Fatigue Life Prediction And Safety Assessment Of Structures Considering Parameter Uncertainties

Civil structures are prone to fatigue damage under external cyclic loads.When the fatigue damage accumulates to a certain extent,structures may suddenly collapse.Therefore,it is of great importance in both theory and practice to study the fatigue lives of civil structures,as well as their fatigue reliability.Presently in the aspect of fatigue life predictions of structures,only damage tolerance methods involving the theory of fracture mechanics take into account the initial defects existing in structures or their components due to manufacture and transportation,which is most close to the engineering reality.However,damage tolerance methods also have some limitations.For example,in fatigue life prediction of structures,once many material parameters are considered without adequate experimental measurement,historical data or finite element simulation should be employed as a substitution.At this moment,approximate values are obtained inducing extra uncertainties,which adds errors to fatigue analysis and life evaluation.Due to it,this thesis incorporates damage tolerance methods with the interval theory.Through defining material parameters as interval parameters,the improved damage tolerance methods become more suitable for practical engineering applications.By this means,a method that can predict the fatigue life intervals has been developed.The main contents and achievements of this thesis are described below.Firstly,the damage tolerance method involving the theory of elastic fracture mechanics has been improved.The interval theory is adopted in the improvement and a concept of fatigue life intervals of structures is proposed.This concept is different from the traditional definition for fatigue life that gives a deterministic value or a mean value.Then a fatigue life prediction method is established based on the improved damage tolerance method.The method can be used to design the fatigue lives of civil structures and to estimate their residual life time.The concept of fatigue life intervals may consider the uncertainties widely existing in engineering structures and thus is practical for real-world structures.Secondly,considering the existence of uncertain factors during the evaluation of structural fatigue reliability,this thesis proposes a fatigue interval reliability model that absorbs the interval theory.The model derives from the fatigue life model from the theory of fracture mechanics.The limit state equation of a structure can be established using the proposed model.Then the interval reliability index can be solved using an optimization algorithm.In this way the fatigue reliability analysis and the corresponding safety assessment of the structure is implemented.Lastly,several reinforced concrete beams have been made and tested under a single cyclic load.The experimental fatigue lives of these beams are compared with predictions given by the proposed methods.The comparison results demonstrate the feasibility and reliability of the proposed damage tolerance method and the fatigue interval reliability model.