Effect Of Stress Ratio On The Corrosion Fatigue Crack Growth Rate Of D36 Steel

With the development and utilization of marine resources in recent years, steel platform has played a more and more important role. But offshore platform is suffered from many severe conditions, including mechanical factors and environmental factors. Corrosion fatigue is a fracture behavior of structures under the combined actions of fluctuating loads and corrosive environment. It is one of the main failure modes for naval architecture. And the offshore platform is prone to corrosion fatigue damage. Therefore, it is essential to study corrosion fatigue properties of materials. In order to reduce and prevent the corrosion fatigue from platform steel, cathodic protection technique has been adopted in many projects. Hence, the study on corrosion fatigue of platform steel under cathodic protection should be paid more attention.With respect to an important mechanical factor-stress ratio and an important characteristic of corrosion fatigue-stable crack growth rate, the effect of stress ratio on fatigue crack propagation rate (FCPR) of D36 steel in air, in sea water, and the effect of potentials on the corrosion fatigue crack rate are mainly studied by the present paper. The following work is carries out:1 The effect of different stress ratios R=0.1,0.3,0.5 on fatigue crack propagation rate (FCPR) of D36 steel in air, sea water was studied with three-point bending experiment. A systematic contrast of da/dN-△K in different environments was made and the following conclusions were made:(1) In air, the results reveal that the da/dN-△K curve moves to lower △K with larger stress ratio in different environments. And an increase in load ratio leads to an slight change in fatigue crack propagation rate in air at a fixed cyclic stress intensity, △K. (2) The fatigue crack propagation rate is accelerated greatly by corrosion environment. However, at higher △K values, the fatigue crack propagation rate is close to the rate in air, and environment effect can be negligible. Meanwhile, the corrosion fatigue crack propagation rate increases with the increase of stress ratio at Pairs regime.2 A range of potentials,-400mv,-600mv,-800mv,-1000mv,-1200mv, was respectively applied to specimens with R=0.1. The da/dN-△K curves under different potentials were contrasted, and the optimum protection potential of -800mv is identified. Both anode polarization (-400mv) and over-cathodic protection (-1200mv) will accelerate crack rate. 3 Stress ratio R was varied from 0.3 to 0.5 at -600mv and -800mv. The result shows that little protective effect is made by cathodic protection potential (-800mv) and hydrogen embrittlement is enhanced with the increase of stress ratio. On the contrary, the protective effect of -600mv is gradually enhanced. So the most ideal method of cathodic protection is keeping crack tip current constant.