bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhanced multi‐dataset transfer learning method for unsupervised person re‐identification using co‐training strategy
Authors: Xian Yuqiao , Hu Haifeng
Published: September 2018

This study proposes progressive unsupervised co‐learning for unsupervised person re‐identification by introducing a co‐training strategy in an iterative training process. The authors’ method adopts an iterative training process to improve transferred models by iterating among clustering, selection, exchange, and fine‐tuning. To solve the problem of transferring representations learned from multiple source datasets, their method utilises multiple convolutional neural network (CNN) models trained on different labelled source datasets by feeding soft labels obtained by clustering on target dataset to each other. The enhanced model can learn more discriminative person representations than the single model trained on multiple datasets. Experimental results on two large‐scale benchmark datasets (i.e. DukeMTMC‐reID and Market‐1501) demonstrate that their method can enhance transferred CNN models by using more source datasets and is competitive to the state‐of‐the‐art methods.
Object detection and tracking under Complex environment using deep learning‐based LPM
Authors: Li Yundong , Zhang Xueyan , Li Hongguang , Zhou Qichen , Cao Xianbin , Xiao Zhifeng
Published: September 2018

Object detection and tracking under complex environment are challenging because of the disturbances induced by background clutter, illumination changes, occlusions and other factors. The bulk of traditional algorithms basically rely on hand‐crafted features, which are not sufficiently robust to a complex environment. Moreover, the processes of detection and tracking are separated, which leads to the overall efficiency not high. In this study, a novel local probability model (LPM)‐based mean shift (MS) algorithm is proposed to integrate object detection and tracking. The main contributions include: (i) a new framework based on the combination of LPM and MS is established for the integration of object tracking and detection. (ii) For object detection, the training and prediction of LPM are built by stacked denoising autoencoders based deep learning. (iii) For object tracking, an MS tracking algorithm leveraging LPM is modified to improve the tracking efficiency under a complex environment. Experimental results demonstrate that the proposed method is superior to the colour histograms based MS and histograms of oriented gradients based MS in terms of robustness and tracking accuracy.
Multi‐view learning for benign epilepsy with centrotemporal spikes
Authors: Yan Ming , Liu Ling , Basodi Sunitha , Pan Yi
Published: September 2018

Benign epilepsy with centrotemporal spikes (BECT) may be the most popular epilepsy to attack children. In recent years, more and more studies have shown that magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI) are promising techniques in distinguishing BECT patients from healthy controls. However, these existing works have suffered from two limitations. On the one hand, they have paid more attention to the brain changes between BETC and healthy controls than developing machine learning methods that can recognize BECT patients. On the other hand, most of the existing approaches extract hand‐crafted features from MRI or fMRI, which cannot obtain the desired performance due to the limited representative capacity of the used features. To address these issues, we propose a novel classification method by fusing the predictions of three different views: hand‐crafted features view, MRI view, and fMRI view. The final result is obtained by passing through those predictions after a fusing neural network. The basic idea of our method is that multiple views could provide complementary information and thus can boost the classification performance. Extensive experiments show that the proposed multi‐view method is remarkably superior to single‐view methods.
Texture‐based feature extraction of smear images for the detection of cervical cancer
Authors: Arya Mithlesh , Mittal Namita , Singh Girdhari
Published: September 2018

In India, cervical cancer is the second most common type of cancer in females. Pap smear is a simple cytology test for the detection of cancer in its early stages. To obtain the best results from the Pap smear, expert pathologist are required. Availability of pathologist in India is far below the required numbers, especially in rural parts. In this paper, multiple texture‐based features are introduced for the extraction of relevant and informative features from single‐cell images. First‐order histogram, GLCM, LBP, Laws, and DWT are used for texture feature extraction. These methods help to recognise the contour of the nucleus and cytoplasm. ANN and SVM are used to classify the single‐cell images either normal or cancerous based on the trained features. ANN and SVM are used on every single feature as well as on the combination of all features. Best results are obtained with a combination of all features. The system is evaluated on generated dataset MNITJ, containing 330 single cervical cell images and also on publicly available benchmark Herlev data set. Experimental results show that the proposed texture‐based features give significantly better results in cervical cancer detection when compared with state of the art shape‐based features regarding accuracy.
Electrical and thermal properties of surface passivated carbon nanotube/polyvinylidene fluoride composites
Authors: Ouyang Yingxiu , Deng Yongmao , Li Dandan , Cai Zhihong , Xu Ruijie , Lei Caihong , Lu Shengguo
Published: September 2018

The composites reported here were prepared through a melt‐rolling mixing process, in which surface passivated multi‐wall carbon nanotube and poly(vinylidene fluoride) were used as the filler and the matrix, respectively. X‐ray diffraction, precision inductance–capacitance–resistance, and laser flash thermal‐diffusivity analyses were employed to characterise the structural, electrical and thermal properties. Results indicate that through the surface passivation of carbon nanotubes, the electrical resistivities of the composites were greatly enhanced, while the thermal conductivities do not change significantly. The electrical properties were discussed in comparison with the Maxwell–Wagner polarisation, universal law, and Gerhardt's method.
COLIDE: a collaborative intrusion detection framework for Internet of Things
Authors: Arshad Junaid , Azad Muhammad Ajmal , Mahmoud Abdellatif Mohammad , Ur Rehman Muhammad Habib , Salah Khaled
Published: September 2018

