PSI - Issue 68

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A. Jiménez et al. / Procedia Structural Integrity 68 (2025) 603–609 Adriano Jiménez et al. / Structural Integrity Procedia 00 (2025) 000–000

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Each of these batches consists of 9 different types of test specimens: three for compression tests to characterize the three orthogonal directions to the printing path (XYZ), and three for three-point bending tests (Fig. 1) evaluating the three existing load planes (XY, XZ, YZ). To assess whether the material behaves in an orthotropically symmetric manner, the latter will also be evaluated by rotating the specimens 90° around their longitudinal axis, resulting in a total of six types of bending specimens and three for compression (Fig. 2). Each of these specimens has been produced at least three times for each of the previously defined batches, reaching a total of 104 specimens manufactured. After the printing of the pieces, they were left to dry at room temperature for at least one week. Subsequently, they were placed in an oven (J.P. Selecta, Barcelona, Spain) at 65ºC for 48 hours and at 105ºC for 2 hours. The sintering of the pieces was carried out in a kiln (Nabertherm Top45, Nabertherm, Lilienthal, Germany) following the firing curve shown in the figure, which consists of three stages: heating at 4ºC/min until reaching 400ºC; 2ºC/min until reaching 750ºC; 1ºC/min until reaching 800ºC, and finally a holding stage of 3 hours at 800ºC. 2.3. Mechanical tests & calculations As mentioned earlier, the mechanical tests performed were three-point bending and unconfined compression. Both tests were conducted using a multi-test machine (MCO-30, Codein S.L., Madrid, Spain), with a maximum load capacity of 300 kN and configured at a constant speed of 1 mm/min. Compression tests and the three-point bending tests were conducted without strain gauges, which only allows for the determination of the compressive and flexural strengths. The three-point bending test procedure followed the European standard EN 843-1, although the dimensions of the specimens were modified from the standard to ensure that each specimen contained several parallel printing lines. The flexural strength value is obtained using the following formula: ! = 3 2 ℎ " Where: • σf = fracture strength, expressed in newtons per square millimeter (MPa) • F = peak force at fracture, expressed in newtons (N) • b = test piece width, expressed in millimeters (mm), calculated as the mean of three determination • h = test piece thickenss, expressed in millimeters (mm), calculated as the mean of three determinations • d = distance between the centers of the outer support rollers (for three-point flexure), expressed in millimeters (mm) The compression test was performed according to the EN 1926:2006 standard using the fragments resulting from the bending tests. The compressive strength was determined using the following formula: = Where: • A = cross-sectional area of the specimen before testing, expressed in square millimeters (mm²) • F = failure load, expressed in Newtons (N) • R = uniaxial compressive strength of the specimen, expressed in Megapascals (MPa) 2.4. Ultrasonic tests Ultrasound testing was selected to assess the material's homogeneity in three orthogonal directions relative to the printing path. The objective of this test was to link the ultrasound data with the mechanical strength and elastic constants obtained from bending and compression tests, marking an initial step in evaluating the quality of ceramic parts made through LDM using non-destructive methods.