PSI - Issue 28

Rhys Jones et al. / Procedia Structural Integrity 28 (2020) 364–369 Rhys Jones/ Structural Integrity Procedia 00 (2019) 000–000

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For crack lengths less than the wall thickness (1.25 mm) the crack has a three-dimensional shape. Consequently, in this regime the Bergman solution [17] for the stress intensity factor ( K ) for a surface rack in a hollow cylinder was used. Once the crack had transitioned to a through-thickness crack the ASM solution [18] for the stress intensity factor solution for a through-the-thickness circumferential crack in a hollow cylinder was used. In order to predict the crack growth history using the small crack equation for annealed AM Ti-6Al-4V, i.e. Equation (3), the value of A , the apparent cyclic fracture toughness, was needed. Fatemi et al. [15] reported that the fracture toughness of this material lays in the range 15-50 MPa √ m. As such, after comparing the crack growth curves given in [15] with that given by Boeing in [19] for crack growth in these LB-PBF Ti-6Al-4V specimens, it was decided to use the value of A = 36.6 MPa √ m given in [19]. The equation used to predict crack growth thus becomes: � � � � � ���� � �� �� � ���� ���� √���� ��� ������ � ���� (4) The resultant computed crack length versus cycles curve is also shown in Figure 2, where good agreement between the measured and computed crack length histories is observed.

10

Measured Computed

Transition to a through thickness solution

a (mm)

1

0.1

70000

75000

80000

85000

90000

Cycles

Fig. 2 Comparison between the measured and predicted crack growth histories for the AM Ti-6Al-4V specimen with a yield stress of approximately 950 MPa.

3. Computing the growth of small cracks in LB-PBF Ti-6Al-4V fabricated via M290 The next problem analysed involves the growth of a surface crack in a machined and annealed LB-PBF Ti-6Al-4V cylindrical specimen, fabricated using an M290 LB-PBF facility, that was subjected to a constant uniaxial fatigue load with a maximum stress of 910 MPa and an R ratio of -1 [15]. The geometry of this specimen was as shown in Figure 1. Unlike the previous specimen, the yield stress of the AM Ti-6Al-4V was approximately 1120 MPa (see Figure 4(a) in [15]). The crack growth data obtained in this test is shown in Figure 3, where the initial crack length is approximately 0.19 mm, and as such is again comparable to the minimum EIDS required in USAF Structures Bulletin EZ-19-01 [5]. This particular AM Ti-6Al-4V also had a significantly greater cyclic fracture toughness ( A ), and as a result the equation