Internet of Things (IoT) represent a network of resource‐constrained sensor devices connected through the open Internet, susceptible to misuse by intruders. Traditional standalone intrusion detection systems (IDS) are tasked with monitoring device behaviours to identify malicious activities. These systems not only require extensive network and system resources but also cause delays in detecting a malicious actor due to unavailability of a comprehensive view of the intruder's activities. Collaboration among IoT devices enables considering knowledge from a collection of host and network devices to achieve improved detection accuracy in a timely manner. However, collaboration introduces the challenge of energy efficiency and event processing which is particularly significant for resource‐constrained devices. In this paper, we present a collaborative intrusion detection framework (COLIDE) for IoT leveraging collaboration among resource‐constrained sensor and border nodes for effective and timely detection of intruders. The paper presents a detailed formal description of the proposed framework along with analysis to assess its effectiveness for a typical IoT system. We implemented the COLIDE framework with Contiki OS and conducted thorough experimentation to evaluate its performance. The evaluation demonstrates efficiency of COLIDE framework with respect to energy and processing overheads achieving effectiveness within an IoT system.
Comparative assessment of two different modes multi‐objective optimal power management of micro‐grid: grid‐connected and stand‐alone
Authors: Razmi Hadi , Doagou‐Mojarrad Hasan
Published: September 2018

This study presents a multi‐objective problem of the optimal power management of micro‐grids, which is based on modified Tribe‐PSO. Minimising total cost and emission over a 24 h time horizon are the objective functions in this problem. For the participation of consumers in the management of micro‐grids, demand response program is used. The scheduling approach is tested on the IEEE standard distribution network with 33 buses and is compared in two modes: grid‐connected and stand‐alone. Simulation results show that the presented power and demand scheduling provide a lower operation cost and more efficient utilisation of the renewable energy sources and the demand response programs.
Adaptive transmission for radar arrays using Weiss–Weinstein bounds
Authors: Greiff Christian , Mateos‐Núñez David , González‐Huici María A. , Brüggenwirth Stefan
Published: September 2018

The authors present an algorithm for adaptive selection of pulse repetition frequency or antenna activations for Doppler and direction of arrival estimation. The adaptation is performed sequentially using a Bayesian filter, responsible for updating the belief on parameters, and a controller, responsible for selecting transmission variables for the next measurement by optimising a prediction of the estimation error. This selection optimises the Weiss–Weinstein bound (WWB) for a multi‐dimensional frequency estimation model based on array measurements of a narrow‐band far‐field source. A particle filter implements the update of the posterior distribution after each new measurement is taken, and this posterior is further approximated by a Gaussian or a uniform distribution for which computationally fast expressions of the WWB are analytically derived. They characterise the controller's optimal choices in terms of signal‐to‐noise ratio and variance of the current belief, discussing their properties in terms of the ambiguity function and comparing them with optimal choices of other WWB constructions in the literature. The resulting algorithms are analysed in simulations where they showcase a practically feasible real‐time evaluation based on look‐up tables or small neural networks trained off‐line.
Improving data interpretation from SHTB tests on ductile metals
Authors: Lindenfeld Avishay , Partom Yehuda
Published: September 2018

This work presents a methodology to calibrate a strength model for ductile metals, based on dynamic tension tests of relatively long Dog-Bone specimens conducted on a Split Hopkinson Tension Bar (SHTB). We address the main difficulties involved in conducting and interpreting such tests, namely the duration of the loading pulse needed to deform long specimens and the non-uniform stress and strain distributions along the specimen due to neck formation. The first issue is addressed by using the waves‘ reflections within the output bar, as explained below. When the first loading (tension) wave does not cause failure of the specimen, a reflected compression wave travels from the specimen‘s bar end to the free bar‘s end. Upon reaching the free end this latter compression wave is reflected again as a second tension wave, which travels back along the bar until it reaches the specimen and loads it the second time. This enables further deformation of the specimen, practically doubling the loading pulse duration without changing the striker‘s length. The second issue is addresses by using full numerical simulations of the experimental setup, including the striker, the bars and the specimen. This way, the full dynamic behaviour of the specimen is taken into account, eliminating the need to consider specimen equilibrium and taking into account the current strain rate in the specimen as it deforms. Hence, model calibration can be done from the very start of plastic deformation and without the need to keep the strain rate constant during deformation. As a result, it is possible to reliably calibrate the strength model considering necking and neck location, as well as plastic heating which is a significant factor in the plastic deformation of ductile metals.
Strain-rate effects associated with the HJC concrete model
Authors: Johnson Gordon , Holmquist Timothy , Gerlach Charles
Published: September 2018

