Strain Time Histories Research Articles

An improved time domain coupled model of Cross-Flow and In-Line Vortex-Induced Vibration for flexible risers

Vortex-induced vibration (VIV) is a complicated fluid-structure interaction problem. In this paper, an improved time domain coupled model to predict Cross-Flow (CF) and In-Line (IL) VIV of flexible risers is proposed, which can take the effect of added mass coefficient into account. The functions of non-dimensional amplitude and frequency based on the forced vibration experimental data are developed to obtain VIV hydrodynamic forces. The inertia force terms caused by added mass are simplified as functions of non-dimensional frequency. For CF VIV, the non-dimensional frequency with a range of [0.125, 0.20] is deemed to be the excitation region, while the lock-in and allocation criterion of IL VIV includes 2St and 3St two excitation regions. Coupling effect of CF and IL VIV is taken into consideration by integrating a magnification model for the IL excitation force associated with CF response amplitude. The excitation forces, added mass forces and damping forces are time-varied and would be updated in each step. To verify the developed model, two test models of flexible risers under uniform current are simulated. The envelopes of RMS displacement, time histories of strain, response frequency spectra and added mass coefficients show good agreement with the measured data.

The nonlinear dynamic behaviour of tapered laminated plates subjected to blast loading

In this study, the geometrically nonlinear dynamic behaviour of simply supported tapered laminated composite plates subjected to the air blast loading is investigated numerically. In-plane stiffness, inertia and the geometric nonlinearity effects are considered in the formulation of the problem. The equations of motion for the tapered laminated plate are derived by the use of the virtual work principle. Approximate solution functions are assumed for the space domain and substituted into the equations of motion. Then, the Galerkin method is used to obtain the nonlinear algebraic differential equations in the time domain. The resulting equations are solved by using the finite difference approximation over the time. The effects of the taper ratio, the stacking sequence and thefiberorientation angleon thedynamic response areinvestigated. Thedisplacement-timeand strain-time histories are obtained on certain points in the tapered direction. The results obtained by using the present method are compared with the ones obtained by using a commercial finite element software ANSYS. The results are found to be in an agreement. The method presented here is able to determine the nonlinear dynamic response of simply supported tapered laminated plates to the air blast loading accurately.

Apt strain measurement technique for impulsive loading applications

The necessity of precise measurement of strain time history for impulsive loading applications has been addressed in the present investigation. Finite element modeling is initially carried out for a hemispherical test model and stress bar assembly to arrive at an appropriate location for strain measurement. In dynamic calibration experiments, strain measurements are performed using two wire and three wire quarter bride arrangements along with half bridge circuit. Usefulness of these arrangements has been verified by analyzing strain signals in time and frequency domains. Comparison of recovered force time histories proved that the half bridge circuit is the most suitable for such applications. Actual shock tube testing of the instrumented hemispherical test model confirmed the applicability of half bridge circuit for short duration strain measurements.

Experimental study on impact behaviour of steel plane tubular frames

This paper experimentally examines the dynamic behaviour of two steel plane tubular frames subjected to impact loading with the same impact energy. One specimen is set up to be hammer impacted on the mid-span section of the top chord between two K-joints, while another is loaded on the mid-span section of the top chord within the middle K-joint. The dynamic response of both frames are described and discussed with emphasis on the effect of impact-loading locations. Based on the experimental results, the key deformation patterns, the time history of impact force, deformation and strain are identified. The two specimens suffer significant local indentation at impacted zone, the deflection of impacted chord between two joints and the global flexural deformation of the whole frame. Moreover, the impact mechanism and energy dissipation is also investigated to evaluate the impact resistance of steel tubular frames. Additionally, based on the plastic hinge line theory, a simplified calculation method of energy dissipation is proposed and verified against the experimental result. The studies in this paper also provide basis data for future anti-impact design of steel tubular structures.

