This paper presents a probabilistic methodology for linear fracture mechanics analysis of cracked structures. The main focus is on probabilistic aspect related to the nature of crack in material. The methodology invol...This paper presents a probabilistic methodology for linear fracture mechanics analysis of cracked structures. The main focus is on probabilistic aspect related to the nature of crack in material. The methodology involves finite element analysis; sta- tistical models for uncertainty in material properties, crack size, fracture toughness and loads; and standard reliability methods for evaluating probabilistic characteristics of linear elastic fracture parameter. The uncertainty in the crack size can have a significant effect on the probability of failure, particularly when the crack size has a large coefficient of variation. Numerical example is presented to show that probabilistic methodology based on Monte Carlo simulation provides accurate estimates of failure prob- ability for use in linear elastic fracture mechanics.展开更多
A comprehensive approach to evaluate the remaining fatigue life and service safety of existing riveted steel bridges is developed in the dissertation, in which the effects of the fatigue failure evolution mechanism, m...A comprehensive approach to evaluate the remaining fatigue life and service safety of existing riveted steel bridges is developed in the dissertation, in which the effects of the fatigue failure evolution mechanism, multi-failure modes of riveted structure, fatigue failure characteristics of riveted multi-elements, riveted build-up multi-members of riveted steel bridges, etc. are included. A system based on the theory of fatigue accumulation damage, fracture mechanics and theory of reliability is established to assess the service safety of riveted steel bridges. The following aspects are investigated in this paper:(1)For the present problem of no adequate traffic load spectrum simulation specifications in Chinese highway and railway bridge design codes, the stress spectrum is measured at site based on the investigation of the statistic traffic data of highway and railway bridges and through the simulation of traffic load with Monte-Carlo method. Analysis is made by comparing field measurement of the stress spectrum with the calculated one of Zhejiang Road Bridge in Shanghai and Ganjiang Bridge of Jingjiu Railway. The result of stress spectrum comparison shows that the simulated traffic load spectrum established in this paper can be used to evaluate the remaining fatigue life.(2)According to the analysis of full-scale fatigue experiments in references, the S-N curve of AASHTO Category D is adopted. Based on investigation of the appropriate fatigue strength category for a riveted bridge member, linear fatigue-damage accumulation analysis approach, commonly known as Miner’s rule, is applied to calculate the fatigue life of riveted bridge members, thus determining a list of inspection priority for riveted members.(3)A fracture mechanics method to evaluate the remaining fatigue life and service safety of riveted steel bridges is proposed in the dissertation. According to the fatigue failure characteristics of riveted member, two simplified fracture mechanics models (SFMM) -CCT and DECT are developed. Due to the deficiencies of SFMM, an alternative riveted member fracture mechanics model (RMFMM) is proposed in the dissertation. The RMFMM model can reasonably incorporate the impact of riveted structure, rivet load etc. on the expansion of fatigue-caused cracks. The finite element programme of plate and shell fracture mechanics is used in calculation and regression to obtain a formula of the geographic modifying factor for RMFMM. SFMM and RMFMM are applied to calculate the fatigue life and inspection intervals of Zhejiang Road Bridge and Ganjiang Bridge. The result of calculation shows that the fatigue life of RMFMM is 35%～60% shorter than SFMM, which indicates that the impact of the rivet load and stress concentration at the rivet hole is significant.(4)In order to show the fatigue failure mechanism of riveted steel bridges, two probability fatigue failure models-brittle and ductile are proposed. Moreover, the corresponding limit-state equation and the basic variables are identified for fatigue reliability analysis, and the parameters of random variables are reasonably quantified. The proposed value of the target fatigue reliability index is 2.0. The dissertation not only presents a single angle steel probabilistic fracture failure model (SAPFFM) of riveted bridge members, but also proposes a double angle steel probabilistic fracture failure model (DAPFFM) for the first time. Furthermore, two reliability analysis programs -SAPFF and DAPFF, based on Monte-Carlo method, are developed to calculate the fatigue failure probability of SAPFFM or DAPFFM. Then SAPFFM and DAPFFM are used to predicate the time-varying fatigue reliability of main truss members in Zhejiang Road Bridge and Ganjiang Bridge. Analysis results show that DAPFFM gives inspection intervals 50%～100% longer than SAPFFM, which indicates that the reasonable choice of probabilistic fracture assessment model could produce remarkable benefit.(5)A system fatigue reliability model of riveted steel bridges is proposed in the dissertation, a basic calculation flow based展开更多
文摘This paper presents a probabilistic methodology for linear fracture mechanics analysis of cracked structures. The main focus is on probabilistic aspect related to the nature of crack in material. The methodology involves finite element analysis; sta- tistical models for uncertainty in material properties, crack size, fracture toughness and loads; and standard reliability methods for evaluating probabilistic characteristics of linear elastic fracture parameter. The uncertainty in the crack size can have a significant effect on the probability of failure, particularly when the crack size has a large coefficient of variation. Numerical example is presented to show that probabilistic methodology based on Monte Carlo simulation provides accurate estimates of failure prob- ability for use in linear elastic fracture mechanics.
