## 3 The Test Results and Discussion
### Cyclic Behavior and Damage Analysis  
The number of semi-cycles until the specimens were fractured are listed in Table 3, which are about the welding material and its base material of welded joints in the steel tubular truss structure. It can be revealed by Table 3 that the cyclic life of the welded metal is less than that of the base metal with the same working condition, which indicates that the speed of damage accumulation of the welding material is faster than that of the steel, in other words, the welding materials accumulate damage more easily than the base materials. The cycling life of the welding materials decreases with the increase of the loading strain amplitude. The higher the strain amplitude level, the faster the degradation rate of the specimens under cyclic loading, and resulting in earlier destruction. Table 3 also shows that the influence of the steel rolling direction on the cycling life of steel is mainly reflected in Q235 steels. The cyclic stress-strain (\(\sigma\)-\(\epsilon\)) curves of welded specimens and steel specimens are shown in Figures 6 and 7, respectively, in which the abscissa axis represents the strain of the specimen, and the ordinate axis represents the stress. It can be seen that the stress-strain constitutive relation of the materials reflected by the hysteretic curves changes with the increase of cyclic cycles under cyclic loading. As can be studied analytically, each of the semi-cycles contains two main stages, that is, the loading stage with curvilinear segment and the unloading stage with approximate liner segment, respectively. There is an apparent difference between the curves of welded materials and steels. The welded material shows obvious damage degradation, that is, the stress amplitudes increase slightly only at the beginning of the loading, then decrease gradually with the increase of cyclic cycles, which indicate that the cyclic softening behavior of the welding material is apparent as the imposed displacement cycles increase, on the contrary, the steel shows cyclic hardening characteristics. During the cycling process, the stress level of the welded materials is higher than that of the steel specimens under the same working condition, but the cycling life of the welded material is less than that of the steel. This indicates that the welded material was hardened to a certain extent  
Figure 5: Damage process of welded specimen in low cycle fatigue test. (**a**) Crack appeared in specimen; (**b**) Crack develop; (**c**) Specim fractured at the maximum main crack. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Specimens**} & \multicolumn{6}{c}{**Welded Specimens**} & \multicolumn{6}{c}{**Steel Specimens**} \\ \cline{2-11}  & **WA1** & **WA2** & **WA3** & **WB1** & **WB2** & **WB3** & **SA1** & **SA2** & **SB1** & **SB2** \\ \hline (1) (0.25\%) & 3091 & 3810 & 3430 & 4351 & 4205 & 4293 & 4321 & 3789 & 4766 & 4703 \\ (2) (0.35\%) & 1865 & 1801 & 1831 & 2223 & 1992 & 1973 & 2109 & 1983 & 2525 & 2607 \\ (3) (0.45\%) & 1089 & 994 & 977 & 1559 & 1165 & 1106 & 1301 & 976 & & 1639 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Number of half cycles of welding material and steel. due to the influence of complex factors such as the welding process with high temperature, but its ability of plastic deformation has been weakened, which makes the welded material more prone to damage accumulation and damage degradation than its base metal. Furthermore, it is suggested that the constitutive relation between the welded metal and its base metal should be considered distinguishingly and the damage accumulation should be considered in the cyclic nonlinear analysis of the direct welded joints of the tubular truss structures. Figure 6 contains the hysteretic curves of the welding materials under different conditions of weld seam types, base metal types, and loading strain amplitudes.