Fig. 1. (a) A comparison of S-N curves of all kinds of Cu polycrystalline foils/wires with the specimen thickness/diameter ranging from 20 μm to 4 mm reported in literatures [26], [27], [28] under the total strain control, (b) LCF (total strain amplitude corresponding to the 104 cycles) and HCF (total strain amplitude corresponding to the 106 cycles) strengths of Cu polycrystalline as a function of the specimen thickness under the total strain control [26], [27], [28], [29], [30], [31], [32], (c) LCF and HCF strengths of Cu polycrystalline with specimen thickness lower than 190 μm as a function of the t/d ratio under the total strain control [27,28,31].

Fig. 2. S-N curves of the (a) CA6NM martensite stainless steel with specimen thickness ranging from 40 μm to 2.5 mm and (b) Ti alloy with the specimen thickness ranging from 100 μm to 2 mm [33,34].

Fig. 3. (a) Normalized HCF strength and (b) normalized LCF strength as a function of the specimen thickness, the t/d ratio is presented by the size of bubble [1,26,[28], [29], [30], [31], [32],35,36], (c) LCF and HCF strengths as a function of the t/d ratio, the black dotted line indicates a critical t/d ratio of 3 as a boundary between the mechanism for crack initiation and crack propagation [33,34].

Fig. 4. Schematic illustration of the effect of specimen thickness (t) on the fatigue properties of conventionally-fabricated metallic materials. The fatigue properties are divided into three regimes. Regime Ⅰ: the grain size (d) dominant, Regime Ⅱ: t/d ratio dominant (the critical t/d ratio of 3 separates the mechanism for crack initiation and propagation) and Regime Ⅲ: the specimen thickness (t) dominant regardless of the t/d ratio.

Fig. 5. Schematic illustration of the flowchart to determine the specimen thickness effect on fatigue properties of AM parts.

Fig. 6. A comparison of (a) yield strength and (b) ultimate tensile strength of as-built specimens with the build thickness of 0.5-2.5 mm and ground specimens with the thickness of 0.5 mm.

Fig. 7. Roadmap for a data-driven qualification platform to evaluate fatigue performance of AM parts using miniature specimens.