Long-term Dynamic Loading Research Articles

Experimental study on deformation characteristics of new prestressed subgrade under static and dynamic loads

An in-deep comprehension of the static and dynamic operational characteristics of prestressed subgrade is essential for its analysis, design, and service performance evaluation. Based on the Buckingham π theory, a novel scaled static and dynamic model test system of the prestressed subgrade has been developed. The structural components, functional characteristics, and working mechanism of the test system were comprehensively elucidated, and a suite of static and dynamic model tests was conducted to study the deformation characteristics of the prestressed subgrade. It is demonstrated that the prestressed steel bars underwent prestress loss due to the additional stress induced creep of the soil elements below and adjacent to the load transfer plates (LTPs). Therefore, it is advisable to over-tension the prestressed steel bars in practical engineering. Upon the application of prestress, the subgrade surface experienced slight uplift deformation, which did not change the geometric shape and smoothness of the subgrade surface and demonstrated that the prestress reinforcement effect could diffuse to the subgrade surface. In the static double-load-plates tests, the prestressed subgrade presented obvious advantages in controlling the subgrade surface settlement and slope lateral deformation compared to the unreinforced subgrade, which could therefore improve the deformation resistance of the subgrade. In the short-term dynamic loading tests, both the acceleration and dynamic displacement of the subgrade approximately linearly decreased with an increase in the prestress, implying that the horizontal prestress had a notable beneficial impact on mitigating the subgrade vibration. Additionally, with the long-term dynamic loading, the prestress reinforcements could significantly restrain the cumulative plastic deformation of the subgrade, with the cumulative deformation decreasing as the applied prestress increased. The developed test system offers viable and implementable technical means for investigating the enhancement mechanism of a prestress reinforced subgrade, and the insights gained from the tests contribute to elucidating the effect of prestress reinforcements on the subgrade’s static and dynamic performance.

Non-linear mechanical behaviour of thermoplastic elastomeric materials and its vulcanizate under tension/tension fatigue deformation by fourier transform rheological studies

Fourier transform (FT) rheology opens up novel frontiers in understanding non-linear mechanical behaviours of polymeric materials under sustained long-term dynamic load. The prediction of the exact point of appearance of a crack in a sample under dynamic mechanical strains of large amplitude and the fatigue analysis has been made possible to such degrees of precision, previously not possible through conventional rheological and mechanical analysis. In this work, the fatigue behaviour of a thermoplastic elastomeric material (TPE) and a thermoplastic vulcanizate (TPV) from thermoplastic polyurethane (TPU) and epichlorohydrin− ethylene oxide−allyl glycidyl ether (GECO) rubber is investigated by Fourier transform studies under oscillatory strain-controlled tensional test to understand the linear and non-linear mechanical behaviour. Fatigue analysis is performed through the fingerprint harmonics of the material’s stress response, i.e., the second and third harmonics, ( I 2/1 and I 3/1) to establish the stress nonlinearity, asymmetry, and the formation of macrocracks. Furthermore, these higher harmonics are fitted with the Neo-Hooke and Mooney-Rivlin model to fundamentally understand the mode of fatigue failure, and a close agreement between theoretical fitting and experimental outcomes is established. Strain-life curves were utilized for the thorough investigation of the fatigue behaviour of the specimens and more than 9-fold enhancement in the strain life of the TPV is observed over TPU at a strain amplitude of ∼1.05 indicating an increasing ductility. Finally, the necessity of FT rheology was further emphasised as it is observed that I 2/1 and I 3/1 harmonics are more sensitive towards fatigue response as compared to the storage modulus and the complex modulus. Henceforth, the FT rheology analysis has enabled the current study to efficiently establish the ductility of each individual specimens and for prediction of the dynamic service lifetime of the developed materials.

