PSI - Issue 17

Jürgen Bär et al. / Procedia Structural Integrity 17 (2019) 308–315 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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it is impossible to determine quantitative results for the dissipative energies from the DFT-evaluation for the investigated materials. On the contrary, the differences in the D-mode evaluation also influence the results for the thermoelastic effect. The E-Mode is influenced by the problems of the D-mode evaluation in two different manners. On the one hand, there is a phase shift, on the other hand, it leads to incorrect values for the amplitude. The phase shift can be used to detect dissipative effects and therefore plastic deformation (De Finis et al (2016)). The differences in the amplitude are more critical. When Lock-In thermography is used to measure elastic stresses (TSA), correct values can only be achieved under pure elastic loading. When the loading is elastic-plastic due to the evaluation errors in the measured values have to be expected. For a correction the contributions of the dissipated energies have to be recognized. Perhaps this is possible when the phase shift is taken into account. This approach will be considered in more detail in further investigations. The differences in the run of T diss for the different stress ratios in copper and between the two materials can be attributed to differences in the plastic deformation. Copper shows a pronounce plasticity and the mobility of dislocations is not hindered by obstacles like precipitates or solute atoms. In case of AA7475 T761 the movement of dislocations is hindered by precipitates and consequently, the dissipation of energy due to dislocation movement is reduced. The latter takes also place in copper under pure tension loading. As reported by Luká š et al (1999) no persistant slip bands are formed during cyclic deformation of copper under positive mean stresses. Obviously the PSB formation and the slip of disloactions within the PSB is responsible for the peak of T diss in compression loading and the resulting additional dissipation of energy. The increase of the specimen temperature T m in case of copper under fully reversed loading conditions can be attributed to the strong increase of T diss under tensile loading especially in the first loading cycle. The increase of T diss in compression is fully compensated and in the 50 th cycle even overcompensated by cooling effects as can be seen in figure 5. At higher frequencies the time for cooling is reduced and the heating of the specimen is increased leading to a faster increase of T m during the experiments. To overcome the problems arising from the DFT evaluation a new evaluation method has to be developed or a correction of the D-mode values obtained from the DFT has to be undertaken. For a correction the real form of the wave signal has to be taken into account, but when this shape changes during cyclic loading a comprehensive solution of this issue seems to be difficult. 1. The DFT evaluation of thermographic measurements on Copper and AA 7475 T761 revealed some distinct differences compared to the true temperature changes caused by the thermoelastic effect (E-Mode) and dissipative energies (D-Mode). 2. The E-mode amplitudes are influenced by dissipative effects resulting in errors in thermographic stress measurements. 3. For correct TSA-measurements probably a correction based on the phase shift of the E-Mode seems to be possible. 4. The run of the temperature change caused by dissipative effects T diss provides interesting information about deformation processes in cyclic loaded metallic materials. 5. A quantitative determination of dissipated energies from the D-mode evaluation is not possible. 6. For a correct determination of dissipated energies in fatigue experiments using Lock-In thermography a new evaluation method has to be developed. 5. Conclusions

References

Bär, J.; Urbanek, R.; 2019. Determination of dissipated Energy in Fatigue Crack Propagation Experiments with Lock-In Thermography, Frattura ed Integrità Strutturale 13 , 563-570. DOI: 10.3221/IGF-ESIS.48.54. Brémond, P.; 2007. New developments in Thermo Elastic Stress Analysis by Infrared Thermography. IV Pan-American Conference for Non Destructive Testing. De Finis, R., Palumbo, D., Ancona, F., Galietti, U.; 2015. Fatigue limit evaluation of various martensitic stainless steels with new robust thermographic data analysis. International Journal of Fatigue 74 , 88–96. DOI: 10.1016/j.ijfatigue.2014.12.010.