Creep Deformation of Fiber Reinforced Plastics-Plated Reinforced Concrete Tensile Members

The problem of long-term creep deformation of reinforced concrete tensile elements strengthened by external fiber reinforced plastic (FRP) plates is studied. Formation of discrete cracks in concrete under tension is taken into account. A kinematic model is used, where relative slips between concrete, steel bars, and FRP plates are considered, governed by viscous interface shear stress–slip laws. Bazant’ solidification theory and exponential algorithm are used to obtain incremental constitutive equations for concrete as well as for steel-concrete and FRP-concrete interface laws. Moreover, cohesive normal stresses across transverse cracks in concrete are considered. The incremental differential system of equations is transformed into a nonlinear algebraic system by a finite difference discretization with respect to axial coordinate. Several numerical examples are presented, concerning both short-term and long-term loadings. It is shown that reinforcing by means of FRP plates or sheets has significant beneficial effects on the behavior of reinforced concrete elements under service loadings because (1) it increases concrete tension stiffening effect and (2) it strongly reduces crack width. The present study shows that these beneficial effects are preserved also in the case of long-term loadings.     

The effects of triaxial stress on void growth and yield equations of power-hardening porous materials

In engineering applications, especially for ductile fracture of materials, nucleation, growth and coalescence of voids have often been observed. Currently there is an increase in interest for the effects of voids on the behaviour of engineering materials. In this paper, by the method of combining micro- and macro-parameters, the effects of triaxial stress on the rates of void growth and yield equations are presented for porous materials with power-hardening. The relations between triaxial stress and the rates of void growth for different n-values and yield equations with different n-values and void volume fractions are discussed. Following results have been obtained: For a porous material with power-hardening, the yield equation can be approximately expressed by an elliptical equation in equivalent stress and triaxial stress. Both the long half-axis and the short half-axis of the elliptical equation are functions of the void volume fraction for a given hardening exponent. The triaxial stress has a strong effect on the growth rates of voids. For linear hardening materials, the relation between the growth rate of voids and the triaxial stress is linear. For elastic/perfectly plastic materials with a small void volume fraction, the growth rate of voids can be described in relation to the triaxial stress with an exponential function. The results from this paper are compared with theoretical results from other researchers for elastic/perfectly plastic materials. A good agreement is shown.     

Pressure-Level Dependency and Densification Behavior of Sand Through Generalized Plasticity Model

Natural soil deposits and man-made earth structures exhibit complicated engineering behavior that is influenced by factors such as the stress level and drainage conditions. The stress conditions within a soil structure vary greatly, ranging from very low to very high values, due to the dead weight, loading and boundary conditions. Saturated sand deposits that exhibit drained conditions under static loading become undrained when subject to earthquake excitations. The Pastor–Zienkiewicz–Chan model has demonstrated considerable success in describing the inelastic behavior of soils under isotropic monotonic and cyclic loadings, including liquefaction and cyclic mobility. This study proposed modifications to the Pastor–Zienkiewicz–Chan model so that effects of stress level and densification behavior are simulated. The proposed model suggested that the angle of internal friction, elastic and plastic moduli are dependent on the pressure levels. Relevant modifications were made to incorporate a power term of mean effective stress on the loading plastic modulus so that a stress-level dependent volume change is obtained in combination with the stress-dilatancy relationship. To better simulate cyclic loading with reference to densification behavior, an exponential term of plastic volumetric strain is included for the unloading and reloading plastic moduli. A total of 11 parameters are needed for monotonic loading, whereas 15 parameters are needed in describing the cyclic behavior. The model simulations were compared with undrained and drained triaxial test results of several kinds of sand under dense and loose states. The predictive capability for monotonic and cyclic loading conditions was also demonstrated.     

Particle reinforcement of ductile matrices against plastic flow and creep

A theoretical investigation is made of the role of non-deforming particles in reinforcing ductile matrix materials against plastic flow and creep. The study is carried out within the framework of continuum plasticity theory using cell models to implement most of the calculations. Systematic results are given for the influence of particle volume fraction and shape on the overall behavior of composites with uniformly distributed, aligned reinforcement. The stress-strain behavior of the matrix material is characterized by elastic-perfectly plastic behavior or by power-law hardening behavior of the Ramberg-Osgood type. A relatively simple connection is noted between the asymptotic reference stress for the composite with the power-law hardening matrix and the limit flow stress of the corresponding composite with the elastic-perfectly plastic matrix. The asymptotic reference stress for the composite with the power-law matrix is applicable to steady-state creep. A limited study is reported on the overall limit flow stress for composites with randomly orientated disc-like or needle-like particles when the particles are arranged in a packet-like morphology.     

