High-Temperature Stress-Strain Behavior of MgO in Compression

Compressive stress-strain curves for several types of polycrystalline MgO specimens were correlated with those for single crystals and analyzed as a function of grain size and grain-boundary character at 1200° and 1400°C for several strain rates. The results for fully dense specimens were explained in terms of grain-boundary sliding and intergranular separation in addition to slip. The modification of grain-boundary nature concurrent with heat treatment for grain growth, caused by residual LUF, was associated with enhanced grain-boundary sliding and intergranular separation. For grain sizes <30 μm, it was concluded that the von Miss criteria for ductility could be relaxed by the Occurrence of dislocation climb and, to a limited extent, by intergranular separation. Yield drop corresponding to dislocation multiplication occurred when grain-boundary sliding was initially promoted. Specimens with a liquid phase of adequate viscosity also indicated plasticity accompanied by high strength. Specimens with clean grain boundaries exhibited ductility and normal strain hardening with no intergranular separation.     

High-Temperature Mechanical Behavior of Fiber-Reinforced Glass-Ceramic-Matrix Composites

Lithium aluminosilicate glass-ceramic-matrix composites reinforced with Sic fibers were tested at 900° and 1000°C in flexural and tensile configurations. The composites showed severe thermomechanical degradation when tested in oxidizing atmospheres. This degradation was found to depend on oxygen partial pressure. For P _{o 2}≳10³ Pa the composites failed in a brittle fashion; a single crack initiated and propagated through the matrix and fibers. At lower P_o2, multiple matrix cracking was observed, and the fibers bridged the matrix cracks.     

Chemical Interactions in High-Temperature Ceramic Composites

The useful lives of many ceramic composites will be influenced, if not controlled, by chemical interactions. Thermodynamic and kinetic calculations are invaluable in evaluating them. Composites may contain as many as four constituents: a fiber, its coating, a matrix, and an external coating. The chemical concerns fall into three categories: (i) chemical compatibility between the various constituents, (ii) internal stability of the constituents, and (iii) environmental stability of the composite. Examples are given for each of these concerns. Also included are the approaches useful for evaluating them.     

Microstructure and High-Temperature Mechanical Behavior of Alumina/Alumina–Yttria-Stabilized Tetragonal Zirconia Multilayer Composites

Layered composites of alternate layers of pure Al₂O₃(thickness of 125 μ m) and 85 vol% Al₂O₃-15 vol% ZrO₂ that was stabilized with 3 mol% Y₂O₃(thickness of 400 μ m) were obtained by sequential slip casting and then fired at either 1550° or 1700°C. Constant-strain-rate tests were conducted on these materials in air at 1400°C at an initial strain rate of 2 × 10^-5 s^-1. The load axis was applied both parallel and perpendicular to the layer interfaces. Catastrophic failure occurred for the composite that was fired at 1700°C, because of the coalescence of cavities that had developed in grain boundaries of the Al₂O₃ layers. In comparison, the composite that was fired at 1550°C demonstrated the ductility of the Al₂O₃+YTZP layer, but at a flow stress level that was determined by the Al₂O₃ layer.     

High-Temperature Mechanical Properties of Mullite Under Compression

The high strength potential of single-phase mullite was investigated under compressive stress-strain and creep testing conditions at 1400° and 1500°C. In single-crystal experiments, no plastic deformation and, hence, no dislocation glide was observed. Polycrystalline mullite was deformed via the Nabarro-Herring mechanism accompanied by grain-boundary sliding and some cavitation. In stress-strain tests, deformation was enhanced by intergranular separation.     

High-Temperature Stability of SiC-Based Composites in High-Water-Vapor-Pressure Environments

Microstructural characterization of boron-containing SiC-reinforced SiC composites exposed at high temperature in high-water-vapor-pressure environments was used to determine surface recession rates and to understand the controlling degradation processes under these conditions. Results showed that composite degradation was controlled by a series of reactions involving the formation of silica, boria, borosilicate glass, and gaseous products. Comparison of results (from characterization of composites exposed at 1200°C and 1.5 atm of H₂O in a laboratory furnace and in the combustion zone of a gas turbine) showed that these reactions were common to both exposure conditions and, consequently, there was little effect of gas velocity on degradation rates of boron-containing SiC/SiC composite materials.     

High-Temperature Strength Behavior of Synroc-C

Comparative measurements have been made of the high-temperature flexure strength characteristics of synroc-C in air and argon environments. The stress–strain curves for synroc show a large deviation from linearity with increasing temperature in both environments, indicating a brittle–ductile transition. Strength is relatively constant at ≤800°C, followed by a discernible increase, with a peak at ∼920°C in air and 940°C in argon, and then a dramatic drop-off. The strengthening response is explored with reference to microstructural changes, in particular oxidation effects, and the implications of the observations are discussed.     

