Probabilistic Fatigue Life Prediction And Reliability Assessment Of Components Under Notch And Size Effects

Fatigue strength analysis,life prediction and reliability assessment of notched components are crucial for ensuring structural integrity and implementing fatigueresistant design of major equipment.The locally non-uniform stress field caused by the geometrical discontinuities is the main area of inducing crack initiation,and the stress concentration phenomenon accelerates crack propagation.In addition,the local stressstrain state of the notched components become more complex and changeable because the lowly stressed area far from the notch root supports the highly stressed area near notch.The microstructure,material composition,defect distribution,stress gradient and other factors in the critical damage region of notched components determine the fatigue damage severity according to the existence of size effect.It is urgent to develop structural design and assessment methods for achieving the fatigue life prediction and reliability assessment of full-size components through the fatigue experimental data of small specimen.Especially,the ratchet effect generated from mean stress would produce the permanent damage in stress concentration area under the asymmetric cyclic loading,which brings great challenges for the structural strength analysis,fatigue life prediction and reliability assessment of engineering components.How to deal with the inevitable problems such as mean stress effect,notch effect and size effect derived from geometrical discontinuities and dimensional changes under asymmetric cyclic loading affected by random factors,and how to develop more resiliently structural integrity and reliability assessment methods are essential for ensuring the safe and reliable operation of equipment.In view of the current deficiencies in the research on probabilistic fatigue life prediction and reliability assessment of notched components,the following work are conducted in this thesis:(1)In view of the superiority of strain energy concept for describing fatigue damage,a notch fatigue life prediction model is proposed through coupling the theoretical stress concentration factor with strain energy concept.And a general probabilistic fatigue life prediction framework for notched components is established based on the proposed model.Finally,fatigue life prediction and reliability assessment of aero-engine compressor turbine disks considering material and load uncertainties are carried out.(2)A probabilistic fatigue life prediction method considering notch and size effects is proposed through combining the strain energy concept and the weakest link theory,which accomplishes the goal of notch fatigue life prediction through the fatigue experimental data of smooth specimen.Especially,a bridge between the fatigue life data of smooth specimen and the local strain energy density distribution of notched components is established through defining a new effective strain energy density damage parameter.In addition,a new probabilistic fatigue life assessment framework is established based on the Weibull distribution.(3)The composite engineering problems of mean stress effect,notch effect and size effect according to geometrical discontinuities and dimensional changes under asymmetric cyclic loading are comprehensively considered,a new fatigue life prediction model and reliability assessment framework are established.A new equivalent strain energy density is defined through coupling the total strain energy density,weight function and strain energy gradient.Finally,the fatigue reliability assessment of notch components considering size effect under different stress ratios is conducted.