# Monotonic and Fatigue Properties of Steel Material Manufactured by Wire Arc Additive Manufacturing
## 3 Results
### Monotonic and Fatigue Properties
#### 3.3.2 Fatigue Properties  
This section describes the results from the strain-controlled fatigue tests in the low cycle fatigue regime as well as the stress-controlled staircase tests to determine the fatigue limit. Figure 10 shows the cyclic hardening/softening curves for three exemplary strain levels (\(\varepsilon_{\rm a}\) = 0.8%, 0.4%, and 0.2%) for each test series of strain-controlled tests. Moderate cyclic hardening is found independently from the strain level within the first load cycles applied. Afterwards, the stress amplitude shows an approximately stabilized behavior. For all specimens, a certain initial shift in the mean stress is observed. A mean compression stress of about \(-\)50 MPa is reduced towards zero as the number of cycles progress. The effect is slightly larger for the specimens that were extracted crosswise but seems to be independent from the interlayer temperature. This can be explained by the residual stresses mentioned above that remain within the microstructure to a small degree, despite the extraction of such small unnotched specimens. Figure 10: Cyclic hardening/softening curves for three exemplary strain amplitudes (semi-logarithmic scale) for interlayer temperature of 150 \({}^{\circ}\)C for lengthwise (**a**) and crosswise (**b**) extraction direction and for interlayer temperature of 300 \({}^{\circ}\)C for lengthwise (**c**) and crosswise (**d**) extraction direction. According to the definitions of the strain-life and cyclic stress-strain curves, Equations (1) and (2), the intercept points \(\sigma^{\prime}{}_{t}\), \(\epsilon^{\prime}{}_{t}\), and K' are located far from the center of the test results, Figure 3. \(\sigma^{\prime}{}_{t}\) and \(\epsilon^{\prime}{}_{t}\) are defined at N = 0.5 and K' at a plastic strain amplitude of \(\varepsilon_{\text{a,pl}}\) = 100%. Therefore, these values are of limited suitability for a comparison between the different data sets. Small errors in the regression have a significant influence on the characteristic values themselves so that dependencies are hard to find. Therefore, the results are compared using the plots of the corresponding curves in Figures 11 and 12. It can be seen that the courses of the cyclic stress-strain curves are almost parallel to each other, and the curves are scattered over an area of approximately 50 MPa. Although the differences in the four curves are not very pronounced, the curves for the interlayer temperature of 300 \({}^{\circ}\)C are below the ones of the interlayer temperature of 150 \({}^{\circ}\)C. Apart from the curve of the interlayer temperature of 150 \({}^{\circ}\)C in the crosswise direction, all strain-life curves seem to fall in one scatter band. The first, however, shows numbers of cycles that are higher compared with the other curves by a factor of approximately 2. The Coffin-Manson approach for the strain-life curve, Equation (1), originally does not contain a fatigue limit (endurance limit) and only applies to the low cycle and high cycle fatigue regime. In order to take the fatigue limit into account, stress-controlled tests were carried out using the staircase method with about 15 specimens each. The resulting fatigue limit values \(\sigma_{\text{a,E}}\), Table 5, were converted from stress to strain by using cyclic stress-strain curves, Figure 11, and therefore may be included in the strain-life curves, Figure 12.  
It can be seen that the fatigue limit of the material manufactured with an interlayer temperature of 150 \({}^{\circ}\)C is slightly lower than that of material manufactured at 300 \({}^{\circ}\)C. However, the differences are within a factor of 1.05, at most. Even though differences between the various interlayer temperatures and extraction directions are determined for both the cyclic deformation behavior and the fatigue strength, no pronounced trend is recognized from these.