Very High Cycle Fatigue

Abstract

In this chapter, an introduction into the experimental challenges of fatigue testing beyond the classical fatigue limit will be presented. For many applications, strength assessment according to the classical durability (N ≤ 107) is no longer sufficient, since increasingly more components are subjected to cyclic loading up to the very high cycle fatigue (VHCF) regime. Recent studies have shown that the effective damage mechanism need not be inferred readily from low- and high-cycle fatigue (HCF) behavior of materials, since competing failure modes resulting from microstructural discontinuities play a prominent role in crack initiation. An identification of the failure-relevant microstructural feature poses a genuine experimental challenge, since from a macroscopic perspective purely elastic deformation is applied. Moreover, a shift from surface to interior crack initiation is observable for several materials. All of these aspects together with the need to test at elevated frequencies to reach numbers of cycles as high as N = 109 call for new experimental strategies. After a brief introduction of the motivation for VHCF testing and a short excursion into the world of VHCF-relevant damage mechanisms, an overview of high-frequency fatigue testing machines and their characteristic features is presented. Particular attention will be paid to the innovative test method of ultrasonic fatigue. A critical discourse of likely influence factors such as frequency and environment in the context of high-frequency testing is given. Finally, particular attention will be drawn to in situ damage monitoring during VHCF test conditions.