Damage Behavior Of High-Speed Railway Ballastless Track And Its Effect On Structure Dynamic Performance

The failure mechanisms of structural damage and its effect on dynamic performance of ballastless tracks in high-speed railway is complicated due to the diversity of component materials, and the complexity of operation environment and train dynamic loads. This research field in China is still on the initial stage and lacks with little theoretical research. Therefore, it is of great significance to explore the damage and cracking mechanisms of ballastless tracks, and to reveal its effect and control limit on the dynamic performance with the aims of improving design theories and maintenance technologies of ballastless track, and of enhancing the safety operation system of high-speed railway. The thesis is funded by the National Key Basic Research Program of China ("973" Program) entitled "Basic research on dynamic performance evolution and service safety of high-speed railway infrastructure (Grant No.2013CB036200)", the China Scholarship Council, and the Cultivation Program for the Excellent Doctoral Dissertation of Southwest Jiaotong University. Research works on damage behavior of high-speed ballastless tracks and its effect on structure dynamic performance as well as its control limit are carried out as follows:1. A detailed review is presented concerning on the damage and failure behavior of main components of ballastless tracks in high-speed railway, and on the dynamic service performance of ballastless tracks at home and abroad, which pointing out the research priorities and contents.2. A nonlinear dynamic model of rail fastening system involved in wheel-rail dynamic analysis is developed, which is represented by an elastic element and a fractional derivative element in parallel with a nonlinear friction element. This model can well take into account the influences of the excitation frequency and of the displacement amplitude. Further, parameters of the model are determined through the experiment test of the static stiffness of rail fastening systems.3. By virtue of the secondary development language of FE software, dynamic models of ballastless tracks involved in wheel-rail dynamic analysis are developed accounting the nonlinearity of rail fastening system and the damage and cracking behavior of ballastless tracks. The method not only can be flexibly employed to establish complicated ballastless tracks with different kinds of damage and crack, but also can ensure reliable dynamic simulation results of ballastless tracks.4. A detailed dynamic interaction model of high-speed vehicles and ballastless tracks is established and the corresponding simulation software CVTDYNA is developed, based on vehicle-track coupled dynamic theory. The validity and reliability of the dynamic model and simulation software are supported by the comparison with the results calculated by commercial software SIMPACK. and the field test in Harbin-Dalian high-speed passenger lines.5. The nonlinear behavior of rail pad depended on the excitation frequency and the displacement amplitude is discussed in detail, and its effect on the dynamic characteristics of wheel-rail systems is investigated as well. A FE model for dynamic analysis of rail fastening clip is created on the basis of the nonlinear contact theory. The nonlinear vibration characteristics of the clip with and without the influence of rail corrugations are studied and compared with field test results, and the control limit of the rail fastening clip in service is investigated. Results show that the stiffness of rail fastener decreases as the rail pad displacement increases, whereas it increases with an increasing excitation frequency; compared with the traditional linear model of rail fastening system, the wheel-rail force is larger at low and medium frequencies, which would tend to provide a more safe result when predicting the damage and failure behavior of railway infrastructures under vehicle dynamic loads; the acceleration of the clip with the influence of rail corrugations can reach ten times as large as that without the influence of rail corrugations rail corrugations; The Vossloh fastening system suggested to be timely repaired and replaced when its static stiffness exceeds 30% of the design value.6. A dynamic analysis model of ballastless tracks with cracks is built up by using extended finite element method (XFEM) on the basis of linear fracture mechanics theory. Variations and statistical characteristics of dynamic stress intensity factors (SIFs) of the through-transverse crack of supporting layer in slab track system are investigated, and the influences of subgrade modulus, crack length, crack angle, friction coefficients on dynamic SIFs are revealed in detail. Results show that the through-transverse crack appears mixed cracking mode primarily consisting of the open mode, together with the slide mode and the tear mode; the distributions of all dynamic SIFs follow approximately Gaussian distributions; lower subgrade modulus, larger crack lengths, and smaller crack angles would lead to an increase in the crack growth rate, speeding up the destruction of the structure; much attention should paid to supporting layer fracture caused by the fatigue crack growth although the crack instability propagation does not occur under one time dynamic loading.7. A statistical damage constitutive model for the cement asphalt (CA) mortar layer is developed considering the strain rate dependence under dynamic load, and comparisons with experimental data support the reliability and validity of the model. By implementing the devised constitutive model into the commercial software ABAQUS as a user-defined material subroutine, the damage evolution and dynamic performance of the CA mortar layer of ballastless tracks subjected to vehicle dynamic load are investigated. The analysis indicates that the proposed model affords capturing the nonlinear response of the strain softening because of the damage development; the C A mortar layer of the slab track exhibits a strong rate sensitivity, an increasing strain rate under vehicle dynamic load will induce a reduction in the damage development, and an increase of the dynamic strength; although a higher strain rate of CA mortar layer with large service damages can suppress the damage development to a certain extent, a large damage increment can be caused by vehicle dynamic load, and it shows an nonlinear increase with the increasing service damage. The dynamic compressive strain increase nonlinearly with the deterioration of the CA mortar layer, and the dynamic compressive stress exhibits a nonlinear decrease with an increasing function area as the CA mortar layer deteriorates.8. A concrete damage plasticity model and a cohesive zone model are introduced to capture damage mechanical behavior of a double-block track-slab and reproduce the interface damage behavior of a slab track, respectively. The damage characteristics and cracking behavior of the track-slab layer and the slab track interface under temperature and vehicle dynamic load are investigated, respectively, and their effects on the structure dynamic response are analyzed as well. The analysis indicates that during temperature drop process, the track-slab occurs damage which weaken the tensile capacity, and severe interface damage and cracking can be identified at both the sides of the slab track interface, while in the temperature rising process, the stiffness of the track slab appears recovery phenomenon due to the fact of that the internal cracks of the track-slab change to be close from open, but the damage level remains unchanged; the increase of shear stresses induced by vehicle dynamic load lead to the development of interface damage, but there is not any increase in the track-slab damage; the dynamic amplitude including displacement and acceleration of track-slab, the dynamic stress of supporting layer and foundation surface are all increased due to the track-slab damage; the interface damages of the slab track cause a significant loss of structural integrity and, therefore, substantially increase the vertical dynamic displacement and acceleration of the slab. Consequently, it will deteriorate the dynamic service performance of the slab track system.9. Concerning the typical damage problems of the CA mortar delamination and void, by considering the randomness of CA mortar void forms and primary mechanical parameters, response surface functions are established in terms of the relationship between dynamic responses of vehicle-tack systems and random variables based on the response surface method and vehicle-track dynamic analysis. The amplification factors of the wheel load reduction rate, the displacement and acceleration of track slab, and the dynamic compressive stress of CA mortar layer, are significantly influenced by the CA mortar void. Taking these amplification factors as the control indexes, the damage assessment level and control limit of CA mortar void are suggested based on the long-term dynamic performance of structures and reliability theory. Results show that the length of CA mortar void is the primary control index when determining the damage control limit of the void; the damage state is evaluated as damage level I when the reliable probability for the amplification factor which is larger than a certain value is 50%, and the damage states under which the reliable probabilities are 30% and 10% are evaluated as damage level ? and ?, respectively; the control limits of the void length for CA mortar at the damage level ?, ? and ? are suggested to be 0.60m,0.85m and 1.05m, respectively, based on the contour line of the response surface function.