Study On Corrosion Fatigue Crack Propagtion Characteristics And Mechanisms Of Offshore Engineering Steel

In the course of service,offshore engineering structures are subject to corrosion and fatigue all the year round,and are prone to corrosion fatigue fracture,resulting in huge economic losses.In the process of corrosion fatigue,the crack initiation stage only accounts for 10%of the whole life cycle.However,the other 90%of the life cycle is in the crack growth stage.Therefore,the research on the characteristics and mechanism of corrosion fatigue crack growth can provide theoretical basis and data support for timely maintenance and residual life assessment of offshore engineering structures.In this paper,the corrosion fatigue crack growth characteristics of DH36Z35 steel pre-cracked samples used for typical offshore engineering were studied under the combined action of seawater environment and mechanical factors.The mechanism of the influence of corrosive environment on fatigue crack growth was explored.The theory and prediction model of corrosion fatigue crack growth were established.Finally the main conclusions can be drawn as followed:(1)A set of seawater corrosion environment simulation system has been independently developed,which coupled with existing fatigue testing machines and electrochemical workstations.It can not only control mechanical factors such as different loads,stress ratios,frequencies,etc.,but also simulate the cyclic flow of seawater and changes in seawater flow rate,temperature,etc.It can also in-situ monitor the electrochemical behavior evolution at the crack tip of fatigue samples.(2)The fatigue crack growth rate in seawater environment is significantly faster than that in air,with an average acceleration ratio of over one time.The higher the seawater flowrate and temperature,the faster the corrosion fatigue crack growth.The combined action of anodic dissolution and hydrogen induced cracking mechanism leads to accelerated growth of fatigue cracks in seawater environments.At the initial stage of corrosion fatigue crack growth,the mechanism of corrosion accelerating fatigue crack growth is mainly anodic dissolution;At the middle stage,corrosion products hinder medium transmission and weaken anodic dissolution;The crack tip forms a closed cell at the later stage,and hydrogen induced cracking dominates corrosion and accelerates fatigue crack growth.(3)In both air and seawater environments,the fatigue crack growth rate of DH36Z35 steel increases with the increase of stress ratio R.In seawater environment,the stress ratio has a more significant impact on the crack growth near the threshold area.Based on this,a Walker modified prediction model(da/dN)cor=CR × Cair ×(ΔK-ΔKth)mair(1-R)n was proposed,which comprehensively considers the effects of stress ratio R and threshold value △Kth.The fatigue crack growth rate of DH36Z35 steel in the air can be used to predict its fatigue crack growth rate in the seawater,which provides a more time-saving and labor-saving way for evaluating the safe service life of marine engineering structures.(4)The loading frequency has little effect on the fatigue crack growth rate of DH36Z35 steel in air,but has a significant impact on it in seawater.The fatigue crack growth rate in seawater increases with the decrease of loading frequency.Comparing the fatigue crack growth rates in seawater and air,corrosion inhibits fatigue crack growth at 30Hz and 10Hz,but accelerates fatigue crack growth at 3Hz and 1Hz.Compared to air environment,the average acceleration rates of fatigue crack growth in seawater environment at 30Hz,10Hz,3Hz,and 1Hz are-33.2%,-18.6%,61.7%,and 99.4%,respectively.The critical frequency ft of corrosion accelerated fatigue crack growth of DH36Z35 steel for offshore engineering is found to be 7Hz.(5)An improved superposition theoretical model of corrosion fatigue crack growth(?),which is strongly correlated with the loading frequency and free corrosion potential is established.Corrosion fatigue crack growth is the superposition of pure fatigue crack growth in air and stress corrosion crack growth in seawater.The stress corrosion crack growth is negatively related to the loading frequency,and the crack growth caused by stress corrosion is strongly correlated with the free corrosion potential.