Force Time Histories Research Articles

Development and validation of a finite element model of a small female pedestrian

Pedestrians are the most vulnerable road user and represent about 23% of the road traffic deaths in the world. A finite element (FE) model corresponding to a 5th percentile female pedestrian (F05-PS) was developed by morphing the Global Human Body Models Consortium (GHBMC) 50th percentile male pedestrian (M50-PS) model to the reconstructed geometry of a recruited small female subject. The material properties of the pedestrian model were assigned based on GHBMC M50-PS model. In model validation, the knee lateral stiffness and force time histories of F05-PS upper body showed similar trends, but softer responses than the corresponding data recorded in post mortem human surrogate (PMHS) tests and linearly scaled to average male anthropometry. Finally, the pedestrian model was verified in a Car-to-Pedestrian Collison (CPC) simulation. The marker trajectories recorded in simulation were close to the data recorded on small PMHS in testing and the model predicted typical knee ligament ruptures. Therefore, we believe the F05-PS model, the first FE model developed based on a female reconstructed geometry, could be used to improve vehicle front-end design for pedestrian protection and/or to investigate various pedestrian accidents.

Numerical Simulation of Low Impact Velocity Behaviour of Polymeric Syntactic Foam

In this study a FE model is prepared for drop weight impact hammer testing of polymeric syntactic foam. The foam is modelled using crushable foam material and hammer is modelled using bilinear material model of LS-DYNA®. A series of simulation is performed by varying density of foam and impact velocity of hammer. Based on the prepared FE model and the force-displacement relation, energy absorption of the foam is computed and compared for three densities and three velocities. A comparative study is presented based on the displacement, reaction force-time history, and forcedisplacement behaviour.

Aerodynamic characterization of barge and spar type floating offshore wind turbines at different sea states

AbstractThe wake dynamics behind a compliant floating offshore wind turbine (FOWT) with two alternative supporting platforms of spar buoy and barge platform are studied numerically. The computational model is based on the large eddy simulation and the use of the actuator disk model of the FOWT rotor in which the circular actuator disk is discretized with an unstructured two‐dimensional triangular mesh in a structured three‐dimensional Cartesian grid of the fluid domain. The wake dynamics and platform motions are calculated for laminar and turbulent inflow conditions. The flow solver is verified through a series of experimental wake measurements done behind a porous disk model and is also cross‐validated against previously published results. The dynamics of the floating spar and barge structures are calculated from wave–structure interaction for three distinctive sea states. The time history of power extraction and wind force for the floating offshore wind turbine are recorded and compared against a fixed horizontal axis wind turbine. The motion of the barge platform is found to induce low‐frequency modulation of the wake, whereas the spar buoy primarily displays similar wake dynamics to the fixed turbine. It is discussed how a single turbine utilizing the spar concept can be more energy efficient under the wave‐induced motion. Moreover, for both laminar and turbulent inflow conditions, due to the large axial oscillation experienced by the barge concept, the turbine wake recovers more rapidly. This suggests that for a tighter spacing of the turbines in a farm, the barge platform concept can be leveraged to obtain higher energy‐capturing efficiency over a fixed area.

Hydrodynamic characteristics of forced oscillation of heave plate with fractal characteristics based on floating wind turbine platform

The heave plate is widely used in the field of marine engineering because of its advantage of increasing damping and added mass. In this paper, the fractal theory is applied to the structure design of the heave plate and a fractal plate has been proposed. With the technology of dynamic mesh and user defined functions (UDF) in Fluent, numerical simulations on forced oscillation of different heave plates have been performed. At different amplitudes of harmonic motion, the hydrodynamic characteristics of solid plate, different fractal order plates (perforation ratios of 5%, 12% and 20%, respectively) and regular ones (different shapes and arrangement of holes with the same perforation ratio of 20%) are compared and analyzed. The time history of vertical force, added mass coefficient (Cm) and damping coefficient (Cd) are obtained for different heave plates. The results show that at the same oscillation period, the Cd value of each heave plate decreases with the amplitude of harmonic motion increases, eventually becoming stable, while the Cm value increases continuously. When comparing different fractal heave plates, the Cm value decreases with the increase of the fractal order, while Cd value increases firstly and then decreases at the same condition. Besides, the Cd value reaches the maximum at the second order. As the hole perimeter becomes higher, the regular ones with the perforation ratio of 20% exhibits a linear increase in damping coefficient with increasing Keulegan-Carpenter Number (KC). The damping coefficient of a fractal plate is obviously larger than that of regular ones, which indicates that the fractal plate has better performance compared to regular ones.

