Investigation Of Three-dimensional Crack Propagation Behavior Of TC4ELI Titanium Alloy

Owing to outstanding mechanical property,the TC4ELI titanium alloy is widely used to fabricate various marine equipment structures.During the in-service period,the intrinsic defects in titanium alloy usually propagate under fatigue loadings,which may lead to high risk for unstable failure.Therefore,the damage tolerance design is gradually adopted for the marine equipment structures to guarantee the long-term safety.However,the existing damage tolerance design principle is based on two-dimensional fracture theory,which is conservative for lightweight design of marine equipment structures.In this thesis,the fatigue crack growth of TC4ELI is systematically tested,in which the obtained results show significant dependences on both the specimen thickness and stress ratio based on the two-dimensional fracture theory.To overcome the shortcomings,the three-dimensional fracture theory is adopted to process the test data,leading to the thickness-and stress ratio-independent fatigue crack growth rate as the true material constants.Then the life and length of fatigue cracks in specimens with different thicknesses under various fatigue loadings are successfully predicted based on the obtained material constants in combination with three-dimensional fracture theory.The main content of this thesis is as follows:1.The TC4ELI titanium alloy specimens with straight through-thickness cracks are tested under different thicknesses and loading stress ratios.Meanwhile,through data processing based on three-dimensional fracture theory,the original fatigue crack growth rate da/d N～ΔK is converted to da/d N～ΔK_eff,which is independent of specimen thickness and stress ratio.Besides,the traditional TC4 titanium alloy is also tested and analyzed to systematically exploit the advantages for the TC4ELI titanium alloy especially for the fatigue crack growth property.2.From the test results,it is found that the crack propagation path of TC4ELI titanium alloy is more zig-zag than TC4 titanium alloy.The fracture morphologies of the two titanium alloy specimens after fatigue tests are quantified using white-light interferometer.Besides,the microstructures of the two titanium alloys are characterized by optical microscope.Finally,the intrinsic mechanism of crack tip deflection is revealed by analyzing the microstructure along the crack propagation path.3.The fatigue crack growth of titanium alloy samples is simulated by using the professional crack-analytical software Zencrack,and the fatigue life of the specimens is predicted based on the three-dimensional fatigue crack growth theory.The simulation results are validated by test data,which shows high-precision prediction of three-dimensional theory for fatigue life of specimens of the titanium alloy with different thicknesses under various stress ratios.The influence of crack deflection on the driving force at the crack tip is analyzed by finite element simulation for different deflection angles.Finally,the micro interaction mechanism of the crack growth effected by grain size and grain boundary are simulated by proposing a two-dimensional polycrystalline model.In this thesis,under the framework of three-dimensional fracture mechanics theory,the fatigue crack growth test,characterization and simulation of titanium alloy specimens with straight through-thickness cracks are carried out,and the fatigue crack growth life of titanium alloy specimens in different states is accurately predicted.The research content of this paper provides theoretical support for the safe operation of marine equipment structures.