# A Displacement Controlled Fatigue Test Method for Additively Manufactured Materials
## 5 Results and Discussion
includes the sensitivity analysis on the monitoring signal. The implementation of the CSM method on other wrought and SLM specimens is shown in Figure 11. To determine the maximum nominal stress, the average of the peak load was calculated up to the end of the nucleation phase. Inserting the average in Equation (4), the nominal stress was calculated. Figure 12 shows the tensile and compressive load response up to the final failure for the wrought specimen displaced at 0.200 mm amplitude. The fatigue life of a specimen in terms of the number of cycles is the sum of cycles during nucleation and propagation stages. Materials demonstrate a higher life cycle at low-stress value. Figure 13 shows the end of the nucleation phase cycle and cycles to failure for fine finished wrought and SLM specimens. The trend of nucleation and propagation presented in the literature [43; 44] supports the results. Both the wrought and SLM specimens demonstrate an increase in the nucleation and propagation cycle as the stress goes low, but for a particular stress value, both the nucleation and propagation cycles for the SLM material is lower than those for the wrought material. A possible reason is that additively manufactured materials have a higher probability of different types of defects. The fracture surface analysis of wrought and SLM specimens is shown in Figure 14. In the SLM specimen, the crack is initiated at the lack of fusion defect near the top surface while surface defect is the crack initiation source for wrought specimens. We observed similar type of defects to be the crack nucleation site for other SLM specimens. The crack starts growing earlier in SLM materials than in wrought materials. Therefore, the nucleation life cycle is less. The presence of other defects such as micro-cracks, pores, and lack of fusion within the volume enhance the propagation rate. Additionally, some authors reported that inter-layer bonding is weak in SLM materials [45; 46]. Therefore, SLM fabricated 304 L SS demonstrate lower fatigue strength than bulk material. Further analysis was performed to evaluate the CSM method in identifying nucleation and propagation phase. Rough finished and notched rough finished wrought specimens were prepared to conduct fatigue test. Rough finished specimens were chosen because these are easy to prepare. Figure 15 shows the notch location in the specimen. Figure 16 illustrates the nucleation and propagation phase identified using the CSM method for the specimens. As we know, notched specimens have higher stress concentration, hence, they fail earlier. The number of cycles decreases more in nucleation phase due to the notch while the influence of notch on the propagation is minimal. This attributes to the proper implementation of the CSM method in identifying the end of nucleation phase. In future, the method will be validated determining the crack length at the end of nucleation phase. Figure 10: Nucleation and propagation stage of a fine finished wrought specimen displaced at 0.200 mm amplitude. Figure 11: Nucleation and propagation stage of fine finished specimens, (**a**) wrought specimen displaced at 0.300 mm amplitude; (**b**) wrought specimen displaced at 0.225 mm amplitude; (**c**) SLM specimen displaced at 0.200 mm; (**d**) SLM specimen displaced at 0.175 mm amplitude. Figure 6:0 Load values for the specimen displaced at 0.200 mm amplitude. Figure 6:1 Fracture surface analysis of fine finished wrought and SLM fabricated materials, (**a**) crack nucleation site; and (**b**) crack propagation site of a wrought specimen, the nominal stress is 411.67 MPa; (**c**) crack nucleation site of SLM fabricated specimen, the nominal stress is 394.76 MPa; (**d**) presence of lack of fusion at the surface initiating the crack; (**e**) presence of lack of fusion within the volume enhancing crack propagation rate. Figure 6:2 End of nucleation phase and cycles to failure for fine finished wrought and SLM fabricated specimens.