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
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Fig. 2 Dimensions of flat specimens in accordance with ASTM E466-07
2.2. Experimental setup and test procedure Tests were carried out on servo-hydraulic testing machine INOVA with loading capacity of 250 kN at room temperature. High cycle fatigue tests were done in resonance at 17 Hz, in accordance with De Finis (2015), under fully reversed tension-compression loading R = -1 at different load steps. Each test begun from low loading amplitude, approximately 10 % of UTS, where UTS of base and degraded material were 720 MPa and 740 MPa, respectively, and continued at each step for 20 000 loading cycles until failure of the specimen. At the end of this amount of cycles, the temperature gradient was almost constant. For evaluation of fatigue limit using thermographic method, at least 8 different loading steps were performed. The IR camera FLIR 8400 was placed in front of a specimen and an aluminium extrusion plate, which reflected the ambient temperature changes during each test. Both objects were sprayed to increase their emissivity to 0.9. Before test, one thermal image was taken by the camera followed by one image every 10 seconds during the test. Temperature of the specimen was measured and recorded as the average temperature at area "A1", see Fig. 3, instead of using a temperature profile along the longitudinal axis of the specimen that differed strongly across the specimen width because of the weld joint. where the temperature distribution is strongly different. The maximum temperature was obtained at the end of each step from average temperature values. This temperature was then subtracted from the ambient temperature, measured at area "A2", see Fig. 3, giving a calculated temperature increase " ΔT average.corr ." that was subsequently used to plot the final graph for fatigue limit estimation. Fatigue limit evaluation was done based on Luong’s method using graphical procedure mentioned above. Graphical estimation of fatigue limit point is given as the intersection of two regression lines that represents two separate trends in ΔT -stress amplitude diagram.
Fig. 3 Specimen temperature area "A1" and reference temperature area "A2" on aluminium plate at the beginning of the test
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