Study Of Marine Used High-strength Steel Fatigue Crack Growth Rate Test And Finite Element Analysis

Not only the Marine engineering structures are complicated, but the working condition is very serious. When there are local cracks exit in the Marine engineering structure, under the certain crack propagation rate, fatigue crack propagation of Marine engineering structure will cause some damage to it. So it needs to study how to keep the Marine engineering structure safe. Therefore, the research of the propagation of the Marine engineering structure rate is a very meaningful work.This study includes four parts:the first part is about experimental studies of the fatigue crack growth rate of steel of marine engineering (Chapter Two); the second part is about relationship between the fatigue crack growth rate and fracture toughness of steel of marine engineering (Chapter three); the third part is about finite-element analysis of the fatigue crack growth rate of steel of marine engineering(Chapter four); the last part is about detail study of the fatigue crack growth rate of steel of marine engineering bases on FRANC2D/L. steel of marine engineering has its own characteristic such as thick, high strength, good fatigue resistance and so on, so steel of marine engineering EH40 was choosed as the object of study in this paper.Firstly, this paper got the formulas of the load which decides the fatigue crack propagation, then made a test of fatigue crack propagation rate about steel EH40.After that,this paper changed the seven point incremental polynomial technique which exited in the standard into five point incremental polynomial technique, programming of the it to analysis the data of the test and getting the formula of fatigue crack propagation rate. The result showed that the fatigue crack propagation rate of steel EH40 locate in the medium-speed area of fatigue crack propagation.Secondly, this paper measured the crack mouth opening displacement (CMOD) through the test of fatigue crack propagation rate, changing it into CTOD, and calculating the equation between the CTOD and the fatigue crack propagation rate da/dN.Simultaneously, this paper tested the crack opening displacement under temperature which below zero, studying the relationship between prefabricated fatigue crack propagation rate and fracture toughness CTOD. The result showed that the fatigue crack propagation had a inverse relation to fracture toughness.Then the fatigue crack growth rate was simulated by two finite element methods which included virtual crack close technique and software of FRANC2D/L respectively. The result showed that the value which got through simulated by finite element methods inosculate to experimental value, and finite element methods has the feature of good precision in simulating fatigue crack growth rate.Finally, based on the theory of couple cohesive zone model(CCZM),this paper study the formation and extension of polycrystal interface on marine used EH40 steel by FRANC2D/L,under nine boundary conditions. Meanwhile, this paper studied the formation and extension of polycrystal interface which contains includes and the polycrystal interface under the fatigue loads. The result showed that the macro fatigue crack propagation revealed the accumulation of damage of polycrystal interface in mesoscopic condition. The study of formation and extension on polycrytal interface in mesoscopic condition laid a foundation to in-depth understanding of fatigue crack propagation in macro condition.