Numerical Simulation Study on the Hydrothermal Characteristics and Dynamic Response of the Embankment–Bridge Transition Zone in Cold Regions

The chain reaction of bridge abutment structure caused by frost heave of the embankment–bridge transition zone filler in cold regions has a great impact on the smoothness of railway lines, which will bring great challenges to the normal operation, safety, and maintenance of rail transit in cold regions under the action of long-term dynamic load. In this paper, the hydrothermal model and dynamic analysis model of the embankment–bridge transition zone are established, and the frost heave deformation rule and dynamic response characteristics of the embankment–bridge transition zone under frost heave conditions are studied. The results show that the effect of the bridge abutment structure on vertical frost heave is mainly concentrated in the range of the bridge abutment, and the vertical frost heave near the abutment is much smaller than other parts away from the abutment. The dynamic response and wheel–rail contact force of the embankment–bridge transition zone are significantly different under non-frost heave and frost heave conditions. Frost heave may cause a higher risk of derailment. This study provides a scientific reference for the design, construction, and safe operation and maintenance of embankment–bridge transition zones in cold regions, and has certain practical engineering significance.

Experimental Study of the Nonlinear Behaviour of Deep-Sea Mooring Polyester Fibre Ropes

Abstract Mooring ropes are essential components of ships and offshore floating structures and they are subjected to cyclic axial loads. This study investigates the evolution of the full-cycle stiffness of fibre polyester ropes under long-term static and dynamic loading. First, the static stiffness characteristics of the ropes, including the rope elongation properties at different stages, shrinkage rates, and creep coefficients after an idle period, are examined under static loads; an empirical formula for static stiffness is established. Second, the dynamic stiffness characteristics of the ropes are investigated under cyclic loads that are typical of platform production operations. The stabilities of the structure under different tensions are compared; the effects of mean tension, tension amplitude, and load cycle on the dynamic stiffness of the ropes are analysed and an empirical formula is established to predict the dynamic stiffness during the engineering design phase. The results of this study can be helpful for the rational design of deep-sea taut-leg mooring systems because they present the evolution of the full-cycle stiffness characteristics of mooring ropes.

Experimental study on wayside monitoring method of train dynamic load based on strain of ballastless track slab

In the long-term service of ballastless track, which constantly bears the cyclic load from high-speed trains, it is important to obtain the long-term mechanical behavior of train dynamic load on the ballastless track. In this paper, a novel wayside monitoring method of train dynamic load is proposed, which takes CRTS III ballastless track as the research object and embeds Fiber Bragg grating (FBG) strain sensors into the track slab. The traditional wayside method installing strain gauges on the rail is used to compared with the sensors embedded in the track slab, which was tested in the field. Then, the time domain and frequency domain information of rail strain and slab strain are analyzed in detail, and the correlation analysis of signals is carried out. Finally, the calculation method of the running trains information and dynamic load is proposed. The results show that the vertical strain level of track slab is low under the action of high-speed train dynamic load. The time history curve shows that the adjacent wheel has obvious coupling effect on the slab strain signal, rather than rail strain. The frequency domain analysis of the signals indicates that the low-frequency components of the rail strain and slab strain have a strong correlation. However, the high-frequency components of train dynamic load are buffered out due to the effect of the fastener system. The mapping relationship between the track slab strain and the dynamic load can be found through calibration test, which provide a novel monitoring method of dynamic train load. The monitoring method can realize the real-time monitoring of train running information and record the long-term train dynamic load cycle law, and provide important parameters for the ballastless track boundary condition model.

Effects of fiber and rubber materials on the dynamic mechanical behaviors and damage evolution of shotcrete under cyclic impact load

Dynamic loads in underground construction environments can significantly damage supporting structures. The mechanical behavior of shotcrete under high strain rate impact has received significant attention. However, few researchers have investigated the performance degradation law, damage evolution process, and failure mode of shotcrete under cyclic impact load. This study utilized the split Hopkinson pressure bar (SHPB) to conduct cyclic impact loading experiments on shotcrete reinforced with copper-plated steel fiber (CSF), basalt fiber (BF), polypropylene fiber (PF), and rubber particles (RP). The addition of reinforcing materials significantly improved the dynamic mechanical properties of shotcrete. Under cyclic impact, the stress-strain curves of reinforced shotcrete basically coincided in the elastic deformation stage. As the content of the reinforcing materials increased, the dynamic mechanical properties of the shotcrete improved. The performance of fiber-reinforced shotcrete improved under the compaction effect caused by the first impact. Only rubber shotcrete sustained 6 impacts under cyclic constant impact pressure. Based on the ultrasonic velocity obtained by non-destructive testing, a cumulative damage evolution model during cyclic impact was established. The incorporation of RP effectively slowed down the damage evolution rate and increased the shotcrete's bearing capacity for cumulative damage. This paper conducted a comprehensive comparative analysis and concluded that the addition of rubber particles at a content level of 5% was the most cost-effective option to enhance the dynamic mechanical properties of shotcrete. The findings provide a theoretical reference for underground engineering supporting structures subject to long-term cyclic dynamic loading.

