## 3 Method  
### Experimental Setup  
Synthetic DIC images from DIC Challenge [11] (DIC Challenge 2.0 Star 6 for PDDO performance evaluation, DIC Challenge 1.0 Sample 12-the same plate with hole as presented in Figure 1) and in-house experimental DIC measurements are considered for the PDDO applications to compute strains. Plate with hole tensile test specimens of dimensions 180 mm \(\times\) 35 mm and hole diameter 10 mm were prepared for experimental DIC measurements. The plate material-DIBOND composite [46] is a sandwich material of total thickness 3 mm, composed of three layers: polyethylene core of thickness 2.4 mm, and aluminum EN AW 5005 external layers of thickness 0.3 mm. The PDDO code was used for strain calculation from the DIC displacement field, and because of this the material properties (Young modulus, Poisson ratio) are not required. No special testing standard was used for the specimen preparation because our purpose was to measure the strains near the hole with DIC rather than testing the material properties.  
The (experimental) strain computation setup is shown in Figure 5. Strain computation stage using PDDO (only this stage was necessary for the strain computation from DIC displacements calculated from DIC Challenge [11] images) is excluded with dashed line in Figure 5. In order to perform a tensile test for experimental DIC measurements, an electromechanical testing machine INSTRON E10000 with maximum force 10 kN was used, and the specimen with hole was loaded in tension at 3 mm/min crosshead speed. The full field displacement and strain measurements were done with the 3D DIC system from Correlated Solutions [2] equipped with two CMOS cameras Basler acA4112 of 12 Mpx resolution, sensor size 14.1 mm \(\times\) 10.3 mm, pixel size 3.45 mm \(\times\) 3.45 mm, and lenses of focal length 25 mm. Adjustable power LED lamp ProfiluxLED1000 with maximum power 185 W was used as light source. A black dots speckle pattern of 0.18 mm size was painted on the specimen surface, previously coated with white paint, using a painting roller from Correlated Solutions. A total of 25 calibration images of a 14 \(\times\) 10 dots calibration plate with 10 mm dots spacing were taken before the experiment to calibrate the DIC system.  
The DIC displacements calculated from experimental and synthetic images were analyzed and based on them the DIC strains were computed by using the DIC software VIC-2D and VIC-3D [40]. The subset size was set to 29 px for experimental DIC measurements and 41 px for synthetic DIC images according to the system-suggested value and then step sizes were set equal to 1/4 of the subset sizes. The other parameters of DIC for all computations were set as default: normalized squared differences correlation criterion between undeformed and deformed speckle, and 6 th order polynomial for the sub-pixel displacement interpolation. The _Exhaustive search_ option was en  
Figure 5: Schematic of the experimental-computational setup.  
initially failed correlation between the tracked points is repeated [40] and more data points can be retrieved.  
A number of 104 images of the DIBOND composite specimen were taken with the DIC system during the in-house experimental testing. The DIC displacements were exported from the VIC-2D or VIC-3D software to calculate strains by applying the PDDO (the DIC-PDDO strains). Moreover, in case of experimental DIC measurements, FE simulation of the specimen was performed in LS-Dyna to confirm the correctness of both in-house experimental DIC and DIC-PDDO strain fields.