PSI - Issue 42

R. Fernandes et al. / Procedia Structural Integrity 42 (2022) 992–999 Fernandes et al./ Structural Integrity Pro edia 00 (2019) 000 – 000

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Fig. 2. Stress amplitude versus number of cycles for the different tested conditions: (a)  a = 1.25%; and (b)  a = 0.5%.

In this case, the saturated state is reached at an early stage representing less than 5% of the entire lifetime. Based on this fact, the mid-life cycle of each test was selected as representative of the stable cyclic stress-strain response. Figure 3 shows the stable hysteresis loops collected at the mid-life for the different material states studied here. As can be seen, the shapes of the hysteresis loops are quite different from case to case which agrees with the analysis conducted from the previous figure. The as-built condition, when compared to the other material states, exhibits higher linear portions in the tensile and compressive branches as well as maximum positive and negative stresses at the same strain amplitude. In the case of the T6 condition, the tensile and compressive tips have the lowest values. The cyclic stress-strain curve is an important tool to describe the stable material response. It can be written in the following form: ∆ 2 ε = ∆ 2 σ E + ( 2 ∆ K σ ' ) 1/n ' (1) where K ’ is the cyclic strain hardening coefficient, and n ’ is the cyclic strain hardening exponent. This curve, also known as the Ramberg-Osgood curve, is generally drawn by connecting the tensile and compressive tips of the stable stress-strain loops collected at different strain amplitudes. In this case, see Figure 3, the fitted functions for the various cases analysed are very close to the mid-life hysteresis loops, either for ascending or the descending branches. Although the differences are slightly higher for the compressive region, it can be concluded that the Ramberg-Osgood model can be used to describe the stable stress-strain response of the AlSi10Mg aluminium alloy manufactured by LPBF.