## 3 Results
### Strain-Controlled Testing  
To ensure the applicability for high temperature material testing, a strain-controlled test was performed on nickel-base alloy Rene 80 at temperatures from 20 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C. Stress-strain hysteresis combined with the change of the specimen surface are given in Figure 6. One recognizes the decreasing slopes of the curves corresponding to the decreasing elastic modulus of the material at increasing temperature. Near 1000 \({}^{\circ}\)C, the material becomes very soft and the stress-strain curves somehow deformed. This is not an artifact of the marker-free measurement, because similar behavior was observed after the application of markers. Figure 5: Force-controlled test on austenitic steel 1.4550 at room temperature with a 10 Hz triangular force-controlled cycle. The optical system (red curve) resolves the turning points as well as the tactile mechanical extensometer (black curve). Figure 6: Strain-controlled test on nickel-base alloy Rene 80 at different temperatures from room temperature to 1000 \({}^{\circ}\)C. The specimen surface changes with increasing temperature. Dependent on the material chemical composition, some materials tend to form oxide layers on the surface at high temperatures, which can adversely affect the surface structure and thus the correlation quality. In Figure 6, the surface becomes darker with temperature increasing from room temperature to about 800 \({}^{\circ}\)C. This darkening is compensated by the normalization in Equation (3), making the correlation results comparable in this regime. At 1000 \({}^{\circ}\)C, oxidation is observable altering the specimen surface. The oxide itself was stable and the surface structure sufficient for marker-free measurements, but the altered structure is not comparable to that below 800 \({}^{\circ}\)C by correlation. We also repeated the marker-free measurements from Figure 6 with a second speckle-painted sample as it is frequently used in DIC. Figure 7 shows stress-strain diagrams from both samples measured at 800 \({}^{\circ}\)C together with the corresponding camera images. The camera images of the marker-free measurement show a higher dynamics in vertical directions than the speckle-painted ones (Figure 7, right side). However, this dynamics in vertical direction is compensated by the line-wise normalization given in Equation (3). The camera we used has a dynamic range of 10 Bit (60 dB) which we found sufficient for measurements of \(\pm\)20 degree from the specular reflex. As the inset shows, the noise was generally lower in the marker-free measurements. The reason for this noise reduction might be that the microstructure of sample exhibits structures on a broad range of spacial frequencies which is only limited by the resolution of the optics. So, the spectrum of these structures is closer to white noise then that of the speckle-painted structures despite their enlarged contrast. As a figure of merit, [PERSON]'s modulus \(E\) was calculated for both measurements. With 162 GPa for the marker-free and 163 GPa for the speckle-painted measurement, it was quite similar. The algorithm described above works for both, marker-free and speckle painted samples. Classical strain-controlled fatigue tests with a duration of over 20 h were conducted to prove the stability of the system. Figure 8 shows the stress response of two experiments with identical conditions. It can be observed, that the resulting stress maxima are nearly the same. The cyclic loading of the specimen leads to crack initiation and crack propagation and thus to a reduction of the cross section which result in a stress drop at the end of the experiment. These cracks are often characterized by full-field strain measurements. Even if the shapes of the samples were similar, their surfaces however can differ greatly. Either by the manufacturing or by the machining process. Measurements on turned, polished, scratched, painted, and oxidized specimen surfaces showed good and reliable results.