Stress Required for Constrained Sintering of a Ceramic-Filled Glass Composite

The stress required for complete constrained sintering of a low-temperature cofirable ceramic-filled glass composite, i.e., borosilicate glass + alumina, has been investigated under uniaxial constant and cyclic loading. The required uniaxial stress to have complete constrained sintering, i.e., zero strain or strain rate in the perpendicular directions, is in the range of 20–250 kPa, which initially increases with temperature, reaches a maximum at 725°–800°C, and then decreases with increasing temperature. The above measured data exhibit a similar trend to those of required stress and sintering potential calculated using the viscous analogy for the constitutive relationship of a porous sintering compact.     

Effect of Crystallization on the Stress Required for Constrained Sintering of CaO–B2O3–SiO2 Glass–Ceramics

The effect of crystallization on the stress required for complete constrained sintering of a low-temperature cofirable CaO–B₂O₃–SiO₂ glass–ceramics has been investigated under uniaxial constant and cyclic loading. As the formation of crystalline phase of wollastonite (CaSiO₃) increases, the uniaxial viscosity of the porous glass–ceramics during firing increases. Moreover, the required uniaxial stress to have complete constrained sintering, i.e., zero strain or strain rate at the perpendicular directions, is in the range of 80–180 kPa, which shows no significant dependence on heating rate and firing temperature. The measured data of required stress are close to those calculated using the viscous analogy for the constitutive relationship of a porous sintering compact.     

Shrinkage-Free Sintering of Low-Temperature Cofired Ceramics by Loading Dilatometry

Sintering with zero radial shrinkage of low-temperature cofired ceramics (LTCC) was investigated by means of loading dilatometry. The results indicate that constant compressive loads tend to produce either decreased or increased radial shrinkages, and this can vary as a function of density, depending on the magnitude of the applied stresses. Coupling the real-time applied uniaxial load to the radial strain of cylindrical samples afforded a sintering scheme where the radial strain was precisely kept near zero (±0.02%). This approach possibly perfects an existing hot-pressing technique for zero-shrinkage sintering technology of LTCC materials.     

树脂基三维机织复合材料拉伸失效模式研究

树脂基三维机织复合材料试件在拉伸过程中,较多试样出现两个断口,其中一个断口出现在夹持过渡段,且呈明显的压缩失效特征。采用显式有限元计算及应力波理论分析方法,对试件断裂后的瞬态位移及应变响应开展了研究,结果显示拉伸断裂后应力波在远端夹持端引起的压缩应变大于静态压缩失效应变,从而导致在夹持端发生二次失效。该结论对采用ASTM D3039开展三维机织复合材料拉伸试验时的有效破坏模式提出了修正建议。     

Thermal and strain rate sensitive compressive behavior of polycarbonate polymer - experimental and constitutive analysis

The mechanical behavior of polycarbonate (PC) polymer was investigated under the effect of various temperatures and strain rates. Characterization of polymer was carried out through uniaxial compression tests and split Hopkinson pressure bar (SHPB) dynamic tests for low and high strain rates respectively. The experiments were performed for strain rates varying from 10 ^?3 to 10³ and temperature range of 213 to 393 K. By conducting these experiments, the true stress–strain (SS) curves were obtained at different temperatures and strain rates. The results from experiments reveal that the stress–strain behavior of polycarbonates is different at lower and higher strain rates. At higher strain rate, the polymer yields at higher yield stress compared to that at low strain rate. At lower strain rate, the yield stress of the polymer increases with the increase in strain rate while it decreases significantly with the increase of temperature. Likewise, initial elastic modulus, yield and flow stress increase with the increase in strain rate while decreases with the increase in temperature. The yield stress increases significantly for low temperature and higher strain rates. On the basis of experimental findings, a phenomenological constitutive model was employed to capture the mechanical behavior of polymer under temperature and loading rate variations. The model predicted the yield stress of polymer at varying strain rate and temperature also it successfully predicted the compressive behavior of polymer under entire range of deformation.     

Strain Dependence of the Uniaxial Compression Response of Vegetable Shortening

Many soft food materials, including vegetable shortening, exhibit complex rheological behavior. For shortening, a precise determination of rheological behavior is necessary to understand its functionality as a food ingredient. Commercial vegetable shortening was subjected to monotonic and cyclic uniaxial compression tests at a wide range of loading rates. The elastic modulus determined from unloading was a function of strain, varying between 740 kPa in the shortening’s strain hardening region to 220 kPa at large strain where perfect plasticity had developed. Visual analysis of shortening specimens during the compression process showed that a rate-dependent stress overshoot was attributable to the development of a shear band following strain hardening. An elastoviscoplastic constitutive model was developed to define the complex rate-dependent compression response of vegetable shortening. Using the fundamental parameters obtained from the different types of compression tests, the proposed model accurately predicted the uniaxial compression response of vegetable shortening over a wide range (three decades) of compression rates. A model with predictive capabilities of large strain properties is desirable because shortening is subject to large strain in essentially all applications.     

