bluuebunny/crossref_metadata_2025 huggingface dataset:
Integrated model predictive and torque vectoring control for path tracking of 4‐wheel‐driven autonomous vehicles
Authors: Ren Yue , Zheng Ling , Khajepour Amir
Published: September 2018

In this study, an integrated path tracking control framework is proposed for the independent‐driven autonomous electric vehicles. The proposed control scheme includes three parts: the non‐linear model predictive path tracking controller, the lateral stability controller, and the optimal torque vectoring controller. Firstly, the upper bound speed limit is regulated based on the known curvature and adhesion coefficient of the road to prevent the tyre saturation. The model predictive controller generates the steering angle and the desired longitudinal force for path tracking. Simultaneously, the lateral stability controller calculates the desired yaw moment to balance the vehicle stability and motility under different situations. Finally, the optimal torque vectoring controller distributes the wheel torques to generate the desired longitudinal force and yaw moment. Three test cases are designed and verified based on a Carsim/Simulink platform to evaluate the control performance. The test results illustrate that the proposed control framework has satisfactory path tracking performance, and the desired balance between vehicle mobility and stability is achieved under different road conditions.
Measurement of dynamic vibration in cycling using portable terminal measurement system
Authors: Xie Ning , Li Hui , Zhao Wenzhong , Ni Ying , Liu Chenwen , Zhang Yi , Xu Zhigang
Published: September 2018

Bicycling is a potential way to improve sustainable urban transportation system. However, the evaluation methods and metrics for bicycle ride quality and slow traffic pavement condition are still in the preliminary stages. The traditional bicycling vibration measurement used instrumented probe bicycle (IPB), which contains several independent devices and obtains scattering data. This method cannot show the detailed pavement condition. A technical challenge for monitoring real‐time bicycling comfort and obtain the pavement condition in detail is to combine the intelligent transportation system (ITS) with bicycles. In this study, two schemes have been proposed to acquire the vibration data based on portable terminal measurement systems. Scheme (1) was based on the IPB using GPS, accelerometers, and sport cameras. The microcontroller was used to integrate several modules to develop scheme (2). Wavelet analysis was introduced to analyse random signal data. Then the pavement condition can be reflected in detail by vibration response. Data derived from the proposed system can be successfully applied to promote the use of bicycles as a reflecting tool for the pavement surface conditions. This method can provide a reference database for pavements’ surface maintenance and for cyclists to choose cycling routes.
PM and PAHs emissions of ship auxiliary engine fuelled with waste cooking oil biodiesel and marine gas oil
Authors: Su Peng‐hao , Geng Peng , Wei Lijiang , Hou Chun‐yan , Yin Fang , Tomy Gregg T. , Li Yi‐fan , Feng Dao‐lun
Published: September 2018

Clean fuels are recommended for ships at berth to reduce air pollutant emissions. This study aimed to evaluate the feasibility of waste cooking oil (WCO) biodiesel application on board with regard to particle matter (PM) and polycyclic aromatic hydrocarbon (PAH) emissions. An experiment was conducted on a marine auxiliary engine for three different fuels: WCO biodiesel, formulation blends with marine gas oil (MGO) and neat MGO. Results revealed that WCO biodiesel could reduce PM and PAHs emissions. WCO exhaust also exhibited differences in PAH profile and phase distribution as compared to MGO, depending on the operation modes and the proportion of biodiesel in the formulation blends. Consequently, WCO biodiesel could dramatically reduce the total carcinogenic potencies related to PAHs of exhausts. Moreover, PAH source recognition pair ratios of tested fuels were observed to deviate from the widely accepted values. This study highlights that WCO biodiesel is a cleaner fuel for operating ship auxiliary engines with respect to PM and PAHs emissions and has the potential to moderate the severe effects of PM and PAHs on an air of coastal areas.
Ensemble classifier for driver's fatigue detection based on a single EEG channel
Authors: Wang Ping , Min Jianliang , Hu Jianfeng
Published: September 2018

Driver's fatigue detection, based on electroencephalography (EEG) signals, is a worthy field of research to study evidence regarding how to exactly pre‐warn and avoid casualties nowadays. In this study, an EEG‐based system of perfect performance and good stability for evaluating driver's fatigue with only one electrode by ensemble learning method is proposed. Given that EEG signals are unstable and non‐linear that using several common entropy measurements to analyse EEG signals is more appropriate including spectral entropy, approximate entropy, sample entropy and fuzzy entropy. In this study, unlike other methods using a single classifier, three ensemble approaches (bagging, random forest and boosting) based on three base classifiers were employed and compared. A driving simulator in this study was used for 12 healthy and adult subjects to perform a continuous simulated driving experiment for 1–2 h. The experimental results show that the proposed method can make use of only one electrode (T6) by gradient boosted DT for driver's fatigue detection, while the average classification accuracy is >94%. The findings of this study indicated that a single EEG channel with optimal ensemble classifier may be a good candidate for usage in the portable system for driver's fatigue detection.
Vehicles detection for illumination changes urban traffic scenes employing adaptive local texture feature background model
Authors: Zhang Yunsheng , Zhao Chihang , Shi Wen , Leng Kaijun
Published: September 2018

