PSI - Issue 28

G. Meneghetti et al. / Procedia Structural Integrity 28 (2020) 1536–1550 G. Meneghetti et al./ Structural Integrity Procedia 00 (2019) 000–000

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Fig. 11. Comparison between calibration curves obtained from different current probes locations and potential probes located at Y PD = 0.5 mm and θ PD = 50°: a) calibration curves; b) sensitivity curves, i.e. derivative of the calibration curves.

2.3. Resistivity: temperature compensation The material electrical resistivity could change due to temperature variation of the environment, caused by plastic strain energy dissipation or heat dissipation from the fatigue testing machine. A normalization of the potential drop signal is required to compensate temperature effects. This normalization can be achieved by using the dual channel DPCD technique. Accordingly, the potential drop active channel ΔV PD , measured across the crack as seen in section 2.2 , is compared to a reference channel ΔV T , measured on the same specimen. As reported by Doremus et al. (2015), this technique can be adopted provided that the temperature is uniform within the specimen. Two configurations are available: (i) a four-probe dual channel technique (Fig. 12b) as suggested by (Van Minnebruggen et al., 2017), (ii) a three-probe dual channel system (Fig. 12c), in which the reference channel is measured between one probe of the active potential channel and a third probe located on the specimen.

a) Tested Specimen

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c)

Crack

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Fig. 12. Different DCPD configuration: a) single channel DCPD setup without in-situ temperature compensation; b) four-probe dual channel DCPD setup and c) three-probe dual channel configuration with in-situ temperature compensation.

From a theoretically point of view, the location of the third probe for temperature compensation can be anywhere on the specimen. For simplicity, in this work the third probe was aligned with the potential probes, i.e. at the same