PSI - Issue 39

Giovanni Pio Pucillo et al. / Procedia Structural Integrity 39 (2022) 700–710 Author name / Structural Integrity Procedia 00 (2019) 000–000

705

6

The da / dN versus ΔK curves have been obtained using the Incremental Polynomial Method (IPM), as recommended by the reference standard (ASTM E647−13 2014) , with which the computation of da/dN is carried out by approximation with a second degree polynomial. To calculate the Stress Intensity Factor range, ΔK , in correspondence of specific crack length values measured by crack gauges, two cases were considered. Indeed, for some specimens only one crack started on the hole edge, and this was true for almost all the duration of the test, as the diagram of Fig. 5-a shows as an example; in this case, the darkened part of the graph was excluded, and a single crack geometry was considered. In the other cases, instead, it was possible to recognize two cracks, as shown in Fig. 5-b as an example. For both cases, the analytical formulation proposed by Bowie (Bowie 1956) was assumed to calculate the SIF. Fig. 6 shows the results of the elaboration of the dataset concerning the same specimens of Fig. 5.

a vs. N - A04 - 400 MPa

CrackGauge_2 CrackGauge_1

CrackGauge_2 CrackGauge_1

a vs. N - A08 - 290 MPa

20

20

a)

b)

15

15

10

10

a [mm]

a [mm]

5

5

0

0

170000 175000 180000 185000 190000

31000 32000 33000 34000 35000

N [Cycles]

N [cycles]

Fig. 5. Crack gauge data concerning not-expanded specimens where one (a) or two (b) fatigue cracks were observed.

da/dN vs. Δ K - A08 – 290 MPa

da/dN vs. Δ K - A04 – 400 MPa

1 E-09 1 E-08 1 E-07 1 E-06 1 E-05 1 E-04 1 E-03

1 E-09 1 E-08 1 E-07 1 E-06 1 E-05 1 E-04 1 E-03

a)

b)

da/dN [m/cycle]

da/dN [m/cycle]

CrackGauge_2 CrackGauge_1

CrackGauge_2

10

100

1000

10

100

Δ K [MPa m^0.5]

Δ K [MPa m^0.5]

Fig. 6. Typical crack growth curves measured with one (a) or two (b) fatigue cracks.

3. Predictive model of fatigue crack growth The procedure and the results Starting from the data acquired with the non-expanded specimens, the parameters C and m of the propagation law proposed by Forman (Eq. 2) have been extracted with MATLAB for the specific loading