## 5 Fatigue Test Procedure and Result  
The fatigue test was commissioned by our co-operator (AECC Beijing Institute of Aeronautical Materials, Beijing, China) and carried out by AVIC Touchstone Testing Innovation (DaChang) Company (Langfang City, Hebei Province, China). The fatigue tester type used in this fatigue test is MTS-250 kN-1 and the test temperature is room temperature. The machine loading control method is stress control and the loading waveform is sine wave of frequency 10 Hz. Due to the limitation of the test conditions, the demand condition of the initial fatigue test of the designed bracket was not used in this fatigue test, in which the maximum magnitude was 50 kN and the minimum magnitude was 5 kN sine load in the vertical direction. The used maximum magnitude was 100 kN and the minimum magnitude was 10 kN sine load in the vertical direction instead. The upper and lower parts of the 3D printed bracket without heat treatment were connected to the fatigue tester by bolts as shown in Figure 15. As the fatigue test was conducted, the number of cycles in which the 3D-printing bracket fracture was 6201. The picture of the bracket fractured during the test is shown in Figure 16. The fatigue life of the parts in engineering are usually presented as S-N curve characteristics and the S-N curve is a negative exponential curve. In paper [23], it is stated that the fatigue properties of parts obtained with 3D-printing are  
Figure 14: New bracket after 3D-printing with EOS M290.  
Figure 13: The mesh diagram of new bracket.  
similar to the parts obtained with traditional manufacturing processes. According to [24], the formulation of the fitted S-N curve of TC4 is given by  
\[\sigma(N)=[1700(N-4900)^{-0.2}+440]\text{MPa} \tag{12}\]  
where \(\sigma(N)\) is the stress amplitude and in this paper von-Mises was used, \(N\) is the number of test cycles. First, it is necessary to verify that Equation (12) is suitable for the new bracket. As shown in Figure 17, the von-Mises stress of main region in new bracket subjected to 100 kN vertical load is about 940 MPa. \(\sigma(N)\) is substituted with 940 MPa in Equation (12) and the number of the test cycles obtained is about 5354. Compared with the practiced fatigue test, the error of calculation is 13.7%, which is within the acceptable range in engineering. When the maximum von-Mises 470 MPa is substituted in Equation (12), it can be inferred that when the new bracket is subjected to a maximum load of 50 kN and a minimum load of 5 kN sine load in the vertical direction, the number of cycles is about \(5.84\times 10^{8}\), exceeding the engineering specifications proposed at the beginning of the bracket design. As the new bracket is manufactured by laser selective melting (SLM), the characteristics of SLM are a good surface finish and a high residual stress, where a good surface finish can improve the fatigue life of the part, while a higher residual stress can have a serious negative impact on the fatigue performance of the material. According to the literature [25], the most desirable heat treatment in the 3D printing manufacturing process is hot isostatic pressing, which can not only relieve residual stresses but also eliminate the hole defects prevalent in 3D printed titanium alloys. Consequently, there is potential for further improvement in the fatigue performance of 3D printed aerospace brackets.  
Figure 16: New bracket fracture during the fatigue test by MTS-250 kN-1.  
Figure 15: New bracket with the actual fatigue tester machine assembly diagram.