Investigation On Thickness Effect Of Fatigue Crack Growth Rate

In large-scale ship and marine structures working in deep sea, thick plates arewidely applied. Their fatigue strength should be concerned even if the welding defectcan be neglected. Fatigue crack growth rate (FCGR) method is regarded as the nextgenerational prediction method for fatigue endurance. It is found that the FCGR willincrease with the increasing thickness of specimen in FCGR test. Therefore, thicknesseffect of FCGR research is of significance in engineering application to prevent theunderestimation of fatigue life.Thickness effect mentioned in this thesis is the phenomenon that stress statesvarying along the thickness direction in through-wall crack front lead to the differentFCGR. If the thickness increases, the plane strain dominant zone throughout thethickness will expand, and the overall FCGR of the specimen rise and approach to theFCGR of plane strain state. In order to investigate this effect, a hybridexperimental-finite element method is applied in this thesis. The research work andresult are as follows:1）Standard FCGR test on3identical high tensile steel Q370qE specimens wascarried out. The threshold value in fatigue crack growth ΔKthand two materialparameters: C and m in Paris‘s formula were obtained in the test. The test shows thatthe through-wall crack tends to be flat when the maximum cyclic load Pmaxapproaches to the limit specified by the standard. Some disadvantages of the standardtest method were pointed out.2）Three kinds of crack: Plane crack, through-wall crack with straight front andthrough-wall crack with curve front were studied based on finite element methodseparately. Mesh size near crack tips of the model, thickness determination in planestress condition were discussed. Calculation method and result of fracture parameterssuch as the stress intensity factor, local constraint factor etc. were investigated. Forcrack with curve front, the influence of non-orthotropic mesh on the stress intensityfactor and local constraint factor was studied. Their data process in non-orthotropicmesh condition was explained.3）Background and state-of-the-art of constraint factor were reviewed at first. Alocal constraint factor was proposed to quantify the stress state along the crack front. The restriction of using constraint factor to qualify the stress state was point outthrough theoretical and FE analysis. Analysis result shows that, apart from stress state,large T stress has significant effect on constraint factor. The phenomenon found in thestandard test was explained using the local constraint factor. The relationship betweenthe crack mouth opening displacement （CMOD） and the global constraint factorwas studied. And a new method of calculating the global constraint factor wasproposed. The CMOD results form measurement and FEM were compared, and thereasons of the considerable gap between measurement and FEM result werediscussed.4）The FCGR test was modified. In the test, a static load method was proposed toobtain a series of crack fronts. After geometric measurement of these fronts, the stressintensity factor and the local constraint factor of these cracks were calculated withFEM. Local constraint factor of crack fronts show that the region around the center ofthe thickness and the surface position maintain in plane strain condition and planestress condition respectively. Based on the above two fracture parameters and thegeometric parameters of these crack fronts, the FCGR of high tensile steel Q370qE inplane stress and plane strain condition was obtained. The modified method and thetraditional standard method were compared. A three dimensional fatigue crack growthsimulation procedure for thick plate based on the result in this thesis was suggested atlast.