Failure Analysis on Auto-lock of Plate Nut

Auto-lock failure occurred to a plate nut in an aircraft. In order to find out the failure cause, macro observation, structure examination and hardness testing were carried out. The results show that the auto-lock failure of the nut was because of the relaxation deformation of the material at the narrowing mouth. The poor microstructure uniformity of the nut, especially the large grains near surface, and a larger amount of residual austenite resulted in the low resistance to stress relaxation. And the poor consistency of the shape and size of the narrowing mouth led to uneven assembly stress. The above two factors directly led to the relaxation failure of the nut. Accordingly, it is proposed that more emphasis should be placed on the following aspects during the manufacturing process of the plate nuts:uniformity of microstructure, shape and size consistency of narrowing mouth, size appropriateness, and microstructure stability under service environment.