Long crack growth and crack closure in high strength nodular cast iron

In the context of wind turbines, highly stressed structural components are often made of EN-GJS-700-2. This high strength nodular cast iron grade is characterized by an almost purely pearlitic matrix resulting in a comparatively low toughness. Therefore and due to inevitable casting defects, a fracture mechanical strength assessment, additionally to a conventional one, is often required. However, it lacks a detailed characterisation of EN-GJS-700-2 in terms of fracture mechanical material parameters. Thus, the objective of this study was to derive a comprehensive description of long crack growth for this cast iron grade. For this purpose, profound experiments were performed on raw material taken from a real planet carrier. Crack growth was analysed under varying loading conditions whereby the impact of the stress ratio and the maximum stress intensity factor was of special interest. The threshold behaviour was described in terms of two threshold values. Relevant crack closure mechanisms were analysed in detail: plasticity, oxidation and surface roughness. The latter one was identified as the most dominating mechanism. Three different approaches to describe crack closure were derived. Its integration into a crack growth law yielded a uniform description in terms of an effective stress intensity factor. To describe crack growth in a less complex way, two other crack growth laws, based on a two-parameter approach respectively on the maximum stress intensity factor, were derived.