Elasto-viscoplastic Material Research Articles

Porosity Effects on the Composite Girder by Rheological Dynamics and FEM

A theoretical model for porous viscoelastoplastic (VEP) materials in the dry state is investigated in this research study. The model is based on the principles of conservation of mass and energy using the rheological dynamic theory (RDT). The model provides expressions for the creep coefficient, Poisson’s ratio, modulus of elasticity, damage variable, and strength as a function of porosity and/or void volume fraction (VVF). The reliability of the proposed model was analyzed by comparing numerical results with experimental ones on hardened concrete. A numerical model was created and analyzed in the commercial software Abaqus and validated by comparison with experimental data obtained by geodetic measurements on a composite wood–lightweight concrete girder. The deflections and stresses of the beam resulting from the influence of concrete creep and porosity were analyzed at the initial moment of time and after 6 years. The results showed that the RDT provided a reliable model for estimating parameters after exposure to long-term loads.

Tensorial harmonic bases of arbitrary order with applications in elasticity, elastoviscoplasticity and texture-based modeling

In continuum mechanics, one regularly encounters higher-order tensors that require tensorial bases for theoretical or numerical calculations. By using results from [Formula: see text] representation theory, we present a method to derive tensorial harmonic bases which unifies existing approaches in one framework. Applying this convention to symmetric second-order tensors results in a second-order harmonic basis which, for example, simplifies the depiction and numerical handling of stiffness tensors for most common material symmetries and reduces the computational effort involved in implementations of incompressible elastoviscoplastic material laws. The same convention can be applied to texture analysis to yield tensorial texture coefficients for both polycrystals and fiber-reinforced composites. Rotations of higher-order tensors are particularly efficient in harmonic bases as both material and index symmetries can be exploited. While special attention is given here to the examples of small-strain material laws and texture analysis, the framework is entirely general and can be used to simplify calculations in other physical contexts involving tensors of arbitrary order and symmetry. A Python implementation of the harmonic basis convention defined in this work is available as a Supplementary Material.

[formula omitted]OWCh: Optimally Windowed Chirp rheometry using combined motor transducer/single head rheometers

Recent advances in rheometry exploiting frequency-modulated (chirp) waveforms have dramatically reduced the time required to perform linear viscoelastic characterisation of complex materials. However, the technique was optimised for ‘separate motor transducer’ instruments, in which the drive motor imposing the strain deformation is decoupled from the torque transducer. Whilst the use of optimised windowed chirps (OWCh) using other rheometers has been recently reported in the literature, no systematic study concerning the use of ‘combined motor transducer’ instruments (in which the motor and transducer subsystems are integrated into a single ‘head’) has been undertaken. In the present study, we demonstrate the use of OWCh rheometry using combined motor transducer/single-head rheometers using a stress-controlled operating principle, thus avoiding the reliance on complicated and instrument-specific feedback control systems that would be required to perform strain-controlled experiments. The use of stress-controlled chirps requires a modification to the established OWCh analysis protocol such that the complex viscosity η∗(ω) is used as an intermediate proxy function for ultimately computing the complex modulus G∗(ω). This approach negates the effect of the strain offset that is inherent to stress-controlled oscillatory rheometry. Secondly, a correction algorithm and operational criteria for identifying inertial artefacts is established before we consider the impact of chirp digitisation on data acquisition. The use of stress-controlled OWCh rheometry (which we term Stress-OWCh, i.e. σOWCh) is demonstrated for a diverse range of material classes including, Newtonian calibration fluids (silicone oil), polymer solutions (polyethylene oxide in water), an entangled polymer melt (polydimethylsiloxane), worm-like micellar systems (cetylpyridinium chloride/sodium salicylate), time-evolving critical gels (gelatin) and aging elastoviscoplastic materials (Laponite®). This novel implementation of chirp waveforms using a single-head rheometer will facilitate the wider adoption of OWCh rheometry and allow the benefits of frequency-modulation techniques to be exploited where separate motor transducer instruments are unavailable/unsuitable.