The Holmquist-Johnson-Cook (HJC) model for concrete was presented in 1993 and has been used extensively since that time. Since then a third invariant effect has been added and the shear modulus has been revised to vary such that Poisson's ratio is held constant. It has always been diffcult, however, to determine the appropriate constant for the strain-rate effect as most of the published data are for the net stress as a function of the strain rate. Because concrete is both pressure dependent and strain-rate dependent, it is necessary to separate the individual effects. Recently strain-rate data for three concrete materials were presented by Piotrowska and others [1, 2], where the data are presented as equivalent stress versus confining pressure for a high strain rate and a quasi-static strain rate. This is the form necessary to determine the appropriate strain-rate effect, and the data show that the strain-rate effect is larger than used in the initial publication of the HJC model, and also that the strain-rate effect is a function of the confining pressure. For lower pressures the strain-rate effect is a factor to be applied to the quasi-static data (which is the effect represented in the original HJC model), but for higher pressures the strain rate effect is better represented by an additive term. With the addition of an another HJC constant (the pressure at which the strain rate effect transitions from a multiplied factor to an additive term) it is possible to more accurately represent the response of concrete under high pressures and high strain rates, and it is possible to compute more accurate results for projectile penetration into concrete targets. The paper presents the modified form of the HJC model, an analysis of the strain-rate effects, and results of penetration computations that are compared to experimental data in the literature.
Mechanical behaviour of graphdiyne film: experimental and molecular dynamics simulation
Authors: Xiao Kailu , Wu Xianqian , Yin Qiuyun , Huang Chenguang
Published: September 2018

AFM experiments and molecular dynamics simulation of rectangular graphdiyne films are performed in this paper. The force-deflection curves are obtained, and the elastic modulus is calculated as 218.5 GPa and 482.615 GPa, respectively. The simulated maximum stress and pre-tension of graphdiyne film are 33.088 GPa and 0.551 GPa, respectively. It is observed that the graphdiyne film fractured in the central point once the indentation depth over the critical depth. Also, the obviously elastic behaviour has found during the loading-unloading-reloading process. The deformation mechanisms and fractured behaviour of the graphdiyne film are discussed in detail during the loading process. Moreover, the effects of various factors including loading speed and indenter radii of the graphdiyne film by the MD simulation are discussed.
Constitutive Modeling of High-Strength Steel Designed for Ballistic Protection
Authors: Zochowski Pawel , Zielenkiewicz Mariusz
Published: September 2018

Constitutive behaviour of a high-strength nanostructured bainitic steel NANOS-BA® was investigated in the article. A few types of mechanical tests under varying strain rates were carried out to determine the values of constants of Johnson-Cook flow and fracture models for the analysed material. In quasi-static conditions (8×10-3 s-1) the material samples were subjected to uniaxial tension and compression tests. In a high strain rates conditions (2,7×103 s-1) the Taylor impact test was carried out. On the basis of the data obtained from mechanical tests the constitutive model of the NANOS-BA® steel was built. It was then validated to determine the extent and degree of compliance between the numerical and experimental observations. The examples of combined numerical-experimental analyses of protective structures containing investigated material were shown. Small differences between the both types of results testifies that determined constants of Johnson-Cook equation may properly reproduce actual behaviour of the material, and the numerical model allows the preliminary evaluation of the effectiveness of armours containing layers of analysed grade of steel.
Numerical Simulation of Crack Initiation and Growth in PBX High Explosive Subject to Compression
Authors: Xicheng Huang , Shangkun Li , Wei Qiang , Gang Chen , Rong Tian , Lixiang Wang
Published: September 2018

PBX solid high explosive exhibits brittle behaviour in uniaxial tension, quasi-brittle in uniaxial compression, and ductile when subject to high confining pressure. Tension cracking is the primary failure mode of PBX quasi-brittle solid, which is the main effect leading to overall failure of structural integrity. One characteristics of brittle or quasi-brittle solids, such as PBXs, is that when subject to overall compressive loading, the tensile cracks can still initiate inside the material due to existence of imperfection within the materials. In present study the extended finite element method is applied to analyze the cracking failure mechanism in the PBX 9502 platelike specimen with cavity subjected to overall compression. The nonlinear constitutive behaviours and failure of PBX under complex stress states were described by means of stress state dependent strength surface, non-associated flow rule and cohesive model the nonlinear behaviors of PBXs, including failure. Analysis indicates the tensile stress around the cavity arises in the specimen although loaded by overall compression, and this local tensile condition leads to cracking initiation. The comparison between simulation results and the experimental data published by LANL[Liu C, Thompson D G. Crack initiation and growth in PBX 9502 high explosive subject to compression. Journal of Applied Mechanics, 2014, 81(10):212-213] shows that they are in agreement with each other on some aspects of crack behaviours, including overall development of crack history and inflexion, crack initiation moment, crack initial speed, etc.
Assessing through-thickness damage propagation, impact tests on layered composite beams - experimental work and numerical simulation
Authors: Vidal-Pérez Ignacio , Eriksen Rasmus , Berggreen Christian , Kepler Jørgen
Published: September 2018

