## 3 Experimental Results and Analysis
### DIC Premonitory Information
At Point a, a large strain began to appear at 535 s, and the formed crack expanded continuously within 535-549 s. Because the crack that did not expand sufficiently was compacted and squeezed at 549 s and the stress reached its peak at 555 s, a large macroscopic  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Specimen** & **Time/s** & **a** & **b** & **c** & **d** & **e** & **f** \\ \hline \multirow{9}{*}{Specimen with a 30\({}^{\circ}\) prefabricated crack} & 497 & 0 & 0 & 0 & 0 & 0 \\  & 499 & \(2.6\times 10^{-19}\) & \(2.05\times 10^{-17}\) & 0 & 0 & \(1.09\times 10^{-17}\) & \(2.23\times 10^{-17}\) \\  & 500 & \(5.14\times 10^{-4}\) & \(8.73\times 10^{-4}\) & \(2.94\times 10^{-4}\) & \(1.21\times 10^{-3}\) & \(5.29\times 10^{-4}\) & \(5.79\times 10^{-4}\) \\  & 501 & \(3.70\times 10^{-4}\) & \(3.76\times 10^{-3}\) & \(6.11\times 10^{-4}\) & \(1.47\times 10^{-3}\) & \(8.86\times 10^{-3}\) & \(6.98\times 10^{-3}\) \\  & with a 30\({}^{\circ}\) & 505 & \(1.04\times 10^{-2}\) & \(6.10\times 10^{-3}\) & \(6.16\times 10^{-3}\) & \(9.32\times 10^{-3}\) & \(2.19\times 10^{-3}\) & \(2.11\times 10^{-3}\) \\  & prefabi- & 510 & \(2.52\times 10^{-2}\) & \(5.25\times 10^{-3}\) & \(1.08\times 10^{-3}\) & \(1.30\times 10^{-2}\) & \(5.22\times 10^{-3}\) & \(4.15\times 10^{-3}\) \\  & 519 & \(2.90\times 10^{-2}\) & \(7.29\times 10^{-3}\) & \(1.85\times 10^{-3}\) & \(1.71\times 10^{-2}\) & \(8.80\times 10^{-3}\) & \(5.98\times 10^{-3}\) \\  & 535 & \(4.22\times 10^{-2}\) & \(8.85\times 10^{-3}\) & \(3.30\times 10^{-3}\) & \(2.33\times 10^{-2}\) & \(1.62\times 10^{-3}\) & \(1.27\times 10^{-3}\) \\  & 540 & \(5.07\times 10^{-2}\) & \(1.13\times 10^{-3}\) & \(3.91\times 10^{-3}\) & \(2.15\times 10^{-2}\) & \(2.31\times 10^{-3}\) & \(1.96\times 10^{-3}\) \\  & 543 & \(5.26\times 10^{-2}\) & \(1.37\times 10^{-3}\) & \(4.18\times 10^{-3}\) & \(2.04\times 10^{-2}\) & \(2.93\times 10^{-3}\) & \(2.59\times 10^{-3}\) \\ \hline \multirow{9}{*}{Specimen with a 60\({}^{\circ}\) prefabri-} & 532 & \(2.56\times 10^{-4}\) & \(7.79\times 10^{-3}\) & \(1.12\times 10^{-3}\) & \(7.46\times 10^{-4}\) & \(9.86\times 10^{-3}\) & \(3.36\times 10^{-3}\) \\  & 534 & \(1.22\times 10^{-3}\) & \(2.64\times 10^{-3}\) & \(7.49\times 10^{-3}\) & \(3.94\times 10^{-3}\) & \(1.39\times 10^{-3}\) & \(3.40\times 10^{-3}\) \\  & 535 & \(2.19\times 10^{-3}\) & \(1.76\times 10^{-3}\) & \(1.08\times 10^{-3 change occurred at Point a at 554 s when the strain curve ended. At Points c and e, the principal strain started to grow slowly and gently at around 535 s, and this trend continued until the final macroscopic failure of the specimen. This demonstrates that the crack, which emerged at Point a (left end of the prefabricated crack), gradually developed and expanded along the a-c-e path. The principal strains at Points d and f changed differently from those at Points c and e. They began to rise slowly at 535 s and started to soar at around 552 s until the macroscopic failure, indicating that the principal strains at Points d and f changed slightly within 535-552 s. As the stress increased and reached the peak at 555 s, the principal strains at these two points changed notably, and large macroscopic deformation could be observed at this moment. Overall, the surface-strain field of a typical pre-cracked specimen is dispersed in the initial stage of loading when the specimen remains in the state of elastic deformation. With the increase in load and stress, the deformation mode gradually transforms from elastic deformation to plastic deformation. At this time, strain concentration occurs at the prefabricated crack on the specimen's surface, and macro-cracks can be observed. With the further expansion of the strain concentration area, the length and opening of the macro-cracks at the two ends of the prefabricated crack increase accordingly until the strain field expands to run through the whole specimen. Finally, penetrating cracks are formed, and tensile-shear mixed failure occurs.