Study On 3D Fatigue Crack Propagation And Residual Life Estimation Of Steel Wires Used In Cables Of Bridge

For large-scale bridge structures under long-term dynamic loads such as wind and vehicle load,the cables of them are exposed to random high-stress.Fatigue failure is a major form of structural failure,and it usually exhibits brittle failure characteristics.Once the important structures such as a large-span bridge is destroyed,it will cause incalculable losses.Therefore,the study of structural fatigue life is of great significance.Recently,researches on fatigue life is mostly carried out by fatigue tests directly or several sample tests combined with rain-flow counting method.Fatigue tests often need a long period of time,while dispersion of the results is large,and the cost is very high.Therefore,a new method to fatigue life prediction for steel components of bridge based on the basic mechanical parameters of materials.Combining cohesive force model theory with real-time rain-flow counting method,we can predict fatigue life of components,while analyze the fatigue crack propagation process.Under the background of subtopic 3,“Mechanical performance evolution of extra-large-span bridge under the action of various factors”(2015CB057703),of “The fundamental research of extra-large-span bridge on safety in design and evaluation”(a national basic project,“973” project),therefore,this thesis focused on the structural fatigue problems with the following researches being carried out:(1)Based on the geometric configuration of cohesive elements,geometric equations,element stiffness matrix,coordinate rotation matrix,constitutive law,and expansion criterion,a 12 nodes element with the thickness of zero initially using bilinear constitutive law is defined in this thesis.The deduced theory has been used to write the user-defined subroutine of the cohesive element through the UPFs interface and the APDL commands tool provided by ANSYS.Combined with a simple example,the effectiveness of the cohesive element was verified.The 12-node cohesive element is a completely quadratic element with higher accuracy,and meshing size can be appropriately increased to reduce the number of elements of the finite element model.The results of the element verification showed that the cohesive elements used in this paper performs well.(2)Several different counting methods were compared by results of stress cycle counting.A method that can extract all stress cycles completely was chosen among the methods above.the real-time stress cycle counting method used in this thesis is improved from the selected method,real-time rain-flow counting method proposed by Musallam,by using dummy peak or valley points.Thus,the stress data information accumulated in the stress stack can be forcibly extracted to empty the corresponding stack,and then the dummy peak or valley point will be removed after the forcibly extraction.Accumulation of stress point in the peak or valley stack can be avoided failure of real time extraction by using this method.Parameters of the equivalent stress cycles of the fatigue stress sequence extracted by the improved real-time rain-flow counting method proposed in this thesis follow a certain mathematical distribution: The stress amplitude approximately obeys the Weibull distribution,and mean value of the stress cycle approximately obeys the normal distribution.(3)An example of fatigue crack propagation and fatigue life prediction of steel wire is also given in this thesis.The analysis results depicted that the method is in good agreement with the test results,which can be applied to analyze the fatigue crack propagation path and fatigue life of bridge steel cables effectively.