Asymmetric tensile and compressive creep deformation of hot-isostatically-pressed Y2O3-doped -Si3N4

The uniaxial tensile and compressive creep rates of an yttria-containing hot-isostatically-pressed silicon nitride were examined at several temperatures between 1316 and 1399°C and found to have different stress dependencies. Minimum creep rates were always faster in tension than compression for an equal magnitude of stress. An empirical model was formulated which represented the minimum creep rate as a function of temperature for both tensile and compressive stresses. The model also depicted the asymmetric creep deformation using exponential and linear dependence on tensile and compressive stress, respectively. Unlike other models which represent either tensile or compressive creep deformation as a respective function of tensile or compressive stress, the model in the present study predicted creep deformation rate for both tensile and compressive stresses without conditional or a priori knowledge of the sign of stress. A statistical weight function was introduced to improve the correlation of the model’s regressed fit to the experimental data. Post-testing TEM microstructural analysis revealed that differences in the amount of tensile- and compressive-stress-induced cavitation accounted for the creep strain asymmetry between them, and that cavitation initiated in tensile and compressively crept specimens for magnitudes of creep strain in excess of 0·1%.     

Effects of uniaxial prestress on the ferroelectric hysteretic response of soft PZT

This paper deals with the influence of preload stress on the ferroelectric hysteretic behavior of piezoelectric ceramics. The polarization and strain versus electric field hysteresis loops were measured for soft lead zirconate titanate (PZT) piezoceramic material under various uniaxial compressive stress preloads of up to −400 MPa. The investigation revealed that the superimposed compression load reduced the remnant polarization, decreased the coercive field, and also had a significant impact on the dielectric and piezoelectric properties. With increasing mechanical load, dielectric hysteresis and butterfly hysteresis became less and less pronounced, as the compressive stress prevented full alignment of the domains and induced mechanical depolarization. The slopes of the polarization and strain curves at zero electric field were measured to evaluate the dependence of permittivity and piezoelectric coefficients on the prestress. The experimental results were interpreted in terms of the non-180° domain switching process under combined electromechanical loading.     

Constitutive modeling and simulation of energy absorbing polyurethane foam under impact loading

The compressive-stress strain response of polyurethane foam under uniaxial compressive impact loading has been studied. The development of a uniaxial constitutive model from strain rate controlled compression tests is detailed. Density and temperature functions have been added to the integral power model proposed by Schwaber, Meincke, and Nagy. The model assumes that the effects of density, temperature, strain and strain rate on stress are separable functions. The model correlated well with actual static compression tests and was used successfully to predict the impact response of energy absorbing polyurethane foam under uniaxial compressive loading.     

Sinter-Forging Characteristics of fine-Grained Zirconia

Powder preforms of zirconia, containing 2.85 mol% yttria, were sinter-forged in simple uniaxial compression at 1400°C by applying constant displacement rates to the specimens. Shear and densification strains and the uniaxial stress were measured as a function of time. In contrast with alumina and silicon nitride, zirconia appears to densify by a dislocation mechanism. As a consequence, the densification rate is linked to the applied strain rather than to the applied hydrostatic pressure; the powder compact requires a critical amount of compressive strain to consolidate to full density, irrespective of the strain rate or the stress at which that strain is applied.     

Measuring Hydro-Static Dependent Constitutive Behaviour of Adhesives Using a Bend Specimen

A simple bend test and associated equations that determine, simultaneously, both the tensile and compressive uniaxial stress-strain behaviour of a bulk adhesive are presented. From this, the ratio of the flow stress in compression to tension (S) can be found. Such data are required if a meaningful elastoplastic stress analysis of an adhesive joint is to be carried out. Experimental results for both the tensile and compressive stress-strain data are obtained for an epoxy specimen using this technique. The tensile data are compared with the results from uniaxial tensile tests on flat dumbell specimens of the same material and the good correlation that is found indicates that this is a reliable technique. Values of the ratio (S) for this adhesive are calculated as a function of both strain hardening and work hardening parameters. These results indicate that this technique complements standard test techniques and provides an elegant method not only of obtaining the ratio (S) but also of deriving uniaxial stress-strain curves.     

