In this research, the effect of pre-fatigue on the impact tensile properties of the laser welded butt joint of high strength steel plates with tensile strength levels of 590MPa and 780MPa (denoted by HR590 and HR780, respectively) were investigated by the split Hopkinson bar apparatus. Quasi-static tensile tests were also performed to examine the effect of strain rate on the strength and the elongation. It was found that the effect of pre-fatigue on the tensile strength of welded butt joints is quite small if the applied stress in the pre-fatigue is less than the apparent yield stress of the welded butt joints. In the results of impact tensile tests for the welded butt joint of HR780 steel, however, the fracture strain of the specimens subjected to high cycle pre-fatigue was smaller than that of the virgin specimens. This may be caused by interaction of a number of cracks introduced by pre-fatigue with high strain rate.

Water cavitation technique can be applied to modify the surface strength of materials as an attractive new peening route. By inducing cavitation of ultrahigh speed water due to water-jet, the numerous impacts induced by the cavitation bubbles impact can produce compressive residual stress on material surface in the similar way as that by shot peening. In the present paper, an automobile part of steel gear shaft with complex shape is processed by water peening process. Compressive residual stress induced by water peening was measured to investigate the surface strengthening effect as a quantitative factor. The residual stresses on the tooth surface of the gear are measured by X-ray diffraction method. The effect of process conditions such as water outlet pressure, standoff distance of the nozzle, and peening time are discussed. To investigate the effect of heat treatment, carburizing-quenching is implemented to the gear shaft. By comparing the residual stresses of the tooth before and after water peening, the effectiveness of water peening process on surface strengthening is verified for a quenched gear shaft.

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In recent automobiles, tailor welded blanks using high strength steel sheets with various combinations of thicknesses and laser welded butt joints are being widely used to reduce weight of vehicle bodies without any decrease of their stiffness and crash performance. It is well known that underfilled weld beads are formed in such butt joints due to large gaps between steel sheets in laser welding process and the underfilled weld beads deteriorate mechanical properties and metal formability. However, the influence of the underfilled weld beads on fatigue properties of the weld butt joints is not sufficiently revealed, even though the fatigue properties are practically important for actual service of automobiles. In addition, the fatigue properties of butt joints with a step cross-section are necessary to build the design standard for tailor welded blanks. Therefore, in this paper, tensile tests and fatigue tests have been conducted for the laser welded butt joints of different thickness steel sheets with various underfilled weld beads in order to clarify their fatigue properties. Also FEM analyses have been performed to study the stress distribution, especially the stress concentration, around the weld metal in the butt joints. It is observed that the butt joints with underfilled weld beads and a step cross-section significantly deteriorate the fatigue strength, although their effects on tensile strength are small.