Research On Fatigue Reliability Of Structural Details In Orthotropic Steel Decks Based On Probabilistic Fracture Mechanics

Fatigue cracking is one of technical bottlenecks perplexing and confronting the application and development of orthotropic steel decks(OSDs)in a long term.The coupling effects of internal and external uncertainties,including material characteristics,structural configurations,manufacturing technique,and stochastic traffic load,result in high complexity of fatigue assessment for structural details and system of an OSD.Existing deterministic methods are incapable of taking into account the influence of randomness from these critical factors,and may give nonconservative fatigue assessment results.Considering the deficiencies of exiting approaches,systematic research focusing on relevant key issues such as fatigue loading,crack propagation,and fatigue reliability model was carried out.The stochastic process and elasticplastic fracture mechanics theories were introduced respectively to investigate the fatigue crack growth characteristics under constant-amplitude and random loading conditions.The probability statistics,fracture mechanics and reliability theory were incorporated into fatigue assessment framework,and approaches for fatigue reliability assessment were proposed based on probabilistic fracture mechanics.These approaches were applied to evaluate the fatigue performance of structural details corresponding typical fatigue failure modes and fatigue effects of component parameters.The major research work and achievements are as follows.(1)Load and stress spectra for fatigue assessment of OSDs.Distribution characteristics of critical parameters relating to traffic load were quantified through probability statistics based on long-term monitoring data of from typical bridges.A program simulating multi-lane stochastic traffic with mixed types of vehicles was wrote to establish fatigue load spectrum.This was incorporated with the stress influence surface obtained by finite element modelling to determine the stress spectra of structural details and corresponding statistical characteristics.(2)Randomness and prediction model of fatigue crack growth.Considering the uncertainty of fatigue crack growth caused by material inhomogeneity,the stochastic process theory was introduced to randomize fatigue crack growth law along the path of the crack.The coefficients of the fatigue crack growth law were derived through statistical analysis of fatigue crack growth data of standard specimens.According to this,a stochastic crack growth model which is appropriate for constant-amplitude cyclic loading,was developed and the rationality of the model was demonstrated.The results indicate that this model agrees well with experimental results,and can properly reflect the fatigue crack growth behavior of structural details in OSDs.(3)Fatigue reliability assessment approach based on probabilistic fracture mechanics under constant-amplitude cyclic loading.According to the stochastic crack growth model,a computational procedure for crack growth and model updating was developed to simulate the dynamic process of fatigue crack growth.The limit state equation of fatigue failure and reliability assessment model based on probabilistic fracture mechanics were proposed to assess the time-variant fatigue reliability of structural details,and the influence of material inhomogeneity was further revealed.(4)Fatigue crack propagation effect and theoretical model under variable-amplitude cyclic loading.The crack growth behaviour(acceleration or retardation)induced by load interaction effects was explored.The incremental crack growth kinetics and elastic-plastic fracture mechanics theories were employed to investigate the crack growth and plastic zone interaction around the crack tip at the small-time scale.A fatigue crack growth formulation under stochastic loads was established,and this model is capable of reasonably describing the crack growth properties of structural details in OSDs under random variable-amplitude loadings.(5)Time-variant fatigue reliability of typical structural details in OSDs under stochastic loads.The limit state equation was established according to the fatigue failure criterion,and the probability statistics was performed to characterize the distribution of random parameters.The fatigue reliability assessment approach under variable-amplitude cyclic loadings was proposed based on probabilistic fracture mechanics to make a quantitative evaluation of time-variant fatigue reliability of structural details as well as the influence of load interaction effects.The research suggests that the results based on this approach are consistent with existing conclusions and fatigue damage of OSDs in service.(6)Reliability-based evaluation for fatigue effects of component parameters.For uncertainties inherent in fatigue problem,the probabilistic fracture mechanics was utilized to take into account the influence of key stochastic factors on fatigue assessment,and the reliability-based approach was present to evaluate the fatigue effects of component parameters.The time-varying fatigue reliability was taken as the index,and the fatigue effects of thickness and matching combination of structural components was assessed from the three levels of fatigue failure modes,structural details and system.