Mechanical property of strain-hardening cementitious composites modified with superabsorbent polymers

In order to improve the tensile property, flexuralproperty and drying shrinkage of strain-hardening cementitiouscomposites （SHCC）, mixtures quantitatively modified withsuperabsorbent polymer （SAP） were investigated. Theuniaxial tensile test, the four-point bending test, thecompressive test, the drying shrinkage test and theenvironmental scanning electron microscope （ESEM） wereemployed to investigate the tensile strain capacity, flexuraldeformation capacity, compressive strength, drying shrinkage,crack width and self-healing of SHCC. The experimentalresults show that SHCC modified with SAP particles exhibitsexcellent ductility and deformability, and the tensile strain isup to about 4.5% and the average crack width is controlledaround 40 μm. Meanwhile, the drying shrinkage of SHCCmodified with SAP particles can reduce by about 60%.Furthermore, the self-healing behavior is observed in thecracks of specimen after three cycles of high-low relativehumidity curing, and the self-healing products can completelyfill the cracks of SHCC specimens modified with SAPparticles. It is, therefore, feasible to produce SHCC materialmodified with SAP particles, while simultaneously retaininghigher material ductility.

Tensile properties of strain-hardening cementitious composites containing polyvinyl-alcohol fibers hybridized with polypropylene fibers

Partially replacing polyvinyl-alcohol(PVA)fibers with polypropylene(PP)fibers in strain-hardening cementitious composites(fiber hybridization)modify certain mechanical properties of these materials.The hybridization based on the introduction of low-modulus hydrophobic polypropylene fibers improves the ductility and the strain-hardening behavior of the cementitious composites containing polyvinyl-alcohol fibers of different types(PVA-SHCC).Pull-out tests indicate that adding PP fibers increases the energy capacity of the hybrid composite with respect to the material containing only PVA fibers under tensile loading,and PP-fiber geometry(i.e.,section shape and length)is a key factor in enhancing the strain capacity.

Kinematic Shakedown Analysis for Strain-Hardening Plates with the C1 Nodal Natural Element Method

This paper proposes a novel numerical solution approach for the kinematic shakedown analysis of strain-hardening thin plates using the C^(1)nodal natural element method(C^(1)nodal NEM).Based on Koiter’s theorem and the von Mises and two-surface yield criteria,a nonlinear mathematical programming formulation is constructed for the kinematic shakedown analysis of strain-hardening thin plates,and the C^(1)nodal NEM is adopted for discretization.Additionally,König’s theory is used to deal with time integration by treating the generalized plastic strain increment at each load vertex.A direct iterative method is developed to linearize and solve this formulation by modifying the relevant objective function and equality constraints at each iteration.Kinematic shakedown load factors are directly calculated in a monotonically converging manner.Numerical examples validate the accuracy and convergence of the developed method and illustrate the influences of limited and unlimited strain-hardening models on the kinematic shakedown load factors of thin square and circular plates.

Mg-6Zn-1Mn镁合金热压缩流变行为及动态再结晶

采用Gleeble-1500热压缩模拟试验机对Mg-6Zn-1Mn合金进行压缩实验,研究了该合金其在变形温度250～400℃、应变速率0.01～10 s-1范围内的流变应力及动态再结晶行为。通过计算加工硬化速率θ得到合金发生动态再结晶的临界应力σc和临界应变εc,并且建立临界值与峰值应力σp、峰值应变εp之间的定量关系,用截线法测量合金压缩后的平均晶粒尺寸。结果表明:Mg-6Zn-1Mn镁合金在高温下塑性变形的热本构方程为:.ε.exp(22919/T)=2.77.σ8.19;合金发生动态再结晶的临界应变随着应变速率的增加而升高,随变形温度的增加而降低,发生动态再结晶的临界条件为:ε>εc=6.648×10-3Z0.06149;各特征变量之间存在如下关系:σc=0.7295σp、εc=0.2639εp;动态再结晶的平均晶粒尺寸dave随温度的升高、应变速率的减小而增大,与Zener-Hollomon参数之间的关系为:dave=2.11×103.Z-0.1378。

