PSI - Issue 24

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

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

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as hydraulic bulge testing Mahabunphachai and Koç (2008), bending Raulea et al. 2001; Kals and Eckstein (2000),punching Kals and Eckstein (2000); Husain, Sehgal, and Pandey (2004). Even for the more common tensile testing, different testing machines were developed for testing of samples with small volume LaVan and Sharpe (1999); Sharpe (2003); Isono, Namazu, and Terayama (2006); Li, Yang, and Wei (2003); Hamid (2016). All these variations in respect of standards indications, arise concerns as to whether tensile behavior measured using such non-standard specimens and techniques are comparable to ASTM standard tests and representative of material properties. The miniaturization of specimen causes the so called “scaling effect”, which implies different material behavior in the microscale compared to the mesoscale and macroscale Klueh (1985). From mechanics point of view, this effect is limited to the strength dependence on cross-sectional area, whilst on a wider perspective may relate not only to specimen size and geometry, but also to other factors, such as micro structural constraints (i.e. grain size and their number in the cross-section), anisotropy due to microstructure and crystallographic texture, micro-structural and chemical inhomogeneity, surface effects and residual stress. 1.2. Use of miniaturized specimen to assess AM mechanical properties Despite some issues related to the usage of miniaturized samples described in the preceding paragraph, their adoption is becoming more popular to characterize additive manufactured materials, especially for metal-based technologies. This is mainly due to the high costs, low productivity of AM systems, together with the high number of samples needed to fully characterize a batch of material. High costs of raw material alone justify the effort of miniaturizing test coupons; moreover, for a single batch of production, to fully characterize material property and assess any eventual directional mechanical properties (e.g. anisotropy), specimens must be produced in different directions respect to build plate, thus increasing the number of needed specimens. The issue of metal consumption reduction is even more important in fatigue characterization, where higher number of specimens in comparison to quasi-static testing must be used. One example of specimen design that allows to dramatically minimize the volume of material needed for fatigue assessment is the one developed by Nicoletto (2017). Referring to quasi static testing, and in particular uniaxial tensile testing, different works study the development of miniaturized specimens in order to assess material mechanical properties. The vast majority of the works try to compare mechanical properties obtained with full size specimens with those determined using a miniaturized geometry. Amongst others, the work of van Zyl et al. (2016), where direct metal laser sintering (DMLS), as-built and stress-relieved miniaturized tensile Ti-6Al-4V (ELI) specimens with different surface qualities were investigated together with full-size specimens. In the case of Karnati et al. (2016), tensile properties of bulk and Selective Laser Melting (SLM) fabricated 304 stainless steel samples, having different gauge length and overall dimensions, were investigated. One of the main targets was to establish tensile testing methodologies and their relation to differing gauge lengths. Another work of Karnati et al. (2017) focuses on the influence of gauge length on miniature tensile characterization of DMLS fabricated 304L stainless steel. In this case, miniature tensile specimens with varying aspect ratios were fabricated and the tensile characteristics measured from these specimens were analyzed to assess the impact of gauge length. The study found no impact upon varying gauge length on yield and ultimate strength measurements, whilst a significant impact was observed on strain measurements. Results confirms the conclusions of other works, as the one of Sergueeva et al. (2009), since analysis of variance reported a significant impact of gauge length on the maximum strain measurements. By increasing its value, maximum strain decreases: this is probably due to the decreasing contribution of localized deformation to maximum strain values. Due to the capacity to catch eventual material property variation using a minimum amount of material, mini specimens were used by Zhou et al. (2018) in a comparative study to evaluate mechanical properties of SLM Ti-6Al-4V fabricated using SLM in normal condition (inert gas filled chamber at slightly greater pressure than ambient) and under vacuum in order to reduce material porosity. The work of Dongare (2012), in the end, showed again good compatibility between obtained mechanical properties from mini specimens developed starting from ASTM E8 / E8M-16a (2016) and the gaining in terms of time and un-wasted material. The specimens proved again to be capable of confirming positional variation in strength values in a laser deposited thin wall.