Tension and fatigue tests are carried out for MIG butt-welded RHA steel(Rolled Homogeneous Armour Steel;Mil-A12560G Classl) in order to investigate the effect of weld penetration on the fatigue life. The specimens are prepared with three kinds of weld penetration; (ⅰ) BG(Complete penetration with back gouging), (ⅱ) NBG(Complete penetration with no back gouging), (ⅲ) PP(Partial penetration). The width of the specimen tested is 0.5", 0.75" and 1.5", respectively.
The microstructure around HAZ(Heat Affected Zone) are optically investigated along with the hardness of the welded material. The residual stress is also measured by various techniques such as the cutting, BN(Barkhausen noise) and X-ray diffraction.
The tensile strength decreases according to the order of BG, NBG, and PP and the fatigue limit decreases according to the order of NBG, BG and PP for 0.5" and 0.75" specimens while the fatigue limit decreases according to the order of PP, NBG, BG for 1.5" specimens. For BG and NBG specimens, the fatigue crack initiates at the toe of the weld. The PP specimens show the fatigue crack initiation at the tip of the unweldede part for relatively low stress, however, they show the multiple initiations at both the toe and the tip of the unwelded part for relatively high stress, leading to the individual propagation and coalescence of the cracks.
The crack propagation rates, da/dN for base material maid in center cracked tension specimen and 0.75"PP specimens respectively are measured and compared with each other. The PP specimens show a lower da/dN than the base material specimens. For the PP specimens crack propagates in an unsymmetrical manner due to the unsymmetrical distribution of the residual stress.
