of secondary in nature and did not cause the failure directly. It assisted in the final (propagation) stage of failure. Other wire failed by overload as a result of over-twisting of its counterpart. Temperature did not play any role in the present failure. Failure mechanism was verified by simulated experimental results on fractographs conducted with the unused wire and obtained by twisting, untwisting and retwisting. Excessively high and unspecified strength of the material and lower toughness (as manifested by Ti–C–N particles–matrix interfacial cracking) might have contributed to the present failure in a secondary way.