Development Of Large-Scale Three-Dimensional Seismic Ground Strain Response Analysis Method and Its Application to Tokyo using Full K Computer

We developed a large-scale three-dimensional ground analysis method aimed at improving the estimation of dynamic ground strain during earthquakes. Using the developed ground modeling and analysis method, a 40 billion degrees-of-freedom unstructured finite element ground model of a 3.25[Formula: see text]km × 3.25[Formula: see text]km area of Tokyo was generated with 0.66[Formula: see text]m sized elements, and its strain time history for ground motion of the 1995 Kobe wave was computed using the full K computer system with 82,944 compute nodes. The obtained strain response showed a complex distribution reflecting the input wave characteristics, surface topography, and underlying ground structure. We also showed the seismic response of 41,675 buildings in the target area computed using the wave at surface as an input. Such a method is expected to be useful for the improvement of seismic design and mitigation of pipelines against anticipated earthquakes.

Operational modal analysis on the hydroelastic response of a segmented container carrier model under oblique waves

Both vertical and torsional mode shapes of a segmented container carrier model towed in a model basin are estimated using the operational modal analysis. The operational modal analysis targets to identify the modal properties of a structure using measured data collected during the operation without any information about the excitation. The proper orthogonal decomposition, which is also known as Karhunen–Loeve decomposition, was used for the extraction of mode shapes, and the random decrement technique was applied for the derivation of free decay signal. The segmented hull model of 10,000 TEU container carrier with the scale of 1/60 was towed in a model basin under head and oblique waves and the strain time histories at the designated locations along the continuous backbone were measured. The sectional load, which was estimated based upon the measured strain, was processed using the proper orthogonal decomposition to obtain both vertical and torsional vibration modes. Finally, the free decay signal of each modes was extracted out of the processed data using the random decrement technique and wet damping ratios as well as damped natural frequencies were estimated.

Dynamic buckling of an imperfect elastoplastic beam subjected to an impulsive load analyzed using transport equations

In this study, we use transport equations to investigate the dynamic buckling of an imperfect elastoplastic beam, including the effect of axial stress waves, when subjected to an impulsive load. Thus, the governing equations for the elastoplastic behavior of the material are extracted using mass, energy, and momentum transport equations. We assume that the passage of the shock wave through the control volume creates five phases with four boundary discontinuities, i.e., the initial elastic phase, initial plastic phase, fluid phase, secondary plastic phase, and secondary elastic phase. Transport equations are used in integral form as well as non-physical variables to eliminate the discontinuity conditions in the governing equations. These equations are also used for continuum modeling of the elastoplastic behavior of the beam under an impulsive load and a continuous model is presented. Finally, the time histories of stress, strain, and velocity wave propagation along the beam are presented, and the results are validated based on comparisons with known solutions.

Nonlinear dynamic analysis of tapered sandwich plates with multi-layered faces subjected to air blast loading

The nonlinear dynamic behavior of simply supported tapered sandwich plates subjected to air blast loading is investigated theoretically and numerically. The plate is supposed to have both tapered core and tapered laminated face sheets and be subjected to uniform air blast load. The theory is based on a sandwich plate theory, which includes von Karman large deformation effects, in-plane stiffnesses, inertias and shear deformations. The sandwich plate theory for plates with constant thickness which have one-layered face sheets found in the literature is developed to analyze the tapered sandwich plates with multi-layered face sheets. The equations of motion are derived by the use of the virtual work principle. Approximate solution functions are assumed for the space domain and substituted into the equations. The Galerkin method is used to obtain the nonlinear differential equations in the time domain. The finite difference method is applied to solve the system of coupled nonlinear equations. The tapered sandwich plate subjected to air blast load is also modelled by using the finite element method. The displacement–time and strain–time histories are obtained. The theoretical results are compared with finite element results and are found to be in an agreement.