Study on the Vehicle-Induced LTAS Law of the Tunnel in Diatomite Area

Due to the weak characteristics of diatomite, excessive settlement may happen under long-term dynamic vibration load and affect the later operation of trains, while there are few researches on the dynamic response of highway tunnel in diatomite yet. Taking the Feifengshan high-speed railway tunnel project as the study case, the impact of different reinforcement parameters on long-term accumulated settlement (LTAS) rule of the highway tunnel in the diatomite area under train load is studied by numerical simulation and layer-wise summation method (LWSM). The results reveal that tunnel LTAS in diatomite area mainly appears in the first and the second years of the running stage of train, and the LTAS in these two years accounts for half of the ultimate stable LTAS. The longer the pile length (PL) and the larger the pile diameter (PD), the smaller the dynamic deviatoric stress (DDS) and LTAS. However, when pile lengths or pile diameter exceed a certain range (PL = 16 m or the PD = 150 mm), the control effect of increasing the PL and PD on the LTAS of the tunnel in diatomite is no longer significant. This indicates that there exist preferred values of the PL and PD. There are few cases of the construction of highway tunnels in the diatomite area before the engineering project in this paper yet. Hence, this paper will provide a reference to the construction of the foundation reinforcement scheme of high-speed railway tunnels in diatomite layer.

Resilient deformation characteristics of subgrade silty filler under intermittent train loading

The intermittent loading method has been proved to be more realistic to simulate the long-term dynamic loading of trains on the subgrade, because the time interval of adjacent trains could be considered in this loading method. The plastic deformation characteristics under intermittent loading have been elaborated in previous studies, but the resilient deformation characteristics have not been reported. By carrying out a series of cyclic triaxial tests with intermittent loading, the resilient deformation characteristics of silty filler under intermittent loading were investigated. The test results indicate that the loading intermittence had a reduction effect on the resilient deformation, and this reduction effect was most significant for incremental collapse samples, followed by the plastic creep samples, and the plastic shakedown samples were the least. There was also a critical resilient strain for this silty filler under intermittent cyclic loading, beyond which the plastic strain developed rapidly to sample failure. A classification criterion for deformation behaviors based on the difference of resilient strain between the first and second loading stages Δεe,1-2, was proposed and then verified by other intermittent loading modes in which the intermittent duration and loading cycles per stage changed. Meanwhile, the limitations of this classification criterion were clarified, and a simpler method to identify the deformation behaviors of subgrade filler was presented based on the stable value of resilient strain under intermittent cyclic loading, which could satisfy the needs of railway engineers and workers to quickly evaluate the railway subgrade in service.

PEMFC performance decay during real-world automotive operation: Evincing degradation mechanisms and heterogeneity of ageing

A long-term dynamic load cycle is performed on state-of-the-art membrane electrode assemblies, aiming to evaluate the degradation mechanisms of Polymer Electrolyte Membrane Fuel Cell under real-world automotive operations. The load cycle, adapted from the stack protocol defined in H2020 ID-FAST European project, includes load, pressure and temperatures cycling. Events that recover the temporary decay are included, specifically procedures classified in short-stops, cold-soaks, long-stops. Operando voltage and current distribution are measured through a segmented hardware, combined to local in-situ electrochemical characterization. Investigation is supported by scanning and transmission electron microscopy analysis, performed at different locations along-the-flow-field. Reversible degradation weights from few to 20 mV and changes local current distribution, mostly at air-inlet, since the dry-out of ionomer. Cycle efficiency decreases of 3%–9%: the largest irreversible performance losses are observed at air-inlet, while middle-region is the least impacted. Cathode catalyst layer and membrane are the most aged components: platinum active surface area drops in 200–400 h, because of electrochemical Ostwald ripening mechanism, and stabilizes around 62%–67% of initial value. Polymer membranes report ageing compatible with mechanical stress that causes localized thinning, increasing hydrogen crossover. Decay of ionomer in the catalyst layer is discussed, which would consistently explain alterations of mass transport resistance.