The growing applications of layered fiber reinforced composite materials lead to the potential use of such lightweight materials for blast and ballistic impact resistant panels. Under such high strain rate loading, through-thickness damage propagation is crucial for the structural integrity of the panels. Experiments performed in composite panels under blast loading exhibited little information for the understanding of the time-line of damage propagation as only post-mortem inspection can be done. Instrumentation may be installed in order to follow the blast but the task reveals itself arduous as all the instruments must be protected against the blast. The sparse information can be valuable for configuration screening purposes, but is not su ffi cient for comparison and validation of numerical models. The work focuses on performing controlled impact experiments in narrow beams filmed side-wise with high-speed cameras. The narrow geometry of the beams leaves the thickness of the specimen exposed to the cameras allowing for a real-time monitoring of the stress waves propagating and for assessing the time-line of through-thickness damage propagation onset. Results showed that polyethylene impactors, given the right diameter, are the most suitable soft impactors. Numerical models including in-plane and out-of-plane damage propagation are being built in order to replicate the experimentally observed damage. The ultimate goal being establishing design rules for such lightweight fiber reinforced panels. It is numerically observed that, the speed propagation of delamination in the longitudinal direction reaches a common constant value for all interfaces.
Numerical Analysis and Experiment for Stress Wave Propagation in Two Connected Cylindrical Bodies with Different Cross-Sectional Area and Same Mechanical Impedance
Authors: Kobayashi Hidetoshi , Seo Yuya , Ogawa Kinya , Horikawa Keitaro , Tanigaki Ken-ichi
Published: September 2018

In this study, the behaviour of elastic stress wave propagating two connected cylindrical bodies was examined using dynamic finite element method (FEM). They consist of two bodies with different cross-sectional area, different Young’s modulus and identical mechanical impedance. It was found that when an incident wave passes through the boundary step between two different cross-sectional areas, a pair of reflected waves which has the same amplitude and opposite sign was observed, despite the same mechanical impedance. This phenomenon appears to be caused by the loading and unloading the boundary section due to the arrival and the passage of incident wave. It was also found that a connection manner to insert the smaller diameter cylinder into the other cylinder with a little length is quite effective for the reduction of the reflected wave, because of the superposition of waves from two edges and control of local deformation. This phenomenon was verified by a series of impact experiments using two cylindrical bodies connected by interference fit.
Shock Tube Testing and Modelling of Annealed Float Glass
Authors: Osnes Karoline , Børvik Tore , Sture Hopperstad Odd
Published: September 2018

Failure of glass is dominantly brittle, and is caused by microscopic flaws randomly distributed on the surface. Fracture mainly initiates in these flaws, and this leads to a high variability in the glass strength, which depends on geometry, boundary conditions and loading situation. Consequently, the identification of the fracture strength, in e.g. finite element analyses, is not straightforward. For rapid loading conditions, as for blast loading situations, the glass strength is generally increased because flaws need time to grow into cracks. The current study aims to identify the probabilistic fracture strength of glass plates under blast loading as a function of the plate?s boundary conditions, geometry and loading by using a newly proposed strength prediction model. To validate this model in some measure, 12 blast tests on annealed float glass were performed in a shock tube. As expected, the tests showed a large scatter in fracture strength. The strength prediction model captured the main trends found in the experimental tests, but a closer investigation of the strain rate sensitivity of glass was deemed necessary. Finally, the results from the strength prediction model were used as input in a simulation of annealed float glass under blast pressure in the finite element program IMPETUS Afea Solver. By use of a node splitting technique, the simulations captured the behaviour displayed in the experimental tests to a great extent.
Prediction procedure for hail impact
Authors: Dousset Simon , Girardot Jérémie , Dau Frédéric , Gakwaya Augustin
Published: September 2018

The constant increase of composite materials’ performances makes them more and more used in recent aircrafts. Structures, as the wings or the fuselage, may suffer from hail impacts that can make critical damages or even perforate them. In order to guaranty the safety of passengers, aircrafts have to be certified and simulations have to demonstrate good agreements with real behaviour of the structures and the hail projectile. The aim of this work is to propose a procedure to analyse the home made manufacturing of the ice generally performed in laboratories, its mechanical characterization and a mechanical model that can predict the time-space profile of the impact force on a rigid structure. Because of the high strain level of the hail during the impact, the Smooth Particle Hydrodynamics (SPH) method will be used. Indeed, the finite elements method needs heavy remeshing that are time consuming to avoid mesh distortion. The SPH is a numerical meshless method that calculates interactions between particles at every time increment. Models available in the literature have been studied and the model of J.D. Tippmann (Tippmann, Kim, et Jennifer D. Rhymer 2013) is chosen. In this paper, the Tippman model is presented with its solving using the SPH. A parametric study is proposed in order to catch the relevant parts of this model. A simple experimental procedure is then proposed to feed the model and the results of impact simulations at different velocities are compared to experimental measurements realized in the laboratory.
Shock wave speed and stress-strain relation of aluminium honeycombs under dynamic compression
Authors: Wang Peng , Zhang Jun , Huang Haiying , Zheng Zhijun , Yu Jilin
Published: September 2018

