PSI - Issue 5

G. Lesiuk et al. / Procedia Structural Integrity 5 (2017) 928–934

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Lesiuk et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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3. Experimental results In order to investigate the potential influence of the electro pulsing on the fatigue lifetimes of specimen, the fatigue test with constant amplitude loading were performed. Before the proper fatigue test, the S-N curve was obtained for notched specimens without electro pulsing. The measurement stand is presented in Fig. 5. The strength tests were performed at the MTS 810 hydraulic pulsator. The spectrum of load was sinusoidal in its character with the constant stress ratio R =0.1 with loading frequency f =25Hz. During the test force, displacement and strain signals were recorded with number of cycles to failure. The obtained results are shown in Fig. 6. After determining the S-N curve (Fig. 6), one level was arbitrary selected for which the force amounted to F max =1100N and at that level another two specimens were studied with a break in the testing when the specimens were subjected to the exposure with current and the tests were continued until breaking. In total, for the focussed region of 1100N, 4 specimens were tested. The specimens were divided in to two groups with numbers 1 and 2. Two group of specimens were subjected to exposure the current under different initial number of loading cycles (1 – after 50 000 cycles, 2 – after 98 000 cycles). This strategy was associated with the potential macro crack formation. After 50 000 cycles there was no macro cracks observed on the contrary to the second group (after 98 000 cycles) where the average crack length a 0 =120  m was detected. An exemplary SEM image of this crack is shown in Fig. 7. After this, the specimens were subjected to current exposure and the fatigue test was continued until breaking.

Fig. 5. Measurement stand: 1 – MTS load cell, 2 – extensometer, 3 – specimen, 4 – hydraulic gripping system, 5 – displacement actuator.