An Adaptive Local Texture Feature Background Model (ALTF‐BM) is proposed to resolve the deficiency in current background models, which are easily contaminated by sudden and gradual illumination changes in complex urban traffic scenes. Based on Weber's law, the authors first develop Adaptive Local Texture Feature (ALTF), calculated over a predefined local region around the pixel employing an adaptive distance threshold, and then the background is modelled on the base of sample consensus scheme using the calculated features. Furthermore, to label the foreground pixels, the difference between the background model and input video frames is then directly compared by ALTF encoding. Finally, the model is updated using the random update policy to adapt to the changing illumination and the dynamic background. The experimental results on real‐world urban traffic videos and the public Change Detection benchmark of 2014 (CDnet2014) show that the proposed ALTF‐BM offers the best performance compared to the other state‐of‐the‐art texture‐based methods, and the average F ‐measures and similarity results of the proposed ALTF‐BM are 0.547 and 0.393 higher than benchmarks on the night traffic‐light sequence, respectively. The encouraging experimental results demonstrate the efficiency of the proposed ALTF‐BM in handling sudden and gradual illumination changes in urban traffic scenes.
Dynamically integrated spatiotemporal‐based trajectory planning and control for autonomous vehicles
Authors: Li Boyuan , Du Haiping , Li Weihua , Zhang Bangji
Published: September 2018

In the literature, the intensive research effort has been made on the trajectory planning for autonomous vehicles, while the integration of the trajectory planner with the trajectory controller is less focused. This study proposes the spatiotemporal‐based trajectory planner and controller by a two‐level dynamically integrated structure. In the upper level, the best trajectory is selected among a group of candidate time‐parameterised trajectories, while the target vehicle ending position and velocity can be satisfied. Then the planned trajectory is evaluated by checking the feasibility when the actual vehicle dynamic motion constraints are considered. After that, the lower level trajectory controller based on the vehicle dynamics model will control the vehicle to follow the desired trajectory. Numerical simulations are used to validate the effectiveness of the proposed approach, where the scenario of an intersection and the scenario of overtaking are applied to show that the proposed trajectory controller can successfully achieve the control targets. In addition, compared with the potential field method, the proposed method based on the four‐wheel independent steering and four‐wheel independent driving electric vehicle shows great advantages in guaranteeing the vehicle handling and stability.
Arbitrary coupling arbitrary phase couplers with improved bandwidth
Authors: Singh Abinash , Mandal Mrinal Kanti
Published: September 2018

In this study, an arbitrary coupling and arbitrary phase coupler with improve bandwidth is proposed. The proposed couplers use a series stub of electrical length θ at all ports which acts as matching element and thereby improves the effective bandwidth of the coupler. A lossless transmission line model is used to obtain design equations and design graphs. Return loss, isolation, amplitude and phase imbalances all four conditions are considered for bandwidth calculation. Two 3 dB couplers with phase difference of 30° and 45° and the third coupler with 4.77 dB coupling and phase difference of 60° are fabricated and measured to validate the proposed topology. Bandwidth improvement is obtained in all the cases. In some cases, the effective bandwidth is more than 100%.
Compact balanced bandpass filter design using miniaturised substrate integrated waveguide cavities
Authors: Ho Min‐Hua , Hou Cheng‐You , Hsu Chung‐I G. , Lee Keh‐Yi
Published: September 2018

The contribution of this work is to propose a compact balanced bandpass filter design using the miniaturised full/half‐mode substrate integrated waveguide cavities (SIWCs) together with the microstrip coupling slot feed. The circuit area of the proposed full‐mode SIWC is only 4.64% of its conventional SIWC counterpart, and the half‐mode SIWC further reduces the circuit area by half. Besides the circuit compactness, the non‐coupling of the microstrip and the slot in the common‐mode (CM) operation provides an excellent CM signal rejection. Prototypes of two‐pole full/half‐mode balanced SIWC BPFs are fabricated and measured to validate the circuit design. Agreements between measured and simulated data are observed. The CM signal rejection levels are larger than 45 dB over a wide‐frequency range with the passband's differential‐mode insertion losses around 1.6 and 0.8 dB for the full‐ and half‐mode designs, respectively.
High energy density polymer nanocomposites with Y‐doped barium strontium titanate nanoparticles as fillers
Authors: Lu Yu‐Chen , Yu Shuhui , Zeng Xierong , Sun Rong , Wong Ching‐Ping
Published: September 2018

