Stress-strain-based Fatigue Criterion Of Materials And Fatigue State Evaluation Method

The engineering structure suffers the fatigue loading during operation and the continuous fatigue loading leads to fatigue damage accumulation in the material and even ultimately cause fatigue failure to the critical parts of engineering structure.Many scholars have made important achievements through material experiments research and the conclusions have been applied to fatigue evaluation in the engineering.Currently,the fatigue failure state of materials is mainly described by fatigue criteria or fatigue envelope line.However,the uncertainty in the selection of fatigue criteria and the limitations in the description of fatigue envelope lines cause the shortcomings of fatigue evaluation methods based on fatigue criteria or fatigue envelopes line in practical applications.The fatigue mechanism of fatigue criteria and fatigue envelopes in the strength theory can be interpreted as the fatigue stress-strain strength that plays a dominant role in fatigue failure.Research on the relationship between fatigue failure and fatigue state based on fatigue stress-strain strength can be available to solve the problems in existing fatigue evaluation.Therefore,this paper focuses on the relationship between fatigue stress-strain and fatigue cycles,and conducted the stress-strain-based fatigue criterion of materials and fatigue evaluation method.The main research results of this dissertation are as follows:(1)Based on the classic fatigue criteria,the basic relationship of fatigue stress-strain strength and fatigue life was deducted,and the mechanism that the fatigue failure is dominated by fatigue stress-strain strength was explained.The stress-strain-based fatigue criterion of materials was established through the static and fatigue failure results.The constraint relationship between fatigue stress-strain strength and geometric shape of fatigue envelope were determined,thereby the fatigue failure of metals,concrete can be uniformly described under different loading ranges,laying the foundation for the study of fatigue state description and evaluation methods.(2)Based on the limit relationship between the proposed criterion and the fatigue state,the fatigue loading surface function was established to describe the fatigue state of materials.The equivalent method based on the equivalent fatigue loading surface equation,characteristic point calibration,and maximum determination coefficient algorithm was proposed by analyzing the law of the fatigue loading surface function value with the fatigue loading,thereby a fatigue state description method based on stress and strain was developed.(3)The solution of the equivalent fatigue cycle number based on fatigue stress and strain is realized,and the equivalent fatigue cycle number corresponding to the fatigue stress-strain state is taken as the main parameter of the fatigue state.The quantitative analysis of the fatigue loading state and the fatigue failure state is completed by comparing the equivalent fatigue cycle number with the fatigue life.Furthermore,the fatigue state evaluation method for materials was proposed to describe the fatigue state of materials.The proposed evaluation method is applied to the beam and bridges,and the effectiveness of the proposed evaluation method is further verified.