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  • Strain Rate Effects on Tensile and Compression Behavior of Nano-crystalline Nanoporous Gold: A Molecular Dynamic Study
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-23
    Yunus Onur Yildiz; Aylin Ahadi; Mesut Kirca

    In this study, strain rate effects on the tensile and compressive properties of nano-crystalline nanoporous gold (nc-NPAu) are investigated by performing molecular dynamics simulations. For this purpose, atomistic models of nc-NPAu structures with three different grain sizes are generated through a novel modeling technique based on the Voronoi tessellation method. Additionally, an adaptive common neighbor analysis (aCNA) is carried out to examine the evolution of the crystal structure. In this way, the deformation mechanisms of nc-NPAu atomistic models are thoroughly investigated. The findings point out that mechanical properties of nc-NPAu specimens such as toughness, ultimate and yield strengths grow at increasing strain rates for both tensile and compressive loadings while their elastic moduli exhibit less significant variations at different strain rates. Furthermore, the study also shows that in addition to dislocation motion, several other deformation mechanisms including grain rotation, grain boundary sliding and grain travelling are observed to be effective for nc-NPAu.

  • A thermodynamically consistent formulation of the Johnson-Cook model
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-22
    Charles Mareau

    The model of Johnson and Cook, which includes a viscoplastic flow rule and a damage criterion, is widely used to describe the mechanical behaviour of metallic materials subjected to severe loading conditions, such as those encountered during fabrication operations or impact. This model has been built on empirical, rather than physical, grounds. The present paper therefore aims at revisiting the model of Johnson and Cook from the view point of thermodynamics with internal variables. The interest of this approach is twofold. First, it provides a guide for the construction of a complete thermomechanical constitutive model, with some constitutive relations not only for the stress tensor but also specific internal energy, specific entropy and heat flux vector. Second, it allows highlighting some possible limitations of the original model of Johnson and Cook. Such limitations can be circumvented with an alternative model, which is described in the present work. For illustration purpose, some applications of both the original and alternative models are presented in the final section.

  • Modeling the effect of physical aging on the stress response of amorphous polymers based on a two-temperature continuum theory
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-21
    Jun Guo; Zhiyun Li; Jianmin Long; Rui Xiao

    Physical aging widely exists in amorphous polymers, which refers to the nonequilibrium structure of amorphous polymers evolving towards the equilibrium state. Aging can significantly influence the thermomechanical properties and subsequently the macroscopic response of polymers. In our recent work, we performed a series of uniaxial compression tests on amorphous thermoplastic poly(ethyleneterephthalate)-glycol (PETG) subject to different annealing temperature and time. The results showed that both annealing time and temperature have a significant influence on the yield strength. Here we apply a two-temperature continuum model to simulate the stress response of PETG with different thermal treatments. The model employs the effective temperature as a variable to characterize the nonequilibrium state. The physical aging is characterized by the evolution of the effective temperature. The effective temperature is also coupled with the viscoplastic deformation to describe strain softening induced by mechanical rejuvenation. The simulation results can reasonably reproduce the main experimental observations with some discrepancies. The possible reasons for the discrepancies between experiments and simulations are also critically analyzed. The theory is also used to simulate the response of amorphous glassy polymers in the creep and strain-rate-switching tests. The model successfully captures the important features of experimental observations in the literature.

  • A comparative study on mechanical behavior and damage scenario of DP600 and DP980 steels
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-21
    Ali Cheloee Darabi; Vinzenz Guski; Alexander Butz; Javad. Kadkhodapour; Siegfried Schmauder

    In this study, the mechanical behavior of two cold rolled commercial dual phase steels (DP) were analyzed on micro and macro scales. First, the anisotropic behavior of these steels were investigated by standard tensile tests in three directions. The results showed that the anisotropy behavior of the fracture strain of DP980 is more pronounced than of DP600. Then, in order to assess the influence on the stress state on the mechanical behavior, four specimens with different stress states were analyzed by ARAMIS approach which is based on digital image correlation (DIC). The ARAMIS results were compared with 3D numerical simulations using the Abaqus/Explicit solver. In this part, the effect of stress state on flow curve and strain distribution in the specimens (i.e. tensile, notched-tensile, shear and bulge specimens) were investigated. To predict the fracture behavior of DP600 and DP980 steels under various loading conditions, the Modified Mohr-Coulomb (MMC) damage model was utilized. A VUMAT subroutine was developed to include a MMC damage model in the 3D models, and good agreement between the numerical and experimental results was observed. Finally, the microstructure failure mechanisms at three different stages (i.e. strain localization, micro-crack initiation and micro-crack coalescence) during uniaxial tensile loading were investigated inside the microstructure of DP600 and DP980 steels using an interrupted in-situ setup. In the results three different damage localizations were observed in both materials, and strain localization in the center of large ferrite phases and at the boundary of ferrite and martensite phases were dominant in DP600 and DP980, respectively. Two different micro-crack initiation mechanisms were observed that were similar in both materials.

  • Dynamic Stress-Strain Response of High-Energy Ball Milled Aluminum Powder Compacts
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-20
    A.W. Justice; M.T. Beason; I.E. Gunduz; W. Chen; S.F. Son

    Ball milling is a bulk powder manufacturing process used in the creation of dispersion strengthened and nanostructured materials. Fundamentally, these powders have not been dynamically characterized in a green state prior to hot consolidation. The understanding of high strain-rate compaction on void collapse and particle interaction for such systems can help the development of predictive models for impact events of porous metallic structures that may be employed as energy absorbers, reactive structures, and intermetallic materials. This study investigates high strain-rate impact of porous green compacts of as-received and high-energy ball milled (HEBM) aluminum powders characterized under dynamic compression using a split-Hopkinson pressure bar (SHPB) in a passive confinement configuration. The plastic deformation of the powder compacts and crush up were shown to be strain-rate insensitive within the strain rate range of 1000-2100 s−1 and as a result, were modeled adequately with a second order P-α model. The as-received aluminum and HEBM aluminum powders appear to have the same strain-hardening coefficient and strength index as solid aluminum after yielding. The respective stress-strain responses of green compacts follow the same trend but differ only in strength as result of porosity and pre-strain experienced prior to dynamic compression. The HEBM powder was found to be twice as strong as the untreated as-received aluminum powder.

  • Poroelastic Effects on Steady State Crack Growth in Polymer Gels under Plane Stress
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-17
    Yalin Yu; Chad M. Landis; Rui Huang

    Fracture experiments on polymer gels are often conducted with thin specimens, which are close to plane stress in two-dimensional models. However, many of the previous theoretical and numerical studies on fracture of polymer gels have assumed plane strain conditions. The subtle differences between the plane stress and plane strain conditions are elucidated in this paper based on a linear poroelastic formulation for polymer gels, including the asymptotic crack-tip fields and finite element simulations of steady-state crack growth in long strip specimens. Moreover, a poroelastic cohesive zone model is adopted to study the rate-dependent fracture process of polymer gels. It is found that, without the cohesive zone model, the normalized crack-tip energy release rate at the fast crack limit is greater than the slow crack limit, suggesting reduced poroelastic toughening for fast crack growth under plane stress conditions, while the two limits are identical under plane strain conditions. With a solvent-permeable cohesive zone for the case of immersed specimens, solvent diffusion within the cohesive zone enhances the poroelastic toughening significantly as the crack speed increases, leading to a rate-dependent traction-separation relation. On the other hand, with no solvent diffusion in the cohesive zone for the not-immersed case, the poroelastic toughening effect diminishes as the crack speed increases. Based on the present study, the intrinsic steady-state fracture toughness of a poroelastic gel can be determined using long-strip pure-shear specimens, which in general is smaller than the applied energy release rate.