Strain-Based Damage-Assessment Method for Bridges under Moving Vehicular Loads Using Long-Gauge Strain Sensing

AbstractA novel damage-assessment method based on long-gauge strain sensors is presented in this paper. This method is not only suitable for rapid diagnosis of structures, but also it can be applied to long-term structural health monitoring of structures. First, theoretical derivation showed that the damage index strain time history area ratio (STHAR) of bridges under moving vehicular load was only in relation to the local bending stiffness, and it was independent of the weight, velocity, and axle numbers of the vehicular load. Second, the influence of vehicular weight, vehicular velocity, axle number, sensor gauge length, and measurement error on the method was investigated by simulation. In addition, a case study of a lab experiment was also examined. The results show that all damages can be localized accurately, and the mean relative errors between the estimated and actual damage extent were 0.7% and 12.3% for simulation and experiment, respectively. Finally, the robustness of the method was verified b...

Evaluation of cyclic fracture in perforated beams using micromechanical fatigue model

It is common practice to use Reduced Web Beam Sections (RWBS) in steel moment resisting frames. Perforation of beam web in these members may cause stress and strain concentration around the opening area and facilitate ductile fracture under cyclic loading. This paper presents a numerical study on the cyclic fracture of these structural components. The considered connections are configured as T-shaped assemblies with beams of elongated circular perforations. The failure of specimens under Ultra Low Cycle Fatigue (ULCF) condition is simulated using Cyclic Void Growth Model (CVGM) which is a micromechanics based fracture model. In each model, CVGM fracture index is calculated based on the stress and strain time histories and then models with different opening configurations are compared based on the calculated fracture index. In addition to the global models, sub-models with refined mesh are used to evaluate fracture index around the beam to column weldment. Modeling techniques are validated using data from previous experiments. Results show that as the perforation size increases, opening corners experience greater fracture index. This is while as the opening size increases the maximum observed fracture index at the connection welds decreases. However, the initiation of fracture at connection welds occurs at lower drift angles compared to opening corners. Finally, a probabilistic framework is applied to CVGM in order to account for the uncertainties existing in the prediction of ductile fracture and results are discussed.

Dynamic experimental studies on lateral behaviour of batter piles in soft clay

A series of small scale dynamic experiments have been carried out on 2 × 1 model aluminum batter pile group embedded in soft clay. The batter angles vary from 0 to 20 degrees. The pile group is subjected to sinusoidal lateral loads of different magnitudes (20–90 N), constituting 20–40% of its static ultimate capacity with the frequency in the range of 1–120 Hz. The time history of pile head displacement and bending strain along the length of pile are measured. It is found that the peak lateral displacement and peak bending strain at resonance region decrease with an increase in the batter angle, particularly for higher magnitude of lateral loads.

Use of fiber Bragg grating array and random decrement for damage detection in steel beam

A growing trend in the use of fiber Bragg grating array to detect structural damage has been observed in recent years. This paper describes the use of fiber Bragg grating optical sensors in an array to identify the location and assess the extent of damage on steel structures. A fiber optical sensing array with eight sensing elements has been designed, fabricated, and applied to measure the time history of strain at different points on a simply supported beam subjected to random loading. The wavelength shifts of the sensors are used to calculate the strain distribution along the beam. The random decrement at each point, for different modes, is extracted from the time history of the responses. The random decrements are compared at different damage ratios to an intact case to identify the existence of damage. Multichannel random decrement is applied to extract excited mode shapes. The mode shapes are then used to determine the location of the damage. The results show that the fiber optical sensor array is a reliable, fast, and accurate tool for the identification and localization of damage by using strain measurements.