Simulation of ultra-low cycle fatigue cracking of coiled tubing steel based on cohesive zone model

An optimal fatigue cohesive zone model (FCZM) was developed for coiled tubing (CT) steel through experiments and finite element calculation. First, aiming at the characteristics of small diameter and thin wall of CT, arc-shaped specimen with single-edge crack was designed and processed. Ultra-low cycle fatigue (U-LCF) tests controlled by displacement were carried out to obtain the variation of both force and crack size with the number of cycles, as well as crack propagation rates corresponding to different crack sizes. Second, FCZM was established by introducing damage into the Park-Paulino-Roesler (PPR) model. Based on the measured time histories of extreme force, the optimal cohesive parameters that described the U-LCF behavior of the CT steel were determined by means of inverse analysis. Third, the optimal FCZM was used to simulate the other U-LCF processes of the CT specimen. The predicted results agreed well with the measured ones. On the one hand, a method of determining the optimal fatigue cohesive parameters is developed based on the time-history of parameter extreme in this paper. On the other hand, an FCZM is provided for life prediction of CT.

Comparative study on analytical and computational aerodynamic models for flapping wings MAVs

ABSTRACTA range of quasi-steady and unsteady aerodynamic models are used to predict the aerodynamic forces experienced by a flapping wing and a detailed comparison amongst these predictions in provided. The complexity of the models ranges from the analytical potential flow model to the computational Unsteady Vortex Lattice Method (UVLM), which allows one to describe the motion of the wake and account for its influence on the fluid loads. The novelty of this effort lies in a modification of the predicted forces as a generalisation of the leading edge suction analogy. This modification is introduced to account for the delayed stall mechanism due to leading edge flow separation. The model predictions are compared with two sets of independent experimental data and with computational fluid dynamics (CFD) simulation data available in the literature. It is found that both, the modified analytical model and the UVLM model can be used to describe the time history of the lift force, in some cases with better results than a high-fidelity CFD model. The models presented here constitute a useful basis for the aerodynamic design of bioinspired flapping-wings micro-air vehicles.

Wind fragility analysis of RC chimney with temperature effects by dual response surface method

Wind fragility analysis (WFA) of concrete chimney is often executed disregarding temperature effects. But combined wind and temperature effect is the most critical limit state to define the safety of a chimney. Hence, in this study, WFA of a 70 m tall RC chimney for combined wind and temperature effects is explored. The wind force time-history is generated by spectral representation method. The safety of chimney is assessed considering limit states of stress failure in concrete and steel. A moving-least-squares method based dual response surface method (DRSM) procedure is proposed in WFA to alleviate huge computational time requirement by the conventional direct Monte Carlo simulation (MCS) approach. The DRSM captures the record-to-record variation of wind force time-histories and uncertainty in system parameters. The proposed DRSM approach yields fragility curves which are in close conformity with the most accurate direct MCS approach within substantially less computational time. In this regard, the error by the single-level RSM and least-squares method based DRSM can be easily noted. The WFA results indicate that over temperature difference of 150°C, the temperature stress is so pronounced that the probability of failure is very high even at 30 m/s wind speed. However, below 100°C, wind governs the design.