  • Nonlinear homogenization for topology optimization
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-16
    Mathias Wallin; Daniel A. Tortorelli

    Non-linear homogenization of hyperelastic materials is reviewed and adapted to topology optimization. The homogenization is based on the method of multiscale virtual power in which the unit cell is subjected to either macroscopic deformation gradients or equivalently to Bloch type displacement boundary conditions. A detailed discussion regarding domain symmetry of the unit cell and its effect on uniaxial loading conditions is provided. The density approach is used to formulate the topology optimization problem which is solved via the method of moving asymptotes. The adjoint sensitivity analysis considers response functions that quantify both the displacement and incremental displacement. Notably, the transfer of the sensitivities from the microscale to the macroscale is presented in detail. A periodic filter and thresholding are used to regularize the topology optimization problem and to generate crisp boundaries. The proposed methodology is used to design hyperelastic microstructures comprised of Neo-Hookean constituents for maximum load carrying capacity subject to negative Poisson’s ratio constraints.

  • Radial instabilities of viscoelastic thin film-elastic substrate system triggered by local pre-stretch: a theoretical solution
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-15
    Xiao Liu; Ying Liu

    Different from an elastic film-substrate system, the buckling mode of a viscoelastic film-substrate system triggered by a local load may change over time. In this paper, the local wrinkling and the stress relaxation process of a Maxwell viscoelastic thin film-elastic substrate system subjected to a sudden homogeneous pre-stretch in a circular area is investigated. The buckling equation for this viscoelastic film-elastic substrate system is derived, and the critical wrinkling condition is given. The wrinkling mode mutation with the time is observed, and the evolution of the wrinkling pattern is presented. The influence of the film modulus and the pre-stretch radius on the mutation time is discussed in detail. The time evolution of the circumferential wrinkling wave number in viscous film-substrate system provides guidance in safety design in soft matter and viscoelastic membrane structures.

  • Identification of the dynamic behavior of epoxy material at large strain over a wide range of temperatures
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-15
    C.A. Bernard; N. Bahlouli; D. George; Y. Rémond; S. Ahzi

    Adhesively bonded joints using epoxy resin are nowadays often replacing welding in offshore applications for safety reasons. During its lifetime, the bonded joint epoxy is submitted to severe environmental and loading conditions, such as humidity, water uptake, thermal aging, and complex loading conditions affecting its mechanical performance. We investigate here the dynamic mechanical behavior of an epoxy resin at large strain over a wide range of temperatures. Analysis of the elastic modulus, yield strain, yield stress and plastic flow as a function of the temperature and strain rate is carried out. Unlike the elastic modulus and the yield stress showing strong sensitivities to the temperature and the strain rate, the plastic flow appears to have a limited sensitivity to the temperature and the strain rate. Numerical modeling is used to determine the yield stress and elastic modulus variations over the glass transition temperature and good agreement is observed between numerical predictions and experimental results.

  • Strain-gradient homogenization: a bridge between the asymptotic expansion and quadratic boundary condition methods
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-15
    Vincent Monchiet; Nicolas Auffray; Julien Yvonnet

    In this paper we deal with the determination of the strain gradient elasticity coefficients of composite material in the framework of the homogenization methods. Particularly we aim to eliminate the persistence of the strain gradient effects when the method based on quadratic boundary conditions is considered. Such type of boundary conditions is often used to determine the macroscopic strain gradient elastic coefficients but leads to contradictory results, particularly when a RVE is made up of a homogeneous material. The resulting macroscopic equivalent material exhibits strain gradient effects while it should be expected of Cauchy type. The present contribution is to provides new relationship to correct the approach based on the quadratic boundary condition. To this purpose, we start from the asymptotic homogenization approach, we establish a connection with the method based on quadratic boundary conditions and we highlight the correction required to eliminate the persistence of the strain gradient effects. An application to a composite with fibers is provided to illustrate the method.

  • Correlation between grain boundary evolution and mechanical properties of ultrafine-grained metals
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-15
    Mohamed Shaat; Adel Fathy; Ahmed Wagih

    We put stiffness and strength of ultrafine-grained metals under scrutiny by means of experimental and analytical mechanics. Samples of Al 1050 were processed by Accumulative Roll Bonding (ARB) at different cycles to give ultrafine-grained microstructures. The microstructure, strength, and stiffness of the processed Al were analyzed by XRD and SEM. An analytical micromechanical model was developed to explain the predicted changes in the mechanical properties of the ultrafine-grained Al. It was revealed that an ultrafine-grained pure metal (e.g., Al) possesses a heterogeneous microstructure with the grain boundary stiffness lower than the grain/subgrain stiffness. This heterogeneous microstructure gave a decrease-after-increase behavior of the overall stiffness of Al during ARB. The grain boundary fraction of the microstructure increased from 1.9% after 2 cycles to 13.4% after 9 cycles of ARB. The stiffness decreased to 0.73 of its maximum value obtained after 2 cycles. With the aim of changing the stiffness and strength of the grain boundary, other samples were processed such that 4%SiC particles were added to Al sheets during the 1st-ARB cycle. The addition of SiC particles gave stiffer and stronger grain boundaries, which enhanced the overall stiffness and strength during ARB. Both the stiffness and strength were observed continuously increasing during ARB up to 484% and 793% after 9 cycles compared to the unprocessed Al, respectively.

  • Progressive damage modelling of composite materials subjected to mixed mode cyclic loading using cohesive zone model
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-15
    J. Ebadi-Rajoli; A. Akhavan-Safar; H. Hosseini-Toudeshky; LFM. da Silva

    Cohesive zone modeling (CZM) has been extensively considered as a powerful method for analysis of initiation and propagation of delamination in composite laminates subjected to cyclic loading. By making a relation between the damage parameter and the loading cycles, damage accumulation can be calculated, and the fatigue life of a component can be estimated. The aim of the current research is to present a new fatigue damage accumulation (FDA) model. The proposed model is developed based on the concepts of CZM and by linking the fracture mechanics to damage mechanics approaches. For this purpose, a developed user material subroutine (USDFLD) is implemented in a finite element software to simulate the initiation and propagation of damage in an interface layer of a composite material subjected to cyclic loading. The method is validated for different loading conditions using Paris law results. Good agreement between the proposed method and the Paris law results is observed.

