## 1 Introduction  
Together with metallurgical factors, the formation of hot cracks during welding is mainly induced by the thermomechanical effect, which characterizes the strain-stress behavior of a material as a function of the temperature. The influence of the structure of the welded component and the stress intensity are also significant drivers of the hot cracking process. A variety of theories consider strain as an essential criterion for hot cracking. According to [PERSON] [1, 2], hot crack initiation is based on strain localisation in the intercrystalline melt film. The tensile strains resulting from the welding process are transferred from the fully solidified region and the dendrites in the solidification region into the interdendritic melt films and accumulate there. Crack initiation occurs close to the solidus temperature as the grain boundaries are covered with thin melt films at this stage, and even small strains are sufficient for crack initiation. [PERSON] [3, 4] defines the 'brittle temperature range (BTR)'. Within this temperature range, the weld metal reaches its minimum ductility during solidification. If the resulting strain exceeds the ductility of the crystallising melt, hot cracking occurs. Based on this basic idea, various authors have experimentally determined the maximum sustainable strain of this temperature range. Although the BTR concept only considers the mechanical component of hot cracking, it is particularly suitable for comparing different materials with regard to their susceptibility to hot cracking. The experimental measurement of strain in the high-temperature range relevant for solidification cracking is usually difficult with the common measuring techniques, such as inductive displacement transducers or strain gauges. Besides the widely used strain gauge measurement method, modern optical technologies are increasingly used for non-contact measurements. Optical methods such as digital image correlation (DIC) do not require mechanical contact for taking measurements, so it is easy to obtain displacement data from the visible range of a digital camera. Unlike the use of strain gauges, these methods therefore have no influence on the processes being measured. [PERSON] et al. [5] were the first to use this technique to measure welding deformations. The strains estimated from the DIC technique were also compared with those obtained from the strain gauge (SG) technique, and the results showed good agreement. Using a digital camera and the normalized correlation algorithm, [PERSON] et al. [6; 7] developed a two-dimensional (in-plane) technique to measure in-situ full-welding deformations. The further development of the measurement technique was reported in [8], where two digital cameras and the stereo imaging technique were used to match the images. [PERSON] et al. [9; 10] employed a similar setup to measure the strain during the gas tungsten arc welding (GTAW) process. [PERSON] et al. [11] also utilized the DIC technique and the controlled tensile weldabilty test (CTW test) to analyze the weld pool near transverse displacements and the influence of this displacement on solidification crack formation. [PERSON] et al. [12] used the free edge test to investigate the hot cracking susceptibility of TRIP steels; in that study, the DIC technique was also utilized to estimate the strain threshold required for hot crack formation. In their investigations, two 30 W LEDs with a wavelength of 450 nm were employed as an illumination source. A similar configuration was also used by [PERSON] et al. [13] to determine the critical strain and strain rate threshold required for hot crack formation during the laser welding of aluminum alloys. The optical flow technique has been used to estimate the motion of pixels in an image sequences [14]. Primarily, this technique is employed to estimate the object velocity and the position of the object in the next image. Moreover, it has found an important utilization in the field of experimental mechanics. [15; 16].