Deformation behavior and dynamic recrystallization of Mg-Y-Nd-Gd-Zr alloy

The characteristics of dynamic recrystallization （DRX） in Mg-Y-Nd-Gd-Zr-RE magnesium alloy were investigated by compression tests at temperatures between 523 and 723 K and at strain rates ranging from 0.002 to 1 s^-1 with maximum strain of 0.693. The strainhardening rate can be obtained from true stress-true strain curves, plots of θ-σ, -（δθ/δσ-）-a and lnθ-σ in different compression conditions were obtained by further study. The critical condition of the onset of DRX process was determined as （（δ/δσ（ δθ/δσ））=0. In the range of the above deformation temperature and strain rate, the ratio of critical stress （σc） to peak stress （σm） and critical strain （εc） to the peak strain （εm） stood at σc/σm=0.62-0.89 and εc/εm=0.11-0.37, respectively. DRX could be observed during hot detormation process, microstructure evolution of the magnesium alloy at different temperatures and strain rates were studied with the aid of optical microscope（OM）, and the average recrystallized grain size was measured by means of intercepts on photomicrographs. It was shown that the average dynamically recrystallized grain size （drew） changed with different deformation parameters, the natural logarithm of the average recrystallized grain size varied linearly with the natural logarithm of Zener-Hollomon parameter; the peak stress changed with the average recrystallized grain size, and the natural logarithm of the average recrystallized grain size varied linearly with the natural logarithm of the peak stress.

Tensile strength prediction of dual-phase Al0.6CoCrFeNi high-entropy alloys

The evolution of the microstructure and tensile properties of dual-phase Al0.6CoCrFeNi high-entropy alloys(HEAs)subjected to cold rolling was investigated.The homogenized Al0.6CoCrFeNi alloys consisted of face-centered-cubic and body-centered-cubic phases,presenting similar mechanical behavior as the as-cast state.The yield and tensile strengths of the alloys could be dramatically enhanced to^1205 MPa and^1318 MPa after 50%rolling reduction,respectively.A power-law relationship was discovered between the strain-hardening exponent and rolling reduction.The tensile strengths of this dual-phase HEA with different cold rolling treatments were predicted,mainly based on the Hollomon relationship,by the strain-hardening exponent,and showed good agreement with the experimental results.

Adiabatic shear localization evolution for steel based on the Johnson-Cook model and gradient-dependent plasticity

Gradient-dependent plasticity is introduced into the phenomenological Johnson-Cook model to study the effects of strainhardening, strain rate sensitivity, thermal-softening, and microstructure. The microstructural effect （interactions and interplay among microstructures） due to heterogeneity of texture plays an important role in the process of development or evolution of an adiabatic shear band with a certain thickness depending on the grain diameter. The distributed plastic shear strain and deformation in the shear band are derived and depend on the critical plastic shear strain corresponding to the peak flow shear stress, the coordinate or position, the internal length parameter, and the average plastic shear strain or the flow shear stress. The critical plastic shear strain, the distributed plastic shear strain, and deformation in the shear band are numerically predicted for a kind of steel deformed at a constant shear strain rate. Beyond the peak shear stress, the local plastic shear strain in the shear band is highly nonuniform and the local plastic shear deformation in the band is highly nonlinear. Shear localization is more apparent with the increase of the average plastic shear strain. The calculated distributions of the local plastic shear strain and deformation agree with the previous numerical and experimental results.

ANALYTICAL EVALUATION OF PERMANENT DEFLECTION OF A THIN CIRCULAR PLATE STRUCK NORMALLY AT ITS CENTER BY A PROJECTILE

The permanent deflection of a thin circular plate struck normally at its center by a projectile is studied by an approximate theoretical analysis, FEM simulation and experiment. The plate made of rate sensitive and strain-hardening material undergoes serious local deformation but is not perforated during the impact. The theoretical analysis is based on an energy approach, in which the Cowper-Symonds equation is used for the consideration of strain rate sensitive effects and the parameters involved are determined with the aid of experimental data. The maximum permanent deflections predicted by the theoretical model are compared with those of FEM simulation and published papers obtained both by theory and experiment, and good agreement is achieved for a wide range of thickness of the plates and initial impact velocities.