  • Thermo-mechanical modeling of a filled elastomer based on the physics of mobility reduction
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-12
    Davide Colombo; Hélène Montes; François Lequeux; Sabine Cantournet

    The addition of rigid fillers to an elastomeric matrix enhances its mechanical properties. This reinforcement effect is primarily due to a filler network structure in which polymer regions between aggregates play the principal role. In this study, a continuum constitutive equation is formulated for polymer behavior under strong confinement conditions. This behavior can be accounted for by a local glass transition temperature that combines the effects of physical interaction and stress softening in a unique viscoelastic formulation. The model reproduces, at a microscopic scale, the processes governing the Payne effect, including the temperature dependence of the viscoelastic behavior of the filled elastomer reinforcement.

  • 更新日期:2020-01-13
  • Topologically Reconfigurable Mechanical Metamaterials with Motion Structures
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-10
    Zhiming Cui; Jaehyung Ju

    Motion structures whose macroscopic topology can be controlled by an internal kinematic mechanism play a new role in the design of mechanical metamaterials. Motion structures with N-fold symmetry show a reconfigurable pattern transformation, providing tunable mechanical properties by topological reconfiguration, not by geometric reconfiguration. The objective of this work is i) to synthesize motion structures from a bar-and-joint framework and ii) to investigate their mobility and symmetry breaking during transformation together with nonlinear structural properties - modulus and Poisson's ratio, switchable stiffness, and bi-stiffness. Two-dimensional (2D) motion structures with N-fold symmetry (MS-N) are synthesized by central scissor links with revolute joints, connected with binary links in the radial direction. Five 2D motion structures - MS-4, MS-6, MS-8, MS-10, and MS-12, are constructed for investigating their mechanical properties together with their transformability. We build analytical models of motion structures on relative density, modulus, Poisson's ratio, and switchable stiffness as a function of transformation, verified with experiments and numerical simulations. By combining the kinematic mechanisms with structural mechanics, this study contributes to expanding the design space of reconfigurable metamaterials.

  • Universal Relations in Coupled Electro-magneto-elasticity
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-09
    Deepak Kumar; Somnath Sarangi; Prashant Saxena

    In the present work, we develop a class of the coupled universal relations with the possible forms of electro-magneto-elastic (EME) deformation families in smart materials. In line with that, we adopt a classical continuum mechanics-based approach following the second law of thermodynamics. More precisely, we first formulate the deformation of an EME continua through the fundamental laws of physics with an amended form of energy function. This amended energy function successfully resolves the physical interpretation of the Maxwell stress tensor under large deformations.Next, we develop the EME coupling type of universal relations through a new inequality Tb−bT≠0 for a class of an EME material parallel to an equation Tb−bT=0 for an isotropic elastic material existing in the literature. Wherein, T and b denote the total Cauchy stress tensor and left Cauchy-Green deformation tensor, respectively. Further, we propose the possible forms of EME deformation families in smart materials for some standard experimental arrangements. At last, we also apply the above findings to a magnetostriction phenomenon in order to check the practical feasibility of the same and a good agreement is achieved successfully.

  • Computational design of shape-programmable gel plates
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-09
    Alessandro Lucantonio; Antonio DeSimone

    Polymer gel plates may be programmed to morph into three-dimensional configurations upon swelling. An effective strategy to control such shape transformations consists in patterning the in-plane cross-linking density of the polymer network to realize non-homogeneous swelling. In general, one needs to solve an inverse problem to determine the shear modulus field that produces a given target shape. Here, we propose a computational framework for the solution of such an inverse problem, which we validate against two benchmark problems, i.e. making cones and saddles from gel disks.

  • Structure and Kinetics of Three-Dimensional Defects on the {101¯2} Twin Boundary in Magnesium: Atomistic and Phase-field Simulations
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-07
    Douglas E. Spearot; Vincent Taupin; Khanh Dang; Laurent Capolungo
  • Variational analysis of the influence of grain shape anisotropy on shear viscosity in Nabarro-Herring-Coble creep
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-07
    Francis Delannay; Laurence Brassart

    The effect of strain-induced grain shape anisotropy on diffusional creep viscosity is analysed in two dimensions via a model representing grains by cylinders with elliptical cross section. Both cases of dominance of grain boundary diffusion and lattice diffusion are considered. Anisotropic creep viscosity is described by two coefficients calculated by considering different loading configurations with respect to the ellipse axes. Upper and lower bounds on these coefficients are obtained using kinematic and statical variational principles and assuming affine velocity, or uniform stress trial boundary fields, respectively. The analysis emphasises the dependence of the viscosity coefficients on aspect ratio and grain boundary viscosity. The difference between the bounds increases with grain elongation. A method is proposed for deriving estimates for the effective viscosity coefficients by coupling the two bounds. The strain hardening effect is analysed. Lattice diffusion contributes less to viscosity anisotropy than diffusion and sliding at grain boundaries.

    Mech. Mater. (IF 2.958) Pub Date : 2020-01-07
    Fatih Uzun; Chrysanthi Papadaki; Zifan Wang; Alexander M. Korsunsky

    The demand on energy generation with low carbon emissions evoked the development of ultra-super critical technology that allows operating steam turbines at high temperature and pressure conditions. However, operating at extreme conditions necessitates careful considerations on structural integrity which is affected by residual stresses. Welding is used for joining of components of steam turbines, but this process causes formation of residual stresses at very complex forms as a result of this process. Careful investigations are necessary to understand distribution of harmful residual stress fields. The eigenstrain theory was previously used for the development of the artificial intelligence based eigenstrain (AI-eig) contour method that allowed advanced modelling of the behaviour of Inconel alloy 740H under thermo-mechanical loading conditions. The models created based on this method are capable of determining the residual stress fields in the whole specimen or in the parts and slices created using electric discharge machining (EDM). In the previous applications of the AI-eig contour method, the determination of the distribution of eigenstrain fields in as-welded and heat-treated specimens was followed by the calculation of volumetric residual stresses. In this study, long- and short-transverse components of the residual strains determined by the AI-eig contour method applied to EDM-cut surfaces of the parts of as-welded and heat-treated specimens were validated using the neutron strain scanning method. The results demonstrate the effectiveness of the integrative modelling approach that enables the determination of eigenstrains in the whole specimen and the calculation of residual strains before and after the machining process.

  • The Effect Of Crystal Anisotropy And Pre-Existing Defects On The Incipient Plasticity Of Fcc Single Crystals During Nanoindentation
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-07
    Mahdi Bagheripoor; Robert Klassen

    Molecular dynamics simulation is used to identify and quantify the initial plastic deformation mechanisms of gold as a model face-centered cubic (fcc) metal in the nanoindentation process. The coupling effects of crystallographic orientation and internal structural defects on the resulting load distribution at the onset of plasticity are investigated to clarify the anisotropic characteristics of material responses to crystallographic orientation. Homogenous defect nucleation is studied by correlating the indentation force-displacement curve with the instantaneous defect structure. In the absence of pre-existing defects, nanoindentation deformation is dominated by nucleation of Shockley partial dislocations regardless of crystal orientation. Various forms of dislocation propagation are observed in different crystal orientations. The elastic-plastic transition point appears later for the [111]-oriented surface than the ones for [001]-, and [011]-oriented surfaces. The relation of hardening and dislocation density shows that conventional Taylor hardening captures the plasticity after a certain amount of indentation depth in the presence of enough dislocation density. The crystal's sensitivity to the presence of internal structural faults is strongly dependent on the crystallographic orientation. Nanoindentation simulations in the presence of sessile dislocation loops in the structure show that the most significant reduction in the pop-in load happens for the [111] oriented sample. Our simulations suggest that the indentation near a defect can lead to small, subcritical events that lead to a smoother "pop- in" at the onset of plasticity. Since internal defects in materials are nearly inevitable, a defect-based model can be useful to understand the stochastic pop- in loads in nanoindentation tests.

