PSI - Issue 18

Markus Berchtold et al. / Procedia Structural Integrity 18 (2019) 532–537 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction ‘‘There is no infinite fatigue life in metallic materials” [1]. Studies on damage mechanism on higher number of load cycles, in the range of up to 1010 cycles and more could well proof the finding published by Claude Bathias and others. Thanks to the development of faster testing technics and the shortening of testing time a large number of basic research activities took place in the recent decades. Ultrasonic fatigue testing systems work on the resonant frequency at about 20kHz, and require relatively small specimens with a specific geometry. Ultrasonic fatigue studies showed that a fatigue limit in the traditional sense does not exist in the gigacycle regime. Cracks may occur subsurface or on the surface, and may start for example from inclusions in the material [1]. Subsequent with higher testing frequency, an old question of fatigue testing is high-lighted and cannot be neglected: “What is the effect of the testing frequency on fatigue life?” Testing on very high testing frequency may lead to different damage mechanism than under real loading condition for example of an engine component. Since inclusions and imperfection play an important role in VHCF the manufacturing process has a significant effect on fatigue life in the Gigacycle regime. Particularly for relatively inhomogeneous materials the testing of material volumes that represents the scatter of the manufacturing process is a concern. In 2014 RUMUL could present a new resonant fatigue testing machine, with a testing frequency of 1000Hz. The dynamic load of maximum 50kN peak-peak is produced with an electromagnetic system, similar to established resonant fatigue testing systems which typically run on testing frequencies from about 40 up to 250Hz. The static portion of the load is provided by two mechanical spindles, the maximum load of the system is +/- 50kN. Any load ratio can be selected. Flat and round specimen types that are normally used in fatigue testing can be used. The new testing machine offers new possibilities for investigations of material properties in the very high cycle fatigue (VHCF) regime. Compared to other systems used in the field of VHCF testing the RUMUL GIGAFORTE provides several advantages. The size of the machine is smaller and energy consumption less compared to a servo hydraulic system. The actually tested material volume is larger than the material volume that is tested on ultrasonic systems. The testing frequency of 1000Hz allows normally continuous testing, without intermittently stopping the test for let the specimen cool down. In the past four years the new testing machine was intensively used for example at the laboratory of the Fraunhofer-Institut fürWerkstoff- und Strahltechnik IWS in Dresden in Germany. It is used for testing material samples and small components as well. Some effects of the 1000Hz testing frequency on the fatigue behavior of the material were observed [2]. Recently the IWS laboratory developed a small salt spray chamber and mounted it on the GIGAFORTE to perform fatigue testing under corrosive atmosphere. 2. Effects of the loading frequency on fatigue life What is the effect of the frequency of an alternating load on fatigue life and fatigue testing? This question is probably present since beginning of fatigue testing in any area of fatigue testing such as axial loading or rotating bending; on room or elevated temperature or corrosive environment. In literature several investigations can be found that show an effect of the testing frequency on fatigue life, particular on very high testing frequencies and relatively high load. For lower frequencies the effects can be neglected very often, however the sometimes unknown magnitude of some effects led to a quite conservative approach of limiting the testing frequency in some areas of fatigue testing. The frequency effects can be divided in three areas: Temperature and environment as extrinsic factors and strain rate as intrinsic factor [2, 3]. The effects may superimpose, and affecting fatigue life in the same or opposite direction. 2.1. Temperature A higher material temperature reduces usually the fatigue life in a similar way as the ultimate strength of a material is related to the temperature. And some materials show a temperature depending crystallographic transformation that affects the material properties and fatigue life. Maintaining the specified temperature range is therefore a basic requirement for fatigue testing.