Statistical Regularization for Identification of Structural Parameters and External Loadings Using State Space Models

AbstractA novel numerical approach is presented, in the time domain, to simultaneously identify structural parameters and unmeasured input loadings using incomplete output measurement only. The identification problem is formulated as an optimization process, wherein the objective function is defined as the discrepancy between the measured and the predicted data, and is solved by a damped Gauss‐Newton method. Because the proposed algorithm is a time domain technique, forward analyses are required to obtain predicted system responses so as to compute the discrepancy. Therefore, we propose an input force estimation scheme in the identification process to complete the task of input‐output forward analyses, for the case of output‐only measurement. The relationship between the unknown input loadings and the output measurement is established through a state space model, which basically formulates an ill‐posed least squares problem. A statistical Bayesian inference‐based regularization technique is presented to solve such a least squares problem. Finally, the proposed approach is illustrated by both numerical and experimental examples using output‐only measurements of either acceleration or strain time histories. The results clearly show the robustness and the applicability of the proposed algorithm to simultaneously identify structural parameters and unmeasured input loadings with a high accuracy.

Critical plane approach to multiaxial variable amplitude fatigue loading

A new critical plane approach based on the modified Manson-Coffin curve method (MMCCM) is presented in this paper for predicting fatigue lifetime under variable amplitude (VA) multiaxial fatigue loading. The critical plane is assumed to coincide with that material plane experiencing the maximum variance of the resolved shear strain. Fatigue damage is hypothesized to be a function of both the amplitude of the resolved shear strain and the so-called critical plane stress ratio. The latter quantity depends on the mean value and the variance of the stress perpendicular to the critical plane as well as on the variance of the shear stress resolved along the direction experiencing the maximum variance of the resolved shear strain. Load cycles are counted from the resolved shear strain time history by using the classic rain flow counting method. Palmgren-Miner’s linear damage rule is applied to estimate cumulative fatigue damage. The accuracy and reliability of the proposed approach is checked by using several experimental data taken from the literature. The estimated fatigue lives based on the new approach are seen to be in sound agreement with the experimental results.

Experimental and numerical studies of non-composite Steel–Concrete–Steel sandwich panels under impulsive loading

Abstract The performance of non-composite Steel–Concrete–Steel (SCS) sandwich panels under impact-induced impulsive loading was experimentally and numerically studied in this paper. Two SCS sandwich panels with different core depths were tested and the impulsive loading was applied in the laboratory by utilizing an inflated high pressure airbag to transfer the impact load from dropped projectile onto the panels. The deformation modes, applied pressures, displacement and strain responses of the panels were determined in the test. A combination of bending and shear deformation mode was observed in the SCS panel with thinner core while there was minimal visible deformation in the thicker panel. The maximum and permanent deformations of the panel with thicker panel were significantly reduced due to the increase in resistance and mass. Debonding between grout core and bottom plate was also noted in both panels during impact from the strain–time histories. Following the test, Finite Element (FE) models were constructed and verified against the test data to simulate the tests. It was shown in the FE analysis that the steel plates absorbed more energy due to its higher strength and ductility as compared to the grout core under the impact-induced impulsive pressure loading.

The Computer Simulation of the Dynamic Buckling of Cyclindrical Shells under Pulse Load

The computer simulation of the dynamic buckling on the cylindrical shell subjected to pulse load was carried out by ABAQUS/Expliect. The value of the rectangle and triangle pulse load is the same and the shape is different, and they are applied on the impact end of the cyclindrical shell. The influence of the load amplitude, the shape of load and the effect of the stress wave on the critical buckling time of cylindrical shell were discussed. The radial displacement curve was obtained which indicates that stress wave propagate along the length of the cylindrical shells, and the curve of strain time history is also acquired which is used to judge bifurcation and give the critical time.

Computer Simulation on the Dynamic Buckling of Initial-Defect Plates under Axial Step Load

Computer simulation on the dynamic buckling of different forms of initial-defect and different boundary conditions plates under axial impact load was carried out by using ABAQUS /Standard.The strain-time history curve of the symmetric point of upper and lower surface is give,they are coincidence before the buckling and suddenly separate when the buckling occur which is called be bifurcation,and the critical buckling time is also acquired. The results indicate that the influence of different initial-defect locations and amplitudes on the dynamic buckling is very great.

