Little to no knowledge exists for fatigue testing of small, thin-walled, component-like specimens with an electrodynamic shaker. Most of the currently used testing approaches for these shakers exploit the amplitude amplification in the resonant case of the specimen. In the course of this work, a testing methodology is developed which enables the testing of small component-like specimens in the VHCF range, independent of their natural or resonant frequency. The basic idea of the resulting test rig is based on the design of a spring-mass system with several components. These components are chosen such that an external excitation induces a mode of vibration in which the specimen is forcibly deformed. Based on this basic idea, a concept study is made in which 5 possible approaches are considered. In the course of this study, the test method, complexity, specimen clamping, and type of applied stress of each concept are examined. These characteristics are evaluated and form the basis for the selection of a test setup. This evaluation is supported with modal and frequency response analyses to find the best variant. The determined concept is then further developed and optimised. This creates a model that can be implemented in reality. The developed test rig consists of an outer and an inner system. The outer system is very rigid and is realised by a frame construction. The frame and the base plate are connected to the specimen by so-called compression springs. The outer system performs the excitation oscillation. The inner system consists of a mass, which is attached to the base plate by springs and into which the specimen is clamped. Due to the small dimensions of the specimen, a sophisticated clamping principle was developed. The inner system is set into resonance by the excitation and starts to oscillate with a strong amplification in vertical direction. This desired eigenmode has a natural frequency of 1670Hz, whereby no spurious modes occur in the surrounding frequency range. An excitation of this eigenmode results in a forced deformation of the specimen, introducing stress into the specimen. According to FEM simulations, an excitation of 50m/s2 with a damping ratio of 0.01 causes a maximum relative displacement in the specimen of 23µm. This induces a notch stress of 1450MPa. The test rig is monitored and controlled by accelerometers, a laser vibrometer and a load cell. Since it is not possible to determine the stress in the cross-section of the small specimens using strain gauges, the relative displacement between the vibrating mass and the base plate is measured contactless during the test using the laser vibrometer. This measured relative displacement has a linear relationship with the notch stress, so that a constant notch stress amplitude can be indirectly controlled. The load cell and accelerometers are used to detect the failure of the specimen.
|Translated title of the contribution||Development of a test concept for high-frequency fatigue testing of small, component-like specimens|
|Award date||25 Jun 2021|
|Publication status||Published - 2021|
Bibliographical noteembargoed until 25-05-2026
- small specimen testing
- fatigue testing