Multiscale Analysis of Multiple Damage Mechanisms Coupled with Inelastic Behavior of Composite Materials

Thermodynamically consistent constitutive equations are derived here in order to investigate size effects on the strength of composite, strain, and damage localization effects on the macroscopic response of the composite, and statistical inhomogeneity of the evolution-related damage variables associated with the representative volume element. This approach is based on a gradient-dependent theory of plasticity and damage over multiple scales that incorporates mesoscale interstate variables and their higher order gradients at both the macro- and mesoscales. This theory provides the bridging of length scales. The interaction of the length scales is a paramount factor in understanding and controlling material defects such as dislocation, voids, and cracks at the mesoscale and interpreting them at the macroscale. The behavior of these defects is captured not only individually, but also the interaction between them and their ability to create spatiotemporal patterns under different loading conditions. The proposed work introduces gradients at both the meso- and macroscales. The combined coupled concept of introducing gradients at the mesoscale and the macroscale enables one to address two issues simultaneously. The mesoscale gradients allow one to address issues such as lack of statistical homogeneous state variables at the macroscale level such as debonding of fibers in composite materials, cracks, voids, and so forth. On the other hand, the macroscale gradients allow one to address nonlocal behavior of materials and interpret the collective behavior of defects such as dislocations and cracks. The capability of the proposed model is to properly simulate the size-dependent behavior of the materials together with the localization problem. Consequently, the boundary-value problem of a standard continuum model remains well-posed even in the softening regime. The enhanced gradient continuum results in additional partial differential equations that are satisfied in a weak form. Additional nodal degrees of freedom are introduced that leads to a modified finite-element formulation. The governing equations can be linearized consistently and solved within the incremental iterative Newton-Raphson solution procedure.     

Mechanical Stabilization of Cemented Soil–Fly Ash Mixtures with Recycled Plastic Strips

An experimental investigation was undertaken to evaluate the mechanical behavior of a soil–cement–fly ash composite, reinforced with recycled plastic strips (high-density polyethylene) that were obtained from postconsumer milk and water containers. The primary motivation for the study was to investigate the innovative reuse of several candidate waste materials in geotechnical and pavement applications. The specific objectives of the research were: (1) to evaluate the compressive, split tensile, and flexural strength characteristics of the material, and (2) to determine the effectiveness of recycled plastic strips in enhancing the toughness characteristics of the composite. Since cement-stabilized materials are weak in tension, the main focus of the experimental program was to conduct a series of specially instrumented split tensile and flexural tests on mixes containing various amounts of cement, fly ash, and plastic strips. For a meaningful comparison of test results, all specimens were prepared at a constant dry density. The standard ASTM C496 procedure for split tensile test was slightly modified by attaching two horizontal linear variable differential transformers (LVDTs) to measure the diametral deformation of the specimen due to compressive loading in an orthogonal direction. This modification enabled the evaluation of the postpeak toughness behavior of the composite. For some specimens, a strain gauge was attached to the middle of the face perpendicular to the loading plane in order to correlate the results with the one found using the LVDTs. All tests were performed with a 90 kN universal testing machine with deformation control. Experimental data show that the soil–cement matrix stabilized with 4% to 10% by weight of fly ash and reinforced with 0.25% to 0.5% (by weight) plastic strips (having lengths of 19 mm or 38 mm) can achieve a maximum compressive strength of 7000 kPa, a split tensile strength of 1000 kPa, and a flexural strength of 1200 kPa. These ranges in strength values are suitable for a high-quality stabilized base course for a highway pavement. To quantify the reinforcing effects in the postpeak region, a dimensionless toughness index is proposed. It is found that the use of fiber reinforcement significantly increases the postpeak load carrying capacity of the mix and thus the fracture energy. It is concluded that the lean cementitious mix containing recycled materials offer a lot of promise as an alternative material for civil engineering construction.     

Modelling and Numerical Simulation of Ductile Damage in Bulk Metal Forming

A complete set of fully coupled constitutive equations accounting for both combined isotropic and kinematic hardening as well as the ductile damage under anisothermal conditions at finite (visco)plastic strain is developed and implemented into the general purpose Finite Element code for metal forming simulation. First, the fully coupled anisotropic constitutive equations in the framework of Continuum Damage Mechanics are presented. Attention is paid to the strong coupling between the main thermomechanical fields as thermal effects, elasto‐viscoplasticity, mixed hardening, ductile isotropic damage and contact with friction. The associated numerical aspects concerning both the local integration of the coupled constitutive equations as well as the (global) equilibrium integration schemes are presented. The local integration is outlined thanks to the Newton iterative scheme applied to a reduced system of two differential equations. For the global resolution of the equilibrium problem, the classical dynamic explicit (DE) scheme with an adaptive time step control is used. A fully adaptive 2D methodology with mesh and loading sequences adaptation based on some appropriate error estimates is used. For 3D simulations only a constant appropriately refined 3D mesh is used. Various 2D and 3D examples are given in order to show the capability of the methodology to predict the ductile damage initiation and growth during metal forming processes.     

