PSI - Issue 46

L. Lücker et al. / Procedia Structural Integrity 46 (2023) 94–98 Lukas Lücker / Structural Integrity Procedia 00 (2021) 000–000

96

3

dimensions for bending fatigue tests are 52 mm × 10 mm × 1 mm. The qualitative degree of damage A Void based on pore areas is given in Fig. 1(a). A pore area of A Void = 302 µm 2 was determined for components with 2α = 90° and A Void = 76 µm 2 for those with 2α = 30° over a representative area of A = 1.29 mm 2 .

2.2. Direct current potential drop (DCPD) measurements A newly developed experimental setup was used for electrical resistance measurements, allowing reproducible and sensitive data due to constant contact pressure and constant measuring point distance. The schematic in Fig. 2 shows the contact points of the electrical current injection (red) and the electrical voltage measurement (blue) on the specimen. The electric current was applied by a Keithley 2602B system source meter and the electric voltage was measured by a Keithley 2182A nanovoltmeter. An innovative conjunction of both systems enables the so-called “delta mode”. In delta mode, the current source applies a highly constant current for a defined time. Thereafter the signal is polarity reversed and then triggers the nanovoltmeter reading at each polarity. As a result, any thermoelectrical offsets, caused e.g. by contacting, can be avoided. Data acquisition and control of the systems were performed by a software specifically programmed for this experimental setup and measurement application. 2.3. Three-point bending fatigue testing For fatigue assessment three-point bending tests were performed on a Shimadzu MMT-500N electromagnetic microforce testing system, Fig. 3. Conventional constant amplitude tests were performed with a load ratio of R = 10, a test frequency of f = 2 Hz and a sinusoidal load-time function. The distance between the bearing points was l = 30 mm and the radius of the bending punch was r B = 4 mm. The tests were stopped reaching the limiting number of cycles N L = 10 6 or a bending angle of  = 25° as failure criteria.

Fig. 2. Measurement contacting points for current injection and voltage tap in high-resolution DCPD measurements.

Fig. 3. Experimental setup for three-point bending fatigue testing.