PSI - Issue 24

Giulia Morettini et al. / Procedia Structural Integrity 24 (2019) 349–359 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 6. Distribution of tensile stress in Y direction for Horizontal specimen.

Going to investigate in more detail the state of stress that is generated due to this deformation, in the Horizontal specimen there is certainly a tensile stress in y direction, that for its particular distribution (illustated in Figure 6) generate a bending distortion of the specime that consequently produces a mean stress that could help the crack to form and propagate. Figure 6 illustrates a qualitative analysis of this state of stress.

4. Discussion

From the experimental results of fatigue life investigation, it is possible to confirm that the fatigue life is influenced by the building direction with which the specimens are fabricated, in particular the Horizontal specimens have a higher mean fatigue limit (121.18 MPa) as compared to the value of the Vertical specimens (91.1 MPa). The Wöhler curves of these two case studies however remain with the same slope, symptom of the fact that the factors that influence this behavior do not depend on the applied load nor on the number of Cycles. Both of the obtained fatigue limits are lower than the fatigue limit of specimens conventionally fabricated, in particular, the Horizontal specimen, that illustrate higher fatigue resistant, present a fatigue limit that is about 60 MPa lower than the as built cast Ti-6Al-4V, Léopold (2015). The different fatigue behaviors, produced by the two-building directions were analyzed involving different aspects. The fracture surface analysis revealed that the horizontal specimen roughness is very high on the overhang face of the specimen, this is both due to manual supports removal and higher number of partially fused particles on the surface. Since the generated heat in the mid-length of the horizontal specimens cannot be transferred as fast as the vertical specimens (where each layer is supported by the previous layers), the sintered layer stays in high temperature for a longer period of time, partially melting the powder particles which are underneath the specimen. Presence of deep micro-notches on the surface of horizontal specimen leads to a shorter fatigue life, due to a high value of initial stress intensity factor (considering the surface defect size as the initial crack). A machining or surface polishing would be necessary to eliminate this stress concentration effect due to the rough surface. A longer life for the machined surface condition is not surprising but could increase the difference between the vertical and the horizontal specimen. The second analysis was done on the residual stress factor. A relative and qualitative comparison of the residual stress between the two oriented specimens revealed that the horizontal specimen has significantly higher residual stresses compared to the vertical counterpart; this concept is in accordance with the principal literature scientific studies Shunyu (2019), Fatemi (2017), Molaei (2018). However, given the particular stress distribution, the deformation state in the horizontal test piece due the distortion after separating the part from platform could also lead to a negative applied moment to the part under axial tensile load which can increase the fatigue resistance during the experimental