With increasing demand of energy saving, -many efforts were made to employ aluminium alloys in the automobile industry. Especially, resistance spot welding has been widely used in the steel metal joining process because of its high productively and convenience. In this paper, spot weldability of 7075-T6 aluminium thin sheets for various welding conditions were examined by series of experiments. The tensile shear strength and microstructure of welded specimens was observed, and The optimal welding conditions were found for each welding conditions.

The welding residual stress should be considered in fatigue stress analysis because it develope during the process of the electric resistance spot welding and it causes bad affect on the fatigue crack initiation and growth at nugget edge of spot welded points. Therefore the accurate estimation of residual stress is crucial. In this study, nonlinear finite element analysis on welding residual stress generated during the process of the spot welding was conducted, and their results were compared with the experimental data measured by X-ray diffraction method. From the results, it was found that welding residual stress existed as tension in the nugget center and as compression around the nugget edge.

In this study, residual stresses of the weldment were calculated by finite element analysis(FEA) and experiment. And, the crack closure behaviour and fatigue crack growth characteristics in field of residual stress of A106 Gr B steel pipe weldment were investigated under various stress ratio. Obtained results are as follows. I) $K_{op}$ was independent of $K_{max}$, and load ratio in fatigue crack growth. 2) In variation of load ratio, the scatter band of crack growth curve was reduced by half considering crack closure. and 3) Neglecting crack closure behaviour, actual fatigue crack growth rate can be underestimated' and Actual fatigue crack growth rate can be overestimated by $K_{res}$, in tensile residual stress field.

The purpose of this study is to examine the characteristics of stress corrosion cracking(SCC) and acoustic emission(AE) signals for the weld HAZ of HT-60 steel under corrosion control in synthetic seawater. Corrosive environment was controlled by potentiostat, and SCC experiment was conducted using a slow strain rate test method at strain rate of 10$^{-5}$ /sec. In order to verify the miroscopic fracture behaviour of the weldment during SCC phenomena, AE test was done simultaneously. Besides, correlationship between mechanical parameters and AE ones was investigated. In case of the parent, reduction of area(ROA) at -0.5V was samller than any other applied voltage such as -0.8V and -1.1V. In addition, reduction of area for the PWHT specimens at -0.8mV was larger than that of the weldment due to the softening effect according to PWHT. In case of the weldment, a lots of events was produced because of the singularities of the weld HAZ compared with the parent.

Fatigue crack propagaion rates and characteristics of the SA516-70 steel which is used for the low temperature pressure vessels, were studied in the room temperature of 1$0^{\circ}C$ and low temperature ranges of -1$0^{\circ}C$, -3$0^{\circ}C$, -5$0^{\circ}C$, -7$0^{\circ}C$ with stress ratio of R＝0.05.