Very high cycle fatigue of steels under torsional loading

Since almost two centuries, scientists and engineers have been intensively investigated why a great number of low repeatedly applied loads can cause material damage and ultimately fracture although all loads are below the static strength. This phenomenon is called fatigue and is one of the most serious problems that cause failure in structures. Even after more than one billion load cycles, in the so-called very high cycle fatigue (VHCF) regime, fatigue failure can occur. Numerous laboratory tests have been conducted to investigate the VHCF behaviour, but most of them are performed under uniaxial loading condition which means that test specimens are repeatedly pushed and pulled until they fail. However, the loading condition of several components in technical practice is not adequately represented by such tests. Coil springs or bearings, for example, are mainly subjected to twisting which leads to cyclic shear loading and additional static mean shear loads are present. Investigations on VHCF properties under cyclic shear loading are very rare so far. Therefore, it is of great technical as well as scientific interest, what damage mechanisms occur under such loading conditions, what is the influence of superimposed static shear load, and what are possible differences and similarities to cyclic axial loading. These questions should be answered in the project, in which the VHCF properties of two different steels under cyclic shear loading are investigated. Most of the experimental work will be performed with a recently developed ultrasonic torsion fatigue testing setup. The ultrasonic fatigue testing technique works by stimulating test specimens to resonance frequency. Approximately 20,000 load cycles are applied within one second which enables to perform tests up to one billion cycles within only hours or days. In comparison, such tests would take months or years using conventional test equipments. However, some experiments will be run in parallel with a conventional servo-hydraulic torsion fatigue test frame. This will allow observing possible influences of the testing technique, notably frequency influences, on the measured cyclic properties. During fatigue failure, most of the lifetime is spent for the evolution of a crack and its propagation. Since the location for crack initiation is very difficult to find during testing, small artificial defects will be introduced into the surface of test specimens. At these defects, cracks initiation will take place and can be observed during the experiment. By the use of the ultrasonic fatigue testing technique, fatigue cracks with extremely low propagation rates per load cycle are observable. With the data obtained, it will be attempt to find a physical explanation for failure or for infinite life during fatigue loading.

For nearly two centuries, scientists and engineers have pondered the question why a large number of low, repeatedly applied loads can cause material damage and ultimately fracture although all loads are below the static strength. This phenomenon is called fatigue and is one of the most serious problems that causes failure of components and structures. Even after more than one billion load cycles - which may be accumulated during decades of operation -, in the so-called very high cycle fatigue (VHCF) regime, fatigue failure can occur. Numerous laboratory tests have been conducted to investigate the VHCF behaviour, but most of them were performed under reversed, axial loading condition, which means that test specimens are repeatedly pushed and pulled until they fail. However, the loading condition of several components in technical practice is not adequately represented by such tests. Coil springs or bearings, for example, are mainly subjected to repeated twisting, which leads to cyclic shear loads that are typically superimposed by static stresses. Investigations on the VHCF properties under cyclic shear loading are very rare so far. Therefore, it is of great interest, both technically and scientifically, to answer the questions which damage mechanisms under such loading conditions occur, what is the influence of superimposed static shear load, and what are possible differences and similarities to cyclic axial loading. The aim of the project was to give further insights into these issues by investigating the VHCF properties of different steels under cyclic shear loading. Most of the experimental work was performed with the recently developed ultrasonic torsion fatigue test setup, that enables to perform tests up to one billion cycles within hours or days. In comparison, such experiments would take months or years using conventional test equipment. Within the project, it was found that the loading condition significantly affects the failure mechanisms. Similarities and differences between the fracture mechanisms under axial and torsional loading could be identified and qualitatively as well as quantitatively described. It has been demonstrated that material-inherent defects such as nonmetallic inclusions are potentially detrimental under a variety of loading conditions, however, the defect sensitivity highly depends on the materials static strength and the specific loading condition. This has been systematically investigated by introducing well-defined artificial defects into specimens made of different high- and ultrahigh-strength steels that were tested under a variety of cyclic loading conditions. Based on the experimental results, prediction equations were developed that enable to calculate critical stresses under different loading conditions. Beside further insights into the basic mechanisms acting during failure caused by VHCF torsional loading, the obtained findings give important indications for the sustainable employment of steels and the development of new high-performance materials.