PSI - Issue 17

Meike Funk et al. / Procedia Structural Integrity 17 (2019) 183–189 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The initial potentials of the two potential probes U i,0 differ in the contact resistance of the welding points. For a comparison of the measured values U i the potentials have to be normalized to the potentials of the undamaged specimen U i,0 : resulting in a relative potential P i : = ,0 ሺʹሻ To display the differences in the crack fronts the differences in the relative potentials according to equation 3 were calculated and displayed in figure 4b. ∆ = 1 − 2 (3) In figure 4a the positions of the potential probes, a straight (parallel to the notch root) and an inclined crack front are sketched up. Figure 4b presents the run of the differences in the relative potentials, as a function of the mean crack length. For the straight crack front of the specimen, shown in figure 3a, only a small difference D P is found, with only a small increase with the crack length. In case of the inclined crack front the difference D P is found to be rising with the crack length, reflecting the increasing skew, which can be seen in Figure 3b. The resulting values D P are considerably higher than those of a straight crack front.

(b)

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specimen 1, straight crack front specimen 2, inclined crack front

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D P= P 1 - P 2

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a mean /w

Fig. 4. (a) Sketch of a broken specimen with marker-loads, positions of the two DCPDs and the secondary notch. (b) Difference of the two normalized potential curves over the mean crack length

2.2. Definition of the location of crack initiation

The crack initiation, as well as the propagation of short cracks are reflected also in the values measured with the two potential probes. When a crack on one side of the specimen is initiated, the corresponding potential probe shows higher values than the other potential probe. This correlation can be represented by the relation of the two potential drops P 1 / P 2 . Is the ratio P 1 / P 2 is greater than 1, the left potential probe measures higher values than the right one. Further on, a positive slope indicates a faster crack propagation on the left hand side compared to the right hand side. In the reverse case, the relation P 1 / P 2 is decreasing. The correlation between the crack propagation, visible on the fracture surface, and the potential drop relation P 1 / P 2 is illustrated exemplarily for one specimen. The fracture surface is shown in Fig. 5a with the notch on the bottom and the crack propagation direction upward. During cyclic loading with 60 MPa, overloads were introduced each 50,000