Viscoplastic Models in Simulation of High Strain-Rate Behavior of Materials

Some specific features of material deformation behavior under impulse loading are considered. Viscoplastic models of materials used in calculations and methods of dynamic testing used to determine their parameters are analyzed. Examples of practical problems are presented (penetration, explosion welding), in the solution of which one should consider the effects associated with high strain rates.     

Investigation on magnetorheological elastomers based on natural rubber

Magnetorheological Elastomers (MR Elastomers or MREs) are a kind of novel smart material, whose mechanical, electrical, magnetic properties are controllable under applied magnetic fields. They have attracted increasing attentions and broad application prospects. But conventional MREs are limited to wide applications because their MR effects and mechanical performances are not high enough. This paper aims to optimize the fabrication method and to fabricate good natural rubber based MREs with high modulus by investigating the influences of a variety of fabrication conditions on the MREs performances, such as matrix type, external magnetic flux density, and temperature, plasticizer and iron particles. Among these factors, the content of iron particles plays a most important contribution in shear modulus. When the iron particle weight fraction is 80% and the external magnetic flux density is 1 T, the field-induced increment of shear modulus reaches 3.6 MPa, and the relative MR effect is 133%. If the iron weight fraction increases to 90%, the field-induced increment of shear modulus is 4.5 MPa. This result has exceeded the best report in the literatures researching the MREs on the same kind of matrix. The dynamic performances of MREs were also experimentally characterized by using a modified Dynamic Mechanical Analyzer (DMA) system. The effects of strain amplitude and driving frequency on viscoelastic properties of MREs were analyzed.     

Dynamic viscoelastic modeling of magnetorheological elastomers

A micromechanics-based viscoelastic constitutive model is proposed to estimate the zero-magnetic-field- and magnetic-field-dependent dynamic shear stiffness and damping behavior for magnetorheological elastomers (MREs). The effect of imperfect interfacial condition between the ferromagnetic particles and the elastomeric matrix on those properties is incorporated in the proposed model. A concept of effective volume fraction of particles is introduced to take into account the particle agglomeration in MREs. The magnetic dipole interaction is further employed to evaluate the magnetic-field-induced increase in shear stiffness of MREs. Numerical simulations are conducted and compared with experimental data to verify the proposed model.     

磁流变弹性体力磁耦合本构关系的研究进展

磁流变弹性体是一类力学特性能够被外磁场可逆调控的新型智能复合材料。将微米尺寸的磁性颗粒填充到橡胶类聚合物基体中制备的磁弹体材料,其模量、阻尼和形变可以由外加磁场快速、连续、可逆改变。目前,基于动力学实验的宏观力学元素组合模型分析方法、微观偶极子力学分析和宏观连续介质力学描述成为分析磁感应多场耦合复合材料本构关系的主要方法。同时,数值模拟也成为研究磁流变材料的颗粒聚集结构演化和磁致伸缩效应的有效手段。本文侧重于介绍磁弹体智能材料力磁耦合的基本理论和研究方法,总结相关研究工作并探讨研究趋势,为磁敏类多功能材料的应用研究提供理论基础。     

Comparison of dynamic compression behavior of single crystal sapphire to polycrystalline alumina

Due to the considerable interest in the shock loading behavior of aluminum oxide whether it is in the polycrystalline phase or in the single crystal phase well-controlled experiments were conducted to probe differences in shock loading behavior between these two materials. Previous studies concluded that the behavior was similar but careful examination of well-controlled experiments has revealed the two materials are different.Although the experimental results appear to have the same behavior in the shock velocity vs. particle velocity plane, they are considerably different in the stress-volume compression plane and evidence is provided that indicates the single crystal remains crystalline up to the stresses imposed for this analysis. This is an extremely interesting observation since it has many implications including developing dynamic material models capable of transitioning between individual grains and polycrystalline material.     

Smart composites of urethane elastomers with carbonyl iron

Smart composites based on carbonyl-iron particles in a polyurethane matrix, known as magnetorheological elastomers (MREs), were manufactured and studied. The influence of ferromagnetic particle content and particle arrangement in relation to an external magnetic field was investigated. Several different elastomers with different stiffnesses were used as matrices. It was found that the structure of a fabricated MRE depends on the viscosity of the matrix before curing and the flux density of the applied magnetic field. Two different magnetic field strengths were used: 0.1 and 0.3 T. The amount of carbonyl iron particles was varied from 1.5 to 33.0 vol%. Scanning electron microscopy technique was used to observe MRE microstructure. The particles’ orientation and their arrangement were also investigated by vibrating sample magnetometer. A correlation was found between MRE microstructure and magnetic properties. Compression tests on cylindrical samples in the presence and absence of a magnetic field showed that a magnetic field increased the stiffness of the material. Additionally the rheological properties of MREs were tested in a magnetic field. It was found that the amount of ferromagnetic particles and their arrangement have a significant influence on the rheological properties of MREs. The highest relative change of storage modulus under 200 mT magnetic field, equal to 282%, was recorded for samples with 11.5 vol% of particles.     