The propagation of layer-wise crushing bands in cellular materials under dynamic impact can be described by the plastic shock wave model. A cell-based finite element model of irregular aluminum honeycomb is constructed to carry out several constant-velocity compression tests. The shock wave speed is obtained by the one-dimensional stress distribution in the specimen along the loading direction. The relation between the shock wave speed and impact velocity is obtained and analyzed. It is found that the relation tends to be linear with the increase of the impact velocity. But the shock wave speed tends to be a constant value with the decrease of the impact velocity. A piecewise model is proposed to describe the dynamic stress-strain relation of aluminum honeycombs based on a piecewise hypothesis of the relation between the shock wave speed and the impact velocity together with the one-dimensional shock wave theory. Different stress-strain relations corresponding to different impact velocity regions and different deformation modes are obtained.
Benefits of Using Lode Angle Dependent Fracture Models to Predict Ballistic Limits of Armor Steel
Authors: Roth Christian C. , Fras Teresa , Faderl Norbert , Mohr Dirk
Published: September 2018

Ductile fracture experiments are carried out at different stress states, strain rates and temperatures on a range of flat Mars 300 steel specimens to calibrate both a plasticity and a fracture model. To predict the onset of fracture a stress state and strain rate-dependent Hosford–Coulomb fracture initiation model is used. Single material impact experiments are performed on targets of homogenous and perforated Mars 300 plates by accelerating cylindrical Mars 300 impactors in a single-stage gas gun. It is shown that the chosen modeling approach allows accurate modeling of the plastic response as well as the fracture patterns.
Tensile Properties of AM Maraging steel
Authors: Church Philip , Reynolds Mark , Gould Peter , Oakley Robin , Harrison Nigel , Williamson Dave , Braithwaite Chris , Taylor Nick
Published: September 2018

Additively Manufactured (AM) materials have great potential for producing graded materials, embedded structures and near net complex shapes. AM maraging steel properties have been compared with wrought maraging steel. The comparison featured interrupted tensile tests over a range of temperatures and strain rates. In addition a specially designed Tensile Split Hopkinson Pressure Bar (TSHPB) has been built to test very high strength metals at high strain rates. The results showed that the AM maraging steel was much more ductile than expected and exhibited significant necking under all conditions tested. All the samples exhibited ductile fracture. Although not as ductile as the wrought material, the AM material could be cost effective through economies of scale for complex components. The microstructure contained inclusions which derived from either the powder or the AM process and thus there is significant potential to improve these materials further. A modified Armstrong-Zerilli model was also constructed for these materials and shown to predict the raw experimental data within experimental error using DYNA3D simulations.
Evolution of penetration mechanism induced by strain rate effect
Authors: Li Gan , Song Chunming , Wang Mingyang
Published: September 2018

According to the dynamic mechanical properties, in the striking velocity range, the strain rate of the projectile and target caused by penetration can reach 104~106/s. The strain rate effect increases sharply and then tends to saturation. During the penetration process, the mechanical properties of the target and the projectile change violently and present serious spatial inhomogeneity, which has a great influence on the penetration effect. In this paper, penetration experiments of granite targets by steel projectiles are carried out in the range of 1200m/s~2400m/s, the crater parameters are measured and the projectiles are recovered to obtain the macroscopic failure pattern of the projectiles and the targets. Based on the dynamic mechanical properties, an interaction model of the projectiles and the target is established, which considers the spatial and temporal distributions of the strain rates during penetration. With this model, the influences of material mechanical behaviour on penetration effect at different velocities are analysed, the formation cause and internal mechanism of penetration effect are discussed, and the influence mechanism of the strain rate effect on the penetration mechanism evolution is also revealed.
Analysis of the localization process leading to fragmentation of dynamically expanded rings
Authors: El Maï Skander , Mercier Sébastien , Molinari Alain
Published: September 2018

A novel extension of the Linear Stability Analysis (LSA) applied to the development of the instabilities in a round bar in dynamic extension is proposed. The history of development of the instabilities and the multi-modal aspect of the perturbations are now considered in this extension. A new statistical approach based on the power spectrum of the perturbation pattern is proposed to evaluate the variability of the spacing between potential sites of necking. This approach has been applied to analyze the LSA results and predictions obtained from numerical simulations of a bar in dynamic extension. It is shown that both analytical and numerical approaches provides good agreement
The Mechanical Response of Pre-Shocked Aluminium Single Crystals
Authors: Millett Jeremy , Gray George. , Whiteman Glenn , Fensin Saryu. , Owen Gareth
Published: September 2018

