A fractographic study has been performed on the fatigue fractured butt-welded joints of an aluminum alloy, A5083-O, frequently used for low temperature applications. Axial load-controlled fatigue tests were conducted either at room temperature or at -196°C; the fracture surfaces of specimens were examined with a scanning electron microscope, and the area proportions of each distinguished characteristic facets, e. g., striation, intergranular or crystallographic decohesion, etc. were evaluated.
The area fraction of crystallographic facets, which takes the maximum in the lower range of stress intensity factor, ΔK, decreased monotonically with an increase in ΔK, and became zero at ΔK≅9MPam1/2. The tendency of this evolution was found to be the same, irrespective of base metal or weld metal, as well as for the position of the facet which could be near or deep under the specimen surface. For higher values of ΔK, the coverage of the striation facets became predominant, and the striation spacing was linearly correlated with ΔK on log-log co-ordinates regardless of the metallurgical differences. Micro-etch pits technic revealed that the crack plane{100} and the propagation direction <100> were the most pertinent, assuring that the striation formation mechanism could be explained by the model proposed by Pelloux.