Dielectric nanocomposites with an enhanced energy density are highly desired in modern electronic and electrical power systems. In this study, the authors developed a new flexible dielectric nanocomposite thin film using Y‐doped barium strontium titanate (Y:BST) nanoparticles as fillers and polyvinylidene fluoride as a polymer matrix. The nanocomposites exhibited a high energy density of 11.07 J/cm3 at 4300 kV/cm with a low filler content. Through the analysis of defect chemistry with X‐ray photoelectron spectroscopy, the dopant Y3+ occupies A‐site of the perovskite structure, which is associated with the generation of Ba&Sr vacancies and oxygen vacancies for charge compensation. As a result, TiO6 2– octahedron distortion occurs, which is responsible for the enhanced dielectric permittivity. Meanwhile, the dielectric loss maintains a low level and the breakdown strength increases at a low filler loading, resulting in improved energy density.
Optimal configuration method based on optimal expected power characteristics for micro power supply and energy storage device
Authors: Xiao Huagen , Yi Xiaoyu , Ding Shiqi , Su Shi
Published: September 2018

In this study, an optimal configuration method based on optimal expected power characteristics for micro power supply and energy storage device is proposed, which takes the minimum average fluctuation rate of the total output power characteristics of renewable micro power supply and microgrid as the optimisation objective, respectively. The proposed method can improve the smoothness of the total output power characteristics of the renewable micro power supplies, and effectively reduce the maximum adjustable power and available capacity of the energy storage devices. At last, the feasibility and effectiveness of the proposed method are verified by the case study results.
Short‐term probabilistic forecasting for regional wind power using distance‐weighted kernel density estimation
Authors: Wang Zhao , Wang Weisheng , Liu Chun , Wang Bo , Feng Shuanglei
Published: September 2018

As the integration of wind power into the power grid increases rapidly, the total output of the regional wind farms has become the concern of the power system operators and market traders. This study proposes a short‐term probabilistic forecast model for this regional application. The uncertainty information provided by the proposed model can help the users make better decisions in the power system. A new distance‐weighted kernel density estimation (DWKDE) method is proposed to forecast the full distribution function of the wind power. Its distance kernel is able to assign different weights to the samples similar to the target point. The beta kernels are introduced to adapt to the double‐bounded characteristic of the wind power density. To further improve the performance of the DWKDE model, a regime‐switching strategy is applied based on the regional wind direction clustering, while a feature selection method of minimal‐redundancy‐maximal‐relevance is provided to determine the proper feature set. A case study of 28 wind farms in the East China is provided to evaluate the performance with the quality measures of reliability, sharpness, and the pinball score. The proposed method is easy to use and performs well according to the results of the evaluation.
Design of unimodular sequence train with low central and recurrent autocorrelations
Authors: Zhao Dehua , Wei Yinsheng , Liu Yongtan
Published: September 2018

This study considers the unimodular sequence train design with low central and recurrent autocorrelation sidelobes. The authors attain the design goal by minimising the integrated energy in central and recurrent autocorrelations with user‐supplied weights. They devise a way to employ a fast Fourier transform in calculating both the objective function and its gradient, such that the considered design problem can be efficiently solved using the general gradient method. Numeric results indicate that the proposed method can effectively suppress the range sidelobes concerned and it can design sequence train of very large sequence length and set cardinality.
Cognitive structure adaptive particle filter for radar manoeuvring target tracking
Authors: Wang Shuliang , Bi Daping , Ruan Huailin , Chen Shiwa
Published: September 2018

The accuracy and efficiency of the particle filter algorithm partly depend on the number of particles used in the estimation. The number of particles is specified beforehand and kept fixed in the process of radar manoeuvring target tracking (MTT). In practice this may be highly inappropriate since it ignores the uncertainty in the models and the varying dynamics of the processes. A cognitive structure adaptive particle filter (CSAPF) algorithm is proposed to increase the accuracy and efficiency by adapting the number of particles online. The name CSAPF is due to the fact that the authors change the number of particles dynamically with radar's real‐time perceptual information entropy for MTT. The authors cognitive structure approach chooses a transmitted waveform to improve tracking accuracy and chooses the lower bound of required particles to improve tracking efficiency. Monte–Carlo simulation results show that the approach has significant improvements over the particle filter with a fixed number of particles and over the fixed waveform particle filter algorithm.
Constant false alarm rate detection of slow targets in polarimetric along‐track interferometric synthetic aperture radar imagery
Authors: Peng Zhang , Jia‐Feng Zhang , Tao Liu
Published: September 2018

To improve the detection performance for ground moving targets with small radar‐cross‐section and low velocity in Along‐Track Interferometric Synthetic Aperture Radar (AT‐INSAR) system, this study explores new methods to elevate the target‐to‐clutter ratio by using full Polarimetric AT‐INSAR (AT‐POLINSAR). Three novel detectors that can achieve Constant False Alarm Rate (CFAR) detection are put forward for AT‐POLINSAR at first, namely, the Suboptimal Interferometric Coherence Detector, the Generalized Polarimetric Whitening Filter and the Dual Detector. Secondly, the analytical expressions of their false‐alarm rates are derived under the Gaussian distribution assumption of the six‐dimensional polarimetric interferometric vector, then the estimation methods of distribution parameters and the calculating methods of detection thresholds as well as the specific implementation schemes of CFAR detection are given. Finally, the proposed detectors are compared with single‐polarimetric AT‐INSAR detectors and full polarimetric log‐likelihood‐ratio test through both the Monte Carlo detection experiments and the scene detection experiments of simulated AT‐POLINSAR imagery, where the results indicate that new methods have rather higher CFAR detection performance and velocity estimation accuracy than current single‐polarimetric detectors.
Generalised polarimetric whitening filter for polarimetric MIMO radar detection
Authors: Yang Yong , Xiao Shun‐Ping , Wang Xue‐Song , Zhang Wen‐Ming , Li Yong‐Zhen
Published: September 2018

