Stress Time History Research Articles

Reducing uncertainty in time domain fatigue analysis of offshore structures using control variates

This study is concerned with time domain fatigue analysis of offshore structures subjected to random waves. The fatigue damage calculated from a single realization of the stress time history is random, thus the mean damage is typically estimated via Monte Carlo simulation (MCS), by averaging over multiple realizations. This approach is time-consuming, because each realization involves a time domain dynamic analysis, and MCS has a slow convergence rate. Variance reduction techniques can improve the efficiency of MCS, but successful implementation necessitate prior information on the system behavior, which is difficult to acquire for this high-dimensional problem. Herein, a method is developed for reducing the variance of the MCS estimator of the damage, based on a new technique known as auto control variates. The control function is constructed via artificial neural network trained from existing MCS data, thus avoiding the need for prior information or additional simulations. The proposed method has several advantages; it is unbiased, and an error estimate is available. Besides, variance reduction is implemented at the post-processing stage; allowing multiple stress locations to be evaluated from the same dynamic simulation results. The case studies include a nonlinear single-degree-of-freedom system under different scenarios, and a full nonlinear model of a floating system. The proposed method enhances the MCS efficiency for all cases, with speedups ranging from one to two orders of magnitude.

Field Test and Numerical Analysis of Dynamic Stress of High-Speed Railway Tunnel Invert in Mudstone

In order to obtain the dynamic stress of invert and its filling in the high-speed railway tunnel under the vibration load of the train, this paper uses the field measurement method to arrange the resistance strain gauge inside the invert of the Lanzhou–Urumqi high-speed railway Fuchuan tunnel. The dynamic stress of the invert and its filling was measured, and the measurement results were verified by numerical analysis, the results obtained by the two were consistent. The results show that under the action of CRH5 electric multiple units, the dynamic stress time history curve of the invert and its filling has 16 peaks, with V-shape on both sides and W-shape in the middle. When the train load speed is constant, the dynamic stress in the tunnel invert and invert filling decreases with the increase of the depth. The dynamic stress at 0.5 m below the track bed is attenuated by about 23.1% compared to 0.2 m, the dynamic stress at 2.0 m below the track bed is attenuated by 67.5% compared to 0.2 m. Tunnel invert and its filling are the main carriers of train vibration load. The dynamic stress attenuation law of the invert and its filling conforms to the exponential function distribution. Through the data fitting, the calculation formula of the dynamic stress in different depths is given. The research results provide a new method for obtaining the dynamic stress of the invert and its filling in high-speed railway tunnel.

Simulation of vertical dynamic vehicle–track interaction using a three-dimensional slab track model

When improving track design, a better understanding of the track’s damage modes is needed, and the railway industry is then dependent on the availability of accurate simulations of the dynamic vehicle–track interaction. In the present study, the vertical dynamic interaction between a travelling railway vehicle and a slab track is simulated in the time domain by using an extended state-space vector approach. A three-dimensional slab track model is launched where the rails are modelled using Rayleigh–Timoshenko beam elements and the concrete panel and roadbed are represented by using either shell or solid finite elements. From the parameterised track model, which is developed in Abaqus using Python scripts, the system matrices are exported to Matlab where the simulation of the dynamic vehicle–track interaction is performed. A complex-valued modal superposition technique is employed, which reduces the computational cost of the simulation. In a post-processing step, calculated wheel–rail contact forces from the dynamic analysis are used as input to the Abaqus track model where various track responses are evaluated. In particular, the time history of principal stresses is determined at critical locations in the concrete panel. Also the influence of the speed of the vehicle on the wheel–rail contact forces, and the influence of a transverse culvert below the track (modelled as a local increase of the foundation bedding modulus) on the track stiffness variation at the rail level, are investigated. A mesh convergence study for a range of track responses has been conducted including investigations of when to use linear or quadratic elements and shell or solid elements. Finally, the presented three-dimensional models have been compared to an alternative two-dimensional model to determine in what situations a two-dimensional model is sufficient.