A damped bipedal inverted pendulum for human–structure interaction analysis

The bipedal inverted pendulum with damping has been adopted to simulate human–structure interaction recently. However, the lack of analysis and verification has provided motivation for further investigation. Leg damping and energy compensation strategy are required for the bipedal inverted pendulum to regulate gait patterns on vibrating structures. In this paper, the Hunt–Crossley model is adopted to get zeros contact force at touch down, while energy compensation is achieved by adjusting the stiffness and rest length of the legs. The damped bipedal inverted pendulum can achieve stable periodic gait with a lower energy input and flatter attack angle so that more gaits are available, compared to the template, referred to as spring-load inverted pendulum. The measured and simulated vertical ground reaction force-time histories are in good agreement. In addition, the dynamic load factors are also within a reasonable range. Parametric analysis shows that the damped bipedal inverted pendulum can achieve stable gaits of 1.6 to 2.4 Hz with a reasonable first harmonic dynamic load factor, which covers the normal walking step frequency. The proposed model in this paper can be applied to human–structure interaction analysis.

Piecewise constant response of underdamped oscillators through suppression of overshoots and undershoots in aerospace, civil, and mechanical systems

When a single-degree-of-freedom underdamped system is subjected to a step function force in order that it eventually acquires a constant displacement, its response shows undershoots and overshoots. Since a step function force is difficult, if not impossible, to apply to many aerospace, civil, and mechanical engineering systems because of their inertias, this paper looks at simple forces that can be generated from a practical standpoint and are not instantaneously applied, that cause an underdamped oscillator to acquire a constant displacement without any overshoots and/or undershoots. These forces are ramped up (or down) over a short duration of time and held constant thereafter. A preliminary approach to the development of such force–time histories is presented by using a force given by a polynomial in time. Open-loop optimal control is next considered, and then closed-loop optimal control. The optimal control problem does not fall within the standard rubric of terminal control and a new approach for doing this is developed. These ideas are then woven into the development of a methodology that allows an undamped/underdamped single degree of freedom system to track a desired piecewise constant displacement time-history using forces that do not need to be instantaneously applied and that generate rippleless responses with no overshoots/undershoots. The methodology is also applicable to classically damped multi-degree-of-freedom systems with underdamped modes of vibration.

Experimental investigation on the response and residual compressive property of honeycomb sandwich structures under single and repeated low velocity impacts

Single and repeated impact tests were carried out on honeycomb sandwich structures. The effect of varying impact energy levels and numbers on impact response, damage parameters, and energy absorption was studied. During these tests, the time histories of contact forces, deflections, and impact energies were recorded in real time. Three-dimensional digital image correlation (3D-DIC) was applied to obtain the dent depths, damage areas, and damage volumes of front and rear surfaces. The residual compressive properties of the above-mentioned structures were determined using compression after impact tests and 3D-DIC. Further, the onset of damage procession and failure modes were analyzed. A finite element model was performed to characterize the impact response of honeycomb sandwich structures. The simulation results was in good agreement with experimental data, which indicated the model could be used for the simulation on aluminum honeycomb sandwich panels subjected to single and repeated low-velocity impacts.

Experimental investigation for characteristics of wave impact loads on a vertical cylinder in breaking waves

In this study, a series of model tests was conducted to investigate the characteristics of wave impact loads and the associated air bubble effects on a vertical circular cylinder with various pressure and force sensors installed in a 2D wave flume. Spilling and plunging waves, generated by the focusing wave method, induced wave impact loads on the cylinder, and these loads were measured by the sensors, which had different natural frequencies and sensing areas. Each test condition was repeated 20 times to verify the repeatability of the impact loads and their effects. The air bubble effects were investigated by analyzing the time histories of the impact forces and pressures, and the effects were related to the natural frequency and sensing area of the sensors. The air bubble effects were observed to increase as the breaking wave developed more at the position of the cylinder. The generated waves and impact loads were also observed to change significantly when even a small noise occurred in the displacement of the wavemaker.

