PSI - Issue 68

Stefan Fladischer et al. / Procedia Structural Integrity 68 (2025) 486–492 S. Fladischer et al. / Structural Integrity Procedia 00 (2024) 000–000

488

3

3

10

SLM04

3.0

SLM05

2

10

SLM06

2.5

1

10

]

2.0

m

SLM04

0

SLM05

10

1.5

SLM06

K [MPa

[nm/cycle]

da

dN

1

10

1.0

2

10

0.5

3

10

0.0

1

0

2

10

10

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

a)

b)

m

[MPa

]

K

crack extension

[mm]

a

Fig. 2. Crack propagation test results of three SENB specimens SLM04, SLM05 and SLM06 at load ratio R = − 1 . 0: (a) da extracted cyclic R-curve data points and fit, fitted parameters are listed in Table 1.

dN over ∆ K records; (b)

Table 1. Cyclic crack resistance curve parametrizations as results of fitting equation (2) to the data sets presented in Fig. 2b.

specimen

∆ K th , ef f

∆ K th , lc

l 1

l 2

v 1

v 2

0 . 864MPa √ m 0 . 864MPa √ m

2 . 541MPa √ m 3 . 201MPa √ m

SLM04 SLM05

0 . 033mm 1 . 180 mm 0.176 0 . 024mm 0 . 594 mm 0.385

0.824 0.615

via the cyclic crack growth resistance curve, also called cyclic R-curve (Pippan and Hohenwarter, 2017).

l i    

ν i · exp  −

∆ K th ( ∆ a ) =∆ K th , ef f +  ∆ K th , lc − ∆ K th , ef f    1 − 2  i = 1

∆ a

(2)

The crack velocity factor F describes the influence of the load ratio for any crack extension ∆ a and incorporates the crack opening function according to Newman (1984). The coe ffi cient C and exponents m and p of equation (1), as well as the parameters describing the cyclic R-curve are material dependent and need to be determined from crack growth experiments. The investigated material is a selective laser melted AlSi10Mg-alloy, that was stress relief heat treated at 300 ◦ C for two hours. A tensile test shows good ductility at a yield strength of R p 0 . 2 = 163 . 7 MPa and a tensile strength of R m = 253 . 7 MPa at a uniform plastic strainof 7 . 27 %. For the determination of fracture mechanical parameters, four crack propagation tests on single edge notched bending specimens were performed, one test at a load ratio of R = 0 . 8 to estimate the intrinsic SIF threshold ∆ K th , ef f and three tests to gather data on the buildup of crack closure at R = − 1 . 0. These results are presented in Fig. 2. The cyclic R-curve parametrization SLM04 presented in Table 1 was used in the simulations. The described short crack growth simulation is applied in an extensive parameter study to investigate interaction e ff ect between co-planar circular cracks of radius 0 . 1mm ≤ a 0 ≤ 2 . 0 mm and distance between cracks a 0 / 4 ≤ d 0 ≤ 3 a 0 , as summarized in Fig. 3, subject to mode I loading. For each crack parametrization ( a 0 , d 0 ), three crack configurations are simulated: the reference case of a single crack, two interacting cracks, and the convex hull formed over the interacting cracks as simplified representation of the coalesced flaw. Short crack growth simulations are performed at varying load levels ∆ σ , following the cyclic R-curve analysis procedure (Tanaka and Akiniwa, 1988; Akiniwa et al., 1997), in order to determine the fatigue limit of maximum non-propagating crack formation ∆ σ NPC . This is implemented as a binary search on the range of load scale factors 2.2. R-curve analysis of interacting circular cracks