Numerical Simulation Study on Lining Damage of Shield Tunnel under Train Load

Under the long-term dynamic load influence of trains, shield tunnel structures are damaged. With the increase in operating number, cumulative damage gradually increases. When cumulative damage increases to a certain value, the tunnel lining produces cracks and loses tensile strength, which leads to tunnel deformation, damage, etc. In serious cases, the tunnel ceases operation, causing traffic accidents and casualties. Based on the finite element software ABAQUS, this paper analyses the change rule of tunnel lining damage under long-term dynamic train load and explores the influence of tunnel buried depth on the change rule of tunnel lining damage. The excitation force function is used to generate a series of dynamic and static loads superimposed by sine functions to simulate the dynamic loads of trains. Load is applied above the tunnel by writing DLOAD subprogram. The results show that the damage of tunnel lining mainly occurs at the arch foot and the structural damage in other places can be neglected. Under the same loading condition, the greater the tunnel lining damage is. Under the same loading conditions, the tunnel lining damage increases with the increase in buried depth. According to the test results, the mathematical expressions of cumulative damage value versus loading times at the location prone to fatigue damage. It provides theoretical reference for safety evaluation and protection of tunnel structure under long-term train load.

A Winkler model of monopile ratcheting under long-term dynamic cyclic loading

Offshore wind turbines (OWTs) experience a range of two-way and partial one-way cyclic load conditions during their operational lifetime. Cyclic loading with a non-zero mean can lead to ratcheting in the soil, which causes the structure to accumulate displacement and rotation over repeated loading, which can compromise the design limit requirements. In this paper a novel ratcheting model is proposed using a Winkler approach with Masing rules and controlled with just one parameter. The model is validated against 5 experimental setups and 17 long term load tests up to 105 cycles, and predicts very well the experimental response, performing comparably with other existing ratcheting models. An OWT from North Hoyle wind farm is also modelled and it is found that the closer the driving frequency is to the resonant frequency, the greater the ratcheting response. This means that for soft-stiff type OWTs, where there is little tolerance between the resonant frequency and driving frequencies, it is crucial to consider the potential of ratcheting during the design phase. The model presented here provides a simple and effective method of predicting future ratcheting with a focus on ease of calibration and implementation into existing numerical analysis tools.

Evaluating characteristics of particles’ surface micro-texture of granular materials based on the spectral analysis method

ABSTRACT Accurately evaluating particle surface micro-texture levels and distribution at different wavelength scales of granular material is critical to optimise service stability of granular material under long-term dynamic loads. Firstly, generalised regression neural network (GRNN) and empirical mode decomposition (EMD) were adopted to impute missing data points and remove the arc-shaped tendency of granular materials’ particle wear raw surface profile. Then, granular materials’ particle surface micro-texture levels and their distribution within constant bandwidth narrow band spectrum and octave/fractional octave band spectrum were obtained with spectral analysis method, which could characterise granular materials’ particle surface properties at different wavelength scales. Fourteen types of parent rocks of granular materials were tested with a modified micro tribological experiment simulating tribological behaviour among particle contact interfaces under dynamic loads. A contrastive analysis with traditionally used surface mean roughness was performed. The results indicate that the micro-texture levels and their distribution obtained in this study can detect the level of the exact texture scales (i.e. 32 and 2 μm wavelengths) significantly influencing the kinetic friction coefficient of granular materials, while surface mean roughness can only represent the global surface property. A high correlation was found between normalised micro-texture level (i.e. 32 μm) and Moh's hardness and coefficient of friction.