The behaviour of metals under mechanical loading, including shock loading conditions is strongly influenced by effects such as impurity levels, grain size, initial dislocation density and texture. The work discussed here is part of a wider study on the effects of orientation of aluminium single crystals to one dimensional shock loading, including the Hugoniot Elastic Limit and spall strength. In this work, specimens with three principle directions (<100>, <110> and <111>) parallel to the loading axis have been shock loaded and recovered under conditions of purely one-dimensional strain, with their post shock response monitored by quasi-static compression tests. Results show that the <100> crystal demonstrates a significant degree of post shock hardening, whilst the <111> crystal shows virtually none, and the <110> intermediate between the two. These results are consistent with the ordering of both the HELs and spall strengths observed in a previous paper, and have been explained in terms of the Schmidt factors.
High Strain-Rate Compressive Properties of Carbon/Epoxy Laminated Composites – Effects of loading direction and temperature
Authors: Nakai Kenji , Fukushima Tsubasa , Yokoyama Takashi , Arakawa Kazuo
Published: September 2018

The high strain-rate compressive characteristics of a cross-ply carbon/epoxy laminated composite in the three principal material directions or fibre (1-), in-plane transverse (2-) and throughthickness (3-) directions are investigated on the conventional split Hopkinson pressure bar (SHPB) over a range of temperatures between 20 and 80 °C. A nearly 10 mm thick cross-ply carbon/epoxy composite laminate fabricated using vacuum assisted resin transfer molding (VaRTM) was tested. Cylindrical specimens with a slenderness ratio (= length/diameter) of 0.5 are used in high strain-rate tests, and those with the slenderness ratios of 1.0 and 1.5 are used in low and intermediate strain-rate tests. The uniaxial compressive stress-strain curves up to failure at quasi-static and intermediate strain rates are measured on an Instron testing machine at elevated temperatures. A pair of steel rings is attached to both ends of the cylindrical specimens to prevent premature end crushing in the 1-and 2-direction tests on the Instron testing machine. It is shown that the ultimate compressive strength (or failure stress) exhibits positive strainrate effects and negative temperature ones over a strain-rate range of 10–3 to 103/s and a temperature range of 20 to 80 °C in the three principal material directions.
High strain rate behaviour of fiber reinforced concrete
Authors: Popovič Miloslav , Buchar Jaroslav , Drdlová Martina
Published: September 2018

The results of dynamic compression and tensile-splitting tests of concrete reinforced by randomly distributed short non – metallic fibres are presented. A Split Hopkinson Pressure Bar combined with a high-speed photographic system, was used to conduct dynamic Brazilian tests. Quasi static test show that the reinforcement of concrete by the non-metallic fibres leads to the improvement of mechanical properties at quasi static loading. This phenomenon was not observed at the high strain rate loading .Some explanation of this result is briefly outlined.
Dynamic behaviour of an earthen material under different impact loading conditions
Authors: Fenu Luigi , Aymerich Francesco , Francesconi Luca , Forni Daniele , Tesio Nicoletta , Cadoni Ezio
Published: September 2018

The dynamic behaviour of earthen materials reinforced with natural fibres is little studied although earth buildings are often built in seismic areas. In this paper the dynamic behaviour of an earthen material reinforced with hemp fibres under different impact loadings has been experimentally investigated. The dynamic response of the material in 3-point bending was investigated through an instrumented dropweight device, while the response in tension and in compression was investigated through a modified Hopkinson bar device. Typical impact response curves for tension, compression and bending impact tests have been obtained. The favourable effect of fibres in dissipating fracture energy under impact loads has been observed in all these types of test.
Strain rate and thermal softening effects in shear testing of AA7075-T6 sheet
Authors: Rahmaan Taamjeed , Zhou Ping , Butcher Cliff , Worswick Michael J.
Published: September 2018

Shear tests were performed at strain rates ranging from quasi-static (0.01 s-1) to 500 s-1 for AA7075-T6 sheet metal alloy at room temperature. A miniature sized shear specimen was used in this work to perform high strain rate shear testing. Digital image correlation (DIC) techniques were employed to measure the strains in the experiments. At maximum in-plane shear strains greater than 20%, the AA7075-T6 alloy demonstrated a reduced work hardening rate at elevated strain rates. At lower strains, the AA7075-T6 alloy showed mild positive rate sensitivity. The strain to localization (using the Zener-Holloman criterion), measured using the DIC technique, decreased with strain rate in shear loading. The strain at complete failure, however, exhibited an increase at the highest strain rate (500 s-1). The current work also focused on characterization of the thermal conditions occurring during high rate loading in shear with in situ high speed thermal imaging. Experimental results from the highest strain rate (500 s-1) tests showed a notable increase in temperature within the specimen gauge region as a result of the conversion of plastic deformation energy into heat.
Experimental investigation of the impact response of novel steelbiocomposite hybrid materials
Authors: Ramakrishnan KarthikRam , Hokka Mikko , Sarlin Essi , Kanerva Mikko , Kouhia Reijo , Kuokkala Veli-Tapani
Published: September 2018

