Study On Damage Mechanism And Life Assessment Of Bridge Suspenders Under Coupled Corrosion-fretting Fatigue

Suspenders,stayed-cables and main cables are crucial load-bearing components in cable-supported bridges.However,some bridge failure and damage cases in recent years reveal that premature damage is frequently observed in bridge suspenders after only a few years of service,especially at the part near the anchor socket.Investigations show that coupled corrosion-fretting fatigue may be a primary reason for the degradation and fracture of wires,and the premature failure of suspenders.Due to the concealed nature of wire damage and the ''domino effect'' of fracture,suspender damage often results in catastrophic accidents,such as bridge collapse.Besides,excessive maintenance and replacement of suspenders result in considerable repair costs,traffic interruption and negative influence on the performance of the bridge.Hence,it is necessary to reveal the failure mechanism of suspenders,accurately simulate the damage evolution and rationally evaluate the life of existing suspenders subjected to coupled action of multiple factors.In this study,the damage mechanism and life assessment of bridge suspenders under coupled corrosion-fretting fatigue are investigated through theoretical analysis,damage inspection,field measurement,laboratory test and numerical simulation.The work conducted in this paper is summarized as follows:(1)The mathematical equations of space helical steel wires were deduced and the geometrical model of multi-strand steel rope was established.A theoretical model was established to analyze the rope subjected to axial loading,and a refined finite element(FE)modeling method and procedure were proposed to develop the FE models of the wire rope cable.Based on the FE simulation,the load-bearing characteristics of the wire rope and contact situations between wires were analyzed.(2)The pattern,evolution and distribution of the suspender cracking were analyzed based on the field inspection of two suspension bridges.According to inspections on the dismantled suspenders,the damage mechanism of bridge suspenders was further investigated.Thereafter,field measurement was conducted on in-service suspenders,and the load-bearing and movement characteristics of suspenders,as well as its local bending deformation and stresses due to the relative movement between the cable and girder,was investigated.Based on the field inspection and measurement of two bridges,it is confirmed that the central clamp suppresses the relative motion between the cable and girder and mitigates the damage of bridge suspenders.(3)A feasible wear fatigue test device was designed and fretting fatigue tests of steel wires were conducted,where the wear and fatigue effects can be applied synchronously.Based on the scar and fracture analyses,the fretting fatigue mechanism of steel wires was investigated.Primary fretting fatigue properties of steel wires under various fretting amplitudes and normal forces were investigated,including friction coefficient,tangential force,contact region and fatigue lives.Numerical simulations are conducted based on the fretting fatigue testing by using a refine finite element model.Stresses in the contact region and the influence of different fretting parameters on the tangential force were analyzed.(4)A corrosion-fretting fatigue test system and the corresponding test method were developed.Corrosion-fretting fatigue tests were conducted under different fretting amplitudes,friction angles and applied current intensities.The influence of wear fatigue parameters and corrosion parameters on the coupled fatigue properties of steel wires were analyzed.Based on the corrosion-fretting fatigue tests and comparison tests,the coupling effect of multiple factors was investigated.(5)A time-dependent degradation model of the steel wire was established,where the fatigue is assumed to be dominant,with corrosion and wear working together.A time-varying reliability calculation method of suspender performance was developed based on the failure index of the steel wire and suspension cable,such as the failure area and the ultimate bearing capacity.To this end,a time-dependent performance analysis and life assessment method of suspenders were proposed,considering the effect of coupled corrosion-fretting fatigue.Finally,a case study was carried out.