Vibration fatigue dynamic stress simulation under non-stationary state

The methods of vibration fatigue stress simulation and strength evaluation under stationary state are quite complete, but the research on vibration fatigue under non-stationary state is insufficient. Based on the theory of structural dynamics and the principle of modal superposition method, the formula for calculating the dynamic stress of vibration fatigue under non-stationary state is deduced in this paper. The complex frequency response function (FRF) of each mode is calculated by finite element program, and the impulse response function (IRF) of each mode is obtained by inverse fast Fourier transform (IFFT) method, so as to realize the decoupling of each mode. Then the principle of modal superposition method is applied to time domain, considering the external multi-input loads of the structure, the dynamic stress caused by each external load can be obtained by convolution calculation based on the load and the corresponding IRF. The final dynamic stress can be obtained by superposition of the dynamic stress caused by each load. So, this method can easily calculate the effect of each mode or each external load on structural damage, which is helpful for the study of structural vibration fatigue failure reasons. Finally, the simulated dynamic stress under non-stationary state is compared with the experimental dynamic stress, and three indicators, which are average maximum stress, structural damage and frequency evolution, are taken into account in the comparison. Compared with the experimental data, the damage error calculated by the stress time history between simulation and experiment is 2.84%, the trend of stress time history obtained by simulation is almost the same as the experimental result, and the frequency evolution process under non-stationary state can also be reproduced well. The comparison results show that the proposed method in this paper can be used as an effective reference for structural vibration fatigue dynamic stress simulation under non-stationary state.

Exploring wall shear stress spatiotemporal heterogeneity in coronary arteries combining correlation-based analysis and complex networks with computational hemodynamics.

Atherosclerosis at the early stage in coronary arteries has been associated with low cycle-average wall shear stress magnitude. However, parallel to the identification of an established active role for low wall shear stress in the onset/progression of the atherosclerotic disease, a weak association between lesions localization and low/oscillatory wall shear stress has been observed. In the attempt to fully identify the wall shear stress phenotype triggering early atherosclerosis in coronary arteries, this exploratory study aims at enriching the characterization of wall shear stress emerging features combining correlation-based analysis and complex networks theory with computational hemodynamics. The final goal is the characterization of the spatiotemporal and topological heterogeneity of wall shear stress waveforms along the cardiac cycle. In detail, here time-histories of wall shear stress magnitude and wall shear stress projection along the main flow direction and orthogonal to it (a measure of wall shear stress multidirectionality) are analyzed in a representative dataset of 10 left anterior descending pig coronary artery computational hemodynamics models. Among the main findings, we report that the proposed analysis quantitatively demonstrates that the model-specific inlet flow-rate shapes wall shear stress time-histories. Moreover, it emerges that a combined effect of low wall shear stress magnitude and of the shape of the wall shear stress-based descriptors time-histories could trigger atherosclerosis at its earliest stage. The findings of this work suggest for new experiments to provide a clearer determination of the wall shear stress phenotype which is at the basis of the so-called arterial hemodynamic risk hypothesis in coronary arteries.

Influences of correlations between biaxial random vibrations on the fatigue lives of notched metallic specimens

In the study, a hybrid frequency-time method is proposed to estimate the fatigue lives of notched metallic specimens under multiaxial random vibrations with different correlations. Firstly, a testing structure is designed and the biaxial random vibration fatigue tests are conducted to study the influences of correlations between two vibration axes on the fatigue lives of the notched specimens. Then, the crack initiation point of the notched specimen is determined by the random vibration analysis with the finite element method (FEM). And the fatigue critical point is determined through the crack initiation point and the point method of the theory of critical distance (TCD). The stress time histories at the fatigue critical point are generated from the stress power spectral density (PSD) matrix by the time domain randomization approach. Finally, the fatigue life estimation is performed by the modified Wöhler curve method (MWCM) at the fatigue critical point. The results show that all the estimated fatigue lives are within the three-time scatter band of the experimental fatigue lives.

A calculation method of interior ballistic two-phase flow considering the compression and fracture process of propellant bed

The fracture degree of propellant charge on the projectile base before it is ignited is the key to study the launch safety of propellant charge and simulate the breech-blow phenomenon of the gun. In this paper, by solving the difficult problem of calculating the gas generation law of fractured propellant charge, an interior ballistic two-phase flow calculation method with mixed-charge considering the fractured propellant charge is established. Through the test method for mechanical situation of propellant charge, the time history of compression stress of propellant charge near the projectile base is acquired, which is loaded into the propellant bed model based on discrete element method(DEM). The compression and fracture process of propellant charge in the 30 mm area near the projectile base at low temperature is simulated. Then the corresponding fractured propellant charge is obtained, and next setting it as the initial condition of the interior ballistic two-phase flow calculation; moreover, the simulation of interior ballistic two-phase flow with mixed-charge considering fractured propellant charge is carried out. And the simulation results are in good agreement with the ballistic performance test results. The research in this paper provides a precise calculation method for the abnormal pressure in chamber caused by fractured propellant charge, which can be used to predict the breech-blow phenomenon of the gun and to guide the design of gun charge structure.

