Time Saving Method for Measuring VHC Fatigue and Fatigue Crack Growth Data with the Ultrasonic Fatigue Technique

AbstractThe principles of the ultrasonic 20 kHz technique are shortly described and its peculiarities explained. Considering the technical requirements, such as an on-line amplitude and frequency feedback control and use of appropriate cooling allow obtaining data of high accuracy and reliability. Life-time data at constant and variable amplitudes, as well as fracture-mechanical values, such as (Δa/ΔN) vs. ΔK curves can be obtained, and especially endurance limit and ΔK thresholds can be obtained in a highly time-saving way. Some examples of performing measurements under superimposed loading conditions are shown. One is on-line observation of specimens during loading and simultaneous mechanical measurements plus fractography after final fracture so that, the correlation of these results allows quantitative information on (Δa/ΔN) and ΔK values in the VHCF regime. Another new technique led to a two-step model of small-crack initiation and propagation from corrosion pits. Investigations on polycrystalline copper likewise rendered interior fatigue-crack formation from persistent slip bands and corroborated that, arrest of small cracks is mainly responsible for several unexpected results in the VHCF regime