PSI - Issue 47

Christoph Bleicher et al. / Procedia Structural Integrity 47 (2023) 478–487 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Nodular cast iron (GJS) (DIN EN 1563 2019) is extensively used in mechanical and plant engineering as well as in automotive and energy technology like wind energy. Due to its wide range of possible material properties with high strength and ductility and the freedom in design of castings, GJS is used for a wide range of highly stressed components. In wind energy application, the weight of the GJS components like rotor hubs, machine carriers, gear boxes and torque arms sometimes exceed 30 tons. For those components with high requirements, especially in terms of fatigue strength and stiffness it is crucial to achieve a more or less perfect microstructure especially in the highly loaded areas. Additionally, driven by the demands concerning performance, improvement of the power-to-weight ratio (t/kW) and the implementation of a lightweight design measures for weight reduction and complexity as well as the afford and costs for production of modern wind energy castings increase.

Nomenclature d / [mm]

diameter of the specimen

E / [GPa] f / [Hz] k / [-] k* / [-] K t / [-] M / [-] N / [-] N f / [-] N i / [-] N k / [-] N lim / [-] P S / [%] R σ / [-] T σ / [−]

Young’s modulus test frequency

slope of the SN curve in the medium cycle fatigue range

slope of the SN curve after the knee point

stress concentration factor mean stress sensitivity

number of cycles

number of cycles to failure number of cycles to crack initiation number of cycles at the knee point

limit number of cycles probability of survival

load ratio under constant amplitude loading for stress

scatter band

σ a,n,k / [MPa] nominal stress amplitude at the knee point σ a,n,Nlim / [MPa] nominal stress amplitude at the limit number of cycles

Nevertheless, discontinuities like shrinkages as well as irregularities in the microstructure, such as dross, blowholes and chunky graphite, can occur during production. In most cases, this leads to the rejection of the components due to a lack of acceptance by the casting user and the certification bodies since the effects on lifetime coming from the defects are not known exactly. Due to the high loss of performance and money foundries try to reduce their component rejects due to those material defects. One possible measure is a reworking of the defect areas by milling and adding of a repair welding in near surface areas. In the context of repair welding for wind turbines, DNV guidelines (2016) require a special qualification of the welding company as well as tested welding procedure approval (WPA or welding process specification WPS) and a registered test report (welding procedure approval record - WPAR) (DNV 2016). According to (DNV 2016), dissimilar welding is not permitted for cyclically stressed components, which is related to a lack of knowledge about the effect of repair welding on the local fatigue strength of the welding. To assess the weldability and the effect of welded zones on fatigue strength thick-walled ferritic EN-GJS-400 18LT, ferritic solid-solution strengthened EN-GJS-450-18 and pearlitic EN-GJS-700-2 were investigated within the research project “nodularWELD” (Schoenborn, 2017) based on welded cast blocks with a thickness of nearly 200 mm by means of stress controlled fatigue tests. While ferritic EN-GJS-400-18LT has a medium strength but high ductility wind turbine manufacturers strive to used high strength grades based on a higher silicon content (EN-GJS-450-18) or even pearlitic EN-GJS-700-2 both having a reduced ductility. For all three materials an appropriate dissimilar welding filler was selected in pretests and extensively investigated by stress controlled fatigue tests at alternating loading,