Recent developments in the production of technical flax fabrics allow the use of sustainable natural fibres to replace synthetic fibres in the manufacture of structural composite parts. Natural fibre reinforced biocomposites have been proven to satisfy design and structural integrity requirements but impact strength has been identified as one of their limitations. In this paper, hybridisation of the biocomposite with a metal layer has been investigated as a potential method to improve the impact resistance of natural fibre composites. The impact response of biocomposites made of flax-epoxy is investigated experimentally using a high velocity particle impactor. A high-speed camera setup was used to observe the rear surface of the plates during impact. Digital Image Correlation (DIC) of the high speed camera images was used for full-field strain measurement and to study the initiation and propagation of damage during the impact. The different modes of damage in the hybrid laminate were identified by postimpact analysis of the section of the damaged composite plate using optical microscopy. The study shows the difference in impact response for different material combinations and configurations. The hybrid construction was shown to improve the impact resistance of the flax composite.
Combined shear/tension testing of fibre composites at high strain rates using an image-based inertial impact test
Authors: Fletcher Lloyd , Van-Blitterswyk Jared , Pierron Fabrice
Published: September 2018

Testing fibre composites off-axis has been used extensively to explore shear/tension coupling effects. However, off-axis testing at strain rates above 500 s-1 is challenging with a split Hopkinson bar apparatus. This is primarily due to the effects of inertia, which violate the assumption of stress equilibrium necessary to infer stress and strain from point measurements taken on the bars. Therefore, there is a need to develop new high strain rate test methods that do not rely on the assumptions of split Hopkinson bar analysis. Recently, a new image-based inertial impact test has been used to successfully identify the transverse modulus and tensile strength of a unidirectional composite at strain rates on the order of 2000 -1. The image-based inertial impact test method uses a reflected compressive stress wave to generate tensile stress and failure in an impacted specimen. Thus, the purpose of this study is to modify the image-based inertial impact test method to investigate the high strain rate properties of fibre composites using an off-axis configuration. For an off-axis specimen, a combined shear/tension or shear/compression stress state will be obtained. Throughout the propagation of the stress wave, full-field displacement measurements are taken. Strain and acceleration fields are then derived from the displacement fields. The kinematic fields are then processed with the virtual fields method (VFM) to reconstruct stress averages and identify the in-plane stiffness components G12 and E22.
Dynamic fracture of a dual phase automotive steel
Authors: Chandran Sarath , Verleysen Patricia , Lian Junhe , Liu Wenqi , Münstermann Sebastian
Published: September 2018

Dynamic testing of sheet metals has become more important due to the need for more reliable vehicle crashworthiness assessments in the automotive industry. The study presents a comprehensive set of experimental results that covers a wide range of stress states on a dual phase automotive sheet steel. Split Hopkinson bar tensile (SHBT) tests are performed on dogbone shaped samples to obtain the plastic hardening properties at high strain rates. A set of purpose designed sample geometries comprising of three notched dogbone tension samples is tested at high strain rates to characterise the dynamic damage and fracture properties under well controlled stress states. The geometry of the samples is optimised with the aid of finite element analysis. During the tests, high speed photography together with digital image correlation are implemented to acquire full field measurements and to gain more insight into the localisation of strains at high strain rates. An experimental-numerical approach is proposed to effectively determine the fracture characteristics of the dual phase steel under extreme conditions. A modified Bai-Wierzbicki model is implemented to assess the damage initiation and subsequent failure. Additionally, the fracture mechanisms are studied utilizing scanning electron microscopy.
Dynamic characterization of tungsten carbide behaviour at very high strain-rates
Authors: Erzar Benjamin , Zinszner Jean-Luc
Published: September 2018

Cemented tungsten carbide, with its very high density and high strength, is known to be the material composing several small calibre armour piercing ammunitions. The impact of a tungsten carbide core projectile onto a high efficiency armour often leads to the fracture of the tungsten carbide core. Thus, the pertinence of material models used in numerical simulations to describe the behaviour and the damage of the target is not sufficient to well predict a ballistic impact. In this work, the GEPI high-pulsed power generator is used to conduct dynamic characterization of the behaviour of a cemented tungsten carbide under both compressive and tensile loadings. The Hugoniot Elastic Limit of this material has been identified (HEL = 5.8 GPa). Moreover, Lagrangian analysis allowed the complete loading path to be identified up to 18 GPa.
Compression behavior and energy absorption capacity of woven flax-epoxy composite under various stain rates
Authors: Hu Jianxing , Yin Sha , Xu Jun
Published: September 2018

