Abstract
In order to obtain useful and reliable data of fatigue strength of 5083 type aluminum alloy and its welded joint for designing of LNG tanks, the fatigue strength was studied experimentally and analyzed statistically including available data in literatures surveyed.Axial zero-to-tension fatigue tests were carried out of 25 mm thick base plate specimens and the transverse butt welded joint specimens whose reinforcements were ground partly, flushed out, and as welded. Rotating bending tests were also carried out to evaluate the fatigue strength of a heavy section aluminum plate, where the specimens were cut out of thickness direction of a 200 mm thick plate, and longitudinal of a 40 mm plate and crosswise to welding direction of the butt welded plate.Present test results show that the fatigue strength of the butt welded joint decreases remarkably due to the reinforcement of the weld, but increases up to about 1.3 times by grinding out partly the reinforcement toes, and reaches to the level of the base metal by flushing out the reinforcement. It is also noticed that the relative fatigue strength Sa/σu of the 200 mm thick plate along the short transverse direction decreases to about 72% of the strength of 40 mm thick plate at Nf = 2 ×106 mainly due to its porous structure.Statistical analysis made on the fatigue strength under axial load including the present data derived fatigue design curves presented as an exponential relation Sa/σu=C (Nf) -k, where the C and k values for the 99% survival were defined with 50% or 95% confidence.Finally fatigue design curves of the 99% survival with 95% confidence were proposed, of which reduction rates of the strength resulted in 2/3, 4/5, and 1/2 for the base metal, the butt joint without reinforcement and with reinforcement respectively. A Pohl diagram (Smax/ σuvs. R) of which Smax/σu is determined by a straight line from Smax/σu at R=0 to that for R=1 in a modified Goodman diagram (Smax/σuvs. Smin/σu) is presented as a design curve which shows the effect of the stress ratio.
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