## 3 Results and Discussion
### Fractographic Analysis  
Figure 10 depicts the clear areas of ductile and brittle fracture surfaces. The central part of the fracture shows the brittle fracture. The crescent, as indicated by the arrow, is the crack initiation site at the circumference of the fracture. The final rupture is slightly eccentric due to the relatively low bending force. The circumference of the fracture shows slow crack propagation.  
Figure 10 shows high loading, and this is indicated by the fact that more brittle fractures are much more prominent than ductile fractures. The valley of fatigue crack propagation is indicated on one side of the circle, indicating the greater bending force. Ratchet marks at several locations around the circumference indicate multiple fatigue origins and a high stress concentration. It is a cup-and-cone fracture where fractures usually occur at the lowest stress level of the notch. Figure 10 reveals the multiple crack initiation sites, which progress slowly towards the center from the circumference. The  
Figure 9: Comparison of fatigue strength at different loading cycles of base metals and welded samples with fillers 4043 and 4047.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sr. No.**} & \multirow{2}{*}{**Load (N)**} & \multirow{2}{*}{**\(\sigma\)=\(\frac{509.55Q}{D}\) (N mm\({}^{-2}\))**} & \multicolumn{3}{c}{**Alternating Stress**} & \multicolumn{3}{c}{**Fatigue Life (No. of Cycles)**} \\ \cline{4-9}  & & & **S=\(\sigma\)\(\times\)\(K_{t}\)** & **Trial 1** & **Trial 2** & **Trial 3** & **Average Cycles** \\  & & & & **= 2.19 \(\times\)\(\sigma\)** & & & & \\ \hline
1 & 470 & 239 & 524 & 215,872 & 274,034 & 244,647 & 244,851 \\
2 & 390 & 199 & 435 & 944,343 & 1,024,538 & 961,987 & 976,956 \\
3 & 335 & 171 & 374 & 1,123,849 & 1,225,455 & 1,169,063 & 1,172,789 \\
4 & 295 & 150 & 329 & 1,439,968 & 1,347,876 & 1,076,507 & 1,288,117 \\
5 & 245 & 125 & 273 & 1,725,789 & 1,489,768 & 1,238,879 & 1,484,812 \\
6 & 195 & 99 & 217 & 1,953,775 & 2,019,875 & 1,578,768 & 1,850,806 \\
7 & 170 & 87 & 190 & 2,676,879 & 2,164,675 & 1,896,437 & 2,245,997 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Experimental results of welded sample with filler AA 4047.  
crack propagation is off center due to a larger bending force then the torsional force. The maximum fibrous zone can be seen due to a fast fracture.  
As shown in Figure 10d, the fracture occurred at the center of AA7075 and in the weld region. The fracture line passes through the fusion region and the bevel part of AA 7075. The crack was initiated from the circumference of the fracture at multiple spots. A compressive stress field was developed at the crack tips, which lessen the crack growth. Ratchet marks or shear ridges can be seen at the center due to multiple crack initiation sites and a high stress concentration condition. Steps are created when multiple crack initiation points are joined [43]. As evident from Figure 10e, three cracks initiated from the lower-right corner and continued towards the center at crescent patches (quench crack). The more threaded portion shows the slow progress of the crack towards the center. The fracture portion indicates low porosity, present at the center of the fracture. The fatigue crack grows slowly at the bottom of the pore. The crack initiation sites are present at the circumference. Hence, a less fibrous zone implies more time will be required for rupturing at low loads.