A relationship between non-exponential stress relaxation and delayed elasticity in the viscoelastic process in amorphous solids: Illustration on a chalcogenide glass

Inorganic glasses are viscoelastic materials since they exhibit, below as well as above their glass transition temperature, a viscoelastic deformation under stress, which can be decomposed into a sum of an elastic part, an inelastic (or viscous) part and a delayed elastic part. The delayed elastic part is responsible for the non-linear primary creep stage observed during creep tests. During a stress relaxation test, the strain, imposed, is initially fully elastic, but is transformed, as the stress relaxes, into an inelastic and a delayed elastic strains. For linear viscoelastic materials, if the stress relaxation function can be fitted by a stretched exponential function, the evolution of each part of the strain can be predicted using the Boltzmann superposition principle. We develop here the equations of these evolutions, and we illustrate their accuracy by comparing them with experimental evolutions measured on GeSe₉ glass fibers. We illustrate also, by simple equations, the relationship between any kind of relaxation function based on additive contribution of different relaxation processes and the delayed elastic contribution to stress relaxation: the delayed elasticity is directly correlated to the dispersion of relaxations times of the processes involved during relaxation.     

Cell nucleation in solid-state polymeric foams: evidence of a triaxial tensile failure mechanism

The mechanism for nucleation phenomenon in solid-state microcellular foams is identified as a solid-state failure process. This process originates at internal flaws within the gas-polymer matrix, where it is induced by the presence of a state of hydrostatic tensile stress within the polymer matrix. The hydrostatic tensile stress is caused by the presence of the saturating gas within the polymer. The nucleation phenomenon is thermally activated at the effective glass transition temperature of the gas-polymer mixture. At this critical temperature, the hydrostatic tensile stress within the gas-polymer mixture is sufficient to cause the polymer matrix to fail, thereby creating a foam cell nucleus. In general, the nucleation sites are observed to be flat, approximately circular, fracture sites. After the appearance of the initial fracture, gas diffuses from the gas-polymer matrix into the fracture. The fracture seam inflates during the growth process, in which growth begins with the appearance of a disk shaped fracture and concludes with an approximately spherical cell. The results and conclusions presented herein suggest a new avenue to explain the cell nucleation phenomena observed in this process.     

On the effects of thermal history on the development and relaxation of thermo-mechanical stress in cryopreservation

This study investigates the effects of the thermal protocol on the development and relaxation of thermo-mechanical stress in cryopreservation by means of glass formation, also known as vitrification. The cryopreserved medium is modeled as a homogeneous viscoelastic domain, constrained within either a stiff cylindrical container or a highly compliant bag. Annealing effects during the cooling phase of the cryopreservation protocol are analyzed. Results demonstrate that an intermediate temperature-hold period can significantly reduce the maximum tensile stress, thereby decreasing the potential for structural damage. It is also demonstrated that annealing at temperatures close to glass transition significantly weakens the dependency of thermo-mechanical stress on the cooling rate. Furthermore, a slower initial rewarming rate after cryogenic storage may drastically reduce the maximum tensile stress in the material, which supports previous experimental observations on the likelihood of fracture at this stage. This study discusses the dependency of the various stress components on the storage temperature. Finally, it is demonstrated that the stiffness of the container wall can affect the location of maximum stress, with implications on the development of cryopreservation protocols.     

Ductile Phase Toughened Brittle Materials

Toughening of brittle materials by the inclusion of ductile phases is governed by several important factors which include ceramic-ductile phase interfacial bond strength, physical and chemical compatibility between ceramic and ductile phase, geometry and mechanical properties of ductile phase. The present understanding of the effect of these factors on toughening is reviewed and clarified. Continuous ductile phases (network, fibre or plate) are found to be more efficient for the toughening of brittle materials than discrete ductile particles. However, ductile particle toughened brittle materials have the advantages of material homogeneity isotropy and particularly better high temperature properties. It has been demonstrated that the influence of interfacial bond strength is determined by the geometry of the ductile phase in the composites. For the comparatively continuous ductile phase, such as ductile network, fibre or plate, comparatively weak inteffocial bond strength can promote partial debonding of the brittle matrix-ductile phase intedece during crack propagation and is beneficial for toughening. For discrete particulate ductile phase toughened brittle materials, the small gauge length of the ductile phase often results in the ductile phase pull-out during crack propagation which is the main limitation to toughening.Thus strong bond strength is required to ensure the bridging of the crack by the ductile phase.The coefficient of thermal expansion (CTE) mismatch between matrix and ductile phase has also been correlated with the geometry of the ductile phase. In most of the ceramic/metal systems,the CTE of the ductile metal phase is greater than that of the ceramic matrix. In the case of ductile network, fibre or plate, the residual stress created by the CTE mismatch can contribute to toughening through its influence on the initial crack opening stress while the bridging of the crack by the ductile phase is still ensured. However, for discrete ductile particles, the residual stress created by CTE mismatch is liable to cause cracks to by-pass the spherical particles, limiting the efficient use of the inherent toughness of the ductile phase. Low-modulus ductile inclusions are beneficial for the bridging of cracks by the ductile phase. Softer, more ductile inclusions are more effective for the toughening of brittle materials by particulate ductile phase     

