PSI - Issue 38

Hendrik Bissing et al. / Procedia Structural Integrity 38 (2022) 372–381 Hendrik Bissing, Markus Knobloch, Marion Rauch / Structural Integrity Procedia 00 (2021) 000 – 000

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1. Introduction Renewable energies including the wind energy are already of major importance and will become even more significant in the future. The development of modern wind turbines pushes innovation and optimisation for tower structures located on- and offshore. For onshore tower concepts with tower heights above 100 m, the main part of the structure is usually made of steel. To guarantee an entire service and operating life of 20 to 25 years, fatigue design is of upmost importance and often decisive for the structural design. In recent years, the price per kilowatt-hour has been decreasing due to competition in the energy market. The economic aspects and technical development are important to make wind energy competitive and to make a substantial and sustainable energy offer to the market demand. A major challenge is to ensure the stability and durability of the tower while reducing material, production, and installation costs. Optimised and advanced concepts can facilitate these requirements, especially for fatigue verifications. The different fatigue approaches applied in engineering practice vary in accuracy and simplicity. The nominal stress approach is easy applicable and relies on experimental results for several notch details considered e.g. in different standards such as EN 1993-1-9 (2012). More recent and advanced approaches are the hot-spot concept and the effective notch stress concept. These concepts consider structural geometry effects by utilising modern numerical and computational methods to achieve an improvement of the fatigue life prediction. The Two Stage Model, another sophisticated approach, is based on two phenomenological effects of fatigue failure, i.e. crack initiation and crack propagation that are superimposed to the total fatigue life. This model allows to consider structural and material inputs explicitly, e.g. local stress concentration and cyclic stress-strain response, as well as approaches for local crack initiation and subsequent fracture mechanics calculations with their effect on service life. 1.1. Brief overview of the Two Stage Model (TSM) The Two Stage Model (TSM) combines the Strain-Life Approach (SLA) and the Crack Propagation Method (CPM). Material characterisation, individual geometry of the notch and load-time function are the main inputs of the SLA (Fig. 1, left). Furthermore, the approach for the transformation from load to notch stress, and the damage parameter can be varied. The effective damage results from closed hysteresis loops with explicit consideration of the local stress-strain behaviour inside the notch. Each complete hysteresis has a designated damage parameter that allows an allocation of the hysteresis to a specific resistance expressed in number of cycles. The entire damage is usually computed by the well-known linear Palmgren-Miner accumulation rule. Details of the TSM are given in Röscher et al. (2019). The CPM comprises also different input parameters and approaches (Fig. 1, right). The material characterisation, the load-time function as well as the geometry and mode of the occurring crack are considered as inputs. Another significant choice can be made for the crack propagation formulation. Different methodologies are available in literature that either neglect or consider parameters and approaches that influence the speed of crack propagation, e.g. Paris, Walker, Forman or NASGRO (Paris et al., 1963, Forman et al., 1967, Walker, 1970, Mettu et al., 1999). Material parameters for the SLA are provided in literature and can also be verified against ones’s own experimental tests. A suitable test method for material characterisation is the Incremental Step Test that provides the cyclic stabilised stress-strain response. The test data can be used to establish the stress-strain curve according to Ramberg and Osgood (1943) and the strain-life curve according to Manson (1960), Coffin et al. (1959) and Morrow (1965). Experimental tests for the determination of the crack propagation formulation are performed using CT specimens. Several results are available in the literature, e.g. Seeger (1996).