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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cycles, being marked in different colors in Fig. 5a and 5b. Additionally, the initial cracks in the notch root are marked in red in Fig. 5a. Besides the two initial cracks in the edges, there are two additionally cracks in between. Within the first 50,000 cycles the first crack is emanating from the secondary notch on the left side. Prior to the second overload, two middle-cracks were initiated in the notch root. Between 100,000 and 150,000 cycles a crack on the right edge has been initiated and all four initial cracks have coagulated to a through-thickness crack with a length of more than 1mm. The crack is propagating through the material and the specimen fails with the next overload at cycle number 200,000. This crack history, extracted out of the fracture surface, can be also identified in the run of P 1 / P 2 over the cycle number. In the beginning of the experiment with no crack, the relation P 1 / P 2 is equal to 1. After 40,000 cycles the relation is rising due to a crack propagating from the secondary notch on the left edge. After the overload at 50,000 cycles, the relation P 1 / P 2 keeps on rising with a changing slope after about 60,000 cycles. It might be, that the two middle cracks were initiated with the overload and started to grow a bit later at cycle number 60,000. Right with the overload at cycle number 100,000 the relation P 1 / P 2 is switching to a negative slope until cycle number 140,000. Obviously the crack in the right edge was initiated and starts to grow. The propagation of this crack seems to get dominant leading to the observed decrease in the relation of P 1 / P 2 . The coagulation is suspected at cycle number 140,000, due to the equal potential drop measurement of both DCPDs and the resulting ratio P 1 / P 2 = 1. After 150,000 cycles the relation P 1 / P 2 keeps obtaining a negative slope. This can be correlated to an inclined crack front with a larger crack length on the right hand side. After cycle number 180,000 the relation P 1 / P 2 is suddenly rising strongly. This phenomenon is characteristically for most of the specimens. With large crack lengths the sensitivity of the DCPD is rising and the relation P i / P j can shift easily to an extreme positive or negative slope without any correlation to the fracture surface.

1,010

1,005

P 1 /P 2

1,000

0,995

0

50.000

100.000

150.000

200.000

cycles

Fig. 5. (a) Fracture surface of the broken specimen. The crack propagation direction is from the bottom to the top. The marker-loads are painted in different colors, the positions of the initiated cracks in the notch root are marked red. (b) A rising curve symbolize a higher value of the left DCPD and vice versa. The cycle numbers of the marker-loads are painted in the same colors than in (a).