A magnesium alloy matrix composite reinforced with metallic glass

Novel light-weight materials of advanced performance are now experiencing global interest due to the strong need to reduce energy consumption in land and air transportation sectors. Here we report on a novel magnesium alloy matrix composite material. The reinforcing phase in the magnesium alloy is a fine dispersion of metallic glass particles. The composite is sintered from the powder mixture of the alloy and metallic glass at a temperature slightly above the glass transition T_g of the metallic glass particles that is close to the Mg alloy’s solidus temperature. At the compaction temperature, the metallic glass acts as a soft liquid-like binder but upon cooling it becomes the hard reinforcement component of the composite. Processing, microstructure and mechanical properties of the composite are discussed.     

Effects of thermal rates on the thermomechanical behaviors of amorphous shape memory polymers

Shape memory polymers (SMPs) are polymers that can recover a large pre-deformed shape in response to environmental stimuli, such as temperature, light, etc. For a thermally triggered (or activated) amorphous SMP, the pre-deformation and recovery of the shape require the temperature of the material to traverse the glass transition temperature T _g under constrained or free conditions. In this paper, effects of thermal rates on the thermomechanical behaviors of amorphous SMPs are investigated. Under uniaxial compression, during a temperature cycle (cooling followed by heating), the stress decreases to zero as the temperature decreases to below the glass transition temperature, and increases to a value larger than the initial stress (termed stress overshoot) as the temperature is raised above the glass transition temperature. These observations are examined by a thermoviscoelasticity model that couples the nonequilibrium structural relaxation and temperature dependent viscoelastic behavior of the material. In addition, using this model, stress-temperature behaviors during temperature cycles with various thermal rate conditions and tensile loading conditions are studied.     

Construction and characterization of a sputter deposition system for coating granular materials

In many modern ceramic or metal matrix composites the interface between the matrix and the reinforcements (particles or fibres) plays an important role. Either no or only weak mechanical bonding is observed or severe reactions between the matrix and the filler during the manufacturing process take place. A method to promote adhesion or to avoid severe reactions is to use coated reinforcements. A uniform film can act as an adhesion promoter, a compliant layer, a reaction inhibitor or a promoter of thermal transport across the interface.The aim of this work was to construct a particle coating system based on magnetron sputter deposition which allows to keep the particles or the granular material in motion during the deposition process to guarantee a homogenous coating on every single particle. As particles to be coated diamond granulates and carbon fibres were investigated. For transparent diamond particles the uniformity of a metallic coating could be evaluated by transmission optical methods and was found to be quite high. Carbon fibres, on the other hand, could only partially be coated due to agglomeration and shadowing effects. The system presented here can be considered as suitable for coating spherical or close to spherical granular matter.     

The recovery properties under load of a shape memory polymer composite material

In many applications, shape memory alloys are being replaced by shape memory polymers as they have some better properties than shape memory alloys. Nevertheless, shape memory alloys can recover under load which shape memory polymers cannot. Shape memory polymers are not capable of giving full recovery even lifting a tiny load. The melting temperature or the glass transition temperature is the transition temperatures to which shape memory polymers are closely heated. Then a deforming force up to a certain position is applied to the heated shape memory polymers. After that shape memory polymer is permitted to cool while keeping it deformed. After the cooling, shape memory polymer obtains the temporary shape which can be recovered by reheating it at the similar transition temperature (glass transition or melting). Consequently, it recovers at its initial state. Shape memory polymer can achieve constrained recovery and unconstrained recovery, nonetheless; under stress, it is partly recovered. In current work, recovery under load has been investigated of an asymmetrical shape memory composite. It is established that it is capable to recover under various loads. Under various loads, it shows full recovery in reference to initial state. The ability to recover under load can be potentially used in diverse applications.     

Micromechanics approach to the indentation hardness of glass matrix particulate composites

A theoretical analysis is made of the indentation hardness of glass matrix, particulate composites. It is hypothesized that glass is an elastic-plastic solid on a microscopic scale. Based upon the Marsh theory of indentation, expressions are formulated for indentation hardness of two-phase composites containing spherical particles. When hard particles are dispersed in a soft glass matrix, the overall hardness depends upon the matrix hardness, the volume-fraction of dispersed phase, the elastic properties of the two phases and also the matrix flow stress. On the other hand, when soft particles are dispersed in a hard glass matrix, the hardness and the elastic moduli vary in parallel with the volume-fraction of dispersed phase. Furthermore, the present analysis indicates that the hardness of a composite is independent of the particle size and interparticle spacing if the volume-fraction of the particles is kept constant. Experimental results of the Vickers hardness of phase-separated glasses as well as published hardness data for a glass-ceramic are used for the verification of the theory. The proposed theory explains well the hardness behaviour of such glass matrix composites in terms of the properties and amounts of the individual phases and the microstructural effects.     