The polarimetric multiple‐input multiple‐output (MIMO) radar utilises both the polarisation and spatial diversities, thus the generalised likelihood ratio test (GLRT) of the polarimetric MIMO radar exhibits better detection performance than the GLRT of the MIMO radar when the secondary data set is large enough. However, the secondary data set is often not enough in reality. In this instance, the good detection performance of the GLRT of polarimetric MIMO radar cannot be guaranteed any more. In this study, the authors propose three kinds of generalised polarimetric whitening filters (GPWFs) to improve the detection performance of polarimetric MIMO radar in the limited secondary data environment. They first present the signal models and the test statistics for the three GPWFs. Then they analyse the detection performance of the three GPWFs by simulation, and find the second GPWF performs the best. They further compare the detection performance of the second GPWF with those of the GLRT and non‐coherent integration (NCI) detector. Simulation results show that the proposed second GPWF outperforms the GLRT and NCI in the limited secondary data environment.
Ground moving target detection algorithm for monopulse‐SAR based on complex monopulse ratio
Authors: Wu Di , Zhu Daiyin , Shen Mingwei
Published: September 2018

Monopulse processing has been proved to be an effective way for airborne synthetic aperture radar (SAR)/ground moving target indication. In the monopulse‐SAR system, moving target detection is applied to a complex ratio diagram, referred to as a complex monopulse ratio diagram (CMRD), generated through a pixel‐by‐pixel comparison between the range‐Doppler (RD) spectra obtained from the difference and sum channels. This study examines the statistics of complex monopulse ratio (CMR) in the Gaussian background. Based on the statistics, a new random variable termed normalised CMR (NCMR) is derived by a linear transform to CMR. It is demonstrated that the conditional probability density function of NCMR, under the null hypothesis (H0), is independent of the clutter‐plus‐noise environment. Motivated by this characteristic, a constant false alarm rate detector using the modulus of NCMR as the test statistic is proposed to detect moving targets. The detection performance is evaluated with a closed‐form expression of the probability of false alarm (PFA) derived. Moreover, the construction of the detector is extended to a so‐called multi‐CMRD form to accommodate even higher detection performance. Experimental results from two groups of real data are displayed to validate the statistical analysis and verify the detection performance.
Deformation of EPS Foam Under Combined Compression-Shear Loading: Experimental and Computational Analysis
Authors: Ling Chen , Ivens Jan , Cardiff Philip , Gilchrist Michael D.
Published: September 2018

Expanded Polystyrene (EPS) foam material is widely used as an energy absorption engineering material. Its compression behaviour, both quasi-statically and dynamically, has been studied widely. However, its mechanical behaviour under combined compression-shear loading is poorly understood due to the difficulty of performing such tests. A novel test rig is presented to perform combined compression-shear loading tests in quasi-static loading conditions. Different densities of EPS foam were tested with this apparatus using a universal Instron testing machine. The compressive and shear stresses were obtained and compared, the results show that the shear stress at yield of EPS foam under combined compression-shear loading is much lower compared with the compressive stress at yield. On the other side, the compressive stress at yield can as high as 40% lower compared with pure compression. The FEA simulations were performed using the Abaqus/explicit 6.14 code and it is found the numerical predictions and experimental results agree closely, which indicates that our FE models exhibit good reliability in predicting the response of EPS foam under such loading conditions.
Three-dimensional constitutive model for the description of high molecular weight semicrystalline polymers over a large range of temperatures and strain rates: Application to Ultra High Molecular Weight PolyEthylene
Authors: Bernard Chrystelle A. , Deplancke Tiana , Lame Olivier , Ogawa Kazuhiro , Cavaillé Jean-Yves
Published: September 2018

Semi-crystalline polymers, and more particularly high molecular weight semi-crystalline polymers, exhibits interesting properties such as wear and impact resistance which contributes to their spreading into several industries and applications. However, because they have very long chains and exhibit high viscoelastic mechanical behavior, they are diffcult to process. It requires high temperature, close to melting temperature, and important compression strength to arrange the chains. However, such process lack accuracy and better understanding of the thermomechanical behavior is needed to improve it. Thus, a three-dimensional model based on the evolution of the microstructure during the plastic deformation have been developed with relative good agreement with experimental results from the literature.
Plate impact shock experiments and numerical modeling of lightweight adobe masonry material
Authors: Sauer Christoph , Bagusat Frank , Heine Andreas , Werner Riedel
Published: September 2018