MODELING OF POROUS DRY MATERIALS USING RHEOLOGICALDYNAMICAL ANALOGY

<p class="ABSTRACT">A theoretical model for porous viscoelastoplastic (VEP) materials under dry conditions is examined based on the principles of mass and energy conservation using rheological-dynamical analogy (RDA). The model provides the expressions for the creep coefficient, Poisson's ratio, modulus of elasticity, damage variable and strength in the function of porosity and/or void volume fraction (VVF). Compared with numerous versions of acoustic emission monitoring developed to analyze the behavior of the total wave propagation in inhomogeneous media with density variation, the RDA model is found to be comprehensive in interpretation and consistent with physical understanding. The reliability of the proposed model is confirmed by the comparison of numerical results with experimental ones on hardened concrete and rocks.</p>

Elasto-visco-plastic flows in benchmark geometries: I. 4 to 1 planar contraction

We present predictions for the flow of elastoviscoplastic (EVP) fluids in the 4 to 1 planar contraction geometry. The Saramito-Herschel-Bulkley fluid model is solved via the finite-volume method with the OpenFOAM software. Both the constitutive model and the solution method require using transient simulations. In this benchmark geometry, whereas viscoelastic fluids may exhibit two vortices, referred to as lip and corner vortices, we find that EVP materials are unyielded in the concave corners. They are also unyielded along the mid-plane of both channels, but not around the contraction area where all stress components are larger. The unyielded areas using this EVP model are qualitatively similar to those using the standard viscoplastic models, when the Bingham or the Weissenberg numbers are lower than critical values, and then, a steady state is reached. When these two dimensionless numbers increase while they remain below the respective critical values, which are interdependent, (a) the unyielded regions expand and shift in the flow direction, and (b) the maximum velocity increases at the entrance of the contraction. Increasing material elasticity collaborates with increasing the yield stress, which expands the unyielded areas, because it deforms the material more prior to yielding compared to stiffer materials. Above the critical Weissenberg number, transient variations appear for longer times in all variables, including the yield surface, instead of a monotonic approach to the steady state. They may lead to oscillations which are damped or of constant amplitude or approach a flow with rather smooth path lines but complex stress field without a plane of symmetry, under creeping conditions. These patterns arise near the entrance of the narrow channel, where the curvature of the path lines is highest and its coupling with the increased elasticity triggers a purely elastic instability. Similarly, a critical value of the yield stress exists above which such phenomena are predicted.

Acid-assisted subcritical blunt-tip crack propagation in carbonate rocks

AbstractSubcritical crack propagation in stressed carbonate rocks in a chemically reactive environment is a fundamental mechanism underlying many geomechanical processes frequently encountered in the engineering of geo-energy, including unconventional shale gas, geothermal energy, carbon sequestration and utilization. How a macroscopic Mode I crack propagates driven by a reactive fluid pressurizing on the crack surfaces with acidic agents diffusing into the rock matrix remains an open question. Here, the carbonate rock is modeled as an elasto-viscoplastic material with the mineral mass removal process affecting the rock properties in both elastic and plastic domains. A blunt-tip crack is considered to avoid any geometrically induced singularity problem and to allow a numerical analysis on the evolution of the chemical field being linked to the micro-cracking activities in front of the crack tip, affecting the delivery of acid. The model is capable of reproducing an archetypal three-region behavior of subcritical crack growth in a reactive environment. The crack propagation exhibits a prominent acceleration in Region III due to a two-way mutually enhancing feedback between mineral dissolution and the degradation process, which is most pronounced in front of the crack tip. With the consideration of initial imperfections in the rock, the macroscopic crack propagation is further accelerated with a secondary acceleration arising due to self-organization of micro-bands inside the chemically enabled plasticity zone.

Supercomputer Simulation of the Chip Separation Process during Dimensional Dispersion of Elastoviscoplastic Materials

Supercomputer Simulation of the Chip Separation Process during Dimensional Dispersion of Elastoviscoplastic Materials

Thermo-rheological properties of xanthan solutions: from shear thinning to elasto-viscoplastic behavior.

The thermo-rheological behavior of xanthan solutions with concentrations spanning a wide range is investigated experimentally. After carefully identifying four distinct regimes of concentration we focused on highly concentrated xanthan solutions. By combining several rheological techniques, it is shown for the first time that such solutions belong to the broad class of elasto-viscoplastic materials by exhibiting both a yield stress and elasticity that manifests around the solid-fluid transition. The soft and weakly entangled structure responsible for the elasto-viscoplastic rheological behavior is controlled by two factors:imposed stress, temperature. Consequently, concentrated solutions of xanthan may yield to either imposed stress or temperature. The systematic analysis of the elasticity mediate solid-fluid transition at various operating temperatures revealed the presence of a novel state termed as "molten solid". A clear relationship between the rheological states and the molecular states (native, denaturated, re-naturated) is established.

