## Materials and Methods
### Creep Test, DIC Application  
The CT testing procedures developed for the present work consisted of applying a tensile load to a single-edge notched specimen. Before the creep test, the ultimate tensile load was determined for the CT specimens. A universal testing machine with an accuracy of \(\pm\)0.5% for destructive ultimate load tests was used (rate of loading 0.3 mm/min). For creep tests, mechanical lever arm creep rigs were used. With these rigs, it is possible to apply constant loading to the specimens and to keep it uniform over a long period. Furthermore, it is not necessary to adjust the stress level during the experiments. The calibration curves are linear; no external energy resources are used. The lever arm ratio of the lever arm creep rig was 1:20. The loading level was adjusted to an accuracy of \(\pm\)2% (determined by an analogue dynamometer with 0.002 kN resolution). The geometry of the test specimens was kept the same for all tests. A single CT specimen was placed vertically in a creep rig, ensuring alignment with the loading axis. Each specimen was fixed (within the creep rig), using two grips (connected with bolts through the \(\diameter 20\) mm drill holes) that transmit the applied tensile loading (Figure 2b). The fastening system was made so that the upper and lower attachment points are hinged (pinned) supports; hence, the specimen is free to rotate, and no additional stresses are caused due to eccentric loading. A more detailed description of the exact creep apparatus (inventory of Riga Technical University) can be found in [45]. Specimens were kept under constant load levels corresponding to 60% of the ultimate tensile load. A tensile dynamometer \(\diameter\Pi\Pi\)-0.02-2 (resolution 0.002 kN, measuring range 0-0.2 kN) was enclosed within the loading rig to monitor precise levels of applied force. The load was applied gradually in five steps and as fast as possible. The modulus of elasticity was determined from the elastic strains that occurred at the beginning of the creep test. According to [PERSON]'s law, the modulus of elasticity was determined by measuring the elastic strain on each loading step. After that, the specimens were kept under a constant load for 50 days. The long-term test was performed in a dry atmosphere of controlled relative humidity in standard conditions t = 20 \(\pm\) 3 \({}^{\circ}\)C, RH = 50 \(\pm\) 5%. Complete creep testing and DIC imaging setup are shown in Figure 2. DIC imaging setup: (**a**) Camera facing the loaded test specimen, camera controlled via digiCamControl free software (FOV visible in the PC display); (**b**) camera tripod with stabilising weight, positioning mechanism, DSLR with macro ring flash, CT specimen with an attached strain gauge. Previously, a similar DIC imaging setup was successfully used by [PERSON]\(\check{\text{c}}\)[PERSON] et al. [37]. In this research work, the propagation of the tensile strain was traced on the surface of the specimens using a digital camera, Canon EOS550D DSLR (APS-C sensor) with the Canon EF-S 60 mm F/2.8 macro lens. The optimal distance between the camera sensor and the specimen was found to be 235 mm, thus FOV was 36 \(\times\) 24 mm. For optimal light conditions, a Meike MK-14 EXT Macro Ring flash was attached to the lens. Due to insufficient sturdiness, extra rubber bands within the connecting junction and self-adhesive tape around the attachment ring were placed to hold the flash in a constant position (otherwise resulting in changing exposure). No additional external/ambient light sources were put to use. The camera was placed on a tripod Manfrotto MT055 XPRO3 (aluminium). For extra vibration resistance, a 2.1 kg weight was hung in the tripod's central node.