## 3 Experimental Results
### Crack Coalescence  
Both the monotonic and cyclic tests showed almost identical coalescence types. For colinear geometry specimens, coalescence occurred due to the internal shear cracks. For non-colinear geometry specimens, coalescence occurred through combinations of internal shear cracks, internal wing cracks and tension cracks. Table 4 summarizes all observed coalescence types. Type I coalescence occurs by the propagation of internal shear cracks in a colinear flaw geometry or an almost colinear flaw geometry. The 30-0-a, 45-0-a, 60-0-a, 45-0-2a, 60-0-2a, 60-0-3a, 60-0-4a, and 30-a-2a geometries show Type I coalescence. The 30-0-2a geometry only shows Type I coalescence in monotonic tests, while Type I coalescence occurs only during cyclic tests within the 45-0-3a geometry. Figure 10: Wing crack propagation angle measurement. Figure 9: An example of secondary cracks (**a**) colinear secondary cracks in the 30-2a-0 geometry; (**b**) horizontal secondary cracks in the 45-a-0 geometry. Type II coalescence is formed by the connection of two internal shear cracks by a vertical or quasi-vertical tension crack. This tension crack is not a wing crack but is instead produced under tension. The 30-a-a, 45-a-a, 45-a-a, 45-a-2a, 60-a-2a, 60-a-3a, 30-2a-a, 30-2a-2a, 45-2a-2a, 30-2a-3a, 60-a-a, 30-2a-a, 45-2a-2a, and 45-2a-3a geometries show Type II coalescence. Type III coalescence can be divided into two classes: Type III-A and Type III-B coalescence. Type III-A coalescence takes place when an internal wing crack from one of the flaws reaches the other flaw. The 60-2a-2a and 60-a-0 geometries show Type III-A coalescence. Type III-B coalescence takes place when an internal shear crack from one of the flaws reaches the internal wing crack of the other flaw. The 60-2a-3a geometry shows Type III-B coalescence. Figure 11: Wing crack propagation angles. (**a**) Upper and lower external wing crack propagation angle. (**b**) Upper and lower internal wing crack propagation angle. (**c**) Upper external and internal wing crack propagation angle. (**d**) Lower external and internal wing crack propagation angle. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type** & **Coalescence Pattern** & **Type of Coalescence Crack** \\ \hline I & **Shear crack** \\ II & **Shear crack** \\ III-A & **On** & **Internal shear and tension crack** \\ III-B & **On** & **Internal wing crack** \\ IV-A & **On** & **Internal wing crack and internal shear crack** \\ IV-B & **On** & **Internal wing crack** \\ IV-C & **  
Type IV coalescence can be divided into three classes: Type IV-A, Type IV-B, and Type IV-C coalescence. Type IV-A coalescence occurs when two internal wing cracks from certain flaw tips reach the other flaw tips. The 30-a-0, 45-a-0, 60-a-a, 30-2a-0, and 45-2a-a geometries show Type IV-A coalescence. Type IV-B coalescence appears when two internal wing cracks from the flaw tips reach the middle of the other flaws and show a different wing crack curvature than in Type IV-A coalescence. The 60-a-0 geometry is a representative example of Type IV-B coalescence. Type IV-C coalescence occurs when two internal shear cracks from the flaw tips reach the internal wing cracks of the other flaws. The 60-2a-0 geometry is a representative example of Type IV-C coalescence. Type V coalescence occurs by a combination of the connection of an internal wing crack that initiates from the internal tip of the upper flaw, an external wing crack that initiates from the external tip of the lower flaw, and Type III-A or Type III-B coalescence. An internal wing crack and an external wing crack propagate until they unite. An external wing crack has a reverse propagation direction from the other external wing crack. The 60-2a-a geometry is a representative example of this type of coalescence. Type VI coalescence occurs by a combination of the connection of two internal wing cracks that propagate until they unite, tension cracks, and internal shear cracks. The first two internal wing cracks initiate from the internal tips of the flaws and propagate until they join.