Application of finite-life frequency sensitivity evaluation methods to UFT of ferritic structural steels

The frequency effect is a commonly encountered challenge in ultrasonic fatigue testing (UFT) of low-carbon, ferritic steels, wherein factors such as the increased strain rate and reduced test duration change the apparent fatigue resistance of the tested material. The usability of UFT for rapid fatigue testing of these materials is therefore limited as the results cannot be directly compared to conventional fatigue results. In this investigation, fatigue curves were evaluated at frequencies of 20Hz and 20kHz for two comparable grades of ferritic structural steels: Q355B and S355JR, using different conventional frequency specimen geometries. Methods to evaluate the frequency sensitivity of the steels based on the finite life regime were adapted from previously proposed models in literature to produce corrected curves and to allow comparison to similar steels in literature. It was found that previously reported results may be overestimating the frequency sensitivity due to the influence of size effects. It was also found that these models are of limited use for producing corrected SN curves based on UFT data.