PSI - Issue 37

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

339

4

3. Results 3.1. Edge crack

Figure 3a shows the crack surface of a specimen with a secondary notch at the edge on the front side. The colored lines represent the crack fronts marked with overloads in an interval of 10,000 cycles. In the first 20,000 cycles the crack propagates only on the frontside of the specimen and along the notch root. This anisotropy in the crack front reflects directly in the run of the relative potentials shown in figure 3b. The relative Potential P front shows the highest values whereas due to the shorter length of the crack on the backside P back exhibits the smallest values. This differences in the relative potentials lead to a clear separation of the potential quotients Q front and Q back (figure 3c). Especially up to 25,000 cycles, where no crack propagation on the backside is visible, the curves differ further and further apart. Between 25,000 and 30,000 cycles a fast crack growth on the backside takes place and the difference between the two curves is decreasing.

2.2

1,05

P front P back P narrow

Q front Q back

2.0

1,04

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

1.0

0,98

0

10.000

20.000

30.000

40.000

0

10,000

20,000

30,000

40,000

cycle number

cycle number

(a)

(b)

(c)

Fig. 3. (a) crack surface of a specimen with a secondary notch at the edge, 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.

The crack initiation can hardly be detected in the run of the relative potentials. Due to the insensitivity of the DCPDM short cracks lead only to a small increase of the potentials (figure 3b) but the small differences in the relative potentials lead to significant changes in the potential quotients (figure 3c) and therefore the crack initiation lifetime can easily detected by the moment when the two curves are beginning to drift apart. 3.2. Center crack In case of crack initiation in the center of the notch a different a nearly perfect symmetrical crack propagation is achieved. Again, the crack fronts marked by overloads introduced in intervals of 10,000 cycles are marked by colored lines. The crack is emanating from the laser notch in the center of the notch root and propagates nearly perfect symmetrical through the specimen. The corresponding run of the relative potentials P front and P back are congruent while the curve of P narrow is slightly below the other two (figure 4b) with a difference increasing with the crack length. The run of the two quotients (figure 4c) is also congruent, indicating that the crack front is nearly perfect symmetrical. The crack surface shows that the crack was initiated within the first 10,000 cycles. In this period the relative potentials show an increase of about 4 % whereas both potential quotients remain nearly constant until the first overload. Due to the symmetrical crack front and the resulting congruent run of the two potential quotients Q front and Q back no conclusions about crack formation can be drawn from this graph.