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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the corresponding long crack equation by setting the threshold term ∆ K thr in Equation (2) to a small value, typically in the range 0.1-0.3 MPa √ m [14] (Here it should be noted that, as shown in [6], for AM parts that are not stress relieved it may be necessary to account for the residual stress field that arises due to the manufacturing process). In [9] this approach was illustrated by considering the small crack da/dN versus ∆ K curve associated with LENS Ti-6Al 4V. The governing equation for sub mm cracks in this material was shown [9] to take the form: � � � � � ���� � �� �� � ��� �� ��� √���� ��� ��� � ���� (3) Structures Bulletin EZ-19-01 states that for structural parts manufactured using AM the most difficult challenge in a durability and damage tolerance certification analysis is to establish an “accurate prediction of structural performance”. Consequently, the present paper focuses on the ability of Equation (3) to compute the crack growth history associated with LB-PBF Ti-6Al-4V alloy manufactured via different AM machines (Renishaw AM250 and EOS M290) with different associated processing parameters (different energy densities) and, therefore, different defect characteristics. Machine-to-machine process parameter variabilities can affect defect formation of the AM materials, even for the same AM method. This, in turn, can significantly affect the number of cycles to fatigue crack initiation (see Figure 9(a) in [15]). In this study, M290 fabricated specimens contained a very small number of defects with no defects larger than about 100 μm, while AM250 fabricated specimens contained a large number of small defects as well as considerable number of large defects up to 1 mm (see Figure 3 of [15]). In the present studies, as required in Structures Bulletin EZ-19-01 [5] for a durability analysis, the initial crack size is taken to be of the same order of magnitude as the minimum allowable EIDS in [5]. 2. Computing the growth of small cracks in LB-PBF Ti-6Al-4V fabricated via AM250 The first problem studied in this paper is the growth of a surface crack in a machined and annealed LB-PBF Ti-6Al 4V cylindrical specimen [16] that was fabricated using an AM250 LB-PBF machine and was subjected to a constant uniaxial cyclic load with a maximum stress of 268 MPa and an R ratio of -1, see Figure 1. The yield stress for the machined and annealed AM Ti-6Al-4V specimen studied was approximately 950 MPa, see Figure 5a and Table 3 in [16]. The crack growth history given in [16] is shown in Figure 2. The minimum crack size shown in Figure 2 is approximately 0.34 mm. This crack size is of a similar order to the minimum EIDS of 0.258 mm required in the USAF Structures Bulletin EZ-19-01 [5] for a durability (economic life) analysis.

Fig. 1 Schematic diagram of the geometry of the cylindrical test specimen, from [15].