  • The distribution regularity of residual stress on a metal surface after laser shock marking
    Mech. Mater. (IF 2.958) Pub Date : 2020-01-03
    Guoxin Lu; Uroš Trdan; Yongkang Zhang; Jeff L. Dulaney
  • Dynamic analysis of carbon nanotube reinforced composite plates by using Bézier extraction based isogeometric finite element combined with higher-order shear deformation theory
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-30
    Vuong Nguyen Van Do; Jun-Tai Jeon; Chin-Hyung Lee

    This study intends to explore the free vibration and dynamic transient response of carbon nanotube reinforced composite (CNTRC) plates by using the Bézier extraction based isogeometric analysis (IGA). Bézier extraction decomposes non-uniform rational B-spline (NURBS) basis functions into the linear combinations of Bernstein polynomial basis through the Bézier extraction operator, hence providing piecewise C0-continuous Bézier elements. The IGA can thus be implemented in the familiar finite element method (FEM) framework. Higher-order shear deformation theory (HSDT) able to remedy the shear locking problem with no use of the shear correction factor is coupled with the variant IGA. The HSDT adopts a hybrid type shape function, and the governing equilibrium equations for the dynamic behavior are formulated by virtue of the Hamilton's principle. The validity of the Bézier extraction based isogeometric approach coupled with the HSDT in obtaining the solutions for the free vibration and dynamic transient response is first evidenced. Compared with the classical FEM based on the first-order shear deformation theory, the proposed IGA method combined with the HSDT is proved to be more accurate, effective and efficient. Illustrative parametric studies are also carried out to further scrutinize the dynamic behavior of CNTRC plates with various reinforcement schemes. The sinusoidally distributed transverse loads with different time-history patterns and harmonic excitation loading are applied for the dynamic transient analysis, and the damping effect on the temporal response is also examined through the Rayleigh damping model.

  • Two-dimensional finite element analysis of frictional sliding between a rigid cylinder and a shape memory alloy half-space
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-30
    Ralston Fernandes; James G. Boyd; Dimitris C. Lagoudas; Sami El-Borgi

    The finite element method is used to simulate frictional sliding contact between a rigid cylinder and a shape memory alloy (SMA) half-space. A set of parametric studies are conducted to investigate the effects of typical SMA phenomenological behaviors such as the influence of temperature on the constitutive behavior, large recoverable strains and the difference between the elastic moduli of the SMA austenite and martensite phases on the sliding response. The simulations show that significant residual stresses are present far away from the contact region for sliding at temperatures below the austenitic finish temperature. The simulations also reveal that the maximum von Mises stress in the half-space during sliding contact decreases for higher values of recoverable transformation strains and for an SMA more compliant stress-induced martensite phase compared to the austenite phase. In contrasting the effects of the energy dissipation mechanisms of a pseudoelastic and a conventional elastic-plastic half-spaces on the sliding response, it is shown that the former continues to dissipate energy after repeated sliding cycles while the latter ceases to dissipate energy upon reaching an elastic shakedown state.

  • Friction between a plane strain circular indenter and a thick poroelastic substrate
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-24
    Yuan Qi; Kristin N. Calahan; Mark E. Rentschler; Rong Long

    This paper presents a computational study on the role of poroelasticity in gel friction. Motivated by recent experimental studies in the literature, we develop a plane strain finite element model to elucidate the contact mechanics between a circular indenter and a thick poroelastic substrate under both normal and shear loadings. Two cases are considered: i) steady state sliding under fixed normal displacements, and ii) relaxation under fixed normal and shear displacements. In steady state sliding, we find that a net friction force can arise even if no intrinsic adhesive or frictional interaction is implemented at the indenter/substrate interface. Such friction force exhibits a non-monotonic dependence on the sliding velocity and peaks at an intermediate velocity. Our model reveals that this friction force is induced by poroelastic diffusion in the gel substrate which can lead to considerable asymmetry in both the contact profile and contact pressure. In terms of relaxation, if the indenter/substrate interface is set to be frictionless, we find that the friction force induced by poroelasticity relaxes to zero with a characteristic time much faster than that of the normal force. When a finite friction coefficient is introduced at the interface, the normalized relaxation curve for the friction force approaches that for the normal force as the friction coefficient increases. These modeling results suggest that poroelasticity can be an important contributing mechanism for gel friction.

  • 更新日期:2019-12-25
  • Multi-scale sensitivity analysis of structural vibration behaviors of three-dimensional braided composites with respect to material properties
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-23
    Xing-Rong Huang; Hao Zhu; Dian-sen Li; Lei Jiang

    Three-dimensional braided composites have a wide range of application in various industries. This article presents a three-step scheme enabling the multi-scale analysis of the vibration properties for three-dimensional braided composites. Firstly, a micro-scale model and meso-scale mechanical model were constructed to predict the stiffness properties of the braided composites using both analytical and numerical methods. Secondly, the macro-scale vibration responses of the braided composites were carefully investigated both from analytical and numerical perspectives, followed by validation with experimental data. Thirdly, sensitivity analysis of the vibration properties, such as the modal frequencies, with respect to the braiding angle, fiber volume fraction, and braided structure was conducted. The results show that the macroscopic vibration properties strongly depend on the braiding angle, fiber volume fraction, and braided structure. Three-dimensional braided composites are quite promising for the integrated design of materials and structures, as their macroscopic structural vibration behaviors can be optimized by properly regulating their microscopic material characteristics.

  • Micromechanical modelling of short- and long-term behavior of saturated quasi-brittle rocks
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-20
    Shuangshuang Yuan; Qizhi Zhu; Lunyang Zhao; Liang Chen; Jianfu Shao; Jin Zhang

    This paper presents a physically-motivated micromechanical damage model for describing short- and long-term behaviors of saturated quasi-brittle rocks under drained condition. Rocks are considered composed of a pores-weakened elastic solid matrix and distributed microcracks. The system free energy is determined with the Mori-Tanaka homogenization method. Two main dissipative mechanisms, inelastic deformation due to frictional sliding and damage by crack propagation, are involved and strongly coupled in both stress-loading phases and creep phases under constant stresses. The effect of pore pressure is taken into account in the damage-friction coupling context. The microcracks induced rock damage is divided into two parts: instantaneous damage related directly to stress-induced microcrack growth, and time-dependent damage caused by subcritical cracking. The short- and long-term strengths of saturated brittle rocks are analytically deduced by damage-friction coupling analysis. By using the long-term strength, the creep parameter is calibrated in terms of a critical stress state identified in experiment. Comparisons between model’s predictions and laboratory tests are performed for two typical quasi-brittle rocks under triaxial compression tests and triaxial compression creep tests. It is shown that the proposed model can capture the main features of the mechanical behavior of quasi-brittle materials.

