*3.4. Computer Tomography*

The characterization of the three specimens in terms of the porosity and cavities was carried out with computer tomography before the tension testing. The results of the porosity analysis of the CT scans are displayed in Figure 11. On the left side, there is the conventional GMAW bar with a comparatively low amount of pores and porosity. The diameter of the pores ranged from 100 to approximately 1000 μm. The pores in the bar welded by the CMT standard process were both small and rare. The highest detected pore diameter measured between 100 and 200 μm. While the size of the pores in the CMT cycle step bar are comparable to the CMT standard bar, the amount of pores or cavities exceeds that of the CMT standard bar many times over. The di fferences in terms of porosity are a result of the weld pool size and the varying degassing behavior during welding. Larger weld pools in combination with short welding times (conventional GMAW) cause solidification pores of a distinct size. Smaller weld pools (CMT cycle step) and longer welding times (CMT standard) are favorable for low pore sizes. However, short welding times in combination with small weld pools lead to a high number of small pores.

**Figure 11.** Computer tomography scans with porosity analysis of the conventional GMAW bar, the CMT standard bar, and the CMT cycle step bar.

#### *3.5. Full Field Strain Measurements*

The material behavior and the influence of the surface topography were examined by full field strain measurements using ESPI during uniaxial tensile tests in the elastic region. To compare the three specimens, the measured strain maps (longitudinal strains) were related to the global longitudinal strain, <sup>ε</sup>*g*, simultaneously measured with the laser extensometer on each specimen. Figure 12 shows a surface photograph with the ESPI measurement area and the related strain map of each specimen at a load of 5 kN.

**Figure 12.** Photograph with the measurement area and local strain maps (longitudinal strain) related to the global strains of (**a**) conventional GMAW, (**b**) CMT standard, and (**c**) CMT cycle step under tensile load.

The ratio from local to global strain (ε/<sup>ε</sup>*g*) allows detection of areas with different material parameters or strong variations in the surface. The local strains in the conventional GMAW bar showed the largest deviations from the measured global strains with a ratio of up to 6 in two distinct areas of the specimen, as displayed in Figure 12a. The CMT standard bar showed a more even distribution with ratios of up to 2.5. In comparison, the CMT cycle step bar showed the most even strain distribution with a maximum ratio of 1.5. While the layered structure was clearly visible in both CMT bars, the conventional GMAW bar showed two concentrations, which can be attributed to strong deviations in the surface topography. Effects of the microstructure or heterogeneous hardness on local strain distribution were not detectable at this load level.

Due to the high local strain concentrations, especially in the conventional GMAW bar, local plastic deformations might have occurred. In the event of a further load increase, the conventional GMAW bar would most likely fail at one of the localizations. This shows that a uniaxial tension test until failure with this topography would not provide representative results about the mechanical properties, except the ultimate bearing load. Therefore, the advanced testing strategy applied here seems to represent a better overall solution. A rated value similar to the strain ratio presented here could later be used as a notch factor corresponding to the different bars. Therefore, these measurement results can also be used to examine the material behavior of WAAM bars in regards to cyclic loading.
