Dynamic cyclic stress analysis of rolling of a diametrically loaded helical test specimen

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 of the spring component, so cyclic stress is applied to the material via a reciprocating rolling motion between two platens. Stress analysis of this arrangement provides the stress range and mean stress necessary for fatigue analysis. A three dimensional finite element dynamic simulation of the test process is applied to a nominal test specimen to determine detailed history of the stress distribution. Numerical challenges are addressed to ensure representative loading conditions are properly applied, and that steady-state response is achieved. Bounding stress ranges are determined, and the implications of the results in fatigue analysis are discussed. It is concluded that response at the ends of the specimns is important in properly evaluating test data.