Failure analysis of aviation torsional springs

Cracks and fractures occur during the assembly process to a type of torsional springs used in the aviation mechanism. Besides visual examination, other experimental techniques used for the investigation are: 1) fracture characteristics, damage morphology and fractography by scanning electron microscopy (SEM), 2) spectrum analysis of covering, 3) metallographic observation of cracks and 4) hydrogen content testing. The results are obtained through the analysis of manufacture process and experimental data. Since no changes of microstructure are found, failures are irrelevant to the material. The cracks and fractures initiate on the inner surface, cracks initiate before the cadmium plating and after the winding. No obvious stress corrosion cracks are found near the crack source region. The opening direction of cracks is consistent with the residual tensile stress of the spring inner surface, and the springs are easy to contact hydrogen media between the spring winding and the cadmium plating. The cracks are caused by hydrogen-induced delayed cracking under the action of the residual tensile stress and hydrogen.