APPLICABILITY INDICATORS FOR THE NONLINEAR MAXWELL-TYPE ELASTO-VISCOPLASTIC MODEL WITH POWER MATERIAL FUNCTIONS AND TECHNIQUES TO CALIBRATE THEM

A physically non-linear Maxwell-type constitutive relation with two material functions for nonaging elasto-viscoplastic materials is studied analytically in order to examine the set of basic rheological phenomena that it simulates, to enclose its application field, to obtain necessary phenomenological restrictions which should be imposed on its material functions and to develop identification and validation techniques. Characteristic features of loading-unloading-recovery curves family produced by the model with two power material functions (with four parameters) under loading and unloading at constant stress rates and subsequent rest are analyzed in uni-axial case and compared to general properties of stress-strain-recovery curves produced by the constitutive relation with two arbitrary (increasing) material functions (theorems 1 and 2). Their dependences on loading rate, maximal stress and material functions exponents are examined. Power functions are the most popular in creep models, elastoviscoplasticity, polymer rheology, hydrodinamics of non-newtonian fluids and simulation of superplastic flow. The analysis reveals several specific properties of theoretic loading-unloading-recovery curves produced by power model with four parameters that can be employed as the model applicability indicators which are convenient for check using test data of a material. They should be checked in addition to general applicability indicators for the Maxwell-type constitutive relation with two arbitrary material functions. A number of effective calibration procedures for the model in the class of power material functions are developed. They are more rapid and effective than general identification techniques for two arbitrary material functions developed previously. The first procedure employs a pair of stress-strain curves at different stress rates, the second one is based on a pair of loadingunloading- recovery curves with various maximal stress values and loading rates and the third one needs only one loading-unloading-recovery curve. The explicit expressions are derived for four material parameters via test data. They enable separate and direct evaluation of the material parameters without error accumulation. Identification techniques versions are considered and their advantages and shortcomings are discussed. The ways to minimize the error using additional tests are proposed.

Time-Scaling Creep in Salt Rocks for Underground Storage

Salt is an elastoviscoplastic material that exhibits time-dependent deformation (creep). Experimental measurements of salt creep behavior help predict underground gas repositories’ long-term geomechanical behavior. Previous time-scaling creep experiments have focused on the axial strain of unconsolidated sands. i.e., time-scaling creep effects under zero lateral strain conditions without describing the creep behavior of the radial strain. In addition, the time-scaling creep of the radial and axial strain has not been investigated in salts. A comparative testing procedure and analysis method was conducted on Spindletop salt plugs using triaxial tests for multistage triaxial tests (MST) and different holding time durations and stress regimes, resulting in time-dependent strain responses (creep tests). The MST showed evolving deformational mechanisms under the mapped yield surface based on the irrecoverable to recoverable strain ratio beginning with crack closure or conformance, plasticity, and ending at early crystal surface failure. Unlike unconsolidated sands, salts showed both time and strain amplitude scaling. The axial and radial strain data show scaling behavior under low and high levels of deviatoric stress separated by a transitional period. The salt showed only an axial creep response at low deviatoric stress distally from the yield surface (one-dimesional (1D) response or zero lateral strain), which indicates negative dilatant deformation or uniaxial compaction. In contrast, the salts showed equal strain amplitude scaling factors both axially and radially at high deviatoric stress proximal to the yield surface (two-dimensional (2D) response or unconstrained boundary condition), which suggests positive dilatant deformation. Microstructural images showed accumulated creep damage under high deviatoric stress associated with parallel planes of dislocation-intergranular slip, microcracking, and compaction-induced dilational strains. The period of scaling is interpreted as regions where a single mechanism is dominating. Strain amplitude scaling for both low and high deviatoric creep stress tests provides inputs for a constitutive model of creep response in understanding the magnitude of mechanical damage associated with time-independent stress-strain curves in salts for the structural integrity of salt caverns during cyclic fluid injection and depletion.

Changes in Residual Stresses in the Vicinity of a Continuity Defect in an Elastoviscoplastic Material under Repeated Loading

The results of calculations of the stress-strain states of an elastoplastic material containing a single cylindrical continuity defect and loaded with external pressure are presented. The hydrostatic pressure on the surface distant from the defect initially increases, remains constant for some time and then gradually decreases to zero. The process of comprehensive compression leaves, after unloading, a formed level and distribution of residual stresses in the vicinity of the continuity defect. The change in such stresses is calculated upon repeated loading and unloading. Intensity and nature of repeated loads are considered identical to the original ones. Calculations are carried out within the framework of the stated one-dimensional problem of the theory of large deformations. The material is assumed to be incompressible. The development of areas of viscoplastic flow in the active part of the process and their attenuation during unloading are taken into account. Changes in the geometric dimensions of the evolving defect are assessed.