Electromechanical properties of CaZrO3 modified (K,Na)NbO3‐based lead‐free piezoceramics under uniaxial stress conditions

Piezoelectric actuators are typically preloaded with a modest mechanical compressive stress during actuation to reduce cracking and allow for operation in the dynamic range. In addition, actuators are required to carry out mechanical work during operation, resulting in a nonlinear relationship between stress and actuation voltage. In fact, mechanical loading can significantly impact the electromechanical performance of lead‐free piezoelectrics. Herein, we report the dependence of electromechanical properties of CaZrO₃ modified (K,Na)NbO₃‐based lead‐free piezoceramics on uniaxial compressive stress, comparing to their lead‐based counterparts. It is demonstrated that increased non‐180° domain switching enhances the strain output at a moderate stress of approximately ?50 MPa from room temperature to 150°C. Larger uniaxial stress, however, is found to suppress ferroelectric domain switching, resulting in the continuous strain and polarization decrease.     

Physical and mechanical properties of soy protein-based plastic foams

Flexible plastic foams using soy protein isolate (SPI), soy protein concentrate (SPC), and defatted soy flour (DFS) were produced by interacting proteins with glycerol-propylene oxide polyether triol (polyol), surfactant, triethanolamine (crosslinking agents), tertiary amine (catalyst), and water (blowing agent). The density, compressive stress, resilience, and dimensional stability of foams with SPI, SPC, and DFS increased as the initial concentration of soy protein increased. The foam density increased with increasing weight percentage of SPI, SPC, and DFS. The resilience values of SPI containing foam increased with the increasing addition of SPI up to a maximum 30% SPI addition. An increase in SPI up to 20% caused an increase in the compressive stress (225 kPa) in comparison to control polyurethane foam (187 kPa). The control foam and foam containing 20% DFS had a similar load-deformation relationship. The foam containing 20% SPI and SPC also exhibited a similar shape, but with a higher compressive stress. The compressive stress of all foams was steeply increased after 55% strain, since the foams completely collapsed upon compression.     

Strain rate mediated microstructure evolution for extruded poly(vinylidene fluoride) polymer films under uniaxial tension

The deformation and fracture mechanism during uniaxial tension under controlled strain rates are investigated for extruded poly(vinylidene fluoride) (PVDF) polymer films at room temperature. It was found that both the longitudinal and transversal film‐samples exhibited pronounced strain rate effect, that is, the yield stress increases while the fracture strain decreases with the increasing of strain rates. For the longitudinal film samples, phase transformation from the nonpolar α‐phase to the polar β‐phase occurs during the uniaxial tension, and the extent of the phase transformation enhances when the strain rate decreases. For the transversal film samples, no phase transformation was detected in all tested strain rates. By combining the stress–strain behavior and the X‐ray results, it can be inferred that the conformational change from α to β phase during uniaxial tension contributes to the higher fracture strain of the longitudinal films than that of the transversal films. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1786–1790, 2007     

Stress Required to Densify a Low-Fire NiCuZn Ferrite Under Constrained Sintering

The effect of uniaxial stress on the densification behavior of a low-fire NiCuZn ferrite during constrained sintering of a multilayer structure of a ferrite tape and a pure alumina tape has been investigated. Compared with free sintering, the densification of ferrite becomes significantly reduced and slowed down under pressure-less constrained sintering. To enhance the constrained densification of ferrite in the temperature range required for free sintering, uniaxial stress applied in the thickness direction is needed. The required uniaxial stress to densify ferrite under constrained sintering to reach a relative sintered density of >95% decreases from 1100–1300 kPa at 900°C to 250–450 kPa at 1000°C. Moreover, no significant grain growth is found when the ferrite is densified under pressure-assisted constrained sintering.     

Strain‐rate and temperature effects on the deformation of polypropylene and its simulation under monotonic compression and bending

Monotonic compressive loading and bending tests are conducted for solid polypropylene (PP) under constant or time‐varying strain‐rates and temperatures of 10, 25, 40°C. The observed compressive stress‐strain responses under constant conditions have revealed that the inelastic deformation behavior is remarkably dependent on loading rates and temperatures of normal use. The examination of such inelastic behavior has indicated that the strain‐rate effects correspond with the temperature effects based on the concept of time‐temperature equivalence. The viscoplastic constitutive theory based on overstress (VBO) has successfully reproduced the experimental responses with stress‐jumping phenomena using the equivalent time. Four‐point bending tests are performed under monotonic loading and holding for PP beams at three different temperatures. The observed deformation behavior has shown that the Bernoulli‐Euler hypothesis is valid. The VBO model and beam bending theory has generated the basic equations for PP beams, showing an analogy with the uniaxial one. In the numerical analysis, the equations are transformed into nonlinear ordinary differential equations with use of Gaussian quadrature for the spatial integrals. The comparison of numerical and experimental results has suggested some modifications for actually loaded moment taking the effect of deflection and friction into consideration. Finally, the numerical calculation has simulated the experimental time‐histories of curvatures fairly well.     

