Failure Analysis of Weldment Crack in Body Structure of a Helicopter

摘要

Leak of nitrogen gas was noticed from a helicopter body structure in-service. Inspection showed that the leak was from a crack located at the end of the welded joint between the parts tube and gusset of the body structure. Scanning electron microscopy showed that the weldment crack was a fatigue crack that had originated at multiple locations from the weld bead. At the end of the weldment between tube and gusset, overlap/overwelding was observed toward the tube side of the weldment. The leg size of the weldment was excessive, and it also showed lack of fusion and penetration. Lack of fusion is discontinuity between weld and base metals, so there is no path for stress to transfer through the weld into the adjoining member. Thus, the stress axis got shifted toward the overlap/overwelded region resulting in bending moment on the weldment and making it vulnerable to fail by fatigue. As an outcome of this investigation, it was recommended to alter the welding sequence of the body structure to avoid any blind welds that can result in lack of fusion and overwelding.