Fatigue Crack Initiation Life Prediction Of Orthotropic Steel Decks Based On Damage Mechanics

As the structural form of deck panels to be widely adopted in modern long-span steel bridges, fatigue problems frequently arise from the combined effect of complex structure, multitudinous weld lines, prominent local stress concentration and overloading of orthotropic steel decks (OSDs). From the perspective of improving the precision, efficiency and convenience to application in engineering of fatigue crack initiation life prediction method, based on the life prediction model of damage mechanics theory, this thesis has predicted the fatigue crack initiation life of diaphragm-rib weld under cyclic loading, and this result was synthetically compared to those to be acquired by fatigue test and local stress strain approach.This thesis has basically accomplished five aspects as following:(1) according to the basic principles, assumptions and analysis process of the current commonly used methods in fatigue crack initiation life prediction, The scope, engineering application, test dependency, defects and improvement measures of these methods were compared and analyzed to provide a reference for selecting the appropriate method of life prediction;(2) on the basis of investigating and analyzing typical fatigue damage parts of OSDs both at home and abroad, fatigue test method was utilized to prediction the fatigue crack initiation life of diaphragm-rib weld in accordance with fatigue destructive testing of full-scale segmental model and division of two-stage model in fatigue life analysis;(3) the nonlinear continuous damage evolution model was introduced and damage equivalent stress concept was employed to convert isotropic damage under the three-dimensional state to uniaxial stress problem, and that the correlative C program was enforced to achieve the nonlinear damage accumulation and fatigue crack initiation prediction with equivalent stress method to take into account coupling effect between damage field and stress-strain field;(4) the modifier formulas of local stress-strain approach has been applied to predict the fatigue crack initiation of diaphragm-rib weld and verify the calculated life to be obtain by equivalent stress method based on damage mechanics.(5) The results of systematically comparing the three methods of fatigue crack initiation life prediction indicates that equivalent stress method has the advantages of small error, low test dependency and convenience for application, so it can be introduced and applied to fatigue life prediction of OSDs as a theoretical analysis method to be reasonable and effective of fatigue crack initiation life prediction.