Development and Validation of a Two-Phase Model for Reinforced Soil by Considering Nonlinear Behavior of Matrix

The paper presents the formulation of a two-phase system applied for reinforced soil media, which accounts for nonlinear behavior of matrix phase. In a two-phase material, the soil and inclusion are treated as two individual continuous media called matrix and reinforcement phases, respectively. The proposed algorithm is aimed to analyze the behavior of reinforced soil structures under operational condition focusing on geosynthetics-reinforced-soil (GRS) walls. The global behavior of such deformable structures is highly dependent to the soil behavior. By accounting for mechanical characteristics of the soil in GRS walls, a relatively simple soil model is introduced. The soil model is formulated in bounding surface plasticity framework. The inclusion is regarded as a tensile two-dimensional element, which owns a linear elastic-perfectly plastic behavior. Perfect bonding between phases is assumed in the algorithm. For validation of the proposed model, the behavior of several single element reinforced soil samples, containing horizontal and inclined inclusions, is simulated and the results are compared with experiment. It is shown that the model is accurately capable of predicting the behavior especially before peak shear strength. The proposed algorithm is then implemented in a numerical code and the behavior of a full-scale reinforced soil wall is simulated. The results of analysis are also reasonably well compared with those of experiment.     

Lateral Resistance of Short Single Piles and Pile Groups Located Near Slopes

Many transmission towers, high-rise buildings, and bridges are constructed near steep slopes and are supported by large-diameter piles. These structures may be subjected to large lateral loads, such as violent winds and earthquakes. Widely used types of foundations for these structures are pier foundations, which have large diameter with high stiffness. The behavior of a pier foundation subjected to lateral loads is similar to that of a short rigid pile, because both elements seem to fail by rotation developing passive resistance on opposite faces above and below the rotation point, unlike the behavior of a long flexible pile. This paper describes the results of several numerical studies performed with a three-dimensional finite-element method (FEM) of model tests and a prototype test of a laterally loaded short pile and pier foundation located near slopes, respectively. Initially, in this paper, the results of model tests of single piles and pile groups subjected to lateral loading, in homogeneous sand with 30° slopes and horizontal ground were analyzed by the three- dimensional (3D) finite-element (FE) analyses. Furthermore, field tests of a prototype pier foundation subjected to lateral loading on a 30° slope was reported. The FE analyses were conducted to simulate these results. The main purpose of this paper is the validation of the 3D elasto–plastic FEM by comparisons with the experimental data.     

Damage Theory Based on Composite Mechanics

A new theory of composite damage mechanics is developed. A material with damage is considered as a composite comprised of two different phases (called matrix and inclusion). Both phases are linearly elastic isotropic materials. The matrix is considered as the intact material, and the inclusion is the damaged material. Three different composite models, Voigt (parallel), Reuss (serial), and generalized self-consistent (spherical), are introduced for three types of damage distributions. These composite models are usually used for initial tangential modulus of a composite material, here we use them for secant modulus of a distressed material. Since the parallel and the serial models represent the upper and lower bounds for stiffness of materials, the composite damage theory obtains the upper and lower bounds for postpeak stress and the level of damage for the material beyond the elastic limit. The spherical model is in between the two bounds. Depending on the “elastic limit” of the inclusion, the theory can be used to describe elastic perfectly plastic behavior, strain hardening, and strain softening. Two different degradations, the linear and exponential degradations of the stress–strain response curve are introduced. The two degradation models are used in two different failure surfaces, i.e., Tresca and Mohr–Coulomb failure surfaces, to predict the postpeak behavior of distressed material.     