Hybrid approach combining modelling and measurement for fatigue damage estimation of welded connections in bridges

The fatigue life of structural steel bridges is governed by the time history of in situ stresses at its fatigue-critical structural details under service conditions. However, these stresses are often not directly and accurately measurable due to the complex geometry of the detail or due to access restrictions. This article proposes a novel methodology to address this challenge. The methodology infers stresses at fatigue-critical locations by combining in situ strain measurements taken further away from a critical location in a full-scale bridge. Strains measured at various points around the physical welded connection are used to compute the forces and moments applied at the connection. These forces are then applied to a finite element model of the connection to predict the stresses that are required to evaluate the hot spot stresses. The developed methodology is illustrated for a welded connection in a full-scale bridge. Results show that the predicted time history of hot spot stress is accurate and much more realistic than those obtained from numerical simulations. Also, the study demonstrates that the proposed methodology is applicable for interpreting measurements from full-scale bridges and can be integrated within a measurement interpretation platform for continuous bridge monitoring.

Probabilistic Fatigue Damage Analysis of Long Span Suspension Bridge Due to Wind Induced Buffeting

In order to consider the fluctuating wind load induced fatigue problem of long span suspension bridge, fatigue reliability formula is modified by assuming the fatigue life is accord with the weibull distribution. Based on the accurate bridge buffeting analysis of time history, the stress time history of components of a suspension bridge in east sea China is simulated, and then the fatigue damages and reliabilities are calculated. The results indicate that the main cables and hangers have enough fatigue reliability under the fluctuating wind load, the fatigue failure will not occur; the stiffening girder has larger fatigue damage, under 40 / (m.s-1) mean wind speed action, the girder of mid-support section’s average fatigue life is only 3.103 years, so the girder’s damage under strong wind action should be taken seriously.

Experimental investigation of stress transients of blasted RC columns in the blasting demolition of buildings

Blasting demolition is the preferred method for safely and efficiently demolishing high-rise buildings, and numerical simulation is very helpful for the design of complicated blasting schemes. In simulations of demolition projects, the explosion processes and blast loadings are usually ignored to save computer time. However, the stress transients of blasted columns at the bottom of buildings are important boundary conditions, which may affect the accuracy of simulation results. Yet, few works have investigated it and provided a detailed model. To investigate the features of stress transients of reinforced concrete (RC) columns, an experiment was conducted in the present study. In a 56-m-high frame structure demolished by controlled blasting technology, a 1000 mm × 700 mm × 4100 mm column was blasted. The collected data include strain-time histories and failure mode of the column. The study shows that explosion-induced stress-time history curves far from the blastholes are jagged-triangular, which may have multiple peaks because of the delay time errors of detonators. After detonation of explosives, the bearing capacity of the column gradually decreases, which takes several milliseconds. When the bearing capacity reduces to less than the initial axial compressive stress in the column, the initial stress releases linearly, which also takes several milliseconds. The movements of the concrete fragments bent the longitudinal reinforcement bars, which induces a strong tensile stress in the residual column; their stress-time history curves are triangular. Based on the experiment results, a stress transient concept model is proposed to detailly describe the stress state of blasted RC columns in demolished high-rising buildings, which can be applied in the numerical simulation of demolition blasting.

Fatigue residual life estimation of jib structure based on improved v‐SVR algorithm obtaining equivalent load spectrum

AbstractFatigue tests of truck crane jib structure have great difficulty, long testing cycle, and high cost. Therefore, with the portfolio strategy for “collection, prediction, measurement and simulation” being introduced, an improved support vector regression (v‐SVR) algorithm is proposed to predict the equivalent load spectrum based on the small measured load spectrum and the fatigue residual life evaluation model is built. First, the v‐SVR algorithm corrected in kernel function, decision function, and parameter optimization is utilized to acquire the equivalent load spectrum. Second, the distribution of critical points is determined by the case‐based reasoning technique. The simulation model of first main stress‐time histories for critical points is established, and the two‐dimensional stress spectrum is obtained by the rain‐flow counting method. Finally, the fatigue residual life of jib structure is estimated by the Forman formula. Taking ZLJ5551JQZ110V truck crane jib structure as an example, the effectiveness of the above method is validated.