Evaluation of coal pillar loads during longwall extraction using the numerical method and its application

It is very difficult to reasonably evaluate the loads acting on coal pillars in longwall panels during the planning of a new pillar system. The application of empirical equations is a common practice in calculating coal pillar loads while designing a new pillar. This paper proposes numerical models for evaluating coal pillar loads. The key of building a successful numerical model for calculating coal pillar loads lies in the fact that the model should represent the redistribution of stress all over the longwall panels and the surrounding areas, and it is especially important to include the characteristics of the stress rebuilding process in the gob areas, which are crucial for the building process of coal pillar loads. Based on the geo-mechanical background of the Baoshan Coal Mine, this paper details the procedures of applying numerical models to the evaluation of coal pillar loads and their local practices. The study results show it is feasible and reasonable to use numerical models to evaluate coal pillar loads.

Dynamic and buckling analysis of a thin elastic-plastic square plate inca uniform temperature field

The nonlinear models of the elastic and elastic-linear strain-hardening square plates with four immovably simply-supported edges are established by employing Hamiltons Variational Principle in a uniform temperature field. The unilateral equilibrium equations satisfied by the plastically buckled equilibria are also established. Dynamics and stability of the elastic and plastic plates are investigated analytically and the buckled equilibria are investigated by employing Galerkin-Ritzs method. The vibration frequencies, the first critical temperature differences of instability or buckling, the elastically buckled equilibria and the extremes depending on the final loading temperature difference of the plastically buckled equillibria of the plate are obtained. The results indicate that the critical buckling value of the plastic plate is lower than its critical instability value and the critical value of its buckled equilibria turning back to the trivial equilibrium are higher than the value. However, three critical values of the elastic plate are equal. The unidirectional snap-through may occur both at the stress-strain boundary of elasticity and plasticity and at the initial stage of unloading of the plastic plate.

A Reverse Numerical Approach to Determine Elastic-plastic Properties of Multi-layer Material Systems with Flat Cylindrical Indenters

In the present study, the indentation testing with a flat cylindrical indenter on typical multi-layer material systems was simulated successfully by finite element method. The emphasis was put on the methods of extracting the yield stresses and strain-hardening modulus of upper and middle-layers of three-layer material systems from the indentation testing. The slope of the indentation depth to the applied indentation stress curve was found to have a turning point, which can be used to determine the yield stress of the upper-layer. Then, a different method was also presented to determine the yield stress of the middle-layer. This method was based on a set of assumed applied indentation stresses which were to be intersected by the experimental results in order to meet the requirement of having the experimental indentation depth. At last, a reverse numerical algorithm was explored to determine the yield stresses of upper and middle-layers simultaneously by using the indentation testing with two different size indenters. This method assumed two ranges of yield stresses to simulate the indentation behavior. The experimental depth behavior was used to intersect the simulated indentation behavior. And the intersection corresponded to the values of yield stresses of upper and middle-layers. This method was also used further to determine the strain-hardening modulus of upper and middle-layers simultaneously.

The mathematic model of tension straightening process of magnesium alloy and experimental validation

The stress-strain curve of bending bar and the stress relax curve of AZ31 was obtained by a tension test using Gleeble-1500.The tension straightening process mainly consisted of the elastic loading-I and unloading stage,the elastic loading-II and unloading stage,and the elastic-plastic loading stage,which were based on the stretch force change during straightening.The circular bar straightening under one-dimensional bending was investigated and assumed to be linear strain-hardening elastic-plastic material.According to the elastic-plastic mechanics theory,the mathematical displacement-force model of a tension straightening process established,on which was based,the predicted displacement of tension straightening for various original deflection was calculated.The tension straightening experiment for AZ31 magnesium was conducted under the guidance of the predicted displacement.The experiment results present good straightness when there is a stress relaxation phenomenon or the temperature of tension straightening is 25℃.

