A new resonance-regulated testing device operating at a frequency of up to 200 Hz has been developed for very high cycle fatigue tests on large-scale cast iron and steel specimens. The device is able to perform tests on axially loaded plate specimens with a thickness of up to 20 mm and a width of up to 40 mm. It allows testing with a maximum amplitude of 70 kN and a stress ratio of 001 ≤ R ≤ 005. In this paper, the working principle of the device is shown. Furthermore, the calibration, pretension, and testing procedures, as well as the results of the first fatigue tests, are presented based on tests with ductile (grade EN-GJS-400-18C-LT) cast iron.

Introduction

Offshore wind turbines are expected to experience up to 109 load cycles during their service life (Seidel, 2010). Thus, the fatigue limit state represents a crucial factor during the design process. In current offshore standards and guidelines (DIN 18088-3 (Deutsches Institut für Normung, 2019), DNVGL-RP-C203 (DNV GL, 2016a)), and DNVGL-ST-0361 (DNV GL, 2016b)), the design S-N curves do not allow for an endurance limit. These S-N curves are mostly derived from fatigue tests up to 107 load cycles. The shape of S-N curves for more than 107 load cycles and the question of the existence of an endurance limit are still of particular interest in ongoing research (Schaumann and Steppeler, 2013; Steppeler, 2014). The investigation of this range requires testing facilities able to perform a very high number of cycles (beyond 1075 within a reasonable time frame. Over the last decades, different servo-hydraulic, resonant, forced-vibration, rotating-bending, and ultrasonic machines for very high cycle fatigue tests have been developed (Stanzl-Tschegg, 2014). The testing frequency of these machines lies between several hundred hertz in the case of servo-hydraulic machines to up to 20 kHz for ultrasonic testing equipment. One of the main limitations of these machines is that only small-sized specimens can be tested. This yields a smaller testing volume, which reduces the probability of encountering large defects. Hence, higher fatigue strength can be measured (Furuya, 2010). Considering offshore wind turbines with some structural components whose wall thicknesses exceed 100 mm, it is necessary to perform tests on large-scale specimens to limit possible scale effects. An improved experimental basis of fatigue tests is vital for the optimization of these structures.

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