Issue 48

J. Bär et alii, Frattura ed Integrità Strutturale, 48 (2019) 563-570; DOI: 10.3221/IGF-ESIS.48.54 563 Focussed on “Crack Paths” Determination of dissipated Energy in Fatigue Crack Propagation Experiments with Lock-In Thermography Jürgen Bär, Ralf Urbanek University of the Bundeswehr München, Germany juergen.baer@unibw.de, http://orcid.org/ 0000-0003-1157-0052 Ralf.urbanek@unibw.de A BSTRACT . Lock-In thermography can be used to investigate elastic stresses and dissipative effects in dynamic processes like crack propagation. The evaluation normally is performed with a Discrete Fourier Transformation resulting in E- and D-Amplitude and phase images. The E-Amplitude images give information about the stress distribution, the D-Amplitude is connected with dissipated energies. The observed changes in the E-amplitude values can be attributed to a change in the stress state at the crack tip due to bending of the specimen caused by the propagating crack. In the crack propagation experiments the maximum value of the D-Mode in the area in front of the crack tip was found to be constant. The appearance of higher harmonic modes in the evaluation raises the question if the Discrete Fourier Transformation is the appropriate method for a quantitative evaluation of dissipative effects. Experiments performed on flat specimens show that the temperature change due to dissipative effects could not be described with a sine wave with the double loading frequency. Therefore, a quantitative determination of dissipated energies using the Discrete Fourier Transformation is impossible. For a quantitative determination of dissipated energies a new evaluation method has to be developed. K EYWORDS . Aluminum alloy; Crack propagation; Dissipated energy; Fatigue; Thermography. Citation: Bär, J., Urbanek, R., Determination of dissipated Energy in Fatigue Crack Propagation Experiments with Lock-In Thermography, Frattura ed Integrità Strutturale, 48 (2019) 563-570. Received: 26.11.2018 Accepted: 20.12.2018 Published: 01.04.2019 Copyright: © 2019 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION hermographic methods are widely used to determine the temperature increase during a fatigue experiment. Different methods were developed to gather information about the fatigue limit and the fatigue lifetime out of the measured temperature changes [1-4]. In case of a propagating crack, the local conditions are changing very fast, therefore a simple temperature measurement is not sufficient. In this case, the Lock-In thermography offers the possibility to gather information about elastic stress fields and dissipated energies. In case of the thermoelastic stimulated Lock-In Thermography, a cyclic change of the temperature is caused by an alternating load according to the thermoelastic effect [5]. T