考虑双周疲劳耦合破坏的钢桥地震损伤评估方法

The low-cycle fatigue failure evaluation method is the greatest and last problem remained unsolved in the field of seismic design system of steel bridges. It is a key issue of taking into account all the unfavorable factors which can decrease the low-cycle fatigue life to solve this problem. This project plan to study the combined fatigue mechanism of both high and low cycle through theoretical analysis and experiments, and investigate the effects of high-cycle fatigue damage accumulated during the bridge service time on the low-cycle fatigue life for its structural steel. Taking the conventional steel material of Q345 as test objects, this project will propose a methodology to predict the coupling fatigue of high and low cycle through the experiments on the base metal specimens, welded steel specimens and notched steel specimens. According to the mechanical properties of the high-cycle fatigue damaged specimens, the reduction regulation of the radii for the yield and boundary surface can be obtained, and then the constitutive relation for damaged structural steel can be optimized correspondingly. Finally, through introducing both the coupling fatigue index and the modified constitutive relation into the numerical models of steel bridges, the impact of accumulated fatigue damage during the service time on the structural seismic performance can be taken into consideration. The achievements expected from this project can be drawn as follows: the proposal of prediction method for combined fatigue damage which can considers the effect of welding, the establishment of constitutive relation considering the material properties degradation, exploration of the seismic performance degradation regulation with the service time for steel bridges. The achievements gained by this project will provide a theoretical basis for the seismic design methodology of steel bridges.

低周疲劳问题是建立钢桥抗震设计体系所必须解决的最后一项重大问题，而有效地评估减少低周疲劳寿命的各种因素是解决该问题的关键。本项目拟通过试验和理论研究揭示钢材双周疲劳耦合断裂机理，考察结构在运营期内所累积的高周疲劳损伤对低周疲劳寿命的影响。以我国常用的Q345钢材为试验对象，通过对母材试件、焊接试件以及刻槽试件进行试验研究，提出可以考虑焊接影响的双周疲劳损伤评估方法；根据高周疲劳损伤后的力学性能试验，获得前期损伤对钢材屈服面和边界面变化规律的影响，以优化现有的本构关系模型。在此基础上，将双周疲劳损伤指标及本构关系引入结构抗震计算模型，研究服役期疲劳损伤累积对钢桥抗震安全的不利影响。通过本项目的研究，拟取得以下成果：提出可以考虑焊接影响的双周疲劳损伤评估方法，建立可以考虑性能退化的本构关系模型，获得钢桥抗震安全性随服役时间的退化规律。研究成果可以为全面建立钢桥抗震设计体系提供理论依据。

桥梁是交通网络的重要节点，强震作用下桥梁的地震安全性是国内外关注的重点问题之一。当前的钢桥抗震设计并不考虑结构的服役历史，均按照全新结构进行抗震验算。由于地震爆发的时刻具有不确定性，应该充分认识到服役历史对结构抗震可能产生的不利影响。为此，本项目分别对钢材高周疲劳损伤累积对其低周疲劳寿命的影响、钢材高周疲劳损伤后的弹塑性滞回本构关系、强震作用下在役钢桥的弹塑性抗震性能及疲劳损伤评估三个方面展开了研究。.本项目针对钢材的高低周疲劳问题及其对钢桥抗震性能的影响开展研究，完成了对钢材的高周疲劳试验、低周疲劳试验以及高低周疲劳耦合试验，提出了考虑前期高周疲劳损伤的钢材低周疲劳寿命评估公式；对钢材的弹塑性本构关系进行了深入的研究，分析了钢材前期的高周疲劳损伤累积对其弹塑性滞回性能的影响，提出了考虑高周疲劳损伤的钢材弹塑性本构模型；采用前述材料试验研究的成果，对钢桥墩的弹塑性滞回性能和在役钢桥的抗震性能进行了分析，对比讨论了在役钢桥墩与新桥墩抗震性能之间的差异；采用实测交通流对桥梁关键传力节点进行影响线加载，统计了结构在服役期内的疲劳累积损伤量，再对桥梁结构进行地震时程反应分析，研究了在役钢桥在强震作用下的高低周疲劳耦合损伤特性。研究结果表明，钢材的高周疲劳损伤降低了其低周疲劳寿命；高周疲劳损伤后，钢材的各项力学指标均会降低；本研究提出的本构关系模型能够考虑材料的前期损伤，与试验吻合较好；在役钢桥的抗震性能随服役时长的增加而逐渐降低，节点在强震作用下的低周疲劳损伤程度随服役时长的增加而增加；对在役钢桥进行抗震安全验算时，需要考虑其服役历史。.本项目研究初步建立了在役钢桥的抗震验算方法，相关研究成果有助于发展和完善钢材的低周疲劳损伤理论及设计，可以为今后实际桥梁抗震设计提供理论基础。

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