In this contribution, we summarize and extend the experimental and numerical investigation of the shock response of lightweight adobe masonry, previously published in [C. Sauer et al., J. Dyn. Behav. Mater. (submitted)]. It is demonstrated that inverse planar plate impact (PPI) experiments are feasible for lightweight adobe. From the obtained free surface velocity time curves, a linear shock velocity vs. particle velocity relation is derived within the measured range of particle velocities. Numerical simulations of these curves show that the employed homogenous numerical model is capable of properly treating the shock response of this porous, inhomogeneous, and low-strength material. This numerical model is then applied to the example of the ballistic impact of steel spheres on targets consisting of one lightweight adobe brick. The experimentally obtained penetration craters are properly reproduced by the simulated target damage. Moreover, we find good agreement of the measured and simulated residual velocities within the presented range of impact velocities.
Dynamic behaviour of core-shell structured Si nanoparticles during lithiation/delithiation cycling at dynamic loadings
Authors: Gao Xiang , Yuan Chunhao , Xu Jun
Published: September 2018

Dynamic behaviours of the core-shell structured (CSS) Si nanoparticles during electrochemical cycling were modelled. A core-shell-structured model that contained Si nanoparticles wrapped with amorphous carbon (a-C) was established. The system was charged and discharged under constant voltage with a dynamic loading applied on the surface of the particle. The volume change and associated stress simulated by the model without external loading were compared with the previous work done by other researchers and matched well. Different imposing moments of the external loading were introduced into this model to see the dynamic effects on the cycling behaviour of the Si particles. The present study illustrate the dynamic behaviour under external loading, which would give guidance for the Si based battery design and help to understand the coupling effects of dynamic loading and electrochemical cycling on Si anodes.
Aluminium plates with pre-formed slits subjected to blast loading
Authors: Granum Henrik , Aune Vegard , Børvik Tore , Sture Hopperstad Odd
Published: September 2018

The dynamic response of thin, perforated aluminium plates subjected to blast loading was studied both experimentally and numerically. Two different blast intensities were used and the plates were pre-cut with four horizontal and vertical slits prior to testing. The applied AA6016-T4 plates had an exposed area of 0.3 m x 0.3 m and a thickness of 1.5 mm. Special focus was placed on the dynamic response and failure characteristics of the plates. Uniaxial tensile tests were conducted in three different directions to determine the material behaviour and material parameters were found by inverse modelling using the optimization tool LSOPT. Finally, numerical simulations were performed in the finite element code Abaqus/Explicit where the plates were uniformly loaded with time-dependent pressure histories from similar tests on massive plates. The material behaviour was assumed to follow the J2 flow theory of plasticity and an uncoupled damage model was used in combination with element erosion to predict material failure. The numerical results were in good agreement with the experimental observations and predicted both the dynamic response and the complete tearing of the centre part of the plates.
Modelling of the behaviour of metal foams under shock compression
Authors: Jacques Nicolas , Barthélémy Romain
Published: September 2018

A theoretical modelling is proposed to describe the shock response of foam materials. This model is based on micromechanical and energetic arguments, and takes into account the contribution of microscale inertia. Within this framework, an analytical expression of the Hugoniot stress-strain curve is proposed for elastic-plastic cellular materials. The predictions derived from the proposed model are in excellent agreement with experimental data for open-cell aluminium foams. The case of viscoplastic foams is also considered.
Experimental and numerical analysis of high and low velocity impacts against neat and shear thickening fluid (STF) impregnated weave fabrics
Authors: Eddine Tria Djalel , Hemmouche Larbi , Allal Abdelhadi , Benouali Abdelkader
Published: September 2018

This investigation aims to study the efficiency of STF impregnated plain-weave fabric made of Kevlar under high and low velocity impact conditions. The shear thickening fluid (STF) was prepared by ultrasound irradiation of silica nanoparticles (diameter ≈30 nm) dispersed in liquid polyethylene glycol polymer. STF impregnation effect was determined from single yarn pull-out test and penetration at low velocity using drop weight machine equipped with hemi-spherical penetrator and dynamic force sensor. Force-displacement curves of neat and impregnated Kevlar were analysed and compared. Also, the STF impregnation effect on Kevlar multilayers was analysed from high velocity impact tests using 9mm FMJ bullet at 390 m/s. After impact, Back face deformation (BFD) of neat and impregnated Kevlar layers were measured and compared. Results showed that STF impregnated fabrics have better energy absorption and penetration resistance as compared to neat fabrics without affecting the fabric flexibility. When relative yarn translations are restricted (e.g. at very high levels of friction), windowing and yarn pull-out cannot occur, and the fibres engaged with the projectile fail in tension that leads to fabric penetration. Microscopy of these fabrics after testing have shown pitting and damage to the Kevlar filaments caused by the hard silica particles used in the STF. Mesoscopic 3D Finite Element models were developed using explicit LS-DYNA hydrocode to account for STF impregnation by employing the experimental results of yarn pull-out tests, low and high velocity impacts. It was found that friction between fibers and yarns increase the dissipation of energy upon impact by restricting fiber mobility, increasing the energy required for relative yarn translations and transferring the impact energy to a larger number of fibers.
Large Deformation Behavior of High Strength Steel Under Extreme Loading Conditions: High Temperature and High Strain Rate Experiments and Modeling
Authors: Li Xueyang , Roth Christian C. , Mohr Dirk
Published: September 2018