Uniaxial ratchetting of polymer and polymer matrix composites: Time-dependent experimental observations

The uniaxial time-dependent ratchetting of polyester resin and glass fiber reinforced polyester resin matrix composites was observed by the stress-controlled cyclic tension–compression with non-zero tensile mean stress and tension–tension tests at room temperature. After the ratchetting of the polyester resin had been observed by the cyclic tests with different loading conditions including some time-related factors, such as stress rate and peak stress hold, the ratchetting evolutions of the continuous and short glass fiber reinforced resin matrix composites were also investigated by the stress-controlled cyclic tests, respectively. It is concluded that: both the polyester resin and its composites present apparent ratchetting deformation, i.e., the ratchetting strain accumulates progressively in the tensile direction during the cyclic tension–compression with non-zero tensile mean stress and tension–tension tests; the ratchetting depends on the applied stress amplitude, mean stress, stress rate and peak stress hold, and the time-dependent ratchetting is obvious even for the continuous glass fiber reinforced resin matrix composites with high fiber volume fraction (such as 40% and 50%); the time-dependent ratchetting of the polyester resin and its composites mainly stems from the viscosity of the polyester resin, while the addition of glass fiber into the resin matrix improves the resistance of the composites to the ratchetting deformation and lowers the time-dependence of the ratchetting simultaneously.     

Mechanical properties of a lithium glass–ceramic matrix (LZSA) reinforced with TiC or (W,Ti)C particles: A preliminary study

Despite their generally low strength and hardness values, glass–ceramics show good potential to be used in structural applications at room temperature instead of other costlier ceramic materials. This work investigates the effect of dispersed hard carbide particles on the sintering behaviour and the mechanical properties of a lithium glass–ceramic. The glass was mixed with 30 wt.% TiC or (W,Ti)C and hot-pressed at 650 °C (30 MPa, 30 min, Ar). The results obtained compare the properties of the composites with those of the parent glass and demonstrate that the addition of hard particles significantly improves the mechanical strength of the glass–ceramic matrix.     

Stress–strain behaviour and abrupt loss of stiffness of geopolymer at elevated temperatures

This paper reports stress versus strain curves of geopolymer tested while the specimens were kept at elevated temperatures, with the aim to study the fire resistance of geopolymer. Tests were performed at temperatures from 23 to 680 °C and after cooling. Hot strengths of geopolymer increased when the temperature increased from 290 to 520 °C, reaching the highest strength at 520 °C, which is almost double that of its initial strength at room temperature. However, glass transition behaviour was observed to occur between 520 and 575 °C, which was characterised by abrupt loss of stiffness and significant viscoelastic behaviour. The glass transition temperature is determined to be 560 °C. Further, the strength reductions occurred during cooling to room temperature. This is attributed to the damage due to brittle nature of the material making it difficult to accommodate thermal strain differentials during cooling phase.     

碳化硅纳米线增强多孔碳化硅陶瓷基复合材料的制备

构建多孔碳化硅纳米线(SiCNWs)网络并控制化学气相渗透(CVI)过程,可设计并获得轻质、高强度和低导热率SiC复合材料。首先将SiCNWs和聚乙烯醇(PVA)混合,制备具有最佳体积分数(15.6%)和均匀孔隙结构的SiCNWs网络;通过控制CVI参数获得具有小而均匀孔隙结构的SiCNWs增强多孔SiC(SiCNWs/SiC)陶瓷基复合材料。SiC基体形貌受沉积参数(如温度和反应气体浓度)的影响,从球状颗粒向六棱锥颗粒形状转变。SiCNWs/SiC陶瓷基复合材料的孔隙率为38.9%时,强度达到(194.3±21.3) MPa,导热系数为(1.9 ± 0.1) W/(m∙K),显示出增韧效果,并具有低导热系数。     

Positive temperature coefficient of resistance effect in hot-pressed cristobalite-silicon carbide composites

Conductive silicon carbide particles were incorporated into an insulating cristobalite ceramic matrix to produce composite materials with a sizeable positive temperature coefficient (PTC) effect. A large drop in resistivity with silicon carbide content at room temperature, or percolation behaviour, was observed. The PTC effect of the composites, which resulted from the thermal expansion of the cristobalite ceramic matrix, was a maximum with five orders of magnitude for the specimen with 25 vol% silicon carbide. The PTC transition temperature of the composites was at 260 °C, which coincided exactly the reversible high-low phase inversion temperature of the cristobalite ceramic matrix.     