Accumulative Plastic Deformation of the Improved Completely Weathered Granite Subgrade in High-Speed Railway

The improvement and reuse of completely weathered granite (CWG) with poor engineering properties for backfilling embankments can help protect the environment and bring great economic benefits. The embankment of high-speed railways demands an extremely low additional subgrade settlement under long-term dynamic loads. To analyze the applicability of the improved CWG used for subgrade filling, a series of laboratory dynamic triaxial and field cyclic loading tests were carried out. On the basis of the test results, a calculation model of accumulative plastic strain considering the dynamic stress ratio and the cement content was established. Using the proposed model and the numerically calculated dynamic stress, the accumulative plastic deformation of an embankment filled with improved CWG under various driving conditions was calculated and discussed. The variation of accumulative plastic deformation of the improved CWG with loading cycles could be described by a power function. The dynamic stress level had a significant influence on the accumulative plastic strain, and the strain in the bottom layer of the subgrade bed was greatly attenuated. Accordingly, most of the accumulative plastic deformation attenuation occurred within the depth range of the subgrade bed. The final total accumulative plastic deformations of the embankment were less than 2 mm (5 mm for the limit value) after 4 million times of high-speed train loadings, proving that the improved CWG can be used for subgrade filling. The influence of the axle load on accumulative plastic deformation was more significant than that of the train speed. The proposed calculation model of the accumulative plastic strain could provide valuable reference for similar railway engineering.

Dynamic response and failure mechanisms of a laser-fabricated flexible thin film strain gauge

Increasing demand for smart devices has spurred the development of advanced sensors with smaller and more adaptable form factors. The integration of thin film technology into sensors such as strain gauges has the potential to reduce their size and allow for the use of new manufacturing techniques. However, fabrication of reliable thin film strain gauges remains a challenge. Here, we demonstrate the manufacture of a 1025 Ω flexible thin film strain gauge using a UV laser patterning process and evaluate its dynamic tensile response. The strain gauges exhibit a sensitivity comparable to conventionally fabricated commercial alternatives, and can reliably survive 106 cycles up to 1750 µε. Above this strain level, several failure mechanisms are identified, with unique electrical responses corresponding to physical damage observed on the strain gauges. These findings provide a guide to diagnose thin film strain gauge failures, demonstrate an unconventional fabrication technique, and show their potential for use in long-term dynamic load sensing applications.

Vertical dynamic response of a floating pile in unsaturated poroelastic media based on the fictitious unsaturated soil pile model

Decades of high-speed railways operation around the world have revealed an increasing number of maintenance issues of pile-supported embankments subjected to long-term dynamic train loads in typical unsaturated soft soil areas. As the physical explanation of vibration characteristics of unsaturated soil piles is a theoretical basis for pile foundation design and pile integrity detection, a developed analytical solution of the vertical dynamic response of a floating pile in homogeneous unsaturated soils is proposed in this work. Based on the fictitious unsaturated soil pile (FUSP) model, the influence of saturation degree on soil shear modulus is considered in governing equations, uncoupled by using potential function and operator decomposition methods. The vertical dynamic response of a floating pile in a frequency domain is derived by employing the pile–soil compatibility conditions and the impedance function transfer method. Time-domain results are obtained by utilizing inverse Fourier transform and the convolution theorem. The correctness of the proposed model is verified by degradation to compare with the two existing solutions. Finally, a parameter study is undertaken to find the most sensitive parameters to the dynamic response of the floating pile in unsaturated soils.

Studies on the validity of strain sensors for pavement monitoring: A case study for a fiber Bragg grating sensor and resistive sensor

To address the problems associated with the validity of pavement sensor measurement, a method of combining indoor experiments with finite element (FE) simulations for strain measurement in asphalt pavement is developed in this paper to analyze the validity of strain sensors for practical measurements. First, correlation analysis and one-way analysis of variance (ANOVA) between the simulated strain and the measured strain of the resistive (R) sensor and the fiber Bragg grating (FBG) sensor are developed, and the results show that the strain simulation of asphalt mixtures with sensors by means of FE simulation is feasible for short-term loading tests; therefore, the simulated strain without sensors is considered the true strain. The FBG sensor is more appropriate than the R sensor for use in the measurement of horizontal strain based on the stability of the regression model. Furthermore, creep experiments and FE simulations with a modified Burgers model are developed, and the results demonstrate that the FE simulation is also effective for long-term dynamic loading tests. Finally, the effect of the ratio of the modulus of the FBG sensor to that of the asphalt mixture on the stability of the regression model is analyzed, and the results suggest that the stability worsens as the modulus ratio increases at the same temperature, which could guide the selection of encapsulating materials. Moreover, the research method could also provide resources for studying the validity of sensors under more complex loading modes.

Maintenance and Inspection of Fiber-Reinforced Polymer (FRP) Bridges: A Review of Methods.

