PSI - Issue 24

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Lorenzo Bergonzi et al. / Procedia Structural Integrity 24 (2019) 213–224 Lorenzo Bergonzi et al. / Structural Integrity Procedia 00 (2019) 000–000

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2. Materials and Methods To perform high performance metal alloys tensile testing on MaCh3D, given the maximum 5kN exercisable force, conventional MaCh3D specimen section Bergonzi et al. (2018) must be reduced in order to achieve higher stress levels: the process of proportional section reduction is therefore described in subsequent paragraphs, starting from the analysis of current standards adopted for AM tensile testing. 2.1. Current standards analysis Main reference standards in performing tensile testing of AM fabricated metal alloys are the same used for mechanical characterization of materials produced using “standard” methods, in particular ASTM E8 / E8M-16a (2016) and EN ISO 6892-1:2016 (2016). Compatibility with AM produced materials has been assessed by Slotwinski and Moylan (2014) and reported in NISTIR 8005 report. The outcome for both standards is that they can be used with guidance, giving the fact that not all kind of geometries reported are producible through AM technology (e.g. wire and sheet). Both standards are reported in many works regarding AM material characterization and round specimens in particular are recommended from machine builders Frey, Shellabear, and Thorsson (2009). The exigence of a dedicated AM standard, regarding not only specimen shape but also preparation, powder metallurgy, machine settings to produce samples etc. is under development thanks to a collaboration between ASTM International Committee F42 on Additive Manufacturing and ISO Technical Committee 261 on Additive Manufacturing. Referring to the present work, ASTM E8 specimens have been used as comparison in determining stress status in reduced section of MaCh3D mini specimen. From ISO 6892 were deduced the criteria to proportionally scale specimen cross section as well as tensile strain rate for experimental testing. 2.2. Proportional section scaling and geometrical constraints As reported in the work of Kumar, Pooleery, and Madhusoodanan (2014) it is generally recognized that in order to compare elongation measurements of different-sized specimens, they must be geometrically similar. The equation shows that for measurement of ductility properties of different-sized specimen the ratio ) = % , % ⁄ for sheet specimen and 0 = % % ⁄ for round specimen must be maintained. In Table 1 ratio for different standards are reported. Table 1. Dimensional relationships of tensile specimens according to different standards. Type of specimen ASTM A370-14 ASTM E8M-16 ISO 6892-1:2016 Round (L 0 /D 0 ) 4 5 5 Sheet (L 0 / √ S 0 ) 2.8-14.1 3.2-14.1 5.65 In this work, miniaturized sheet specimens are developed maintaining a constant value of ) = 5.65 as specified by ISO 6892. Other geometrical constraints are described in the work of (Jung et al. 1996) as well as in ISO 6892 and also refers to the minimum number of grains that must be present in specimen thickness. Principal recommendations are reported in following equations: ⁄ > 3 (1) ⁄ > 0.5 (2) 0 ⁄ ≥ 1.25 (3) ? > % + 1.5, % (4)