A new test method to obtain biaxial tensile behaviors of solid propellant at high strain rates

A new test method was proposed and applied for studying the biaxial tensile behaviors of hydroxyl-terminated polybutadiene (HTPB) propellant at high strain rates. The biaxial tensile stress responses of the propellant at room temperature and at different strain rates (0.40–85.71 s^?1) were obtained through the use of biaxial tensile strip samples, a new designed aluminum apparatus and a uniaxial Instron testing machine. A high-speed camera and scanning electron microscop (SEM) were employed to observe the biaxial tensile deformation and the damage of HTPB propellant under the test conditions. The results indicated that strain rate could remarkably influence the biaxial tensile behaviors of HTPB propellant. The effect of strain rate on the characteristics of stress–strain curves, mechanical properties and fracture mechanisms was consistent with that in uniaxial tension. However, the biaxial weakening of HTPB propellant was obvious. The strain at biaxial maximum tensile stress was between 10 and 30 % lower than that at the corresponding uniaxial case. Finally, the correlations between the fracture mechanisms and the mechanical properties of HTPB propellant, stress state and the damage of HTPB propellant were discussed. The damage of the propellant under the biaxial tensile test was less serious than that under uniaxial tension at the same strain rate. In addition, continuously increasing strain rate could change the fracture mechanism of the propellant under the biaxial and uniaxial tensile tests. In this investigation, the dominating fracture mechanism of HTPB propellant changed from the dewetting and matrix tearing at lower strain rate to the particles fracture at higher strain rate.     

Sphere flattening with application to the moderate plastic compaction of regular sphere assemblages

The problem of flattening a rigid/perfectly plastic sphere under a uniaxial compressive loading is solved. This solution gives approximate relations between contact force and contact area analogous to the Hertz classical solution for an elastic sphere. It is used as the basis for studying the moderate plastic compaction of arrays of spheres. The simple cubic and hexagonal close-packed arrangements, representing the extremes of initial density, are treated for various deformation conditions. Pressure—porosity expressions are derived and are used in several comparisons with available experimental data. The theory shows, as expected, that the work required to achieve a given compressive strain is less in the simple cubic arrangement than in the hexagonal close-packed arrangement, and less for uniaxial compression than for biaxial or triaxial compression.     

Compressive behavior of C/SiC composites over a wide range of strain rates and temperatures

The mechanical behavior of two-dimensional (2D) carbon fiber reinforced silicon carbide (C/SiC) composites is investigated at both quasi-static and dynamic uniaxial compression under temperatures ranging from 293 to 1273 K. Experimental results show that temperature and strain rate dramatically affect the compressive behavior of 2D C/SiC composites. If the temperature is below 873 K, the compressive strength increases with rising temperature. The reason is that the release of thermal residual stress enhances the compressive strength and this enhancement is more significant than the strength degradation due to the high temperature induced oxidation. In contrast, when the temperature rises above 873 K, the compressive strength decreases as temperature rises due to the stronger effect of oxidation induced strength degradation. Moreover, the degradation of compressive strength at strain rate of 10⁻⁴/s and temperatures above 873 K is much more obvious than those at higher strain rates, and the strain rate sensitivity factor of compressive strength increases remarkably at temperature above 873 K. Post-deformation observation shows that failure angles and fracture surfaces are also strongly dependent on testing temperature and strain rate. The change of interfacial strength at high strain rate or high temperature is responsible for the variations.     

Visco-elasto-plastic constitutive model of adhesives under uniaxial compression in a range of strain rates

Determining the constitutive model of adhesives enables the prediction of the mechanical response of hybrid structures in which the adhesive is used. In this study, the stress–strain behavior of a two-component structural adhesive was first measured in uniaxial compression experiments at the strain rates ranging from 0.001 to 2600 s⁻¹ for developing a constitutive model. It was found that the compressive response, including elastic modulus, yield strength, and plastic flow stress, is substantially influenced by the strain rate. In plastic deformation, the strain rate sensitivity is not constant but varies with the rate; moreover, strain softening and hardening dominate the plastic deformation at low and high strains, respectively. A visco-elasto-plastic constitutive model was then proposed for the adhesive, integrating the strain rate sensitivity that was quantified by empirical equations. The model which was validated can reliably represent the strain rate dependent compressive behavior of the adhesive. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48962.