スポット溶接継手の曲げ疲労強度と冷間加工によるその改善

抄録

The spot welded specimen employed in the present study was made of mild steel sheet 1.6mm thick and lap-jointed with 2 spot welds in one row. The stress distribution on the surface of the spot welded specimen under uniform bending was investigated with brittle coating and semiconductor strain gages. The stress measured at the situation adjacent to the welds in longitudinal direction was 1.3 times higher than the nominal stress calculated on the single sheet section.
The reversed plane bending fatigue tests were made with the spot welded specimen and the plain specimen of the same sheet. The fatigue fracture of as-welded specimen was found to initiate from the inner surface adjacent to the welds. The endurance limit of as-welded specimen was 1/1.4 of the plain specimen. After the cold press working around the welds with rings and punches, the location of fatigue fracture moved to the single sheet part and the fatigue strength became equal to that of the plain specimen. However, in the cold worked specimen which fractured at the single sheet part, the non-propagated crack was also found at the situation adjacent to the welds.
By the X-ray stress measurement, the residual stress at the part adjacent to the welds of as-welded specimen was recognized as tension having the value near the yield stress of the sheet. After the cold press working, the residual stress in the same situation was converted into compression, not inducing the work hardening of the material around the welds.
The fatigue cracks adjacent to the welds were initiated at the same cycles regardless of the sign and quantity of residual stress. The residual stress, however, played an important role upon the propagation of the cracking. Under the compressive residual stress by cold press working, the rate of crack propagation was found to be suppressed extensively.