Performance of structural insulated panels with rigid skins subjected to windborne debris impacts – Experimental investigations

Structural insulated panels (SIPs) are high-performance building materials used as building envelope for residential, industrial and light commercial construction. When the building envelope is impacted by windborne debris in cyclonic area, the debris might perforate the panel resulting in dominant openings. Dominant openings in the building envelope might cause differential internal pressurization and result in the damage to the whole building structure. In the present study, the dynamic response of SIPs with Extended Polystyrene (i.e., EPS) foam core sandwiched by two rigid skins of Oriented Strand Board (i.e., OSB) or fiber cement board were experimentally investigated by using a pneumatic cannon system for windborne debris impact. The failure and damage modes under various projectile impact scenarios were observed and compared. The performances of the SIPs were examined quantitatively in terms of the projectile penetration length, and deformation and strain time histories of the back skin. The effects of various specimen configurations, impact locations, projectile impact velocities and boundary conditions on their performance were studied. The penetration resistance capacity of the SIPs against windborne debris impact was analyzed.

Assessment of Fatigue Behaviour of Orthotropic Steel Bridge Decks using Monitoring System

During the last decades orthotropic steel decks have become a widely used component of steel bridges, however high numbers of fatigue cracks have been recently detected in these structure types. It is well known, that the orthotropic decks are sensitive to fatigue damage, therefore safe fatigue design is important for the safety assessment of bridge structures. The fatigue damage assessment and life prediction of steel bridges can be carried out by using online structural health monitoring systems (SHM). By using the SHM system, it is possible to detect deterioration, determine anomalies and assess the safety level which will allow optimum maintenance strategies. On-structure long-term monitoring systems have been already successfully implemented on bridges all around the world in order to detect cracks just like on the orthotropic steel deck structure of M0 Háros Danube highway Bridge in Hungary. The purpose of the current research program is the investigation and analysis of the fatigue behaviour of this orthotropic steel bridge deck and the evaluation process improvement of the data rows collected by the monitoring system. Additional laboratory tests are carried out at 6 large scale test specimens at the Budapest University of Technology and Economics, Department of Structural Engineering to determine the fatigue resistance of closed section longitudinal stiffeners. In parallel finite element model is developed to simulate the fatigue behaviour of the analyzed components and to investigate its fatigue behaviour by numerical analysis. The aim of the current study is to analyze and verify the structural behaviour of the orthotropic steel deck on the basis of the executed test results, numerical simulations and the strain-time history data from the online structural health monitoring system implemented on the studied bridge.

Strain-threshold- and frequency-dependent seismic simulation of nonlinear soils

A one-dimensional equivalent linear method (EQL) is widely used in estimating seismic ground response. For this method, the shear modulus and damping ratio of inelastic soil are supposed to be frequency independent. However, historical earthquake records and laboratory test results indicate that nonlinear soil behavior is frequency-dependent. Several frequency-dependent equivalent linear methods (FDEQL) related to the Fourier amplitude of shear strain time history have been developed to take into account the frequency-dependent soil behavior. Furthermore, the shear strain threshold plays an important role in soil behavior. For shear strains below the elastic shear strain threshold, soil behaves essentially as a linear elastic material. To consider the effect of elastic-shear-strain-threshold- and frequency-dependent soil behavior on wave propagation, the shear-strain-threshold- and frequency-dependent equivalent linear method (TFDEQL) is proposed. A series of analyses is implemented for EQL, FDEQL, and TFDEQL methods. Results show that elastic-shear-strain-threshold- and frequency-dependent soil behavior plays a great influence on the computed site response, especially for the high-frequency band. Also, the effect of elastic-strain-threshold- and frequency-dependent soil behavior on the site response is analyzed from relatively weak to strong input motion, and results show that the effect is more pronounced as input motion goes from weak to strong.