Fatigue analysis of stay cables on the long-span bridges under combined action of traffic and wind

This paper presents a framework for the fatigue analysis of stay cables under combined action of traffic and wind. The North Channel Bridge (NCB) of Hangzhou Bay Bridge in China was used as an example for demonstration. The bridge model and the stress influence surface of stay cables were established first and then the coupled wind-vehicle-bridge system was developed. Considering the multiple random vehicles on the bridge, a simplified approach was proposed and verified for the stress calculation of stay cables under the combined loads. This simplified approach using the stress influence surface and considering the vehicle as a whole is much more efficient than the coupled dynamic analysis. The databases under individual load were then established to generate the stress time histories of all the stay cables. Finally, the fatigue analysis was performed and the cumulative fatigue damage considering the random traffic and wind was calculated. The results show that the combined actions of different loads should be considered in the analysis. An increase of the equivalent stress range and a decrease of the stress cycles are also found with the increase of daily traffic volume.

A comparative study of impact behaviour between natural flax and glass FRP confined concrete composites

This research addresses the dynamic compressive properties of flax fibre reinforced polymer (FFRP) and glass fibre reinforced polymer (GFRP) confined concrete under impact loadings. The investigations were performed by conducting impact tests using a high-speed servo-hydraulic machine, with the impact-induced strain rates ranging from 0.2 s−1 to 30 s−1. Plain concrete (PC), FFRP-PC, and GFRP-PC specimens were tested. The influences of FRP confinement and strain rate on compressive strength, stress and strain time histories were studied. The failure processes of the specimens were analysed via evaluating the high-speed camera photographs. The empirical relationship between the corresponding dynamic increase factor (DIF) of compressive strength and strain rate for PC, FFRP-PC and GFRP-PC were proposed based on experimental data. Experimental results of FFRP-PC and GFRP-PC specimens were compared to assess the confinement effectiveness of the FRP.

Corrosion fatigue analysis of stay cables under combined loads of random traffic and wind

After the failure of their protection system, stay cables may subject to environmental corrosion and fatigue loading simultaneously. To study the cable safety during the bridge service life, a general framework was proposed in this paper to estimate the corrosion fatigue life of stay cables under the combined action of random traffic and wind. A simplified approach integrated with the wind-vehicle-bridge system was first introduced to calculate the stress time histories of the stay cables. Based on the traffic data and wind records at the bridge site, the procedure to predict the stress ranges of bridges in service was applied to the North Channel Bridge (NCB) of Hangzhou Bay Bridge as an example and the most critical stay cable was determined. The time-variant corrosion fatigue model of the stay cable was then established considering the deterioration of the steel wires and the corrosion distribution inside the cable. Three levels of corrosion rates (low, medium, and high) were also defined in the model for different environmental conditions, and the corrosion fatigue analysis of the stay cable was finally conducted. The results show that the coupled effects of corrosion and fatigue greatly reduce the lifetime of the wire, and the fatigue resistance of the stay cable in the bridge service life is insufficient under certain corrosion rates.

Characterization of adhesive joints under high-speed normal impact: Part II – Numerical studies

This two-part article presents an experimental method with numerical verification of the characterization of adhesive bonds under normal impact loading. Part I presented the experimentally measured performance of static and impact behavior of two adhesives (a methacrylate and an epoxy). Part II presents the numerical modeling of the static and dynamic tests performed in Part I. The finite element simulations are used to determine approximate dynamic material modeling parameters. The simulations are also useful in estimating certain critical performance aspects of the adhesive for the proposed impact test such as the estimation of contact force time histories, the dynamic stress time history profiles within the adhesive, and to the various energy components in the adhesives and adherends.

Seismic Response and Damage Mechanism of the Subway Station in Rock and Soil Strata

Numerical simulations are carried out to study the seismic response of a subway station in rock and soil strata. In addition, during the numerical simulation, Drucker-Prager yield criterion and the constitutive model for concrete are introduced to be able to adequately describe the yield failure of surrounding rock and damage behavior of concrete. Based on the numerical results, the time histories of principal stresses, distribution of principal stresses at particular moments, the plastic deformation of surrounding ground and the damage of structure are fully studied. According to results and discussion, it can be concluded that the largest tensile stress generally occurs at corners including the spandrel, top of sidewall and the bottom of the sidewall. In addition, large shearing deformation is observed at the interface of rock and soil layer, due to the incongruous deformation. Structure damage is mainly observed at corner area in symmetrical distribution.

