PSI - Issue 2_A

José A.F.O. Correia et al. / Procedia Structural Integrity 2 (2016) 3272–3279 Correia et al. / Structural Integrity Procedia 00 (2016) 000–000

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Figures 6 and 7 exhibit the fatigue crack propagation rates of the 6028-T6 aluminium alloy, for the two notch geometries tested with stress R -ratio, R=0.1. The scatter level, for the CT geometry with circular notch is higher than observed for the standard CT geometry. Nevertheless, both sets of results show a very good correlation. The threshold values of stress intensity factor range was estimated using the K -decreasing procedure proposed in ASTM E 647 standard. The results are presented in Table 3 and it is possible to observe that the values of Δ K th are higher for the CT geometry with initial circular notch when compared with the results of the standard CT geometry with V notch. Table 4 presents the Paris’s law constants determined through a linear regression analysis performed on the experimental data log( da/dN ) versus log( Δ K ). The experimental results of the crack growth tests, for the two CT geometries under study, exhibit similar values, resulting similar Paris’s law constants. Despite the two types of CT geometries resulted in similar fatigue crack growth rates in the fatigue crack propagation regime II, the fatigue crack propagation thresholds resulted significantly distinct between the two types of CT specimens investigated in this work. The circular notch CT specimen resulted in significantly higher crack propagation threshold.

Table 3. Threshold values of the stress intensity factor range from the tested CT specimens

Regime II of the Paris law

Notch Geometry

Δ K

Ref. Specimen

R=  min /  max

th

Δ K min

Δ K max

P-01 P-02 P-03 P-05 P-06 P-07

286.51 **

398.28 **

273.21 **

310.42 312.71 311.57 438.21 398.85 424.06 420.37

865.32 786.63 825.98 747.33 814.17 765.43 765.64

229.82 263.45 246.64 412.22 365.33 348.00 375.18

V notch

0.1

Average

Circular notch

0.1

Average

*  K in N.mm -1.5 ** excluded test of the analysis due to overload

Table 4. Constants of Paris Law obtained for the tested CT specimens.

Notch Geometry

da/dN = C.  K m

R 2

Ref. Specimen

R=  min /  max

C

m

P-01 P-02 P-03 P-05 P-06 P-07

5.9097E-18 ** 3.2642E-12 8.3275E-11 3.0519E-11 1.1256E-09 6.5816E-12 3.1208E-12 1.1551E-11

5.3067 **

0.8807 **

3.0192 2.4813 2.6477 2.8654 3.0026 2.7845 2.062

0.9830 0.9852 0.9744 0.8914 0.9643 0.9492 0.9327

V notch

0.1

P-02+P-03

Circular notch

0.1

P-05+P-06+P-07

* da/dN in mm/cycle and  K in N.mm -1.5 ** excluded test of the analysis due to overload