The influence of particle content on the equi-biaxial fatigue behaviour of magnetorheological elastomers

The equi-biaxial fatigue behaviour of silicone based magnetorheological elastomers (MREs) with various volume fractions of carbonyl iron particles ranging between 15% and 35% was studied. Wöhler curves for each material were derived by cycling test samples to failure over a range of stress amplitudes. Changes in complex modulus (E¹) and dynamic stored energy during the fatigue process were observed. As for other elastic solids, fatigue resistance of MREs with different particle contents was shown to be dependent on the stress amplitudes applied. MREs with low particle content showed the highest fatigue life at high stress amplitudes while MREs with high particle content exhibited the highest fatigue resistance at low stress amplitudes. E¹ fell with the accumulation of cycles for each material, but the change was dependent on the particle content and stress amplitude applied. However, each material failed in a range suggesting a limiting value of E¹ for the material between 1.22 MPa and 1.38 MPa regardless of the particle content and the magnitude of the stress amplitude. In keeping with results from previous testing, it was shown that dynamic stored energy can be used to predict the fatigue life of MREs having a wide variation in particle content.     

An analysis of impact-induced deformation and fracture modes in amorphous glassy polymers

The finite deformation response of a planar block of polymer material subject to impact loading is analyzed using two constitutive models for glassy polymers, a reference Drucker–Prager type model and a physics-based macromolecular model, supplemented by a phenomenological model for craze initiation and widening. Full transient finite element analyses are carried out using a Lagrangian formulation of the field equations. The analyses allow an assessment of possible failure mechanisms under dynamic loading and the ability of the different models to predict such behavior. The results highlight the effect of the stress–strain behavior of polymers, notably the post-yield softening and large strain hardening, on localization of plastic flow. This behavior is adequately captured only by the macromolecular model.     

Creating novel granular mixtures as proppants: Insights to shape,size, and material considerations

Fragmentation of proppant particles in a pack subjected to compressive loading results in a loss of load bearing capacity. Addition of ductile particles to a brittle particle pack reduces particle fragmentation. Computational models simulating confined compression of a proppant pack with a mixture of brittle and ductile particles are developed. The effect of soft particle material, shape, and size on the fragmentation behavior of the brittle particle in a proppant pack is studied. The results showed that larger, nonuniform particles lead to higher incidence of particle fracture. More efficient pack compositions are proposed for further study and development.     

Systematic investigation and modeling of piezoelectric interaction with loading structures

Piezoelectric actuators are spreading their applications in a variety of engineering systems, e.g., a compression mechanism with large force and high-precision motion systems. A systematic investigation of the dynamics of piezoelectric actuators that interact with loading structures is becoming requisite. This article discusses the dynamic model by merging constitutive equations of the piezoelectric material and the dynamics of loading structures. The dynamic behavior of piezoelectric actuators that interacts with loading structures depends on the stiffnesses of both entities. Two of the most common operating conditions of the actuator, based on its interaction with the loading structures, are classified and considered in this study: fixed–free and free–free conditions. The proposed dynamic models are subsequently validated on real mechanisms with satisfactory results. It is shown that the models are capable of capturing the dynamic interaction between the actuators and loading structures.     

Transient analysis of the DSIFs and dynamic T-stress for particulate composite materials—Numerical vs. experimental results

The fracture behavior of particulate composite materials when subjected to dynamic loading has been a great concern for many industrial applications as these materials are particularly susceptible to impact loading conditions. As a result, many numerical and experimental techniques have been developed to deal with this class of problems. In this work, the fracture behavior of particulate composites under impact loading conditions is numerically studied via the two most important fracture parameters: dynamic stress intensity factors (DSIFs) and dynamic T-stress (DTS), and the results are compared with the experimental data obtained in Refs. [1,2]. Here, micromechanics models (self-consistent, Mori–Tanaka, …) or experimental techniques need to be employed first to determine the effective material properties of particulate composites. Then, the symmetric-Galerkin boundary element method for elastodynamics in the Fourier-space frequency domain is used in conjunction with displacement correlation technique to evaluate the DSIFs and stress correlation technique to determine the DTS. To obtain transient responses of the fracture parameters, fast Fourier transform (FFT) and inverse FFT are subsequently used to convert the DSIFs and DTS from the frequency domain to the time domain. Test examples involving free–free beams made of particulate composites are considered in this study. The numerical results are found to agree very well with the experimental ones.     