Fluid Dynamics of Flow Around Side-by-Side Arranged Cylinders

Flow past four side-by-side identical square cylinders arranged normal to the flow have been found to show interesting and important flow features which are very difficult to get through experiments. Lattice Boltzmann method (LBM) is used for numerical simulations of two-dimensional (2D) flow around four side-by-side arranged cylinders. In this study, the Reynolds number (Re) is chosen to be 60, 80, 100, 120 and 140 and the spacing ratio g* (= g/D, where D is the size of cylinder and g is the distance between the cylinders) is set at 0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2, 2.5, 3, 3.5 and 4. Based on the flow characteristics, seven distinct and unique flow regimes are identified for different ranges of Re and g*. Physical features of each flow regime such as wake structures, vortex dynamics, gap flow behavior, time histories of lift coefficients, shedding frequencies and hydrodynamic forces are thoroughly discussed. The Reynolds numbers strongly affect the flow, especially at 0 ≤ g* ≤ 2, in terms of vortex-shedding frequency. A significant secondary frequency is also found other than the primary frequency in the base-bleed and flip-flopping flow regimes. It is observed that for g* ≥ 2.5 primary shedding frequency strongly affects the flow dynamics and the mutual interaction of the wakes behind the cylinders decreases with an increase in the Reynolds number. The Strouhal value is same for the outer and inner cylinders in inphase–antiphase weak interaction flow regime and different for base-bleed and flip-flopping flow regimes. In inphase asynchronous weak interaction flow regime, the Strouhal number is same for all four cylinders.

Experimental and numerical investigation on impact and post-impact behaviours of H-shaped steel members

The impact and post-impact behaviours of H-shaped steel members are investigated experimentally and numerically in this paper. The drop-hammer impact tests and the axial compression tests after impact were firstly performed on thirteen specimens, in which two boundary conditions and four impact energies (drop height ranged from 1.04 m to 2.50 m) were used. Experimental results such as failure patterns, impact force–time histories, permanent deformations and post-impact axial load-lateral deflection curves were presented. Subsequently, finite element models were established using ABAQUS software. The impact test and the axial compression test after impact were reproduced using the explicit and implicit algorithms in ABAQUS, respectively. Finally, case studies were carried out using the validated FE models to assess the residual load-carrying capacities of impact-damaged H-shaped steel members in industrial plants. The results showed that the boundary condition had an influence on the failure pattern and impact force response during impact. Within the parameter range of this work, the residual load-carrying capacity of H-shaped steel members was primarily affected by the global deformation at the mid-height induced by impact.

Behavior and design of reinforced concrete building columns subjected to low-velocity car impact

Exposure of the columns on the first floor to lateral vehicular impact poses a great threat to the structural stability of buildings. In most cases, these columns may not be designed to resist the lateral impact force caused by the car-column collision. The dynamic impact force will cause deformation, reduce the bearing capacity of the column, and can cause catastrophic failure. In this study, the effect of a car impact on an axially loaded building column is evaluated by numerical simulation. An experimentally verified car model with a mass of 2203 kg and an axially loaded square reinforced concrete (RC) column model have been used for the numerical simulations. The results of the numerical analysis showed that the column-foundation joint is highly affected by the lateral impact force when the impact velocity is more than 30 km/h. A shear fracture occurs at the joint when the speed exceeds 40 km/h for columns with dimensions 400 mm or less. The axially loaded RC column has a higher impact resistance than columns without axial loads. From the impact force-time histories, the equivalent static forces (ESFs) have been calculated for the different ranges of impact velocities and different sizes of columns. It is found that the calculated ESF values are higher than the recommended values given by Eurocode 1. New proposed guidelines for the estimation of ESF and design recommendations are provided for the lateral impact load caused by a veered car on a building column.

Lift Regulation During Transverse Gust Encounters Using a Modified Goman–Khrabrov Model

This paper presents a framework for the estimated state regulation of the lift force for a maneuvering wing during a transverse gust encounter. An expression for the effective angle of attack of the wing is derived and used to construct a modified Goman–Khrabrov (GK) model for wing–gust encounters. Experiments were carried out in the University of Maryland water towing facility to identify parameters in the modified GK model and to evaluate its ability to represent gust encounters of varying gust strengths. Using this gust-encounter model, an observer-based feedback controller for pitch actuation is constructed to regulate the coefficient of lift of a wing during a transverse gust encounter. Closed-loop control simulations provided pitching kinematics that mitigated the transient force response. Gust encounter experiments were performed using the control signal from simulation as an open-loop control signal during an encounter with a known gust. The lift force time histories of the gust encounter experiments with control show effective mitigation of the peak lift force. The wing experienced transient forces that merit further investigation, including an initial transient peak and a later undershoot that may be due to uncertainty in the start of the gust region.