On crack closure of precipitation hardened steels in aqueous solution

Fatigue crack propagation tests were carried out in air and in a 3.5 pct NaCl aqueous solution under cathodic potential of −0.85 V (Ag/AgCl) for aged-hardened high strength steel (Ni−Al−Cr−Mo−C steel). the emphasis in the study was placed on the crack closure behavior of age-hardened materials in air and in the NaCl aqueous solution. The degree of crack closure in air was dependent on the behavior of plastic deformation such as inhomogeneous or homogeneous slip under mixed modes I and II. The underaged material containing coherent precipitates with the matrix had a higher crack opening load in air, compared with the overaged steel containing incoherent precipitates with the matrix. The degrec of crack closure of the underaged material in the NaCl aqueous solution was lower than that in air and was similar to that of overaged materials in the NaCl aqueous solution. It was shown that the decreased crack closure level for the underaged material resulted from accelerated fatigue crack growth under mode I due to hydrogen embrittlement in the aqueous solution.     

Modeling the Thermal Fields of Deposited Materials During the Spray Rolling Process

The technology of spray rolling can be applied to manufacture strips with a uniform cooling rate and a high production rate. The cooling behavior of the spray-rolled material prior to rolling contact was studied using mathematical models, tracing the accumulation of multi-layers with respect to time. Thermal history, elastic–plastic, and friction behavior of the material were considered in the complicated rolling process. The developed model had a good agreement with experimental results with potential to be utilized for prediction of the spray-rolled material thermal profile. Results show that the temperature of deposited materials prior to/or during rolling and the total equivalent plastic strain distribution in the deformation zone of deposited materials during rolling increase with increasing roller preheating temperature, the initial droplet temperature, and the mass flux distribution of the spray cone. Moreover, the deposit thickness and enthalpy remaining in the deposit are found to be the dominant influencing factors on the thermal field of deposited materials during the spray rolling process.     

Deformation and fracture of miniature tensile bars with resistance-spot-weld microstructures

Plastic deformation of miniature tensile bars generated from dual-phase steel weld microstructures (i.e., fusion zone, heat-affected zone, and base material) was investigated up to final rupture failure. Uniaxial tensile true stress-strain curves beyond diffuse necking were obtained with a novel strain-mapping technique based on digital image correlation (DIC). Key microstructural features (including defects) in each of these three metallurgical zones were examined to explore the material influence on the plastic deformation and failure behavior. For weld fusion zones with minimal defects, diffuse necking was found to begin at 6 pct strain and continue up to 55 to 80 pct strain. The flow stresses of the weld fusion zones were at least twice those of the base material, and fracture strains exceeded 100 pct for both materials. The heat-affected zones exhibited a range of complex deformation behaviors, as expected from their microstructural variety. Only those fusion zones with substantial defects (e.g., shrinkage voids, cracks, and contaminants) failed prematurely by edge cracking, as signaled by their highly irregular strain maps.     

Cyclic creep and cyclic deformation of high-strength spring steels and the evaluation of the sag effect: Part I. Cyclic plastic deformation behavior

The cyclic creep and cyclic plastic deformation behavior of two commercial suspension spring steels of high hardness levels, namely, SAE 9259 and SAE 5160, were studied under different testing conditions of cyclic peak stress and cyclic stress ratio. The experimental results indicate that both the cyclic stress ratio and cyclic peak stress have strong, but complicated, effects on the cyclic creep and cyclic plastic deformation behavior of these materials. It has also been found that the addition of silicon can increase the resistance of these steels to cyclic creep and cyclic plastic deformation, although the extent of this increase is also related to other cyclic deformation conditions. A transition in the relationship between the total plastic strain range and the cyclic stress ratio (R) has been detected at approximately R=0.5. The mechanism of such a transition is explained by the operation of cross-slip during the unloading process of cycling. Moreover, a cyclic softening behavior of these spring steels in the quench-tempered condition was also detected and is attributed to the activation and reorganization of obstacle dislocations introduced into the steels during the process of martensitic transformation. More importantly, this study has indicated that parameters such as the cyclic creep strain, the cyclic creep rate in the secondary creep stage, and the total cyclic plastic strain range can better reflect, and should be used to depict and characterize, the sag behavior of spring steels as well as other materials. Finally, the effect of silicon on sag behavior, in comparison with the results from the Bauschinger-effect test, has also been discussed through the influence of Si on carbide formation and distribution.     

Expressions for predicting the elasticity modulus of materials reinforced by second-phase grains

In the present study, expressions for predicting the elasticity modulus of the materials reinforced by the second-phase grains, which may be referred to as the granular composite for the sake of simplicity, are developed based on the solution of the overmatching problem in elastic mechanics. Taking the voids or defects in materials as the second phase with zero elasticity modulus, one can easily obtain the expressions for predicting the elasticity modulus and the threshold of the elastic percolation failure of materials containing voids. The values of the elasticity modulus of the granular composite and the materials containing random voids and the values of the threshold for the elastic percolation failure of the materials with voids predicted by using the above-mentioned expressions are in good agreement with results given in available literature.     

