Study On The Corrosion Fatigue Crack Propagating Of High-strength Steel In Seawater

Marine structures suffer the coupling effect of corrosion and fatigue in seawater. The damage is severer than one in any single case. With the development of deep-water oil and gas, high-strength steels are more necessary to be applied. As well it should be payed more attention to the corrosion fatigue crack propagating (CFCP) as part of health assessment during lifetime of this material. Due to its complex mechanism and phenomenon, together with the different environment, loading and material property factors, the theory of CFCP is hard to be accepted academically and the method to solve the problem is still limited. It is required to study on the essence of CFCP and put forward a new model applied to engineering.In this paper, based on the mechanism of CFCP which related to stress corrosion crack (SCC), anodic disillusion and hydrogen induced cracking as the main theories of SCC can describe the effect on the acceleration due to corrosion environment. High-strength steel is more sensitive to hydrogen induced cracking. Based on the essence of the hydrogen induced cracking, in this paper, firstly it is introduced the experimental phenomenon and empirical formula of hydrogen enhanced localized plasticity and then give an example. Meanwhile calculate the crack tip hydrogen concentration distribution by numerical simulation. Because the hydrogen has an important influence on the parameters of traction separation law (TSL), it is conducted the simulation of stress corrosion crack with the cohesive element through ABAQUS with the cohesive zone model (CZM) to get the maximum tensile stress decreasing ratio with different concentration of hydrogen. With the influence of hydrogen diffusion, the ability to resist damage of material will weaken, so as to accelerate the crack tip cell damage, thus accelerate the crack propagating.At the same time, this article considers the anodic dissolution amount of crack tip after each cycle loading, as well as the hydrogen induced cracking. Because materials with different yield stress have different sensitivity to these two phenomena, a reasonable relevant expression of stress corrosion cracking is given.Finally, this paper presents a new model of corrosion fatigue crack growth in considering of stress corrosion cracking and purely mechanical fatigue crack growth. On the traditional Paris superposition model, consider the competition of the two, and introduce the concept of environmental factors, which is related to amplitude of stress intensity factor and fracture toughness of material and loading frequency. To a certain extent, it reflects the corrosion influence on fatigue crack growth. And the experimental dates of different yield stress of material obtained from other paper are well consistent with the results of the new model, which has an engineering significance to some extent.