  • Computational micromechanics model for the analysis of fiber kinking in unidirectional fiber-reinforced polymers
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-20
    M. Herráez; A.C. Bergan; C.S. Lopes; C. González

    A computational micromechanics (CMM) model is developed to analyze fiber kinking, which is a failure mechanism that takes place in fiber-reinforced composites when they are loaded under longitudinal compression. The CMM model consists of a single AS4 carbon fiber with an initial misalignment embedded in an 8552 polymer matrix. The deformation of the model is governed by periodic boundary conditions (PBC). The relatively simple CMM model enables the evaluation of the role played by initial misalignment of the fiber, shear yielding of the matrix and fiber-matrix debonding. A novel microscale experimental technique devoted to the characterization of the longitudinal compressive strength of the fibers, Xcf, is developed. By exercising the model and comparing it with several models in the literature, the nonlinear shear response of the composite lamina is shown to play a fundamental role not only in the prediction of the compressive strength Xc, but also during the post-peak regime in terms of residual stress σr and fiber rotation φ. Finally, the influence of the fiber-matrix interface damage (not considered in most other fiber kinking models) on the fiber kinking phenomenon is assessed through a parametric study.

  • Optimisation based material parameter identification using full field displacement and temperature measurements
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-19
    Lars Rose; Andreas Menzel

    A material parameter identification is presented for a fully thermo-mechanically coupled material model based on full field displacement and temperature measurements. The basic theory of the inverse problem is recapitulated, focusing on the choice of the objective function, proposing a new formulation, and explaining in detail the necessary numerical treatment of experimental data during the pre-processing of an identification. This includes the handling of the intrinsically different data sets of displacement (Lagrangian type) and temperature (Eulerian type). Experimental data is obtained by means of a Digital-Image-Correlation (DIC) as well as by a thermography system and three algorithmic boxes are provided for the necessary pre-processing. The experimental setup is discussed, measured data presented and analysed. From this setup, a successive approach to the identification process is motivated. Based on the experimental observations, a thermo-mechanically coupled material model is chosen, the required constitutive relations summarised and the material parameters interpreted. For the fixed choice of model and experiments, the inverse problem is solved. A very good fit was obtained for both the displacement and the temperature field. Results are interpreted and remaining errors discussed.

  • Multiscale microsphere modelling of open-cell metal foams enriched by statistical analysis of geometric parameters
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-19
    T. Bleistein; M. Reis; X. Cheng; C. Redenbach; S. Diebels; A. Jung
  • Numerical derivation of a normal contact law for compressible plastic particles
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-19
    B.D. Edmans; I.C. Sinka

    A new contact law is proposed to describe the behaviour of plastically compressible particles. The law was derived from contact simulations in which a general continuum constitutive model, the von Mises Double Cap (VMDC) model, was introduced to represent the particle material behaviour, allowing distinct dilatory, shearing and densification plastic flow regimes. Elastic and plastic properties were prescribed as functions of density. Parametric studies were conducted covering the parameter space of published experimental data for a range of pharmaceutical powders and granules. The analysis showed plastic zones corresponding to the three flow regimes developing within the particle, with size, shape, location and onset conditions being dependent on the strength ratios of the constitutive model. The contact law established combines an initial quasi-linear region followed by an exponential hardening region, arising from the initiation, growth and hardening of plastic zones, and the development of dense and stable load-bearing structures. The outcome of these studies is a new contact law, relationships for predicting contact law parameters from material parameters for both loading and unloading, and guidelines for the analytical treatment of plastic compressibility in particle contact. The contact law can be employed in discrete element and homogenisation models to predict macroscopic properties of porous granular materials, while the analytical framework and qualitative findings can be used in the design of granules.

  • X-ray computed tomography investigation of dilation of mineral-filled PVC under monotonic loading
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-19
    Sindre Nordmark Olufsen; Arild Holm Clausen; Dag Werner Breiby; Odd Sture Hopperstad

    The deformation-induced dilation of mineral-filled polyvinyl chloride (PVC) is investigated by means of polychromatic X-ray absorption tomography (XCT). Axisymmetric notched tensile specimens are strained to prescribed elongations using a tensile test apparatus and subsequently scanned by XCT after relaxation. During straining, surface deformations are quantified by digital image correlation (DIC) and contour tracking. The influence of stress triaxiality and strain rate on dilation is investigated by using tensile specimens with three different notch radii, all strained to three deformation levels at two different nominal strain rates. Pronounced reduction of density, corresponding to an increase of volume, is observed for all specimens in the XCT scans, but markedly different relative density distributions are measured for the three geometries. The accuracy of the polychromatic XCT density estimates is evaluated against data obtained with monochromatic synchrotron radiation computed tomography (sr-XCT), and a comparison to surface deformation-based dilation estimates is presented.

  • 更新日期:2019-12-18
  • Simulations of deformation and fracture of graphene reinforced aluminium matrix nanolaminated composites
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-12
    Youshuo Song; Yue Ma; Ke Zhan

    The mechanical properties of rGO/Al composites, considering the damage and failure behaviours, are studied by finite element analysis in this paper. A numerical 3D structural model of the bulk reduced graphene oxide (rGO) nanosheets with two different lateral sizes (0.23 μm and 1.1 μm) is created based on AFM images. A representative volume element with 3D realistic microstructure was used to represent the selected composite microstructure. The 3D program developed can predict the elasto-plastic response and fracture behavior of the actual rGO/Al composites. Based on the 3D structural models, the simulated stress-strain curve and the fracture morphology were verified by the experimental results. The numerical results can be used to provide insight into the deformation and fracture mechanisms of rGO/Al composites.

  • Study on Size-Dependent Vibration and stability of DWCNTs subjected to moving nanoparticles and embedded on two-parameter foundations
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-11
    Mostafa Pirmoradian, Ehsan Torkan, Davood Toghraie

    Parametric resonance is an important phenomenon that may be evinced in applying carbon nanotubes for the delivery of nanoparticles. This paper aims to investigate dynamics instability of double-walled carbon nanotubes (DWCNTs) surrounded by elastic medium and excited by a sequence of moving nanoparticles. The DWCNT is modeled as two Euler-Bernoulli beams interacting between them through van der Waals (vdW) forces. Based on Eringen's nonlocal elastic theory to consider the small-scale effects, the governing equations are derived by using Hamilton's principle. All inertial terms of the moving nanoparticles are taken into account. In addition, the van der Waals force between the constitutive atoms of the moving nanoparticle and those of the nanotube is considered. By utilization of the Galerkin method, the partial differential equations (PDEs) of motion are reduced to couple ordinary differential equations with time-varying coefficients describing a parametrically excited nanosystem. Then, an incremental harmonic balance (IHB) method is implemented to calculate the instability regions of the DWCNT. The results show that considering the vdW effects, increasing the amplitude of the static axial tensile force, reducing the amplitude of axial oscillating force, and increasing the stiffness of the elastic medium improve stability of the system. A comparison between the results with those reported in the literature is performed to verify the precision of the presented analyses.

