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Manganese sulfides (MnS) are nonmetallic, ductile inclusions with high melting temperature (1610 °C) which improve the machinability and retard the grain growth in steels, in addition of contributing to avoid cracking during hot working. In this paper, the effect of manganese sulfides on the fatigue life of the vanadium micro-alloyed forging steel 38MnVS6 is discussed. Force-controlled fatigue tests are performed on small sized specimens until the crack occurs. The fatigue life of the forged material, presented by Wöhler curves, is considerably reduced at high levels of the nominal stress amplitude compared to the wrought material. Moreover, it is evident that the presence of longer and thinner particles of MnS reduces the scatter band of Wöhler curves and decreases the fatigue strength of the material. This paper presents a first attempt to find a relation between the shape and content of manganese sulfides due to the forging process and the fatigue life of the material.

The cyclic material behavior is investigated, by strain-controlled testing, of 8 mm thick sheet metal specimens and butt joints, manufactured by manual gas metal arc welding (GMAW). The materials used in this investigation are the high-strength structural steels S960QL, S960M and S1100QL. Trilinear strain-life curves and cyclic stress-strain curves have been derived for the base material and the as-welded state of each steel grade. Due to the cyclic softening in combination with a high load level at the initial load cycle, the cyclic stress-strain curve cannot be applied directly for a fatigue assessment of welded structures. Therefore, the transient effects have been analyzed in order to describe the time-variant material behavior in a more detailed manner. This should be the basis for the enhancement of the fatigue life estimation.

Fatigue testing is known to be time consuming and expensive. Therefore, it should be the main target of fatigue research to accelerate the derivation of fatigue properties. Depending on the required properties, strain- or load-controlled fatigue tests have to be performed. Carrying out load-controlled fatigue tests is necessary to derive the influence of mean stresses and notches on the fatigue strength and fatigue life of different materials and joining technologies. In the case of material samples, increasing test frequencies could be a proper way to accelerate the fatigue testing, as long as the increased test frequencies have no influence on the resulting fatigue life. In the case of strain-controlled fatigue tests, it is not possible to increase the test frequencies in order to accelerate the fatigue tests. Therefore, the Incremental Step Test, which allows the derivation of the cyclic stress-strain curve with only one test, was introduced.

Lightweight design in the automotive industry is not always combined with the usage of alternative materials like composites. Even high strength steels have high potential for reducing the weight for lightweight design. For the forging industry a new steel is developed, which enables the TRIP-effect (Transformation Induced Plasticity) for forging parts. This material effect is already well known and used for steel sheet structures. The TRIP-effect is based on the structure of the TRIP-material containing retained austenite, which has the possibility to form residual stresses due to the austenite-martensite transformation under cyclic loading. Beside static properties, the dynamic and cyclic material behaviour has a high importance for parts in the automotive industry. So, for lightweight design, a focus has to be on fatigue behaviour under service loads including overloads for an optimal weight reduction.

To reduce the weight and to increase the power as well as to enable the utilization of nodular cast iron components, e.g. for wind turbines and heavy industry parts, locally higher stresses need to be withstood by the material. This becomes crucial, when additional overloads influence the structure of thick-walled components causing high local elastic-plastic deformations. In this case, the cyclic, elastic-plastic material behavior and its development under cyclic loading are important points to be considered during component design. To assess the material’s local elastic-plastic material behavior, strain-controlled fatigue tests were performed under alternating loading, Rε = -1, with unnotched specimens removed from cast blocks as well as from a hub and a planet carrier of wind turbines, made of EN-GJS-400-18U-LT, EN-GJS-700-2, ADI-800 and ADI-900.

The fatigue life approach is the main topic of structural durability. Improved methods for the numerical fatigue analysis should be based on experimental results. In some fields of material testing progress in research are very hard to achieve. Especially the regime of amplitudes below the knee point of the SN-curve with a huge number of load cycles to failure is one of these challenges with respect to fatigue tests. With standard testing devices, 108 to 1010 cycles cannot be achieved in a reasonable time span because of their low and limited testing frequencies or their inflexible control systems concerning variable amplitude loading. For this reason, a new piezo based testing facility has been developed by Fraunhofer LBF which is capable to master this challenge. Built up with a high performance piezo actuator and a specially designed high frequency load frame this testing facility enables test frequencies up to 1.000Hz and locking forces of 10kN.

For the design of thick-walled nodular cast iron components, fatigue assessment, especially in the context of local imperfections in the material, is a challenging task. Not only the cyclic material behavior of the sound baseline material, but also the cyclic behavior of materials with imperfections, such as shrinkages, dross and chunky graphite, needs to be considered during the design process of cast iron components. In addition to this, new materials, such as solid solution strengthened alloys, offer new possibilities in lightweight design, but need to be assessed concerning their fatigue strength and elastic-plastic material behavior. If a safe and reproducible fatigue assessment for any component cannot be performed and a secure usage is therefore not given, the cast components are generally rejected, leading to a loss of additional material, energy and money for recasting the component.