PSI - Issue 77

Karel Trojan et al. / Procedia Structural Integrity 77 (2026) 537–542 Karel Trojan / Structural Integrity Procedia 00 (2026) 000–000

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The samples were analysed in both the axial A and perpendicular T directions, see Fig. 1. The irradiated volume was defined by experimental geometry, the effective penetration depth of the X-ray radiation (approx. 4–5 µm), and the pinhole size 1 × 1 mm for A direction and 2 × 0,5 mm for T direction. 2.2. High cycle fatigue testing A high cycle fatigue experiment was conducted using the INOVA FU-O-250-1620-V1 test rig, operating at a frequency of 15 Hz with a stress ratio R = –1. The applied stress amplitude ranged from = 360 to 800 MPa, and the test was terminated at a runout of 2 million cycles. The resulting S-N data were fitted using a Wöhler curve (1), from which key fatigue parameters were extracted, including the fatigue strength coefficient and the fatigue exponent, characterizing the material's resistance to cyclic loading. = ′ (1) 3. Results and discussion From the macroscopic RS results, see Fig. 2, it is clear that the depth profile varies significantly between individual samples, even though the RS values on the sample surface are identical within the margin of error for both samples in the A direction. With increasing distance from the surface, the RS of the sample with standard preheating increases significantly, while for the sample with higher preheating, it decreases. At a depth of 130 µm, the difference reaches 500 MPa, which is half the typical yield strength declared by the 3D printer manufacturer and powder supplier. In the T direction on the surface, the sample with a preheating temperature of 40°C (SP) shows more favourable compressive RS, and the sample with a preheating temperature of 120°C (SP120) exhibits tensile stresses. However, within the monitored depth, the situation is reversed. Based on these results, it can be stated that a higher platform temperature during printing reduces the high tensile RS in the surface layers and that, in order to correctly describe the residual stress state, we cannot limit ourselves to surface analysis using X-ray diffraction.

Fig. 2. Depth profile of macroscopic RS in the direction of printing for samples with different building platform preheating.

Fig. 3 shows the depth profile of the FWHM parameter, which is the width at half of the observed diffraction maximum. From the increasing value of FWHM, it could be said that in the analysed volume the crystallites (coherent diffracting domains) are smaller and/or the microstrain (deformation of individual crystallites) is higher. The comparison clearly shows that the size of the crystallites in the surface layers of sample SP120 is smaller and/or the microstrain is higher. Conversely, for sample SP, we observe a decrease in the FWHM parameter, which indicates that the crystallites are larger and/or the microstrain is lower. As in the previous case, if we limited ourselves to analysing only the surface, we would not be able to describe such a significant difference.