Supercomputer simulation of chip separation process during dimensional dispersion of elastoviscoplastic materials

Aspects of leather waste processing for their use in a dispersed state in the preparation of composite materials for various purposes are considered. It is shown that to reduce the relaxation and creep of the material, it is necessary to increase the cutting speed. A method is developed for dimensional dispersion with a claw-shaped impact-cutting tool, the complex movement of which is provided by a high-frequency vibration drive. In the LSTC LS-Dyna software for continuum mechanics modeling, a model of the process of dispersing leather waste is developed. Keywords: chip formation, modeling, leather waste, dimensional dispersion, impact cutting tool, elastoviscoplastic medium. sergeevys@susu.ru, sergeev-sv@list.ru, mr.maks.134@gmail.com

Hydrodynamic interaction between coaxially rising bubbles in elastoviscoplastic materials: Equal bubbles

Hydrodynamic interaction between coaxially rising bubbles in elastoviscoplastic materials: Equal bubbles

Experimental and numerical investigations for compressive behavior of porous hydroxyapatite bone scaffold fabricated via freeze-gel casting method

Artificial bone scaffolds with a porosity of ≥64% were manufactured using hydroxyapatite (HA). The Freeze-gel Casting method using tert-butyl alcohol (TBA) as a solvent was adopted to maintain proper mechanical strength. The manufactured HA scaffold had excellent internal connectivity owing to the continuous connection of the long columnar pores and arrangement of the constant pore walls. For the HA scaffold, compression tests at different strain rates under uniaxially compressed loads were performed, and the compressive behavior according to the strain rate was analyzed. A maximum yield stress of 2–3 MPa was observed, and the overall compressive behavior was divided into elastic, plateau, and densification sections, which were different from the brittleness behavior of ordinary ceramics and similar to that of polymer materials. The compressive behavior of the HA scaffold was successfully simulated by applying the Frank–Brockman–Zairi constitutive model used to simulate elasto-viscoplastic material behavior. Seven material parameters belonging to the constitutive equation were proposed for the HA scaffold by adopting a deterministic approach for material parameter identification. Models can be developed from methods for simulating the mechanical behavior of a porous ceramic scaffold to predict the long-term mechanical behavior of the scaffold inserted in vivo, which may help in bone defects recovery.

Chaboche viscoplastic material model for process simulation of additively manufactured Ti-6Al-4 V parts

For the estimation and further optimization of the residual stress and distortion state in additively manufactured structures during and after the wire arc additive manufacturing (WAAM) process, thermomechanical simulation can be applied as a numerical tool. In addition to the detailed modelling of key process parameters, the used material model and material data have a major influence on the accuracy of the numerical analysis. The material behaviour, in particular the viscoplastic behaviour of the titanium alloy Ti-6Al-4 V which is commonly used in aerospace, is investigated within this work. An extensive material characterization of the viscoplastic material behaviour of the WAAM round specimen is carried out conducting low cycle fatigue (LCF) and complex low cycle fatigue (CLCF) tests in a wide temperature range. An elasto-viscoplastic Chaboche material model is parameterised, fitted, and validated to the experimental data in the investigated temperature range. Subsequently, the material model is implemented in the thermomechanical simulation of a representative, linear ten-layer WAAM structure. To finally determine the effect of the fitted material model on the estimation accuracy of residual stress and distortion, simulation results using the standard material model and the elaborated Chaboche model from this study are compared to experimental data in the substrate. The thermomechanical simulation with the Chaboche model reveals a better agreement with the experimental distortion and residual stress state, whereby the standard material model tends to an overestimation. The estimation accuracy with respect to the maximum distortion is improved from an error of 60% with the standard model to an acceptable error of about 6% using the elaborated model. Additionally, the estimated residual stress state shows a sound agreement to the experimental residual stress in the substrate.

A microstructure-based three-scale homogenization model for predicting the elasto-viscoplastic behavior of duplex stainless steels

A new microstructure-informed three-scale homogenization scheme for elasto-viscoplastic heterogeneous materials is developed. It is applied to predict the mechanical behavior of cast duplex austenitic-ferritic (AF) steels, which are widely used in primary loop of pressurized water reactors (PWR). At the first scale (microscale), the elasto-viscoplastic behavior of single crystals for both phases is modeled using linear elasticity and a viscoplastic crystal plasticity model. An “affine” type formulation based on the first moments of stresses is applied to the inelastic non-linear part of the deformation. Then, at the second scale (mesoscale), an EBSD-informed two-phase austenite/ferrite laminate structure (LS) model is developed with {110}-type habit planes (HP) in ferrite and a Kurdjumov-Sachs orientation relationship (KS-OR) between both phases. At the third scale (macroscale), the model considers a single ferritic primary grain as an aggregate of spherical two-phase laminate structure domains corresponding to the 24 KS-OR variants. The elasto-viscoplastic self-consistent scheme (EVPSC) is used through the Translated Fields (TF) method to obtain the effective behavior at this scale. The TF-EVPSC scheme is also used for an ensemble of primary ferritic grains to identify materials parameters from experimental tensile curves corresponding to as received and aged specimen. From EBSD measurements, the crystallographic data are thoroughly analyzed to physically feed the three-scale model. The results are discussed in terms of stress/strain responses in ferrite and austenite, which are correlated to the different KS-OR variants and HP orientations. The effect of primary ferritic grain crystallographic orientation and aging is also studied regarding monotonic and cyclic tests to study the possible origins of backstress in this material.