Natural fiber reinforced composites serving as building block for structural parts are highly desired due to weight reduction in automotive industry. In the current study, the compressive behavior and energy absorption capacity of woven flax-epoxy composite were experimentally investigated under various strain rates. These tested cubic specimens were manufactured by the vacuum assisted resin infusion (VARI) process and tailoring operation. Quasi-static and dynamic experiments were conducted in a hydraulic servo test machine and a Split Hopkinson Pressure Bar (SHPB), respectively. Preliminary dynamic experimental results revealed significant strain rate sensitivity of woven flax-epoxy composite. The ultimate stress increased by 61.2% from 185.2 MPa to 298.6 MPa and 25.9% for specific energy absorption when the strain rate increased from 0.003/s to 2800/s. Results may serve as a guidance for the further investigation of flax fibers reinforced composites in automotive industry.
Dynamic Deformation Behaviour and Dislocation Substructure of AZ80 Magnesium Alloy over a Wide Range of Temperatures
Authors: Lee Woei-Shyan , Chou Cheng-Wen
Published: September 2018

The high strain rate deformation behaviour and dislocation substructure of AZ80 magnesium alloy are investigated at strain rates of 8x102 s-1, 1.5x103 s-1 and 2.2x103 s-1 and temperatures of-100 ºC, 25 ºC and 300 ºC using a compressive split-Hopkinson pressure bar system. The flow stress, work hardening coefficient, strain rate sensitivity and temperature sensitivity all increase with increasing strain rate or decreasing temperature. Moreover, the dynamic deformation behaviour is well described by the Zerilli-Armstrong hcp constitutive equation. Transmission electron microscopy observations show that the dislocation density increases with a higher strain rate or a lower temperature. Finally, the flow stress varies linearly with the square root of the dislocation density in accordance with the Bailey-Hirsch model.
Static and dynamic response of ultra-fast annealed advanced high strength steels
Authors: Vercruysse Florian , Castro Cerda Felipe M. , Petrov Roumen , Verleysen Patricia
Published: September 2018

Ultra-fast annealing (UFA) is a viable alternative for processing of 3rdgeneration advanced high strength steels (AHSS). Use of heating rates up to 1000°C/s shows a significant grain refinement effect in low carbon steel (0.1 wt.%), and creates multiphase structures containing ferrite, martensite, bainite and retained austenite. This mixture of structural constituents is attributed to carbon gradients in the steel due to limited diffusional time during UFA treatment. Quasi-static (strain rate of 0.0033s-1) and dynamic (stain rate 600s-1) tensile tests showed that tensile strength of both conventional and UFA sample increases at high strain rates, whereas the elongation at fracture decreases. The ultrafast heated samples are less sensitive to deterioration of elongation at high strain rates then the conventionally heat treated ones. Based on metallographic studies was concluded that the presence of up to 5% of retained austenite together with a lower carbon martensite/bainite fraction are the main reason for the improved tensile properties. An extended stability of retained austenite towards higher strain values was observed in the high strain rate tests which is attributed to adiabatic heating. The extension of the transformation induced plasticity (TRIP) effect towards higher strain values allowed the UFA-samples to better preserve their deformation capacity resulting in expected better crashworthiness.
In-situ Experiments to Capture the Evolution of Microstructure During Phase Transformation of Titanium Under Dynamic Loading
Authors: Morrow Benjamin M. , Jones David R. , Rigg Paulo A. , Gray George T. , Cerreta Ellen K.
Published: September 2018

Under sufficient stresses, such as during dynamic loading, titanium experiences a phase transformation from hcp alpha phase to hexagonal omega phase. Omega phase is often retained in the microstructure after unloading, and has a strong influence on subsequent mechanical properties. Simulations suggest there are multiple pathways and underlying mechanisms for this transformation. Due to the incredibly short timescales involved, experimental measurements for model validation have been difficult. However, new capabilities at the Advanced Photon Source have enabled diffraction measurements during plate impact experiments to study the evolution of titanium during transformation. These high-rate data allow us to probe the mechanism and kinetics of phase transformations in new ways. Recent results will be presented and compared to post-mortem characterization of soft-recovered shocked specimens. Comparisons are made with previous tests where material was shock-loaded and soft recovered for microstructural analysis. Together these techniques create a consistent picture of material behavior during the shock-induced ff–! phase transformation in titanium.
Elastic Behavior of Zirconia under Ramp Compression
Authors: Luo Binqiang , Mo Jianjun , Wang Guiji , Tan Fuli , Zhao Jianheng , Sun Chengwei
Published: September 2018

Dynamic properties of single crystal cubic zirconia were investigated under magnetically driven ramp wave compression on pulsed power generator CQ-4. Photonic Doppler Velocimeter (PDV) was employed to measure the free surface velocities of stepped samples. The elastic particle velocity and Ramp Elastic Limit (REL) of <100> cubic zirconia under ramp wave compression are 475 m/s and 25.4 GPa respectively, which are much higher than shock results reported in literature. The <110> cubic zirconia was loaded to maximum particle velocity 460 m/s and pressure 20.8 GPa in elastic region, but no distinct elasticplastic transition was observed. Inverse characteristic method was used to process the velocity data. The results show that the Lagrangian sound speed increases linearly with particle velocity in elastic region, and there exists distinct orientation effects.