  • High cycle fatigue behavior of magnesium matrix nanocomposite at elevated temperatures
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-10
    A.H. Jabbari, H. Delavar, M. Sedighi

    In this article, the high cycle fatigue (HCF) behavior of AZ31B/1.5 vol.% Al2O3 nanocomposite was investigated at elevated temperatures. To understand the influence of nano-sized reinforcing particles, the results were compared to those of monolithic AZ31B samples. The effect of reinforcing particles and temperature was studied on tensile, compressive, and HCF behaviors of the specimens. According to the results, the nanocomposite showed improved mechanical properties in comparison to the monolithic alloy at all temperatures. The results of HCF tests at 100°C and 200°C revealed that the enhanced ultimate tensile strength in the composite is the most important factor in modifying the HCF behavior, especially at the higher temperature. Unlike the tensile and the compressive tests, increasing the temperature from 100°C to 200°C reduces the impact of the reinforcing particles on the fatigue behavior of the composite, which could be attributed to the formation of magnesium oxide on the samples surfaces.

  • Evaluation of the effect of adding carbon nanotubes on the effective mechanical properties of ceramic particulate aluminum matrix composites
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-09
    Changjiang Nie, Hengxue Wang, Jing He

    In this study, a high-performance hybrid aluminum matrix composite (HAMC) reinforced with ceramic particles and carbon nanotubes (CNTs) is analyzed. A novel multi-step micromechanical approach, based on the Mori-Tanaka model and the generalized method of cell, is proposed in order to predict the effective elastic modulus and Poisson's ratio of the HAMCs. The influences of volume fraction, aspect ratio, waviness shape, alignment and agglomeration of CNTs, and ceramic particle volume fraction on the mechanical properties of CNT/ceramic particle-reinforced HAMCs are explored. Moreover, the role of aluminum carbide (Al4C3) which may be formed at the CNT/matrix interface in the mechanical behavior is micromechanically investigated. It is found that adding a small amount of CNTs into the microscale ceramic particle-reinforced aluminum matrix composites (AMCs) can significantly improve the effective mechanical properties of the resultant HAMCs. As compared to HAMCs, which contain the randomly dispersed CNTs, the alignment of CNTs into the HAMCs leads to a higher level of mechanical properties. However, the waviness and agglomeration of CNTs can decrease the HAMC elastic modulus. According to the obtained results, the CNT/matrix interfacial interaction may have significant effects on the effective properties of the particulate hybrid metal-based composites. The elastic properties estimated by the micromechanical model are compared to those measured by the experimental method. The outcomes of this research suggest that these hybrid metal-based composites, containing CNTs, have significant potential in diverse engineering applications as compared to the conventional metal-based composites.

  • Mechanical Modification of Bacterial Cellulose Hydrogel under Biaxial Cyclic Tension
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-07
    Xing Gao, Emrah Sözümert, Zhijun Shi, Guang Yang, Vadim V. Silberschmidt
  • Spall Strength of Steel-Fiber-Reinforced Concrete Under One-Dimensional Stress State
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-05
    Ji-ye ZHANG, Hui-qi REN, Feng HAN, Gui-juan SUN, Xing WANG, Qiang ZHAO, Lei ZHANG

    A specially designed large-scale Hopkinson bar (φ100 mm) with hollow aluminum alloy bar as transmitted bar was used to measure the dynamic spall strength of steel-fiber-reinforced concrete (SFRC). The experimental results indicate that the steel fibers significantly enhance the spall strength of the SFRC. The enhancement of the spall strength from the steel fibers is linear with the product of the fiber influencing coefficient (α), the slenderness ratio (l/d), and the fiber volume fraction (Vf). Under dynamic loading, the value of α is higher than that observed under quasi-static loading, which is due to the enhancement of the dynamic bond strength between the fiber and the cement matrix. Furthermore, undulated fibers bring about a higher α than hooked fibers. Finally, an empirical formula is built to estimate the dynamic spall strength of SFRC under one-dimensional stress. The prediction was shown to be consistent with the experimental results. The steel fiber enhanced spall strength can be regarded as an important guideline in the design of high-performance concrete for improving the penetration resistance of protective structures.

  • Phase-field modeling of multivariant martensitic microstructures and size effects in nano-indentation
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-04
    Mohsen Rezaee-Hajidehi, Stanisław Stupkiewicz

    A finite-strain phase-field model is developed for the analysis of multivariant martensitic transformation during nano-indentation. Variational formulation of the complete evolution problem is developed within the incremental energy minimization framework. Computer implementation is performed based on the finite-element method which allows a natural treatment of the finite-strain formulation and of the contact interactions. A detailed computational study of nano-indentation reveals several interesting effects including the pop-in effect associated with nucleation of martensite and the energy-lowering breakdown of the symmetry of microstructure. The effect of the indenter radius is also examined revealing significant size effects governed by the interfacial energy.

  • Effects of Cu/graphene interface on the mechanical properties of multilayer Cu/graphene composites
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-04
    Weixiang Peng, Kun Sun

    Molecular dynamics simulations are performed to investigate the effects of graphene on the mechanical properties in multilayer Cu/graphene composites under uniaxial tension. It is found that both of zigzag and armchair graphene can improve the mechanical strength of multilayer Cu/graphene composites. The enhanced efficiency is concerned with chirality and interlayer thickness of graphene. The Cu/graphene interface has a great effect on the dislocation nucleation and propagation in the plastic deformation. Firstly, the interface can act as a resource of dislocation emission. This is due to the high stress concentrated on the interface caused by lattice mismatch and shear modulus mismatch between Cu and graphene, which can reduce the energy of nucleation. The interface stress of armchair graphene is more evident than the zigzag graphene. Secondly, the dislocations are confined by the impenetrable interface during the propagation process, which leads to intense interaction between dislocations and interface. Both the confinements and interactions are responsible for high stress required during the propagation process. A confined layer slip (CLS) model is established to predict the strength of multilayer composites in quantification. After the fracture of graphene, the dislocations penetrate through the interface of Cu and graphene and the composites would neck and fracture around the region.

  • Role of critical interfacial shear modulus between polymer matrix and carbon nanotubes in the tensile modulus of polymer nanocomposites
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-03
    Yasser Zare, Kyong Yop Rhee

    The main purpose of this paper includes the definition of critical interfacial shear modulus (Gc) between polymer matrix and carbon nanotubes (CNT) and the investigation of its roles in the effective interphase properties and tensile modulus of polymer nanocomposites. Halpin-Tsai model is developed to determine the roles of “Gc”, interfacial shear modulus (Gi) and interphase properties in the tensile modulus of nanocomposites. The calculations of the developed model for the modulus of various samples are compared to the experimental results. Furthermore, the effects of all parameters on the tensile modulus of nanocomposites are explained to support the developed model. The original Halpin-Tsai model overestimates the tensile modulus of the samples, but the predictions of the developed model properly agree with the experimental data. The high “Gc” and poor interphase modulus cannot reinforce the nanocomposites. Also, CNT radius (R) > 15 nm considerably weaken the modulus of nanocomposites, but the modulus grows about 500 % at R = 5 nm and interphase thickness (t) = 25 nm demonstrating the most important roles of these parameters in the modulus of nanocomposites.

