*3.6. Mechanical Properties*

The second stage of testing consisted of uniaxial tension tests until failure with a further machined specimen geometry, comparable to conventional tensile testing. For each type of bar, one specimen was tested until failure. A laser extensometer was used to measure the integral strains during the test on the length of the transversal section (10 mm). Figure 13 shows the obtained engineering stress–strain curves for all three specimens. The resulting stress was calculated based on the 4 mm diameter in the predefined measurement section.

**Figure 13.** Stress–strain diagram for all three specimens.

The stress–strain curves show a very ductile material behavior with large ultimate elongations, especially for the CMT bars. A clear yield point was also recognizable for all three specimens as is expected from the used unalloyed weld metal. All mechanical properties, as listed in Table 3, were calculated in accordance to DIN EN ISO standards [42]. Table 3 also includes the corresponding properties of the weld metal data sheet from the welding wire used.


**Table 3.** Mechanical properties of the three bars and the welding wire.

1 Due to the geometry of the specimen, the elongation at break is not comparable to standard tensile test specimens [35].

All the bars showed a linear elastic material behavior with a modulus of elasticity ranging from 180 to 198 GPa. The yield stress, *ReH*, for all three bars and the ultimate tensile stress, *Rm*, for the conventional GMAW bar is lower than nominal values from the corresponding weld metal data sheet. For both CMT bars, the *Rm* lies in the desired range. In comparison, the conventional GMAW bar shows the lowest values in terms of strength properties. The results are in agreemen<sup>t</sup> with findings from metallographic investigations and hardness tests.

## *3.7. Fracture Surface Imaging*

After destructive testing of the bars, the fracture surfaces were examined by help of scanning electroscope micrographs. Figure 14 shows the fracture surface of each bar in two different magnifications. The macroscopic images of all the specimens depict significant area reductions resulting from plasticity. This effect is most distinctive at the specimen welded by conventional GMAW. However, all fracture surfaces show dimples at high magnification. On the surface of the conventional GMAW specimen, there are also artefacts, which may be silicate segregations. In comparison to the GMAW specimen, the CMT specimens show fracture surfaces with indications for multiple crack initiation sites. A rougher surface indicates more ductile fracture behavior.