Fiber-reinforced polymers (FRPs) are materials that comprise high-strength continuous fibers and resin polymer, and the resins comprise a matrix in which the fibers are embedded. As the technique of FRP production has advanced, FRPs have attained many incomparable advantages over traditional building materials such as concrete and steel, and thus they play a significant role in the strengthening and retrofitting of concrete structures. Bridges that are built out of FRPs have been widely used in overpasses of highways, railways and streets. However, damages in FRP bridges are inevitable due to long-term static and dynamic loads. The health of these bridges is important. Here, we review the maintenance and inspection methods for FRP structures of bridges and analyze the advantages, shortcomings and costs of these methods. The results show that two categories of methods should be used sequentially. First, simple methods such as visual inspection, knock and dragging-chain methods are used to determine the potential damage, and then radiation, modal analysis and load experiments are used to determine the damage mode and degree. The application of FRP is far beyond the refurbishment, consolidation and construction of bridges, and these methods should be effective to maintain and inspect the other FRP structures.

Experimental Investigation of Influences of Traffic Load on the Church of the Assumption of the Virgin Mary in Stará Boleslav

Long-term dynamic loads represent a serious factor which compromise the safety and durability of historical buildings. Though the daily traffic may cause only low level vibrations, which an undamaged structure could safely resist, extreme values of traffic loads over longer periods can be critical having as a consequence the initiation of cracks or the growth of existing ones, such that they may endanger the structure. Good maintenance including control measurements or monitoring can aid in early damage detection and timely planning of remedial actions. The preceding statements are supported by an investigation of the effects of ambient vibrations mainly due to traffic loads on the Basilica of the Assumption of the Virgin Mary in Stará Boleslav. Attention is aimed at a structural fault, its rehabilitation and prognosis for the future.

Evaluation of the structural and waterproof performance of a precast and assembly underpass under long-term surface dynamic loads

With the development of rapid underground construction, precast assembly underpass (PAU) has been used as an alternative way to replace the traditional cast-in-place underpass. However, only limited research was conducted to investigate the behaviour of longitudinal joints on the ultra-shallow buried structure under long-term surface dynamic loads. Although the underpass segments may not be damaged under dynamic loads, the unexpected deformation of longitudinal joints can lead to the safety risk of connecting bolts and waterproofing. This paper aims to evaluate the mechanical behaviour and waterproof performance of PAU under dynamic loads. Firstly, the full-scale numerical model was carried out, and a relative circumferential deformation index of longitudinal joints is proposed on the basis of the yield strength of connecting bolts. Then, a 1/10 scaled physical model test is designed to evaluate the long-term accumulative deformation by simplifying the ground surface vehicle loads into a simple cosine wave. Moreover, based on the results of the scaled model test, a relative radial deformation index is suggested by conducting the waterproof strip shear tests. The relative circumferential and radial deformation values at the longitudinal joints are recommended less than 15 and 30 mm, respectively, which is to satisfy the standards of both structural safety and waterproof. The results were also applied successfully to a tunnelling project.

Long-term prediction of fatigue crack growth in ballastless track of high-speed railway due to cyclic train load

This paper proposes a vehicle-ballastless track coupled model with consideration of concrete cracks in high-speed railways through finite element method. Cracked Euler beam elements are used to model the concrete slab and the supporting layer with through-transverse cracks. In addition, an iterative method is proposed to estimate the fatigue crack growth of concrete under long-term train dynamic load on the basis of the Forman model. The vehicle-ballastless track coupled model with concrete cracks is employed to obtain the crack-tip stress intensity factor (CTSIF) of concrete under the condition of initial crack depth. The CTSIF induced by a high-speed vehicle is adopted in the Forman model to calculate the crack growth rate. The crack depth is updated by the number of passing vehicles in each iteration step, which is determined by a crack growth increment threshold. The dynamic characteristics of the vehicle-ballastless track coupled system are re-calculated based on the updated crack depth. Then, the effects of the crack depth, the stiffness of CA mortar and subgrade on the CTSIF of the concrete slab and the supporting layer are investigated in detail. The fatigue crack propagation process and residual service life under different initial crack depths of the supporting layer are discussed. Some practical conclusions are drawn, which can provide some theoretical guidances for the routine maintenance of slab tracks.