  • Effective Elastic-Plastic Response of Particulate Two-Phase Composite Materials Under Multi-axial Loading
    Mech. Mater. (IF 2.958) Pub Date : 2019-12-02
    Hong Teng

    In recent papers (Teng, 2014 and 2018), an incremental elastic-plastic double-inclusion model was developed to determine the effective elastic-plastic response of two-phase composites of spherical or aligned spheroidal particles. For composites of purely elastic particles and elastic-plastic matrix of von Mises yield criterion with isotropic strain hardening the double-inclusion model was formulated through the use of an isotropic matrix tangent stiffness tensor. The use of the isotropic tangent stiffness tensor, however, is only suitable for uniaxial loading or pure shear, but not for multi-axial loading. For in general the elastic-plastic tangent stiffness tensor of the matrix is inherently anisotropic, and during plastic deformation normal stress (strain) increment and shear strain (stress) increment are coupled, as a result of the anisotropy of the tangent stiffness tensor. In the current paper, the original incremental elastic-plastic double-inclusion formulation presented in the previous papers (Teng, 2014 and 2018) for composites of elastic spherical or aligned spheroidal particles and elastic-plastic matrix is modified by adding anisotropic corrections of stress increment defined in terms of the difference between the anisotropic and isotropic tangent stiffness tensors of the matrix. The resulting improved incremental double-inclusion model can be applied to multi-axial loading. Comparison of the model predictions to the results of the direct approach using representative volume elements containing many particles shows that the improved incremental elastic-plastic double-inclusion formulation is capable of predicting the effective elastic-plastic response of two-phase composites of spherical or aligned spheroidal particles under multi-axial loading.

  • On the controllability of a creasing singularity in a nonlinear elastic circular sector
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-29
    P. Ciarletta

    The deformation of a circular sector into a full self-contacting circle can be sustained in all homogeneous, isotropic, incompressible materials by surface tractions alone. In this class of nonlinear elastic materials, this works investigates the controllability of such a peculiar mapping having uniform constant strains and a creasing singularity. By performing a perturbative analysis based on small–on–large incremental methods, we determine the critical conditions for the normal traction load to trigger a morphological transition from the circular ground state to an elliptic shape. Such predictions are given for neo-Hookean, Gent and polynomial material models to illustrate how both geometrical and physical nonlinearities concur to this elastic instability.

  • Stress-regime-dependence of inelastic anisotropy in forged age-hardening aluminium alloys at elevated temperature. Constitutive modeling, identification and validation
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-28
    Konstantin Naumenko, Elisabetta Gariboldi, Rostyslav Nizinkovskyi

    Many structural materials exhibit stress-regime-dependent anisotropy of inelastic responses. Examples are age-hardening aluminium alloys forgings. Experimental creep curves indicate that inelastic strain rates depend significantly on the loading direction within the power law regime, while in the power law breakdown range the anisotropy is weak and can be neglected. The aim of this paper is to analyze anisotropic behavior of forged AA2014 alloy and to develop a constitutive model to describe inelastic response under multi-axial stress state. Microstructural observations suggest that the anisotropy is primarily caused by elongated grains and coarse particles on grain boundaries. To account for inhomogeneous inelastic deformation and stress redistribution in different microstructural zones a composite model is developed. Constitutive equations for inelastic-hard and inelastic-soft constituents and anisotropic rules for mechanical interactions between them are elaborated. Hardening/recovery and overageing processes in grain interiors are characterized by internal state variables and kinetic equations. The model is calibrated against families of creep curves for two loading directions in a wide stress and temperature ranges. For the validation, creep tests under the loading with the angle of 30∘ relative to the longitudinal grain axis as well as tensile tests are simulated by the model and results are compared with experimental data.

  • Effect of electron beam irradiation on mechanical properties of unsaturated polyester/nanoclay composites reinforced with carbon and glass fibers
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-28
    Seyed Mohammad Razavi, Seyed Javad Ahmadi, Peyman Rahmani Cherati, Mina Hadi, Seyed Amir Reza Ahmadi

    Following our previous work on morphology, corrosion resistance and thermal properties of unsaturated polyester (UP)/nanoclay composites, the effect of electron beam (EB) irradiation on mechanical properties of UP/nanoclay composites with various nanoclay contents is investigated in this work. Moreover, influence of EB irradiation with doses of 100, 500 and 1000 kGy on tensile properties and hardness of nanocomposite was examined. FTIR spectroscopy was employed to examine the effect of irradiation on chemical structure of polymer. EB irradiation up to 500 kGy led to the improvement of tensile strength and hardness of the samples. The best mechanical properties were observed for the nanocomposite with 1 wt.% nanoclay content. As the selected nanocomposite, UP/nanoclay(1wt.%) composite was reinforced by carbon and glass fibers and the effect of EB irradiation on the mechanical properties of the prepared UP/nanoclay/fiber composites was studied. When compared to the fiber-reinforced UP without nanoclay, presence of nanoclay particles in UP/nanoclay/fiber composites resulted in the better mechanical properties and higher radiation resistance. Hence, the UP/nanoclay/fiber composite with proper amount of nanoclay has been shown to be a dependable choice for structural applications in which the material is exposed to ionizing radiations.

  • Thermo-mechanical behavior prediction of shape memory polymer based on the multiplicative decomposition of the deformation gradient
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-27
    Wei Zhao, Liwu Liu, Jinsong Leng, Yanju Liu

    During the service of shape memory polymer (SMP), the thermal-mechanical cycle is a necessary process. Meanwhile, the influence of the viscoelastic properties of SMP, such as rate-dependent behaviors, can not be ignored during the application. Consequently, a constitutive model based on the multiplicative decomposition of the deformation gradient is proposed in this work, which can clearly describe the viscoelastic behavior and capture the thermal-mechanical cycle process of SMP. By introducing the “phase transition” concept, SMP is assumed as a composite composed of glassy phase and rubbery phase, and the volume fractions of each phase vary with temperature. Furthermore, according to the different mechanical behaviors of the glassy phase and rubbery phase, two different constitutive structures that can describe the strain sensitivity of SMP are developed to represent the mechanical response. The developed model was verified by simulating a series of experiments, including strain sensitivity tests and thermal-mechanical cycle experiments.

  • Mechanical response and plastic deformation of coherent twin boundary with perfect and defective structures
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-27
    Liang Zhang, Wei Mao, Mao Liu, Yasushi Shibuta
  • Hot deformation behavior of nano-sized TiB reinforced Ti-6Al-4V metal matrix composites
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-22
    Yuankui Cao, Yong Liu, Yunping Li, Bin Liu, Rongjun Xu

    In situ Ti-6Al-4V/TiB composites with different contents of TiB were prepared by SPS. The flow behavior of the composites deformed at temperatures of 900∼1200°C and strain rates of 0.001 to 10s−1 was studied. The constitutive equations and processing maps were established. Results show that the composites have fine microstructures, and nano-sized TiB whiskers distribute homogeneously in the α+β titanium matrix. The activation energy of the composites is low due to the fine microstructures, resulting in a good workability. The processing maps suggest that the instable deformation occurs at low temperatures and high strain rates. The flow localization band of Ti matrix, cracking and debonding of TiB whiskers are the reasons for the instable deformation. Dynamic recrystallization has been found in composites deformed at low strain rates, accounting for the stable deformation.