Development of a novel fatigue damage model for Gaussian wide band stress responses using numerical approximation methods

A significant development has been made on a new fatigue damage model applicable to Gaussian wide band stress response spectra using numerical approximation methods such as data processing, time simulation, and regression analysis. So far, most of the alternative approximate models provide slightly underestimated or overestimated damage results compared with the rain-flow counting distribution. A more reliable approximate model that can minimize the damage differences between exact and approximate solutions is required for the practical design of ships and offshore structures. The present paper provides a detailed description of the development process of a new fatigue damage model. Based on the principle of the Gaussian wide band model, this study aims to develop the best approximate fatigue damage model. To obtain highly accurate damage distributions, this study deals with some prominent research findings, i.e., the moment of rain-flow range distribution MRR(n), the special bandwidth parameter μk, the empirical closed form model consisting of four probability density functions, and the correction factor QC. Sequential prerequisite data processes, such as creation of various stress spectra, extraction of stress time history, and the rain-flow counting stress process, are conducted so that these research findings provide much better results. Through comparison studies, the proposed model shows more reliable and accurate damage distributions, very close to those of the rain-flow counting solution. Several significant achievements and findings obtained from this study are suggested. Further work is needed to apply the new developed model to crack growth prediction under a random stress process in view of the engineering critical assessment of offshore structures. The present developed formulation and procedure also need to be extended to non-Gaussian wide band processes.

Numerical Simulation of Steel Fiber's Crack Resistance on Cement-based Composites

In this paper, the cement base and steel fiber are modeled separately through a separate model. The cement base uses a concrete damage plastic model; while the steel fiber imports the programmed program into the simulation software to generate randomly distributed steel fibers, and the steel fiber uses a truss unit. After the separation model is modeled, it is combined with the extended finite element to simulate the cracks of the steel fiber reinforced cement-based beam with I-shaped notch under the action of three-point bending, and the crack propagation until the cement-based beam is completely broken. According to the simulation results, the load-deflection and load-crack opening displacement curves of the cement base were drawn; the crack evolution process was tracked; various fracture parameters were calculated; and the stress time history curves of the steel fibers in different regions were drawn. The results show that: steel fiber can effectively improve the fracture energy and fracture toughness of the cement base, and play a toughening and crack resistance effect on the cement base; the crack resistance of steel fiber is mainly produced by the steel fiber in the middle span and the crack area. The steel fibers at both ends of the beam have little effect on the cement base.

Fatigue Life Evaluation Method for Foundry Crane Metal Structure Considering Load Dynamic Response and Crack Closure Effect

To compensate for the shortcomings of quasi-static law in anti-fatigue analysis of foundry crane metal structures, the fatigue life evaluation method of foundry crane metal structure considering load dynamic response and crack closure effect is proposed. In line with the theory of mechanical vibration, a dynamic model of crane structure during the working cycle is constructed, and dynamic coefficients under diverse actions are analysed. Calculation models of the internal force dynamic change process of dangerous cross-sections and a simulation model of first principal stress-time history are established by using the steel structure design criteria, which is utilised to extract the change of first principal stress of danger points over time. Then, the double-parameter stress spectrum is obtained by the rain flow counting method. The fatigue life calculation formula is corrected by introducing a crack closure parameter that can be calculated by the stress ratio and the effective stress ratio. Under the finite element model imported into Msc. Patran, crack propagation analysis is performed by the growth method in the fatigue integration module Msc. Fatigue. Taking the metal structure of a 100/40t-28.5m foundry crane with track offset as an example, the accuracy of calculation results and the feasibility and applicability of the proposed method are verified by theoretical calculation and finite element simulation, which provide a theoretical basis for improvement of the fatigue resistance design of foundry cranes.

Reliability on ProSHAKE 2.0 in the Assessment of One Dimensional Ground Response Analysis through Verification Example

It is understood from the recent destructive earthquakes, topography, nature of the bedrock and geometry of the soil deposits are the prime factors that made modifications to the underlying earthquake motion. The influence of such confined soil states on the strong earthquake motion plays a significant task in accessing the uniqueness of ground action. In this paper, the response of the soil layers to the earthquake action of the bedrock directly under it is determined. The analysis is done through frequency domain approach. Pro-shake software 2.0 is used to arrive the reliability of the ground response study. A wide variety of output parameters such as time histories of acceleration, velocity, displacement, shear stress, shear strain, response spectra and maximum amplitude of various parameters with depth are plotted and the other scalar parameters such as peak acceleration, peak velocity, peak displacement, RMS acceleration, arias intensity, predominant period and bracketed duration was computed.