Effect of twin-roll casting parameters on microstructure and mechanical properties of AA5083-H321 sheet

Thepurpose of the present paper is to study the mechanical propertiesand microstructureof the twin-roll cast and cold rolled AA5083 aluminum alloy sheet in strain-hardened H321 temper. To reach this goal, first, a sound surface slab of 8.90 mm thick and 1260 mm wide was cast by a 15°; tilt back twin roll caster at a casting speed of 490 mm/min. After homogenization at 520 ℃, the product was cold rolled to two thicknesses of 6.30 mm and 3.85 mm with an intermediate annealing at 370 ℃ and final stabilization at 180 ℃. Opticalmicroscopyand scanning electron microscopy （SEM） investigations of the as-cast state depicted the segregation of intermetallic particles mainly in grain boundaries which wasthe cause of grain removal observed in the fracture surface of tensile test samples. In addition, mechanical properties indicated an increase in total elongation after homogenization heat treatment dueto the elimination of the grain boundary segregations. Finally, it was observed that the properties of the 3.85 mmthick sheet were consistent with the H321 temper requirements according to ASTM B 290M standard due to applying sufficient cold reduction during cold rolling stage.

Finite element implementation of strain-hardening Drucker-Prager plasticity model with application to tunnel excavation

This paper presents a finite element implementation of a strain-hardening Drucker-Prager model and its application to tunnel excavation.The computational model was constructed based on the return mapping scheme,in which an elastic trial step was first executed,followed by plastic correction involving the Newton-Raphson method to return the predicted state of stresses to the supposed yield surface.By combining the plastic shear hardening rule and stress correction equations,the loading index for the strain-hardening Drucker-Prager model was solved.It is therefore possible to update the stresses,elastic and plastic strains,and slope of the yield locus at the end of each incremental step.As an illustrative example,an integration algorithm was incorporated into ABAQUS through the user subroutine UMAT to solve the tunnel excavation problem in strain-hardening Drucker-Prager rock formations.The obtained numerical results were found to be in excellent agreement with the available analytical solutions,thus indicating the validity and accuracy of the proposed UMAT code,as well as the finite element model.

Strengthening of the concrete face slabs of dams using sprayable strain-hardening fiber-reinforced cementitious composites

In this study,sprayable strain-hardening fiber-reinforced cementitious composites(FRCC)were applied to strengthen the concrete slabs in a concrete-face rockfill dam(CFRD)for the first time.Experimental,numerical,and analytical investigations were carried out to understand the flexural properties of FRCC-layered concrete slabs.It was found that the FRCC layer improved the flexural performance of concrete slabs significantly.The cracking and ultimate loads of a concrete slab with an 80 mm FRCC layer were 132%and 69%higher than those of the unstrengthened concrete slab,respectively.At the maximum crack width of 0.2 mm,the deflection of the 80-mm FRCC strengthened concrete slab was 144%higher than that of the unstrengthened concrete slab.In addition,a FE model and a simplified analytical method were developed for the design and analysis of FRCC-layered concrete slabs.Finally,the test result of FRCC leaching solution indicated that the quality of the water surrounding FRCC satisfied the standard for drinking water.The findings of this study indicate that the sprayable strain-hardening FRCC has a good potential for strengthening hydraulic structures such as CFRDs.

