Effect Of Load Frequency On Corrosion Fatigue Crack Rate Of D36 Steel

The synergistic effect of cyclic loading and corrosion on the offshore steel structure, can lead to its corrosion fatigue damage, which is one of main failure modes of offshore engineering components. The main life of the offshore steel platform is the fatigue crack propagation stage, which accounts for 90% of the whole structure service life, while the percentage of crack initiation stage is only 10%.This dissertation studies the propagation rate of corrosion fatigue cracks of D36 steel in seawater using three-point bending specimens under different potentials and load frequencies, and obtains the relationship between the crack growth rate (da/dN) and the range of stress intensity factor â–³K. The research contents and results show as follows:(1) The fatigue tests were conducted under polarization potential of -800mV and-1200mV, at load frequencies of 0.2Hz,1Hz,5Hz, 10Hz, respectively. The results show that: the corrosion fatigue growth rate increase with the decrease of frequency in seawater; At the frequencies above 1Hz, the rate is approximately the same; However, high frequency has effect on the rate when the cracks propagate to a certain extent;(2) The effect of potential on corrosion fatigue crack propagation rate of D36 was studied. Four levels of load frequency mentioned above were imposed to the specimens in air seawater, and polarization potentials of-800mV and-1200mV. The results show that: cathodic protection does not perform very well at high frequency, but restrains the propagation of effectively at low frequency. Under 0.2Hz curves of specimens in seawater and cathodic protection intersect obviously. The effect of cathodic protection reduces as â–³K increases.(3) The mechanisms of corrosion fatigue crack propagation of D36 at different frequencies were researched. Fracture surfaces of specimens in seawater and -800mV potential under frequencies of 0.2Hz, 1Hz and 5Hz were amplified 5000 times by a scanning electron microscope. The results show that corrosion fatigue crack propagation of D36 steel mainly results from anodic dissolution, and hydrogen embrittlement emerges at 0.2Hz and 1Hz; Cathodic protection does not inhibite anodic dissolution effectively at high frequencies.