Plasticity and fracture experiments are carried out on flat smooth and notched tensile specimens extracted from DP800 steel sheets. A split Hopkinson pressure bar testing system equipped with a load inversion device is utilized to reach high strain rates. Temperature dependent experiments ranging from 20°C to 300°C are performed at quasi-static strain rates. The material exposes a monotonic strain hardening behaviour with a non-monotonic temperature dependency. The rate-independent material behaviour at room-temperature is described with a non-associated Hill’48 plasticity model and an Swift-Voce strain hardening. A machine learning based model is used multiplicatively to capture the rate and temperature responses. A good agreement between measured and simulated force-displacement curves as well as local surface is obtained. The loading paths to fracture are then extracted to facilitate further development of a temperature dependent fracture initiation model.
A mesoscopic model for compression of granular materials
Authors: Rejovitzky Elisha
Published: September 2018

The design of protective structures often requires numerical modeling of shock-wave propagation in the surrounding soils. Properties of the soil such as grain-grading and water-fraction may vary spatially around a structure and among different sites. To better understand how these properties affect wave propagation we study how the meso-structure of soils affects their equation of state (EOS). In this work we present a meso-mechanical model for granular materials based on a simple representation of the grains as solid spheres. Grain-grading is prescribed, and a packing algorithm is used to obtain periodic grain morphologies of tightly packed randomly distributed spheres. The model is calibrated by using experimental data of sand compaction and sound-speed measurements from the literature. We study the effects of graingrading and show that the pressures at low strains exhibit high sensitivity to the level of connectivity between grains. At high strains, the EOS of the bulk material of the grains dominates the behavior of the EOS of the granular material.
Design and Optimization of Dynamic Test Samples for Ductile Damage Assessment
Authors: Sasso Marco , Mancini Edoardo , Cortese Luca , Nalli Filippo
Published: September 2018

The present research aims at assessing and comparing the damage evolution in a structural steel, mainly used in pipeline applications, both under quasi-static and dynamic conditions. Accordingly to the core of the literature related to plastic damage modelling, two key parameters must be controlled in the tests: the stress triaxiality and the Lode angle, both depending on the stress state. Either strongly affect the material strain to failure. Hence, different specimen geometries are needed to test the material in the desired ranges of these parameters. In this work, three kinds of geometries typically used in static tests, i.e. round and notched cylindrical, and thin rectangular, have been considered and adapted to an available Hopkinson bar facility. The shape of the specimens (diameter, fillet/notch radius, thickness, gauge length) and the incident pulse intensity have been studied within a multi-objective optimization scheme, in order to achieve similar strain rates for the three kinds of tests, with nearly constant time histories of strain rate, triaxiality and Lode angle during deformation. More specifically, the adopted solutions permitted to achieve an average strain rate of 3500 s-1, with varying triaxialities from 0.5 to 1.2, Lode angles from 0.5 to 1 and strains to failure from 0.8 to 1.5.
Numerical analysis of damage kinetics in aluminium under axisymmetric loading
Authors: Tyupanova Olga , Nadezhin Sergey , Duday Pavel , Ivanovsky Andrey
Published: September 2018

Based on the analysis of the results of two R-Damage experiments, this paper provides a variant of a relation to simulate the spall damage recollection under the effect of weak compression waves. This relation closes the kinetic fracture model, considering both growth and recollection of damage. Using this model makes it possible to simulate adequately (with good quantitative agreement) the sequence of the processes “shock-wave compression – spall fracture – convergence of spall layer – recollection of damaged medium”, including the dynamics of the compacting wave arrival to the internal surface of the hollow cylindrical targets since it passed through the damaged media.
Dynamic crack arrest capability of some metallic alloys and polymers
Authors: Manar Gunasilan , Mat Jali Norazrina , Longère Patrice
Published: September 2018

We are here interested in the crack arrest capability under impact loading of metals and polymers used as structural and/or protection materials in aerospace engineering. Kalthoff and Winkler-type impact tests are carried out to that purpose on high strength AA7175 aluminum alloy and shock resistant polymethyl methacrylate (PMMA). Impact tests are carried out at impact velocities ranging from 50 m/s to 250 m/s and high speed camera is used to record the different steps of the failure process. For AA7175, early Mode II shear failure followed by late Mode I opening failure are seen. The premature ductile failure of the alloy is shown to result from a preceding stage of dynamic localization in the form of adiabatic shear bands. Impact tests on shock-resistant PMMA evidence the brittle feature of the material failure. It is notably shown that the higher the impact velocity (in the range 50-100 m/s) the larger the number of fragments. Moreover, depending on the impact velocity, changes in the crack path and thus in the mechanisms controlling the PMMA dynamic fracture can be seen.
Hybrid composite sandwich panels under blast and impact loading
Authors: Rolfe Emily , Arora Hari , Hooper Paul A. , Dear John P.
Published: September 2018