A novel long short-term memory neural-network-based self-excited force model of limit cycle oscillations of nonlinear flutter for various aerodynamic configurations

Due to the strong nonlinear properties of the entire flutter process [including growth stages, decay stages and steady limit cycle oscillations (LCOs)], the generalization of the self-excited force model for the entire flutter process of bluff bodies is a very critical issue. This paper proposes a self-excited force model based on a novel long short-term memory (LSTM) neural network to simulate the entire flutter process for various leading-edge configurations. To obtain the dataset, the nonlinear flutter with the growth stages, decay stages and steady LCOs for different leading-edge aerodynamic configurations is investigated by a series of two-degrees-of-freedom spring-suspended sectional model tests. Based on the measured flutter responses and self-excited forces in the tests, a data-driven self-excited force model is proposed on the basis of a novel LSTM neural network. Considering that the nonlinear dynamic system is very sensitive to the accuracy of the model parameters, the Newmark-$$\beta $$ method is incorporated into the LSTM networks and forms a closed-loop process to restrain the error amplification effects and improve the model robustness, that is, the response calculated from the output of LSTM (the self-excited force) by the Newmark-$$\beta $$ method is set as the input of LSTM at the next step rather than the measured response. To validate the performance of the proposed model, the flutter critical wind speed, time history of oscillation and self-excited forces, steady-state oscillation amplitude and flutter development process predicted by the model are compared with the test results. The comparison indicates that the predicted results agree well with the test results, meaning that the proposed model has high accuracy, generalization and robustness in describing the nonlinear characteristics of the flutter for various aerodynamic configurations.

Multihazard fragility assessment of steel‐concrete composite frame structures with buckling‐restrained braces subjected to combined earthquake and wind

SummaryEngineering structures may inevitably be subjected to multiple natural hazards (such as earthquakes and winds) during their life cycles. This paper presents an efficient multihazard fragility methodology based on the structural demand models. The approach is applied to two steel‐concrete composite frame structures (SCCFSs), with and without buckling‐restrained braces (BRBs), aiming to evaluate the effect of BRBs on controlling the structural responses and fragilities under the combined earthquake and wind loads. In total, 120 earthquake records are selected, and 120 sets of wind drag force time histories are simulated by considering the spatial variation along the height of the exemplar building. The combined “earthquake–wind” events are stochastically assembled, in which the intensities of these two hazards are modeled using the Monte Carlo simulation. The OpenSees platform is employed to calculate the dynamic responses of the SCCFSs with and without BRBs under simultaneous earthquake and wind loads. The goodness of fits of the first‐, second‐, and third‐order polynomial in predicting the structural demand are evaluated, and the optimal polynomial is employed to generate the multihazard fragility surfaces at different damage states. The numerical results indicate that the structural responses and fragilities under the combined earthquake and wind are higher than those under an individual hazard, while the influencing extent varies with the relative intensities of these two hazards. The impact of multiple hazards and the control effect of BRBs on the structural responses and fragilities are systematically quantified and discussed in details.

Behavior of API 5L X56 submarine pipes under transverse impact

The performance of API 5L X56 submarine pipes subjected to transverse impact is investigated through both drop weight impact tests and finite element simulations. To study the improvement of resistance to transverse impact for pipe-in-pipe specimens, corresponding single-layer pipe specimens under impacting load were also tested for comparison. From the experimental tests, the failure mode, the impact force-time history curve, the displacement-time history curve, the strain-time history curve and the impact force - displacement curve of all specimens were obtained. The pipe-in-pipe specimens are found to have superior performance to resist impacting loading compared to single-layer specimens because smaller global bending deformation and local indentation were observed in them. In addition, the cross-section deformation rate, R, was proposed, and the local indentation length of the specimen was divided into three zones based on experimental observation and numerical simulation. A comprehensive comparison of the length and the depth of local indentation in the pipe-in-pipe specimens shows that the local indentation decreases and the impact resistance increases due to the presence of the inner tube. Finally, the post-peak mean force (Pm) and the energy absorption capacity (EAC) were used to evaluate the energy dissipation mechanism of different specimens. The results also indicate that the pipe-in-pipe specimens have better energy absorption performance.

