## 3 Experimental Results and Analysis
### Fatigue Test Analysis  
According to the national standard GB/T15248-2008 (metal material axial constant amplitude low-cycle fatigue test method) in the lifting method for a high-cycle fatigue test, the results are shown in Table 4. This was after multiple tests to obtain multiple sets of cyclic-stress and cycle-number relationship data.  
In the fatigue-test data of Table 4, the experimental stress ratio \(\mathrm{R}=S_{\mathrm{min}}/S_{\mathrm{max}}=0.1\). When plotting the S-N curve, stress amplitude \(\mathrm{S_{a}}=(S_{\mathrm{max}}-S_{\mathrm{min}})/2\) and fatigue life N are usually used as variables in the S-N curve. Fatigue life N is usually in the form of logarithmic coordinates, while stress amplitude is in the form of linear coordinates and logarithmic coordinates. This paper uses the form of logarithmic coordinates, because there is an approximately linear relationship between logN and logS\({}_{\mathrm{a}}\) in a large N-value interval. Fatigue data for base metal and welded joints are matched by Equation (1).  
\[\mathrm{C}=\sigma_{\mathrm{a}}^{\mathrm{k}}*\mathrm{N} \tag{1}\]  
where \(\mathrm{S_{a}}\) is the stress amplitude, N is the number of fatigue cycles, k is the fatigue-strength coefficient, and C is the fatigue-strength coefficient defined when the number of cycles N = 1. In order to facilitate linear fitting, the logarithms of the two ends of Equation (1) are obtained in Equation (2):  
\[\mathrm{log}\mathrm{C}=\mathrm{k}\mathrm{log}\sigma_{a}+\mathrm{log}\mathrm{N} \tag{2}\]  
According to Table 3, the S-N curve of the 6082 aluminum alloy T-weld joint was drawn, in which the abscissa is logN and the ordinate is logS\({}_{\mathrm{a}}\). The data of Table 3 were substituted and linear fitting was performed by using Origin software to obtain Figure 7.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{\begin{tabular}{c} **Maximum Stress** \\ **(S\({}_{\mathrm{max}}\)/MPa)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Stress Amplitude** \\ **(S\({}_{\mathrm{s}}\)/MPa)** \\ \end{tabular} } & \multirow{2}{*}{**Life Cycle**} & \multirow{2}{*}{\
\begin{tabular}{c} **Logarithm of** \\ **S\({}_{\mathrm{a}}\)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Logarithm of** \\ **Life Cycle** \\ \end{tabular} } \\ \hline
1 & 70 & 31.5 & 44,986 & 1.498 & 4.653 \\
2 & 60 & 27 & 789,367 & 1.431 & 5.897 \\
3 & 55 & 24.75 & 329,704 & 1.394 & 5.518 \\
4 & 50 & 22.5 & 641,012 & 1.352 & 5.807 \\
5 & 45 & 20.25 & 738,243 & 1.306 & 5.868 \\
6 & 42.5 & 19.125 & 456,113 & 1.281 & 5.659 \\
7 & 40 & 18 & 2,525,146 & 1.255 & 6.402 \\
8 & 37.5 & 16.875 & 3,441,430 & 1.227 & 6.533 \\
9 & 35 & 15.75 & 10,000,000 & 1.197 & 7 \\
10 & 32.5 & 14.625 & 10,000,000 & 1.165 & 7 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Fatigue-test data.  
Figure 6: Hardness distribution of T-type joints on the cross plate: (**a**) T-type joint transverse hardness; and, (**b**) T-type transverse-point joint hardness test.  
From the linear fitting results in Figure 7, it can be obtained that the life of the 6082 aluminum alloy T-welded joint increases significantly with the decrease of stress amplitude. When the stress level increased, the life of the welded joint rapidly decreased. The linear formula that is obtained by fitting is:  
\[\text{log N}=-0.13023\text{ logS}_{\text{a}}+\text{log}\,125.5 \tag{3}\]  
It can be estimated by Equation (3) that the fatigue limit of the 6082 aluminum alloy T-weld joint is 37.6 MPa, and, when it exceeds this limit, the T-weld joint breaks. Analysis of the 6082 aluminum alloy T-weld joint with fatigue fracture found that all fracture locations occurred in the weld, indicating that the weld is a weak position for the welded joint.