Dynamic frequency-dependent fatigue damage in metals: A state-of-the-art review

Abstract

Ultrasonic high-frequency fatigue testing differs from conventional low-frequency fatigue testing in strain rate and cyclic loading. Ultrasonic fatigue has a resonance frequency of 20,000 Hz and a load frequency below 100 Hz. A large change in cycles per second may affect material fatigue and cyclic strain rate. Comparative studies show that test technique and loading frequency affect fatigue life and strength. Over the past decades, researchers have discovered the main causes of the frequency effect on fatigue characteristics, the mechanisms behind this effect on materials, and the frequency effect on various materials structures. This review paper summarizes our knowledge of dynamic frequency-dependent fatigue damage in various metals. The strain rate effect, time-dependent environmental impact, and temperature effect of frequency on fatigue properties are highlighted. This review article examines dislocation interaction, crack initiation, crack propagation, fatigue strength, and fatigue life. Load frequency affected the fatigue strength of some low-strength steels, nickel-based superalloys, and pure polycrystalline coppers but not most high-strength steels, aluminum, or titanium alloys. The frequency effect on fatigue strength increased due to the strain rate hardening effect and increasing frequency. Plastic deformation cannot follow high-frequency loading in ultrasonic testing because dislocation velocities are slower than sonic speed and fatigue crack initiation and propagation are tightly connected to local plastic deformations. This causes fatigue property changes. The ultrasonic high loading frequency caused brittle, intergranular, and crystallographic crack initiation, while the conventional low loading frequency caused ductile, transgranular cracking.