From Hertz to Kilohertz: Evaluating Ultrasonic Fatigue as a Surrogate for Conventional Fatigue Testing in a Cast Al-Si Alloy

Ultrasonic fatigue (USF) testing enables accelerated, high-frequency cyclic loading, reducing experimental time by several orders of magnitude compared to conventional servo-hydraulic fatigue (CF) testing. However, the validity of USF as a surrogate for CF data generation is in contention, as prior studies report inconsistent agreement in fatigue lives for identical materials. In this work, the frequency-dependent fatigue response of a cast Al-Si alloy was systematically evaluated to assess the mechanistic and practical differences between USF and CF loading conditions. First, extensive S-N fatigue testing by CF and USF was conducted for alloys produced with varied pore populations while controlling for heat generation and specimen geometry effects. The fatigue life was found to vary with the test mode and defect population, and quantitative fractography of the crack-initiating defects and early growth mechanisms was conducted. These studies were complemented by novel investigations of spatial strain distributions using high-speed digital image correlation, strain rate sensitivity using variable-rate split-Hopkinson bar testing, and environmental influences by crack growth rate testing at CF and USF frequencies. These experiments elucidate the effect of fatigue frequency on the operative damage mechanics and are discussed in the context of the validity of USF as a surrogate for CF testing.