Abstract
Lock-In-Thermography is a common technique for determining local stresses in cyclic-loaded components. This so-called Thermographic Stress Analysis (TSA) uses the cyclic change of the temperature, which results from the elastic stress by the thermoelastic effect. To reduce the noise in the measured temperature signal, the evaluation is performed using a Discrete Fourier Transformation. Brémond [1] and Sakagami [2] have shown that in case of plastic deformation a second mode, coupled with the double loading frequency appears in the DFT-evaluation. This part is known as the so-called D-Mode and is assigned to dissipative energies. Investigations of Bär et al. [3] have shown that there are discrepancies in the elastic and dissipative temperature changes between the Lock-In evaluation and an analytical evaluation, showing that the Lock-In evaluation is just qualitatively but not quantitatively correct. In this study a detailed study of the temperature changes in the vicinity of a crack in AISI 316L was undertaken. A determination of temperature-force hysteresis was carried out on a precracked specimen. The comparison of temperature-force hysteresis based on a frequency synthesis of the different modes of the Lock-In evaluation with an analytical evaluation showed only an approximate match. The compensation of the thermoelastic effect with force proportional local stress changes uncovers the dissipative temperature behavior at the crack. A simple isolation of the stress change or rather the dissipative temperature changes out of the E Mode and D-Mode is not generally possible. Crack opening and compressive loads cause thermoelastic temperature changes, which affect the E-Mode as well as the D-Mode and higher harmonic Dk-Modes.
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