Two‐scale FE‐FFT‐based thermo‐mechanically coupled modeling of elasto‐viscoplastic polycrystalline materials at finite strains

AbstractDue to the general pursuit of technological advancement, structural components need to meet increasingly higher standards. In order to optimize the performance behavior of the used materials, detailed knowledge of the overall as well as microscopic material behavior under certain mechanical and thermal loading conditions is required. Hence, we present a two‐scale finite element (FE) and fast Fourier transformation (FFT)‐based method incorporating finite strains and a thermo‐mechanically coupled constitutive model for elasto‐viscoplastic polycrystalline materials. Assuming that the length scale of the microscale is sufficiently smaller compared to the length scale of the macroscale, we consider the macroscopic and microscopic boundary value problem as two coupled subproblems. The macroscopic boundary value problem is solved utilizing the finite element method. In each macroscopic integration point, the microscopic boundary value problem is embedded as a periodic unit cell whose solution fields are computed utilizing fast Fourier transforms and a Newton‐Krylov solver. The scale transition is performed by defining the macroscopic quantities via the volume averages of their microscopic counterparts. In order to demonstrate the use of the proposed framework, we predict the macroscopic and microscopic fields of a polycrystalline material within a numerical example using an efficient and accurate FE‐FFT‐based two‐scale method.

Calculations of Large Nonisothermal Deformations of Elastoviscoplastic Materials

Calculations of Large Nonisothermal Deformations of Elastoviscoplastic Materials

Peridynamics methodology for elasto-viscoplastic ductile fracture

This paper presents a novel peridynamics (PD) methodology and implementation approach for predicting the ductile fracture of elasto-viscoplastic materials. The proposed analysis model consists of three components: 1) an elasto-viscoplastic constitutive model based on modified Bodner-Partom (BP) constitutive theory; 2) bond damage prediction based on Wang’s unified damage theory; 3) crack growth prediction using the PD critical bond stretch criterion. The PD model for BP materials is realized under the ordinary state-based PD framework to define the bond-wise relationship between deformation state and force state. The use of a unified constitutive theory endows the present model with the characteristic of no yield criterion. In addition, the non-local nature of the PD framework allows discontinuities to naturally occur, thereby ensuring the present model can capture the initiation and propagation of ductile fracture. Simulations of the standard tension tests and mode-I compact tension tests were carried out to demonstrate the use of the PD model for BP materials in practical problems. Particle-wise indicators, representing the intensities of plasticity, damage and fracture, were extracted for post-processing. An in-depth analysis shows that the presented PD model for BP materials can effectively reproduce ductile fractures at the macro-scale, allowing a general PD description of ductile behavior, which brings a new outlook for the application of PD methods.

A multi-material ALE model for investigating impact dynamics of pile-soil systems

The dynamic pile-soil interaction is one of the most important and challenging problem in geomechanics, especially when geometrical, material-related, and dynamic contact-related nonlinearities exist. Geometrical nonlinearity is a fundamental obstacle to the popular Lagrangian finite element methods (FEM) that have been utilized in other areas of geomechanics research. This paper introduces a Multi-Material Arbitrary Lagrangian-Eulerian (MM-ALE) model for predicting the pile-soil interaction and forces during lateral dynamic impact events. The study used the MM-ALE method in conjunction with an elasto-viscoplastic soil material model that included strain softening and an elasto-plastic steel material model that incorporated strain rate effects to examine pile response during lateral vehicle impacts. The model was validated using large-scale, dynamic impact test data for piles embedded in soil, thus confirming its capability to simulate the complex dynamic impact pile-soil interaction. The effects of impact velocity, embedment depth, and soil strength on impact forces, energy dissipation, and impulse response of a pile-soil system were subsequently investigated. It was determined that average impact force, Favg, and impact velocity, are proportional to one another as Favg∝v2 regardless of pile embedment depth. This relationship provides evidence for the existence of inertial or hydrodynamic drag forces in the pile-soil system during lateral impacts similar to a solid object passing through a fluid.