PSI - Issue 34

Rhys Jones et al. / Procedia Structural Integrity 34 (2021) 39–44 Rhys Jones/ Structural Integrity Procedia 00 (2021) 000 – 000

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of this paper is to compare the da/dN versus ∆ K curves associated with AM Scalmalloy with those of two commonly used aerospace aluminium alloys, viz: AA7050-T7451, and AA7075-T6. USAF Structures Bulletin EZ-19-01 states that the most difficult challenge facing the adoption of AM parts is to obtain an “accurate prediction of structural performance specific to DADT” . Similarly, the US Army Directive 2019 29 requires the consideration of advanced manufacturing in all aspects of a system’s life cycle, i.e. both in ab initio design and in through-life sustainment. Unfortunately, the da/dN versus ∆ K curves associated with AM materials can be strongly dependent of the AM process, and can also be highly anisotropic, see Illiopolous et al (2020) and Jones et al (2021). However, as shown in the above two references, and also in Jones et al (2018), and Iliopoulos et al (2018), this variability vanishes when crack growth is expressed as per the Hartman-Schijve crack growth equation, viz: = [Δ ] (1) Here a is the crack length/depth, N is the number of cycles, D is a material constant, p is another material constant, that is often approximately 2, and the crack driving force  κ is as suggested by Schwalbe: Δ = [ ∆ − ∆ ℎ √{1− ℎ / } ] (2) Here, K is the stress intensity factor, K max and K min are the maximum and minimum values of stress intensity factor seen in a cycle, ∆ K = ( K max - K min ) is the range of the stress intensity factor that is seen in a cycle, ∆ K thr is the “effective fatigue threshold”, and A is the cyclic fracture toughness. As explained in Jones (2014) , the terms ∆ K thr and A are best interpreted as parameters that are chosen so as to fit the measured da/dN versus ∆ K data. The ability of the Hartman Schijve equation to represent the growth of both long and small cracks in AM materials is discussed in the recent review paper by Sanaei and Fatemi. 2. Crack growth in Scalmalloy Scalmalloy® is a second-generation aluminum-magnesium-scandium (Al-Mg-Sc) alloy that was developed by Airbus as a high-strength aluminum alloy for selective laser melting (SLM). Unfortunately, whilst there are a number of studies that present strain life curves for AM Scalmalloy the work of Schmidtke is, to the best of the author’s knowledge, currently the only study to present the da/dN versus Δ K curves that are needed for a damage tolerance analysis. This study investigated Scalmalloy specimens built using selective laser melt (SLM). The specimens were then heat treated at temperatures ranging from 300 ◦ C up to 325 ◦ C for a duration of 4-8 hours, and then hipped. Tests were then performed at R ratio’s of 0.1 and 0.7. The resultant da/dN versus Δ K curves are shown in Figure 1. Figure 1 also presents the da/dN versus Δ K curves given by Jones et al 2014 for AA7050-T7451 and by Forman et al for AA7075-T7351 for tests with R ratio’s of 0.1 and 0.7 . Figure 2 presents the da/dN versus Δκ curves for the se Scalmalloy tests along with the corresponding curves given in Jones et al 2014 for AA7050-T7351, and in Jones et al (2020) for AA75-T7351. The values of the terms Δ K thr and A used in Figure 2 for the SLM Scalmalloy tests are given in Table 1. Figures 1 and 2 reveal that crack growth in SLM Scalmalloy is very similar to that of crack growth in AA7075 T7351. We also see that crack growth in SLM Scalmalloy can be accurately represented by the Hartman-Schijve crack growth equation and that the R ratio effect on crack growth can be captured by allowing for changes in the threshold term Δ K thr .