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NDT.net Issue - 2019-03 - Articles
NDT.net Issue: 2019-03
Publication: 10th International Symposium on NDT in Aerospace, Oct 2018, Dresden, Germany (AeroNDT 2018)
Session: Applications
ARTICLE

Determination of the nonlinear elastic constants in a surface treated layer for aero-engine disk residual stress measurement

Sergey Gartsev13, Marek Rjelka16, Bernd Köhler150, Andreas Mayer22
1Fraunhofer Institute for Ceramic Technology and System (IKTS)231, Dresden, Germany
2Hochschule Offenburg2, Offenburg, Germany

Abstract: A common method to enhance the fatigue life of components is the introduction of compressive residual stress in the components’ surfaces which suppresses the surface crack growth under (cyclic) tensile load. One major application is the surface treatment of blade discs in jet engines, produced of titanium and nickel based superalloys. Up to the moment, the presence of compressive stress can only be evaluated either during production by reference samples (Almen stripes) or with destructive (borehole drilling)/laboratory (x-ray) methods. One of the difficulties in surface layer stress estimation by the acoustoelastic effect is that the surface wave velocity is not only influenced by the stress via the acoustoelastic effect but also directly by microstructure changes due to plastic deformation (cold work). The goal of the paper is the investigation of an independent way to determine the nonlinearity of the plastically deformed surface layer at the samples itself, as a prerequisite to correctly identify the stress contribution to the velocity change. Fortunately, beside the acoustoelastic effect nonlinear wave interactions like harmonics generation and wave mixing offer a way to assess the nonlinear material properties. The concept of the present work is to use these interactions to get the third-order elastic constants (TOEC), which are linearly related to the sought acoustoelastic constants. Surprisingly, there are very few publications about the nonlinear interaction at surfaces of isotropic materials. Therefore, as a first step, we have to solve the forward problem of predicting the efficiency of harmonic generation and wave mixing for known TOEC. This is followed by the design of an appropriate experiment and the development of inversion strategies for extracting the TOEC from the measurement data. In this work the current progress on the method development is presented. The possible interactions of surface guided waves and surface skimming bulk waves are described and formulae for the efficiency of harmonic generation and mode mixing are derived. First results for surface and bulk wave mixing with known second- and third-order elastic constants are shown. The concept of the inversion procedure for TOEC determination from the measurement data is introduced.


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