  • 更新日期:2019-11-22
  • Influence of thermal recovery on predictions of the residual mechanical state during melting and solidification
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-21
    Martin Kroon, MB Rubin

    A thermomechanically consistent Eulerian plasticity model with work hardening is adopted for studying the residual mechanical state resulting from loading at elevated temperatures. The isotropic plasticity model includes the standard effect of thermal softening as well as specific modeling of thermal recovery. The model parameters and functions were calibrated to data for an austenitic stainless steel 316L. The model is applied in two numerical examples: a case of uniaxial tension and a circular disk that is exposed to a temperature load. The influence of thermal recovery is examined for each example by comparing the response of the complete model with thermal recovery to that when thermal recovery is omitted. The results of the second example indicate the importance of modeling thermal recovery for accurate prediction of residual stresses for problems dealing with melting and solidification.

  • 更新日期:2019-11-21
  • Compaction of Machining Chips: EXTENDED EXPERIMENTS AND MODELING
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-17
    Naseer M. Abbas, Xiaomin Deng, Anthony P. Reynolds

    The process of compaction of machining chips inside a cylindrical processing chamber is investigated. This process can be either an independent process or the first step in the Friction Extrusion Process (FEP) where the consolidated chips are softened due to frictional heating and are turned into a wire through an extrusion hole in the die. The current study provides an extension to our previous study in order to determine experimentally the Poisson's ratio as a function of the relative density during compaction. This determination is done through measurements of strains on the outer surface of the processing chamber containing the machining chips. The elastic theory of a thick-walled cylinder under internal pressure is employed to relate the surface strain measurements to stress and strain states at the processing chamber-chips interface. Also, the loading machine compliance effect is removed from the compaction and uniaxial test results whereas in the previous work it is implicitly assumed that the loading machine is rigid so that the measured axial deformation is taken to be entirely due to the deformation of the compacted chips. The porous elastic-plastic material model developed previously is extended to model the mechanical behavior of the chips during the compaction process. Simulation predictions of the axial stress-strain curves are performed and the predicted data are found to have a good agreement with experimental data.

  • Thermo-mechanical Stability of Single-layered Graphene Sheets Embedded in an Elastic Medium under Action of a Moving Nanoparticle
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-16
    Mostafa Pirmoradian, Ehsan Torkan, Nasir Abdali, Mohamad Hashemian, Davood Toghraie

    Using an energy-based method, this paper sought to analyze dynamic stability and parametric resonance of single-layered graphene sheets (SLGSs) embedded in thermal environment and elastic medium while carrying a nanoparticle moving along an elliptical path. In order to present a realistic model, all inertial effects of the moving nanoparticle are taken into account in the dynamic formulation of the system. Equations governing the transverse vibrations of the embedded SLGS are obtained using the Hamilton's principle. Small-scale effects based on the Eringen's nonlocal elasticity theory are considered in deriving the motion equations. The equations governing the reduced model are calculated based on the Galerkin method. To calculate the instability boundaries, the energy-rate method is applied on the ordinary differential equations (ODEs) governing the system oscillations. The effects of nonlocal parameter, the nanoparticle motion path radii, SLGS length-to-width ratio, temperature change of the thermal environment, stiffness of the elastic medium and boundary conditions of SLGS on the parametric instability regions are examined. The results show that these parameters influence the system stability, so that a decrease in the nonlocal parameter, the SLGS length-to-width ratio and the nanoparticle motion path radii and also an increase in the stiffness coefficients of the elastic medium improve the system stability. The model presented in this paper is validated by comparing the observations with those published in previous studies.

  • Ideal strength of nanoscale materials induced by elastic instability
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-14
    Duc Tam Ho, Soon Kim, Soon-Yong Kwon, Sung Youb Kim

    The ideal strength of a defect-free material, which is the stress causing a material failure, is one of the fundamental mechanical properties. In this study, we investigate ideal strengths of some face-center cubic nanostructures using molecular statics simulations and an elastic stability criterion. The simulation results show that ideal strength depends strongly on loading direction, loading mode (tension or compression), side surface orientation, shape of cross-section, and size. Consequently, nanostructures can exhibit the “smaller is stronger” trend, the “smaller is weaker” trend, and the “size-independent strength plateau” trend. Our semi-analytic model for prediction of ideal strengths of nanostructures is in good agreement with molecular statics simulation results.

  • 更新日期:2019-11-13
  • Nanoscale mode-III interface crack in a bimaterial with surface elasticity
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-11
    Ying Yang, Zhen-Liang Hu, Xian-Fang Li

    This paper studies a mode-III nanocrack at the interface between two bonded dissimilar materials under antiplane shear loading. The classical elasticity incorporating surface elasticity is applied to solve a mixed boundary value problem associated with an anti-plane shear interface crack. The influence of surface elasticity on the crack-tip field for a nanoscale mode-III crack is analyzed. By use of the Fourier transform, the problem is reduced to a set of hypersingular integro-differential equations. The displacement and bulk stress jumps are expanded as the Chebyshev orthogonal polynomials and the Galerkin method is used to approximately determine the singular elastic field near the interface crack tips. Consideration of surface elasticity does not cause the disappearance of crack-tip singularity. A usual inverse square-root singularity is derived near the crack tips. The influences of surface elasticity on the stress intensity factor are examined and displayed graphically. The surface residual stress does not alter the stress field for a mode-III interface crack.

  • Experimental study of strength properties of SLA resins under low and high strain rates
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-09
    Danuta Miedzińska, Roman Gieleta, Ewelina Małek

    A study of the material properties of resins used for stereolithography rapid prototyping is presented in the paper. The samples were prepared in accordance with producers' guidelines and loaded in two ways: quasi-static and with the use of a Hopkinson bar. Different strain rates were considered in this way. The researched material showed significant differences when responding to the different strain-rate loadings. The presented work will be used in numerical constitutive modeling of SLA raisins.

  • Bioinspired toughness improvement through soft interlayers in mineral reinforced polypropylene
    Mech. Mater. (IF 2.958) Pub Date : 2019-11-09
    Johannes Wiener, Florian Arbeiter, Abhishek Tiwari, Otmar Kolednik, Gerald Pinter

    The effects of soft, polymeric interlayers on a brittle, mineral reinforced polymer matrix are investigated. Interlayers made of a standard polypropylene (PP) and a soft type of PP are introduced into matrix materials of either highly or moderately mineral particle reinforced PP. Single edge notch bending experiments are performed to characterize the fracture toughness of these composites. The experimental J-integral Jexp is used to describe the fracture toughness of the investigated materials. The multi-layered materials are compared to the homogeneous matrix material. A modified plotting technique is applied to more distinctly demonstrate the effects of soft layers on Jexp as a function of the crack extension Δa. The fracture toughness is evaluated and the slope of the J-Δa curves is used as a qualitative measure of crack growth resistance. In addition, the crack growth rate is recorded. The results show improvements in fracture toughness of almost twenty times of the matrix material, provided the material combination is chosen properly. This increase in fracture toughness is achieved due to a crack-arresting effect in the soft layers, which is followed by an energy-expensive crack re-initiation step.

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上海纽约大学William Glover