Naval vessels may undergo high strain rate loading, including impact, wave slamming and blast loading. Predicting the behaviour of composite sandwich structures to such loading is complicated, hence representative experiments are required. Two panels with hybrid carbon-and glass-fibre skins were fabricated and subjected to full-scale air blast loading. The panels were 1.7 × 1.5 m in size and were subjected to a 100 kg nitromethane charge at a stand-off distance of 15 m. 3D Digital Image Correlation (DIC) was implemented behind each of the panels to record the full-field out-of-plane displacement of the panels. In addition, the panels were instrumented with foil strain gauges on the front skins to record the response of the panel side in contact with the blast. The results revealed that the combination of glass-and carbon-fibre improves the blast resilience when compared to previous blast testing on panels with exclusively glass-fibre or carbon-fibre skins. However, the order in which the glass-and carbon-fibre layers were arranged did not have a significant effect on the overall panel performance. In addition, panels with the same hybrid skins were subjected to high velocity impact testing. An aluminium projectile with 25 mm diameter was fired at small scale panels (160 × 160 mm) using a laboratory gas gun at a velocity of 78 ms−1. 3D DIC was again used to record the out-of-plane displacement of these panels. In contrast to the blast experiment, the impact results showed that the order in which glass-and carbon-fibres were arranged did affect both the out-of-plane displacement and damage to the panels. The least damage occurred when glass-fibre layers were placed on the outermost layers impacted by the projectile.
Development of “Dropkinson” Bar for Intermediate Strain-rate Testing
Authors: Song Bo , Sanborn Brett , Heister Jack , Everett Randy , Martinez Thomas , Groves Gary , Johnson Evan , Kenney Dennis , Knight Marlene , Spletzer Matthew
Published: September 2018

A new apparatus – “Dropkinson Bar” – has been successfully developed for material property characterization at intermediate strain rates. This Dropkinson bar combines a drop table and a Hopkinson bar. The drop table is used to generate a relatively long and stable low-speed impact to the tensile specimen, whereas the Hopkinson bar principle is applied to measure the load history with accounting for inertia effects in the system. In addition, pulse shaping techniques were applied to the Dropkinson bar to facilitate uniform stress and strain as well as constant strain rate in the specimen. The Dropkinson bar was used to characterize 304L stainless steel and 6061-T6 aluminum at a strain rate of ~600 s−1. The experimental data obtained from the Dropkinson bar tests were compared with the data obtained from conventional Kolsky tensile bar tests of the same material at similar strain rates. Both sets of experimental results were consistent, showing the newly developed Dropkinson bar apparatus is reliable and repeatable.
Dynamic Compressive Behaviour of Closed-Cell Foam Materials Using Load-Measuring Apparatus with Opposite Load-Cells
Authors: Yamada Hiroyuki , Tateyama Kohei , Ogasawara Nagahisa , Watanabe Keiko
Published: September 2018

It is necessary to evaluate the mechanical properties of foam materials at wide range of strain rates, since these materials have the strain rate dependence of strength. In this study, we evaluated the dynamic compressive behaviour of the closed-cell foam materials using the load-measuring apparatus with opposite load-cells, which is applying the drop-weight testing machine. In order to measure large deformation at dynamic strain rate range, universal rate range load-cell, which can reduce the influence of the reflected stress wave, was used. In addition, load equilibrium can evaluate using opposite load-cells which consist of movable (drop-weight) and stationary load-cells. In this study, the commercially available ALPORAS closed-cell aluminum foam was used. From the results of quasi-static tests, three deformation processes of elastic response, plateau deformation and densification were confirmed in stress-strain relation. Within the set of experiments, the closed-cell aluminum foam was locally deformed from cells with low strength and the stress variation occurred during plateau deformation, regardless of the strain rate. In addition, it was clarified that the stress equilibrium was not achieved at the dynamic strain rate. This is thought to be a phenomenon caused by local deformation of cell structure.
Image-based high strain-rate testing for the characterization of viscoplasticity
Authors: Bouda Pascal , Langrand Bertrand , Notta-Cuvier Delphine , Markiewicz Eric , Pierron Fabrice
Published: September 2018

The present work aims at identifying an elastic-viscoplastic material behaviour over a wide strain and strain-rate range (up to 0.1 and 1000 s-1 respectively), using the so-called Virtual Fields Method. To define the experimental campaign, a design process has been set. This relies on the numerical optimization of the setup - notably the specimen shape ? with respects to user-defined criteria. Finally, the selected configuration ensures an accurate and robust identification of material parameters.
Dynamic properties of stainless steel under direct tension loading using a simple gas gun
Authors: Bragov Anatoly , Konstantinov Alexander , Kruszka Leopold , Lomunov Andrey , Filippov Andrey
Published: September 2018

