PSI - Issue 48

America Califano et al. / Procedia Structural Integrity 48 (2023) 238–243 Califano et al / Structural Integrity Procedia 00 (2019) 000 – 000

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Data collected through the BFS and the two CGs have been compared and calibrated by means of the back-face strain compliance method proposed by Newman and Johnston (Newman et al., (2011)). In detail, they developed a relation for monitoring the crack-length-against-cycles valid for the standard C(T) specimens and for a wide range in crack-length-to-width ( a / W ) ratios. The so-called Newman-Johnston equation is the following: = 0 + 1 + 2 2 + 3 3 + 4 4 + 5 5 (1) where: = (√ 1 +1) (2) = | ( ) | (3) being E is the Young ’s modulus, ε is the strain measured by the BFS, W is the width, B is the thickness and P max the maximum load applied during the test. The coefficients for 0.2 < a/W < 0.8 are: A 0 = 1.0343, A 1 = -2.8098, A 2 = 4.1335, A 3 = -23.694, A 4 = 76.371, A 5 = - 3.25. The Eqs. (1)-(3) can be calibrated using the data coming from the BFS and the data coming from the CGs, in order to find the material’s Young’s modulus E . 3. Results and discussion As aforementioned, specimen n. 10 was used to carry out a metallographic analysis of the specimen surfaces. The macrographs for the xy , yz and xz (defined through the coordinate system in Fig. 1) planes are reported in Figs. 3a, 3b and 3c, respectively, with a scale of 50 microns.

Fig. 3. Macrographs of the printed full specimen (n. 10 in Fig. 1) in the (a) XY plane, (b) YZ plane, (c) XZ plane.

It can be noticed that the macrographs in the yz and xz planes are more heterogeneous if compared to the macrograph of the xy plane. As a matter of fact, being z the building direction (Fig. 1), long and slender grains are obtained along z-axis during the manufacturing process and, therefore, they are clearly visible in yz and xz planes. The selected C(T) specimens were subjected to fatigue tests, which ended as soon as each specimen reached the total failure. As an example, a picture of a fractured specimen is reported in Fig. 4.