Ultrasonic fatigue of cast aluminium under cyclic compression and of superelastic Nitinol in the synchrotron

• Purely compressive ultrasonic fatigue tests of cast aluminium are demonstrated. • Numerous slip bands and shear cracks cause VHCF failure under pure cyclic compression. • Mean stress sensitivity is strongly overestimated by the FKM guideline. • Elastic strains in superelastic Nitinol are similar at low and ultrasonic frequency. • In-situ synchrotron ultrasonic tests reveal the progress of fatigue damage in Nitinol. Ultrasonic fatigue testing is used for the first time to study the very high cycle fatigue (VHCF) properties of cast aluminium AlSi8Cu3–T6 under purely cyclic compression loading (load ratio R = –∞). Formation of numerous shear cracks and disruption of large material volumes caused failures under cyclic compression, whereas casting porosities were preferential sources of cracks leading to early failures at load ratios R = –1 and R = 0. The Goodman straight line approximation well predicts the mean stress sensitivity for cyclic tension, whereas the FKM guideline overestimates it for cyclic tension as well as cyclic compression. In–situ ultrasonic fatigue tests in the synchrotron have been performed with superelastic Nitinol. Specimens were subjected to a preloading procedure which led to a multiphase austenitic − martensitic condition. Lattice deformation over an ultrasonic cycle as well as under quasi static condition confirmed similar elastic deformation at 0.1 Hz and 18.3 kHz. Progress of fatigue damage with increasing numbers of ultrasonic cycles was observed in–situ with XRD patterns from the multiphase area. Formation and growth of austenitic bands in formerly martensitic areas was found that led to a progressively increasing specimen stiffness and an increase of stresses in displacement-controlled ultrasonic fatigue tests.