Hybrid Monte Carlo Simulation of Stress-Induced Texture Evolution with Inelastic Effects

A hybrid Monte Carlo (HMC) approach is employed to quantify the influence of inelastic deformation on the microstructural evolution of polycrystalline materials. This approach couples a time explicit material point method (MPM) for deformation with a calibrated Monte Carlo model for grain boundary motion. A rate-independent crystal plasticity model is implemented to account for localized plastic deformations in polycrystals. The dislocation energy difference between grains provides an additional driving force for texture evolution. This plastic driving force is then brought into a MC paradigm via parametric links between MC and sharp-interface (SI) kinetic models. The MC algorithm is implemented in a parallelized setting using a checkerboard updating scheme. As expected, plastic loading favors texture evolution for grains that have a bigger Schmid factor with respect to the loading direction, and these are the grains most easily removed by grain boundary motion. A macroscopic equation is developed to predict such texture evolution.     

Plastic-containing materials as alternative reductants for base metal production

Shredder residue materials are produced after the removal of ferrous and non-ferrous fractions from end-of-life electronic equipment. Despite the high plastic content and metal value in the ash, high percentages of these materials are currently sent to landfills. In this study, the potential of utilising shredder residue material and other plastic-containing materials as reducing agents was studied. Plastic-containing materials were co-injected with coal into a zinc-fuming furnace in Boliden-Rönnskär smelter. The data obtained from the trial, such as the data from the chemical analysis of the slag and the steam production, are discussed. The observations indicate that plastic-containing material can replace up to 1?ton?h^?1 of coal without a significant decrease in the zinc reduction rate.     

Mechanical behavior of cast particulate SiC/AI (A356) metal matrix composites

Mechanical tests were carried out to study the deformation behavior of particulate SiC-reinforced Al (A356) matrix composites produced through direct casting using the molten metal mixing method. The matrix alloy-Al (A356) was also tested as a control material for comparison. The elastic constant and yield strength of the composite material were found higher than those of the control alloy, but the ultimate tensile strength (UTS) and the ductility were lower. The Tsai-Halpin equation was found applicable for calculating the elastic constant if an average particle aspect ratio could be determined. The strain-hardening behavior of the tested composite material appeared very different from that of the control alloy. The high strain-hardening rate in the early stage of plastic deformation of the composite was rationalized by the interaction between the hard particles and the ductile metal matrix; on the other hand, the low hardening rate recorded from intermediate strain amplitude to fracture was attributed to the early coalescence of voids and other microdamages. Particle-matrix interface debonding, particle cracking, and void for-mation in the metal matrix were considered to be responsible for the low ductility. Deformation asymmetry of the composites was noticed, not only through the Bauschinger effect, but also through the difference in virgin specimens’ yield stresses in tension and compression.     

Bc路径等径角挤压SiC_p/Al基粉末材料的显微组织及力学性能

以SiCp/Al基复合粉末材料为研究对象,在250℃下采用粉末包套-等径角挤压工艺沿Bc路径成功将粉末颗粒直接固结成高致密度的块体细晶材料。结果表明:复合粉末材料成分分布均匀性和致密度在等径角挤压强烈的剪切细化作用下效果显著。初始SiC平均粒径为13.69μm,复合粉末初始相对密度为0.75,经过3个道次等径角挤压后,得到相对密度达0.97接近完全致密,SiC颗粒得到一定程度细化且分布均匀的细晶组织,平均显微硬度高达75HV,约为工业致密纯铝的2.2倍,初始SiC颗粒的尖锐棱角特征也得到明显改善。压缩性能测试表明,挤压后SiCp/Al基复合材料表现出明显优于工业纯铝的变形行为特征。     

High strain rate deformation of Mo and Mo-33re by shock loading: Part II. Rates of defect generation and accumulation of plastic strain

The kinetics of plastic flow in Mo and Mo-33Re under shock loading conditions has been examined by characterizing the substructure quantitatively and relating these observations to defect generation rates and the dynamic strength of the materials. The finite dislocation generation rates, apparently limited to the order of 10 21 m⁻² s⁻¹ result in a maximum plastic strain rate on the order of 10 6 s⁻¹ and a maximum dislocation velocity of ∼3 x 10 2 m s⁻¹. The maximum shear stress imposed on the materials is estimated to be ∼μ/50, at short pulse durations. The estimated shear stresses, dislocation velocities, and production of dislocations as a function of strain are found to be consistent with conventional deformation mechanisms, and require no novel material behavior such as homogeneous nucleation of dislocations or supersonic dislocation velocities.