An Evidence Basis for Future Equestrian Helmet Lateral Crush Certification Tests

The aim of this study is to determine what loads are likely to be applied to the head in the event of a horse falling onto it and to determine by how much a typical equestrian helmet reduces these loads. An instrumented headform was designed and built to measure applied dynamic loads from a falling horse. Two differently weighted equine cadavers were then dropped repeatedly from a height of 1 m (theoretical impact velocity of 4.43 m/s) onto both the un-helmeted and helmeted instrumented headforms to collect primary force–time history data. The highest mean peak loads applied to the headform by the lighter horse were measured at the bony sacral impact location (15.57 kN ± 1.11 SD). The lowest mean peak loads were measured at the relatively fleshier right hind quarter (7.91 kN ± 1.84 SD). For the heavier horse, highest mean peak loads applied to the headform were measured at the same bony sacral impact location (16.02 kN ± 0.83 SD), whilst lowest mean peak loads were measured at the more compliant left hind quarter (10.47 kN ± 1.08 SD). When compared with the un-helmeted mean values, a reduction of 29.7% was recorded for the sacral impact location and a reduction of 43.3% for the lumbosacral junction location for helmeted tests. Notably, all measured loads were within or exceeded the range of published data for the fracture of the adult lateral skull bone. Current helmet certification tests are not biofidelic and inadequately represent the loading conditions of real-world “lateral crush” accidents sustained in equestrian sports. This work presents the first ever evidence basis upon which any future changes to a certification standards test method might be established, thereby ensuring that such a test would be both useful, biofidelic, and could ensure the desired safety outcome.

Response Ratio Development for Lateral Pendulum Impact with Porcine Thorax and Abdomen Surrogate Equivalents.

There has been recent progress over the past 10 years in research comparing 6-year-old thoracic and abdominal response of pediatric volunteers, pediatric post mortem human subjects (PMHS), animal surrogates, and 6-year-old ATDs. Although progress has been made to guide scaling laws of adult to pediatric thorax and abdomen data for use in ATD design and development of finite element models, further effort is needed, particularly with respect to lateral impacts. The objective of the current study was to use the impact response data of age equivalent swine from Yaek et al. (2018) to assess the validity of scaling laws used to develop lateral impact response corridors from adult porcine surrogate equivalents (PSE) to the 3-year-old, 6-year-old, and 10-year-old for the thorax and abdominal body regions. Lateral impact response corridors were created from 50th adult male PSE pendulum lateral impact T1, T14, and L6 accelerations and pendulum impact force time histories for the thorax and abdomen testing performed. The ISO 9790 scaling technique using length, mass, and elastic modulus scale factor formulas were used in conjunction with measured swine parameters to calculate scale factors for the PSE. In addition to calculation of pertinent test scale factors, response ratios for the pendulum impact tests were calculated. The scaling factors and response ratios determined for the porcine surrogates were compared to the already established ISO human lateral pendulum impact response ratios to determine whether a consistent pattern over the age levels described for the two sets of data (human and swine) exists. The actual lateral impact pendulum data, for both thoracic and abdominal regions, increases in magnitude and time duration from the 3-year-old PSE up to the 50th male PSE. This increase in magnitude and time duration is comparable to the human response corridors developed based on an impulse-momentum analysis and the elastic bending modulus derived from human skull bone. This pattern in the human impact response corridors was observed in the response ratio values and the swine response data. Based on the current study's findings, when utilizing the elastic modulus of human skull bone presented previously in research, thoracic and abdominal lateral pendulum impact response of PSE follows the general scaling laws, based on the impulse-momentum spring-mass model. The thoracic and abdominal lateral pendulum force impact response of PSE also follows the human scaled impact response corridors for lateral pendulum impact testing presented in previous research. The overall findings of the current study confirm, through actual swine testing of appropriate weight porcine surrogates, that scaling laws are applicable from the midsized-male adult down to the 3-year-old age level using human skull elastic modulus values established in previous research.