PSI - Issue 7

R. Prochazka et al. / Procedia Structural Integrity 7 (2017) 315–320 R. Prochazka, J. Dzugan, P. Konopik/ Structural Integrity Procedia 00 (2017) 000–000

319

5

100 105

7.164

11 12

10.088

20 °C

70 °C

Fig. 5Thermal image of a specimen during a test. Left - at the beginning, center - at fatigue limit and right – at the end of the test

Fatigue limit evaluation of both materials based on graphic method is done in Fig. 6. Even though, the results of UTS show same results for both materials, the endurance limit differs a lot. This value for base material is more than four times higher in comparison with degraded material. Apparently, in the case of welded joints, techniques using thermal dissipation should start at loading levels lower than 10 % of UTS, and increments per step should remain relatively small due to the very low fatigue limits of such degraded materials. Thanks to high sensitivity of infrared instruments, the recognizable temperature change is at about 0.1 K; the detection of thermal radiation is, therefore, very precise and suitable especially for any application concerning whole components or test specimens of small geometries. Also, it allows the measurement of temperature change below yield point, where the increase in temperature is difficult to distinguish, and, consequently, improves consistency of the thermal data.

Fig. 6 (a) Graphical estimation of fatigue limit of base material and (b) weld joint

4. Conclusions The aim of this work was to estimate the fatigue behavior of duplex steel as base material and thermally degraded material by standard high cycle technique and very progressive thermography one. This kind of steel is characterized by its high toughness and rapid temperature growth at relatively small loading amplitudes and test frequencies. These limitations ultimately prevented testing at higher test frequencies than 30 Hz, prolonging the time required to obtain the classical S-N curve three times. In such case, the use of rapid thermographic method decreases the material and testing time demands to the minimum. In other words, the potential of this method is to reduce the time consumption of the tests from 1 month to 5 hours which plays huge role in field of material research.