Figure 7 shows the test results correlated the von Mises’ equivalent strain amplitude Δεeq/2 with the failure life Nf. Figure 8 shows the generated hysteresis loops for 0.5Nf and the maximum stress behavior during the tests in each test condition.
The results (Fig. 7) of fatigue tests under PP clearly show that the data scatter is not so drastic and were within the factor of 3 bands. Moreover, they were no difference between heat treatment and no heat treatment. It is considered the temperature for heat treatment was not sufficient to remove residual stresses. However, the data of PP HT (Δεeq = 1.3 %) exhibited lower failure lives and out of the factor of 3 bands. In Fig. 8 (a), (b) and (e), the hysteresis loops and the maximum stress amplitude at Δεeq = 1.3 % were compared. The hysteresis loops indicated the almost same elastic deformation. The maximum stress amplitude for NHT remains almost constants. Then probably, the crack initiated from the vicinity of Nf, and the failure occurred very quickly after the crack initiation. On the other hand, for HT, the maximum stress amplitude decreased instantly from around 0.3Nf. Therefore, the crack was considered to initiate from 0.3Nf, and after some crack propagation, the failure took place.
Failure life (Fig. 7) under CI was reduced compared to PP resulting in 10% decrease in fatigue life. The scattering of data was also very small; however, it cannot be confirmed due to limited data (the presence of the only one data of CI NHT (Δεeq = 0.5%)), the reduction in failure life for NHT cannot be seen. For the other material such as stainless steel and carbon steel, in case of low strain level, the failure life is the almost same as PP, but it is not clear for this study due to limited data. In Fig. 8 (c), (d) and (f), the hysteresis loops and the maximum stress amplitude of Δεeq = 0.5% were compared. In case of low strain level, the hysteresis loops also exhibited the almost same elastic deformation. The maximum stress amplitude for two data was similar in deformation behavior. Although the overload was seen in the data of HT, it is estimated that there is almost no effect because it could be adjusted in 10 cycles. Therefore, why this data shows the longer life than other data remains as question. The previous test results are also shown in Fig. 7 [1]. In the previous study data, failure lives were longer than that of this study. The specimens of the previous study were the same material (Ti-6Al-4V) but were fabricated with a different manufacturing condition. Based on the above-mentioned, the internal defects were investigated to quantify the size of defects.
4.2 Internal Defects
Figure 9 compares the internal defects between the previous study and this study. In the previous study, the small defects were observed inside, but in this study, the internal defects depend on the observation site. AM components have usually the defects inside, and fatigue strength of the material depends on size and density of the defect; however, the defects size does not affect the fatigue life above a certain size because the results of fatigue tests (Fig. 7) shows a clear correlation regardless of various defect size. It is necessary to investigate the dependence of fatigue strength reduction on defect size.
4.3 Fracture Surface
Figure 10 shows the pictures of fracture surface after fatigue tests, and the observation surface is perpendicular to the specimen axis. It is difficult for main crack initiation site to be analyzed as for specimens of this study. The cracks were initiated from the boundary of partly melted defects regardless of strain range and strain paths, and the stress concentration in the defects causes premature rupture in comparison to the results which were obtained with the same material [1]. From Figure 10 (a2) and (b2), the crack propagation patterns were divided into the first phase (from the crack initiation site to the crack middle site) and the second phase (form the crack middle site to the crack end site) during test. In the first phase of the test, the propagation path was different due to strain paths, and it is possible that high stress existed evidenced by rougher fatigue fracture surfaces. In PP, the crack propagated simply, but in the CI, it became complicated. Basically, due to the change of the principal direction of stress and strain during the cycle in CI, maximum shear stress plane is changed in the cycle. Thus, there is a possibility for the crack to initiate and propagate to many directions. In the second phase of the test, fatigue fracture surfaces were with the similar pattern, and the crack propagation seems to be suppressed. It is estimated that a local non-propagating effect under multiaxial stress took place. Thus, it is possible that the change of the crack propagation pattern in each phase of the test is a characteristic of AM components with numerous defects, and it is noticeable under non-proportional multiaxial loading.
From this study, the fatigue strength is absolutely inferior to that of the previous study and traditionally manufactured components; however, there is a possibility that AM technology is used for the mechanical structure because the non-propagating effect of cracks was verified under non-proportional multiaxial loading.