磁流变弹性体调频吸振器的研制

传统的动力吸振器吸振频带较窄,限制了稳定性和吸振效果的提高,影响了其应用范围.而磁流变弹性体是一种剪切模量可由外加磁场可控的智能材料.本文利用磁流变弹性体作为动力吸振器的弹性元件和阻尼元件,并采用移频调谐的控制方法,设计了一种可调频的动力吸振器.对其进行的动力特性的研究表明,该吸振器移频范围较宽,在较宽的吸振带宽内具有较好的吸振效果.     

Influence of particle breakage on the dynamic compression responses of brittle granular materials

The dynamic compression responses of dry quartz sand are tested with a modified spilt Hopkinson pressure bar (MSHPB), and the quasi-static compression responses are tested for comparison with a material testing system. In the experiments, the axial stress–strain responses and the confining pressure of the jacket are both measured. Comparison of the dynamic and the quasi-static axial stress–strain curves indicate that dry quartz sand exhibits obvious strain-rate effects. The grain size distributions of the samples after dynamic and quasi-static loading are obtained with the laser diffractometry technique to interpret the rate effects. Quantitative analyses of the grain size distributions show that at the same stress level, the particle breakage extent under quasi-static loading is larger than that under dynamic loading. Moreover, the experimental and the theoretical relationships of the particle breakage extent versus the plastic work show that the energy efficiency in particle breakage is higher under quasi-static loading, which is the intrinsic cause of the strain-rate effects of brittle granular materials. Using the discrete element method (DEM), the energy distributions in the brittle granular material under confined compression are discussed. It is observed that the input work is mainly transformed into the frictional dissipation, and the frictional dissipation under dynamic loading is higher than that under quasi-static loading corresponding to the same breakage extent. The reason is that more fragmentation debris is produced during dynamic breakage of single grains, which promotes particle rearrangement and the corresponding frictional dissipation significantly.     

Computational simulation of thermally sprayed WC–Co powder

WC–Co cemented carbides are a class of hard composite materials of great technological importance. They are widely used as tool materials in a large variety of applications that have high demands on hardness and toughness, including mining, turning, cutting and milling. The HVOF (high velocity oxygen fuel) technology has been very successful in spraying wear resistant WC–Co coatings with higher density, superior bond strengths and less decarburization than many other thermal spray processes, attributed mainly to its high particle impact velocities and relatively low peak particle temperatures. The degree of decomposition and bond strength is directly related to relevant particle parameters such as velocity, temperature and state of melting or solidification. These are consecutively related to process parameters such as powder particle size distribution, carrier gas flow rate, and fuel type employed. To obtain detailed particle data important for thermal spraying, mathematical models are developed in the present paper to predict the particle dynamic behavior in a liquid fuelled HVOF thermal spray gun. The particle transport equations are coupled with the three-dimensional, chemically reacting, turbulent gas flow, and solved in a Lagrangian manner. The melting and solidification within the particles as a result of heat exchange with the surrounding gas flow is solved numerically. The in-flight characteristics of WC–Co particles are studied and the effects of carrier gas parameters on particle behavior are examined. The results demonstrate that WC–Co particles smaller than 5 μm in diameter undergo melting and solidification prior to impact while most particles never reach liquid state during the HVOF thermal spraying. The flow rate of carrier gas has considerable influence on particle dynamics as well as deposition on substrate. At higher flow rate the powder particles are redirected further away from the substrate center, while smaller flow rate results in better heating, higher impact velocity and deposition closer to the substrate center.     

基于颗粒动力学演化的磁弹体力磁耦合数值模型

基于颗粒动力学演化的磁致微观结构建立了横观各向同性磁弹体（MREs）三维几何模型,在考虑了磁场和变形耦合作用的基础上,依据当前MREs研究较热的两种磁颗粒作用模型构建了颗粒的控制方程,从而建立MREs多颗粒的力磁耦合数值模型,从细观角度研究MREs的力磁耦合性能。数值模型和剪切实验对比表明,点偶极子作用力模拟的MREs磁流变效应远低于实验数据,而多极作用力在量级上更接近实验数据。基于构建的数值模型,还详细探究了磁感应强度和颗粒浓度对磁致剪切模量的影响,模拟结果和实验趋势吻合较好,颗粒体积分数在20%附近时,相对磁流变效应达到最大。     