The combined experimental and theoretical approach was applied to the study of high-speed deformation and fracture of the 1810 stainless steel. The material tests were performed using a split Hopkinson pressure bar to determine dynamic stress-strain curves, strain rate histories, plastic properties and fracture in the strain rate range of 102 ÷ 104 s-1. A scheme has been realized for obtaining a direct tensile load in the SHPB, using a tubular striker and a gas gun of a simple design. The parameters of the Johnson-Cook material model were identified using the experimental results obtained. Using a series of verification experiments under various types of stress-strain state, the degree of reliability of the identified mathematical model of the behavior of the material studied was determined.
A novel experimental method to characterise the shear strength of concrete based on pre-stressed samples. A comparison with existing techniques
Authors: Forquin Pascal , Abdul-Rahman Reem , Saletti Dominique
Published: September 2018

Mode II shear fracture process can be observed in plain-concrete structures exposed to intensive loadings such as shock, blasting or projectile-impact. It is the reason why the quasi-static and dynamic shear fracturing properties of concrete need to be investigated. In previous works performed by Forquin and coauthors a passive confining cell was applied to PTS (Punch-Through-Shear) specimens in a similar way than in quasi-oedometric compression tests where metallic ring are used as passive confinement. However the change of confining level during the sample loading constitutes a main drawback of this technique. In the present work a novel testing method is proposed based on a pre-stressing of the sample. To do so, the concrete specimen is inserted into pre-stressed metallic cell. During the unloading stage confining stresses are transferred to the ligament of the sample. In a second stage, a differential displacement applied to the central part of the sample toward the side parts produces a shearing of the ligament. The experiments performed in static and dynamic conditions (SHPB testing) allow the shear response of concrete to be determined over a wide range of strain-rates. In the present communication, the testing technique and obtained results are compared to the obtained data with the previous experimental method.
Inertial Impact Tests to Identify the Plastic Properties of Metals
Authors: Davis Frances , Fletcher Lloyd , Pierron Fabrice
Published: September 2018

The behavior and failure mechanisms of materials often change at high strain rates (> 100 1/s) when compared with their quasi-static response. These differences are critical when designing structures or components that will be subjected to impact or blast loads. The recent progress in ultra-high speed imaging and full-field measurement techniques provides a unique opportunity to improve the quality of high strain rate test data. The objective of the current work is to design and validate an experimental technique to identify the elastoplastic material properties of metals. The methodology uses an ultra-high speed camera and the grid method to obtain time-resolved full-field deformation data as impact induced stress waves propagate in a sample. The virtual fields method is then used to inversely identify the plastic properties of the specimen. The results for five aluminum 6082-T6 impacted at 50 m/s are presented.
Characterizations of dynamic material properties on compact pulsed power generator CQ-4
Authors: Wang Guiji , Luo Binqiang , Zhang Xuping , Cai Jintao , Chong Tao , Zhao Jianheng , Chen Xuemiao , Tan Fuli , Sun Chengwei , Liu Cangli , Wu Gang
Published: September 2018

Over last two decades, the techniques of magnetically driven quasi-isentropic compression and launching high velocity flyer plates based on pulsed high current generators have being extensively used to do dynamic material experiments under extreme conditions, such as high pressure, high temperature and high strain rate. A compact pulsed power generator CQ-4 was developed to do quasi-isentropic compression experiments of materials at Institute of Fluid Physics of CAEP, which can deliver maximum peak current of about 4 MA to short-circuit loads and produce approximate 100 GPa pressure on the metallic samples. On CQ-4, several types of dynamic material experiments have being conducted for equation of states, phase transitions, constitutive relationships, micro-structure evolutions of matter under quasi-isentropic compression and shock loadings. Meanwhile the dynamic behaviors of solid plastic bonded explosives and their components have also being researched for better understanding the interaction of explosive components under stress waves and the hot spot originations and evolutions mechanism of PBX explosives under dynamic loadings. Several typical applications in dynamic material properties were shown in this paper to exhibit the capabilities of CQ-4.
Effects of microstructure on the dynamic strain aging of ferriticpearlitic steels at high strain rates
Authors: Mardoukhi Ahmad , Rämö Jari , Vuoristo Taina , Roth Amandine , Hokka Mikko , Kuokkala Veli-Tapani
Published: September 2018

This paper presents an experimental study of the effects of dynamic strain aging on the mechanical behavior of selected high carbon and chromium-manganese steels in dynamic loading condition. In ferritic-pearlitic steels, the dynamic strain aging is typically caused by carbon, nitrogen, and possibly some other small solute atoms. Therefore, the thermomechanical treatments affect strongly how strong the dynamic strain aging effect is and at what temperature and strain rate regions the maximum effect is observed. In this work, we present results of the high temperature dynamic compression tests carried out for two different ferritic-pearlitic steels, 16MnCr5 and C60, that were heat treated to produce different microstructure variants of these standard alloys. The microstructures were analyzed using electron microscopy, and the materials were tested with the Split Hopkinson Pressure Bar device at three different strain rates at temperatures ranging from room temperature up to 680 °C to study the effect of the heat treatments and the resulting microstructures on the dynamic behavior of the steels and the dynamic strain aging effect. The results indicate that for both steels, a coarse grain structure has the strongest dynamic strain aging sensitivity at small plastic strains. However, at higher strains, all microstructures show similar strain aging sensitivities.