Application of Torsional Fatigue Testing of Helical Springs to Explore the Effect of Mean Stress on Inconel X-750 Fatigue Resistance

Abstract In a particular nuclear application, separation between structural components is maintained by a helical spring such that the separating load bears across the diameter of the spring coils. Relative motion between the structural components due to changing load and temperature is accommodated by rolling of the spring. This rolling motion while under radial load results in cyclic loading of the spring material. Fatigue analysis of the cyclic loading must take into consideration the material degradation due to the unique operating environment, so testing of ex-service material is required. Standard fatigue test specimens are not possible due to the small dimensions and helical form of the spring component. A fatigue test system has been developed in which cyclic stress is achieved by applying torsional load with respect to the axis of the helical spring. This load results in a pure bending state of stress in the cross-section of the coils. The relationship between applied load and stress is achieved analytically through curved beam analysis. The fatigue test has previously only been applied with fully reversed load cycles with zero mean stress. In this work, test with non-irradiated specimens are done with mean stress controlled to be nonzero such that the effect mean stress on the S-N curve can be quantified. The outcome of the tests demonstrates that the test method is effective in producing valid fatigue data over a range of mean stress and alternating stress. Also, the test results add to the fatigue data available for Inconel alloy X-750.