Influence Of Loading Waveforms On Corrosion Fatigue Crack Growth For D36 Steel

In seawater, effect of corrosion and cyclic loading on the offshore steel structure leads to the corrosion fatigue damage of steel. Corrosion fatigue is a complex problem related to metal material science, mechanical, chemical, electrochemical metallurgy science. The main factors affecting corrosion fatigue are, loading frequency, stress waveform, stress ratio, the electrode potential, corrosion medium temperature, pH value and oxygen content and so on.The corrosion fatigue crack propagation tests of D36 steel were carried out in natural seawater under different waveforms and potentials. The relationship between corrosion fatigue crack growth rate da/dN and stress intensity factor amplitude â–³K was obtained. The mechanism of corrosion fatigue crack propagation of D36 steel under different waveforms in seawater is discussed.The influence of loading waveforms on corrosion fatigue crack propagation was mainly studied for D36 steel in natural seawater at different applied potentials. The main research contents and the results are as follows:(1) The KIC was calculated using three-point bending specimens with fatigue pre-cracking based on "Standard test method for plane-strain fracture toughness of metallic materials".(2) The corrosion fatigue crack growth tests of D36 steel at different potentials in nature seawater solution was conducted. The results indicate that the da/dN exhibited a minimum value at the applied potential of -800 mV (SCE), in other words,-800 mV (SCE) is the best protection potential.(3) The fracture surfaces under triangular and square waveforms were examined in detail using a scanning electron microscope. Results show that, in seawater, the corrosion fatigue crack propagation for D36 steel is controlled by anodic dissolution under triangular waveform and controlled by hydrogen embrittlement unde.r square waveform.(4) Corrosion fatigue crack propagation rate of D36 steel was investigated in nature seawater under different waveforms. The results indicate that da/dN under sinusoidal waveform is the same as that under triangular waveform and lower than the value under square waveform in natural seawater. The acceleration of square waveform on da/dN appears mainly in the lowâ–³AK range.(5) Experiments of CFCGR of D36 steel under different waveforms at the anode polarization potential and cathodic protection potential was conducted. The results show that, the accelerating action of square waveform on da/dN was greater under cathodic protection condition, Whereas, the acceleration of square waveform on da/dN reduces under anodic polarization solution.