Failure Analysis of Weldment Crack in Body Structure of a Helicopter

The body structure of a military helicopter is made of complicated cluster welded joints. In-service, the tubes of the body structure are kept charged with nitrogen gas at a designated pressure. Leak was noticed in the body structure after about 83 hours of service since new against stipulated overhaul life of 1600 hours. Inspection showed the exact location of leak was at a welded joint between tube and gusset which in-turn was welded to gear box mounting plate. The tube and gusset were made of Cr-Mo-V steel of specification 15CDV6 and their chemistry, hardness and microstructure (heat-treatment condition) met the specified requirements. At the location of the leak, penetrant inspection revealed presence of crack at the end of the weldment between the tube and gusset. Scanning Electron Microscopy showed that it was a fatigue crack that had originated at multiple locations from the weld bead and had propagated through the wall thickness of the tube from outer to inner diameter. At the end of the weldment between tube and gusset, overlap/overwelding was observed towards the tube-side of the weldment. Because of this, the leg of the weldment towards the tube-side was about four times the thickness of the tube. In addition to this, about half of the leg of the weldment towards the tube-side showed lack of fusion and penetration. Hence, this area of the weldment was incapable of withstanding the service stresses. Thus, the weld quality as well as the weld geometry at the end of the weldment between the tube and the gusset was not adequate/proper and this was responsible for the crack/leak in the body structure. It was also recommended to investigate the sequence of joining of tube, gusset and mounting plate in the manufacture of cluster welding at the affected area to prevent its reoccurrence.