Thermal shock cracking in a metal-particle-reinforced ceramic matrix composite

We study thermal crack shielding and thermal shock damage in a double-edge cracked metal-particle-reinforced ceramic matrix composite subjected to sudden cooling at the cracked surfaces. Under severe thermal shocks, the crack will grow but will be bridged by the plastically stretched metal particles. A linear softening bridging law is used to describe the metal particle bridging behavior. An integral equation of the thermal crack problem incorporating the bridging effect is derived and the thermal stress intensity factor at the bridged crack tip is calculated numerically. It is found that the thermal stress intensity factor is significantly reduced by the metal particle bridging. While the crack growth in thermally shocked monolithic ceramics is unstable, the composite can withstand sufficiently severe thermal shocks without failure.     

The effect of temperature on the behavior of the interphase in polymeric composites

The single-filament fragmentation method for measuring the fiber/matrix stress transfer was used for the identification of interphase perturbations. This technique is based on the measurement of the fiber length resulting from the multiple fracture of a single fiber embedded in a resin specimen during tensile loading. A series of single-fiber fragmentation experiments was conducted over a wide range of temperatures on the AS4-carbon-fiber/Epon-828/PACM20-epoxy-resin system. Critical aspect ratios, the magnitude of which is considered to be inversely proportional to the square root of the matrix modulus, showed a significant increase from ambient to elevated temperatures, at temperature levels much lower than the glass transition point of the bulk matrix. This increase was consistent with the existence of an interphase of lower glass transition temperature than the bulk matrix. A three-concentric-cylinder elastic model was employed to correlated the effect of material properties.     

Effects of reinforcement content and shape on cavitation and failure in metal-matrix composites

Cavity development in alumina-reinforced aluminium composites during tensile loading at room temperature has been monitored using microstructural studies and precision density measurements. The materials examined were based on commercial purity aluminium, reinforced with 10 and 20 volume% of short fibres, conventional angular particles or spherical particles, produced by a thermal spraying process. The composites were made by a powder blending and extrusion route, involving no liquidation of the matrix and leaving arrays of fine oxide particles aligned parallel to the loading direction. In all cases, stable voids were found to form well before final failure. Significant void contents were developed earlier in the test for the higher reinforcement content and when fibres were present. However, extensive voiding, corresponding to approximately hemispherical voids being formed at most fibres or particles, occurred in all cases before final failure. Voids tend to form adjacent to the reinforcement, most readily when it is elongated in the direction of applied stress and when it has a relatively flat surface normal to the stress axis. Sharp corners do not themselves appear to be favoured sites for cavity formation. Consistent with this, cavities can form with spherical particles, although their incidence is somewhat less than with angular particles, presumably because of the absence of elongated shapes and surfaces which are actually flat. A simple model is proposed which allows prediction of the failure strain for a given reinforcement volume fraction and aspect ratio. This is based on the constraining effect of the reinforcement on plastic deformation in adjacent regions of matrix and the contribution of cavitation to the observed strain. Fairly good agreement is observed between the predictions of this model and the experimental data.     

Effect of transient stress on acoustic emission behaviour during firing of dental porcelain

Acoustic emission activity was measured in the glass transition range of dental porcelain during firing. Transient and residual stresses in porcelain during cooling from a temperature higher than porcelain sag point and during re-heating of the tempered porcelain were calculated by computer simulation using a viscoelastic stress analysis. The detected acoustic emission event was discussed with the relative rules of the simulated transient stresses. High acoustic emission activity was detected at the temperature where the internal stress faded away for heating and build up for cooling. The low-level acoustic emission pulses were only detected in the following conditions: (1) in the temperature range where porcelain behaved like an elastic solid; (2) at temperatures higher than the deformation point of porcelain; (3) with a re-heating process of the porcelain without tempered stress. From these results, it was concluded that elastic energy is released related to transient stress in porcelain during viscoelastic deformation and can be detected by the acoustic emission method. The acoustic emission method is considered to be helpful in non-destructive testing in order to understand transient stress due to viscoelastic deformation of glassy materials in heat treatment.     

Deformation-induced volume damage in particulate-filled epoxy resins

In this investigation the microstructural deformation and damage mechanisms in an epoxy resin filled with silica flour are investigated. The unfilled resin exhibits bulk shear yielding in both compression and tension which is localized into fine bands by the addition of the particulate filler. In addition, the filled material stress-whitens in compression and at lower rates and higher temperatures in tension. The shear yielding is recoverable upon heating for a time above the glass transition temperature; however, the stress whitening is irreversible. The addition of filler particles introduces several damage mechanisms including particle-matrix debonding, particle fracture and matrix microcracking. In the following report, these damage mechanisms are characterized microstructurally in terms of the rate, temperature and type of loading and their relative contributions to stress whitening.