文摘A comprehensive approach to evaluate the remaining fatigue life and service safety of existing riveted steel bridges is developed in the dissertation, in which the effects of the fatigue failure evolution mechanism, multi-failure modes of riveted structure, fatigue failure characteristics of riveted multi-elements, riveted build-up multi-members of riveted steel bridges, etc. are included. A system based on the theory of fatigue accumulation damage, fracture mechanics and theory of reliability is established to assess the service safety of riveted steel bridges. The following aspects are investigated in this paper:(1)For the present problem of no adequate traffic load spectrum simulation specifications in Chinese highway and railway bridge design codes, the stress spectrum is measured at site based on the investigation of the statistic traffic data of highway and railway bridges and through the simulation of traffic load with Monte-Carlo method. Analysis is made by comparing field measurement of the stress spectrum with the calculated one of Zhejiang Road Bridge in Shanghai and Ganjiang Bridge of Jingjiu Railway. The result of stress spectrum comparison shows that the simulated traffic load spectrum established in this paper can be used to evaluate the remaining fatigue life.(2)According to the analysis of full-scale fatigue experiments in references, the S-N curve of AASHTO Category D is adopted. Based on investigation of the appropriate fatigue strength category for a riveted bridge member, linear fatigue-damage accumulation analysis approach, commonly known as Miner’s rule, is applied to calculate the fatigue life of riveted bridge members, thus determining a list of inspection priority for riveted members.(3)A fracture mechanics method to evaluate the remaining fatigue life and service safety of riveted steel bridges is proposed in the dissertation. According to the fatigue failure characteristics of riveted member, two simplified fracture mechanics models (SFMM) -CCT and DECT are developed. Due to the deficiencies of SFMM, an alternative riveted member fracture mechanics model (RMFMM) is proposed in the dissertation. The RMFMM model can reasonably incorporate the impact of riveted structure, rivet load etc. on the expansion of fatigue-caused cracks. The finite element programme of plate and shell fracture mechanics is used in calculation and regression to obtain a formula of the geographic modifying factor for RMFMM. SFMM and RMFMM are applied to calculate the fatigue life and inspection intervals of Zhejiang Road Bridge and Ganjiang Bridge. The result of calculation shows that the fatigue life of RMFMM is 35%～60% shorter than SFMM, which indicates that the impact of the rivet load and stress concentration at the rivet hole is significant.(4)In order to show the fatigue failure mechanism of riveted steel bridges, two probability fatigue failure models-brittle and ductile are proposed. Moreover, the corresponding limit-state equation and the basic variables are identified for fatigue reliability analysis, and the parameters of random variables are reasonably quantified. The proposed value of the target fatigue reliability index is 2.0. The dissertation not only presents a single angle steel probabilistic fracture failure model (SAPFFM) of riveted bridge members, but also proposes a double angle steel probabilistic fracture failure model (DAPFFM) for the first time. Furthermore, two reliability analysis programs -SAPFF and DAPFF, based on Monte-Carlo method, are developed to calculate the fatigue failure probability of SAPFFM or DAPFFM. Then SAPFFM and DAPFFM are used to predicate the time-varying fatigue reliability of main truss members in Zhejiang Road Bridge and Ganjiang Bridge. Analysis results show that DAPFFM gives inspection intervals 50%～100% longer than SAPFFM, which indicates that the reasonable choice of probabilistic fracture assessment model could produce remarkable benefit.(5)A system fatigue reliability model of riveted steel bridges is proposed in the dissertation, a basic calculation flow based