Evaluation of Strain Hardening Parameters

The plane-strain compression test for three kinds of materials was carried out in a temperature range between room temperature and 400℃.Theσ-εcurves and strain-hardening rate at different temperatures were simulated and a reasonable fit to the experimental data was obtained.A modified model created by data inference and computer simulation was developed to describe the strain hardening at a large deformation,and the predicted strain hardening are in a good agreement with that observed in a large range of stress.The influences of different parameters on strain hardening behaviour under large deformation were analysed.The temperature increase within the test temperatures for stainless steel 18/8 Ti results in dropping of flow stress and strain-hardening rate.For favourableγ-fibre texture to obtain high r,the cold rolling was applied at large reduction.In the experimental procedure,the X-ray diffraction test was carried out to compare the strain hardening and microstructure under large deformation for a bcc steel(low carbon steel SS-1142).The results indicate that the high strain-hardening rate possibly occurs when the primary slip plane{110}is parallel to the rolling plane and the strainhardening rate decreases when lots of{110}plane rotate out from the orientation{110}∥RP.

Preparation of Self-compacting Ultra-high Toughness Cementitious Composite

A self-compacting ultra-high toughness cementitious composite （UHTCC） reinforced by discontinuous short polyvinyl alcohol （PVA） fibers, which exhibits self-compacting performance in the fresh state and strain-hardening and multiple cracking behavior in the hardened state, was developed through controlling flow properties of fresh mortar matrix at constant ingredients concentrations determined by micromechanical design and ensuring uniform fibers dispersion. The superplasticizer was utilized to adjust its flow properties in the fresh state. A series of flow tests, including deformability test, flow rate test, and self-placing test, were conducted to characterize and quantify the fluidity performance of fresh mortar matrix and self-compactability of fresh UHTCC. It is revealed that the utilization of superplasticizer is efficient in producing the fresh mortar matrix with desirable fluidity and the resulting self-compacting UHTCC. In addition, results of four point bending tests on the developed self-compacting UHTCC confirm the insensitivity of mechanical performance of self-compacting UHTCC to the presence of external vibrations as well as the flexural characteristics of deformation hardening and multiple cracking.

Dynamic Plastic Analysis of Ship-Platform Collision

This paper studies intensively the problems of ship-platform collision. The ship and platform are treated as one structural system connected with spring elements and then motion equation of the collision system is established. A nonlinear force-displacement relationship is derived for the simulation of local dent in a hit member and the yield surface of a dented tubular section is developed to consider the reduction of load carrying capacity of hit members. Large deformations, plasticity and strain-hardening of the beam-column element are taken into account by combining the elastic large displacement analysis theory with the plastic node method. The effect of the hydrodynamic forces acting on the platform, the rubber fender the property of the local dent and the buckling behavior of beam-column on collision are analyzed. The numerical simulation of the nonlinear dynamic response is carried out by Wilson theta method with updated Newton-Raphson iteration. And the numerical example of the dynamic response of a offshore platform in ship-platform collision is also present.

Analysis of Deformation in a High Entropy Alloy Using an Internal State Variable Model

Deformation in the model high entropy alloy CoCrFeMnNi is assessed using an internal state variable constitutive model. A remarkable property of these alloys is the extraordinarily high strain hardening rates they experience in the plastic region of the stress strain curve. Published stress-strain measurements over a range of temperatures are analyzed. Dislocation obstacle interactions and the observed high rate of strain hardening are characterized in terms of state variables and their evolution. A model that combines a short-range obstacle and a long-range obstacle is shown to match experimental measurements over a wide range of temperatures and grain sizes. The long-range obstacle is thought to represent interactions of dislocations with regions of incomplete mixing or partial segregation. Dynamic strain aging also is observed at higher temperatures. Comparisons with measurements in austenitic stainless steel show some common trends.

Application of the Mechanical Threshold Stress Model to Large Strain Processing

Large-strain deformations introduce several confounding factors that affect the application of the Mechanical Threshold Stress model. These include the decrease with the increasing stress of the normalized activation energy characterizing deformation kinetics, the tendency toward Stage IV hardening at high strains, and the influence of crystallographic texture. Minor additions to the Mechanical Threshold Stress model are introduced to account for variations of the activation energy and the addition of Stage IV hardening. Crystallographic texture cannot be modeled using an isotropic formulation, but some common trends when analyzing predominantly shear deformation followed by uniaxial deformation are described. Comparisons of model predictions with measurements in copper processed using Equal Channel Angular Pressing are described.