idw – Informationsdienst Wissenschaft
Nachrichten, Termine, Experten
Nachrichten, Termine, Experten
The Fraunhofer Institute for Nondestructive Testing IZFP carries out research and development activities in the field of nondestructive testing processes along the entire materials value chain. For customers in the automobile, aerospace, rail, energy, construction and agriculture industries, the institute offers a wide range of NDT expertise and technologies. From June 13 to 17, 2016, our researchers present an ultrasonic inspection method in order to detect and monitor preload in bolting at the 19th World Conference on Non-Destructive Testing (WCNDT) in Munich.
The detection and monitoring of preload in bolting is an omnipresent need in structural mechanics without sufficient solution. A detailed study about the preload in bolts and its detection with a combination of different types of ultrasonic waves is presented. The accuracy of preload measurement based on relative runtime variations between different wave types was investigated. For this purpose a system was built to adjust defined stress states in bolts and correlate them with ultrasonic runtime measurements.
In a first step the influence of stress on every single type of wave and the repeatability of runtime measurement in the bolt were determined. This led to a good estimate of the measurement error and defines the minimum and the step size of preload detection. Additionally the bolt’s microstructure caused by the production process and the thread geometry are very important factors for the interaction of ultrasound with the bolt. Taking these boundary conditions into account, it is possible to define a material based acoustoelastic parameter which enables detection of stress levels in bolts of the same material and dimension. The necessary verification of this special acoustoelastic parameter is also possible with the preload-adjustment-system.
The mentioned system provides full control over ultrasound generation, measurement and data handling in combination with precise mechanical measurement of the applied load. In the following investigations a comparison of parameters between different bolt diameters and lengths were performed to quantify dimensional effects and their contribution to the measurement error. Regarding the potential use in on- or offline monitoring systems a comprehensive database is crucial to investigate bolts of different materials and dimensions as well as easy-to-use GUI.
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