Experimental investigations on normal mode nodes as support positions of a resonant testing facility for bending fatigue tests

Large-scale fatigue testing is very important to the research on scale effects, which occur in large cyclic loaded structures, such as wind turbine towers. However, such experimental testing has a very high energy consumption. As an efficient alternative, this paper presents a new resonant testing facility for large-scale specimens under cyclic bending loads. The facility works as a 4-point bending test, in which the specimen is supported in the nodes of its first normal bending mode, where theoretically no reaction forces occur. Two counter-rotating imbalance motors with excitation frequencies near resonance generate a harmonic force acting on the specimen. Experimental trial fatigue tests on a steel pipe as a specimen were carried out, in order to validate the new testing setup. A great decrease in the support forces was reached by placing the supports at the normal mode nodes. Additionally, the behavior of the support forces under varying positions and excitation frequencies was also investigated. In summary, the resonant testing method combined with the supports at the normal mode nodes offers an efficient and energy-saving testing setup for large-scale fatigue tests.