Analytical approach to the strain rate effect on the dynamic tensile strength of brittle materials

An explicit mathematical expression for the dynamic load-carrying capacity of brittle materials under dynamic tensile loads is derived based on a kind of structural-temporal failure criterion [1] and the one-dimensional longitudinal plane wave propagation model. It is shown that the dependence of the dynamic load-carrying capacity on the strain rate can be determined only by the static material parameters such as tensile strength, density, incubation time, critical failure length and constitutive constants, which verifies that the well known strain rate effect on material strength can be considered as an structural rather than material behavior, as pointed out by Cotsovos and Pavlovi? [2] recently. Moreover, it is found that, under constant strain rate, the dynamic load-carrying capacity depends also on the amplitudes of imposed boundary loads, which explains, to a significant extent, the scatter that characterizes the available experimental data. Furthermore, the derived expression can also be used as a foundation of theoretical analyses on other problems involving the strain rate effect such as dynamic size effect, dynamic failure of quasi-brittle materials and composites.     

High strain rate mechanical behavior of seashell-mimetic composites: Analytical model formulation and validation

Seashells (esp. Nacre, or mother-of-pearl) have attracted a substantial attention of the scientific community over the last decade. What makes it as a role model for bio-inspiration is its strength and specifically, superior toughness owing to the synergistic effect of its inorganic and organic components and its unique architecture at its most elementary level, i.e., brick and mortar architecture. Note that preponderance of the studies in literature is limited to quasi-static loading scenario. This is intriguing given the fact that both these materials are fundamentally used as a basic structural material for synthesizing and in orchestrating complex load bearing/shielding structures against predatory impact/crush attack. In our current investigation, an attempt has been made to predict the strength of these types of composites under dynamic regime by development of an analytical model, followed by experimental verification under high-strain rates using Split Hopkinson Pressure Bar (SHPB). Two types of ceramic–polymer composites were tested which were prepared via freeze casting technique. A reasonable agreement has been observed between the analytically predicted and experimentally observed values. Manifestation of dynamic self-stiffening behavior by these types of composites will find particular relevance in application of these types of composites as protective material when subjected to ballistic impact. Understanding the mechanical behavior of these types of composites under dynamic rates of loading will provide insights on the applicability of hybrid bio-inspired composites as protective materials.     

取向磁场对钴颗粒填充硅橡胶磁流变弹性体动态黏弹性的影响

为了研究取向磁场强度对磁流变弹性体（MRE）动态黏弹性的影响规律及影响机制,采用溶剂热法制备球状钴颗粒,SEM和XRD表征结果显示,其粒径为1~2 μm,呈密排六方结构。以硅橡胶为基体,以钴颗粒为填充相,分别在0 mT、480 mT、1 154 mT取向磁场强度下制备MRE,并在不同工况下测试其动态黏弹性。实验结果表明,Co颗粒填充的MRE微观结构的有序性随取向磁场强度增大而增加,其储能模量G'、损耗模量G ″和磁流变效应也随之提高;当取向磁场强度增大到一定程度,由于有序结构趋于稳定,动态黏弹性随取向磁场的变化较小。     

Recent developments in modeling the hot working behavior of metallic materials

Three distinct trends in the modeling of hot working behavior of metallic materials are identified. These are respectively (a) analytical and numerical methods to characterize the material flow behavior and related macroscopic properties, (b) metallurgical models to predict microstructural parameters at each step in a multi-step processing operation, and (c) hot workability models to predict optimum conditions of temperature and strain rate for a single step deformation. Important features of the models are critically examined and some possible directions for future work indicated.     

Comparing the mechanical performance of synthetic and natural cellular materials

This work compares the mechanical performance of agglomerated cork against synthetic materials typically used as impact energy absorbers. Particularly, the study will focus on the expanded polystyrene (EPS) and expanded polypropylene (EPP).Firstly, quasi-static compression tests are performed in order to assess the energy storage capacity and to characterize the stress–strain behavior cellular materials under study. Secondly, guided drop tests are performed to study the response of these materials when subjected to multiple dynamic loading (two impacts). Thirdly, finite element analysis (FEA) is carried out in order to simulate the compressive behavior of the studied materials under dynamic loading.Results show that agglomerated cork is an excellent alternative to the synthetic materials. Not only for being a natural and sustainable material but also for withstanding considerable impact energies. In addition, its capacity to keep some of its initial properties after loading (regarding mechanical properties and dimensions) makes this material highly desirable for multiple-impact applications.