PSI - Issue 47

R. Nobile et al. / Procedia Structural Integrity 47 (2023) 176–184 R. Nobile et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Before performing fatigue tests, a static test was carried out under displacement control with a velocity of 0.5 mm/min, to evaluate the breaking load (F max = 63 kN), taken as a reference for the subsequent tests (Nobile et al. (2022)). Fatigue tests were carried out on a servo-hydraulic axial/torsional testing machine INSTRON model 8850 with an axial maximum load capacity of ± 250 kN and connected by a computer equipped with Instron Wave Matrix software. Force control tests were performed in tension-tension load with a sinusoidal waveform, a stress ratio R = F min /F max = 0.1 and a load frequency of 9 Hz. Table 1 shows the experimental parameters used in the tests.

Table 1. Testing schedule parameters for the tested specimens.

Amplitude of load [kN]

Number of cycles to failure (Nf)

ID Specimen

Maximum load [kN]

Minimum load [kN]

A6 A3 A5

48 50 52

4.8

21.6 22.5 23.4

137464 126225

5

5.2

37336

2.2. Electrical resistance monitoring procedure The experimental setup for real-time electro-mechanical resistance measurements during fatigue tests basically includes a programmable DC power supply and a digital data acquisition module (Fig. 2a). Additionally, a type T thermocouple was applied to the specimen surface near the hole in order to monitor the temperature throughout the duration of the fatigue test. During the tests, it was verified that the temperature read by the thermocouple (Type T) was approximately the same of the average temperature detected around the hole, inside the measuring electrodes, using an infrared camera. To measure the electrical resistance, a direct current 0.1 mA was injected into the specimen through two external contacts realized with conductive copper tape, while the resistance is determined by measuring the voltage between two internal contacts in the typical four-wire setup. The specimen was also isolated from the testing machine by means of two insulating tabs (Fig. 2b).

(a) (b) Fig. 2. (a) Experimental setup for electrical resistance measurements; (b) Instrumented specimen with insulation tabs and electrical contacts (four-wire technique). Figure 3a-b shows the electrical measurement circuit adopted in this study. The internal contacts in copper conductive tape have been arranged at a distance of 53 mm, while the external contacts for the injection of current have been positioned at the ends of the specimen (Fig. 2b). Raw resistance data was recorded in real-time for the entire duration of the test with a time interval of 4 seconds. The acquired measurements were subsequently processed with adequate sampling intervals dependent on the expected fatigue life and averaged over a time interval of approximately 22s.