PSI - Issue 37

Jürgen Bär et al. / Procedia Structural Integrity 37 (2022) 336–343 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

341

6

1,05

P front P back P narrow

2.2

Q front Q back

1,04

2.0

1,00 Potential quotient Q i 1,01 1,02 1,03

1.4 relative Potential P i 1.6 1.8

1.2

0,99

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 1.0

0 10.000 20.000 30.000 40.000 50.000 60.000 70.000 0,98

cycle number

cycle number

(a)

(b)

(c)

Fig. 6. (a) crack surface of a specimen with a secondary notch at a quarter of the thickness, the crack front is clearly asymmetrical; (b) Run of the three relative potentials, P front , P back and P narrow are clearly separated due to the asymmetric crack-front; (c) the Quotients Q front and Q back are clearly separated. 4. Discussion The investigations have shown that the multiple potential drop measurement provides detailed information about the crack initiation and crack front in fatigue tests on single edge notched specimens. The applicability of this method to detect crack initiation and to generate information about the shape of the crack front will be discussed in more detail below. 4.1. Crack initiation As already mentioned above, the sensitivity of the DC-potential drop method is increasing with the crack length and, therefore, this method is excellent for investigating the propagation of long cracks, but only suitable to a limited extent for investigating crack initiation and short crack propagation. In addition, the potential is also influenced by temperature fluctuations, making it difficult, if not impossible, to detect the formation of cracks.

1.01

1.06

P front P back P narrow

Q front Q back

1.05

1.01 relative Potential P i /P i,0 1.02 1.03 1.04

1.00 Potential quotient Q i

1.00

0.99

0.99

0

5,000

10,000

15,000

0

5,000

10,000

15,000

cycle number

cycle number

(a)

(b)

Fig. 7. (a) Relative potentials in the beginning of the fatigue test (detail from figure 6b); (b) Potential quotients in the beginning of the fatigue test (detail from figure 6c).

Figure 7a shows an enlarged section of the run of the potentials of the specimen shown in figure 6. In this plot the crack initiation is difficult to determine. At the beginning all 3 potentials rise nearly simultaneously. This increase can either be caused by crack initiation and propagation, but also by an increasing sample temperature. By calculating the