Influence of Coupling Agents on Melt Flow Behavior of Natural Fiber Composites

The influence of coupling agents on the melt rheological properties of natural fiber composites has been investigated in this work using capillary and rotational rheometers. Scanning electron microscopy was also employed to supplement the rheological data. It was found that molecular weight and molecular weight distribution of the polymer matrix and coupling agent characteristics influence the filler wetting and the melt flow properties of the filled composites. Generally, low molecular weight and narrow molecular weight distribution polyethylene matrix provides relatively larger increase of the viscosity of the composites. Coupling agents tend to increase the resistance to shearing, but wall slip effects may interfere with the measured values, especially at very high filler loadings. Entrance pressure loss in capillaries is also influenced by polymer matrix and coupling agent used.

     

拉压不同模拉伸硬化GRC矩形梁弯曲性能研究

对拉压不同模拉伸硬化(THDM)模型矩形梁进行分析,推导了弯矩方程,其可以退化为弹性(E)、弹塑性(EP)、弹性硬化(EH)、拉压不同模弹性(DM)、拉压同模拉伸硬化(THSM)模型矩形梁弯矩方程.用ABAQUS模拟不同模型的四点弯曲试验,THDM模型挠度与玻璃纤维掺量为5％ 的GRC试验挠度最接近.相同最大拉应力对应的THDM模型的弯矩大于E模型的弯矩.     

High-Temperature Deformation of SiC-Whisker-Reinforced MgO-PSZ/Mullite Composites

The effect of 33.5 vol% SiC whisker loading on high-temperature deformation of 1 wt% MgO-38.5 wt% zirconia-mullite composites was studied between 1300° and 1400°C. At strain rates of 10⁻⁶ to 5 × 10⁻⁴/s the creep resistance of zirconia-mullite composites without SiC reinforcement was inferior to monolithic mullite of similar grain size. Analysis of the results suggested that the decreased creep resistance of mullite-zirconia composites compared to pure mullite could be at least partially explained by mechanical effects of the weaker zirconia phase, increased effective diffusivity of mullite by zirconia addition, and to the differences in mullite grain morphology. With SiC whisker reinforcement, the deformation rate at high stress was nearly the same as that of the unrein-forced material, but at low stress the creep rates of the SiC-reinforced material were significantly lowered. The stress dependence of the creep rate of unreinforced material suggested that diffusional creep was the operative mechanism, while the reinforced material behaved as if a threshold stress for creep existed. The threshold stress could be rationalized based on a whisker network model. This was supported by data on other whisker-containing materials; however, the threshold stress had a temperature dependence that was orders of magnitude higher than the elastic constants, leaving the physical model incomplete. The effects of residual stresses and amorphous phases at whisker/matrix interfaces are invoked to help complete the physical model for creep threshold stress.     

High-Temperature Fatigue Behavior of Polycrystalline Alumina

The strength and fatigue behavior of a 99.5% polycrystalline alumina were measured as a function of temperature. Both the strength and fatigue behavior remained essentially constant up to 500°C; from 800° to 1100°C the strength and fatigue resistance decreased markedly and at >1100°C macroscopic creep was observed. It is believed that the decrease in strength and fatigue resistance is caused by a grain-boundary glassy phase enhancing subcritical crack growth. Proof-testing at room temperature was effective in improving the strength distributions at both room temperature and 1000°C; however, at 1000°C it was not effective, due to crack growth during the proof test. The good agreement between proof-test results and fracture-mechanics theory indicates that the same flaws control the strength at room temperature and at high temperatures.     

双螺杆挤出炸药模拟物的流场模拟分析及实验研究

为解决含超细粉体炸药模拟物中各组分的均匀分散混合问题,采用双螺杆挤出机试验及ANSYS有限元流场模拟分析,首先用平板流变仪测得炸药模拟物在特定温度下的流变参数,运用ANSYS的Fluid Dynamics模块对炸药模拟物在同向双螺杆挤出机的混合挤出流场进行模拟,以累积最大拉伸速率、加权平均剪切应力表征混合能力的大小。基于模拟结果,用同向双螺杆挤出机对炸药模拟物的混合流场进行实验,对经历不同混合流场的炸药模拟物进行SEM扫描电镜分析。结果表明,大导程和反向的螺纹元件能够提供较大的剪切和拉伸作用,有利于炸药模拟物的分散混合。     

Models of High-Temperature, Environmentally Assisted Embrittlement in Ceramic-Matrix Composites

The intermediate-temperature oxidation embrittlement, or 'pest,' effect found in ceramic-matrix composites (CMCs) is shown to have features analogous to stress corrosion cracking. The behavior involves crack growth upon oxidation of the fibers or the fiber coatings to form an oxide that weakens the fibers. It has reaction- and diffusion-controlled regimes. The former occurs at low stresses. The latter occurs at higher stresses. It is controlled by oxygen ingress through the matrix cracks. There is also a crack growth threshold. Expressions for the crack velocity above the threshold are derived as well as the stress dependence of the rupture life.     

Product Design of Cohesive Powders – Mechanical Properties,Compression and Flow Behavior

The fundamentals of cohesive powder consolidation and flow behavior are explained using a reasonable combination of particle and continuum mechanics. By the model “stiff particles with soft contacts”, universal models are presented which include the elastic‐plastic and viscoplastic particle contact behavior with adhesion, load‐unload hysteresis and thus energy dissipation, a history‐dependent and a nonlinear adhesion force function. With this as the physical basis, incipient powder consolidation, yield and cohesive steady‐state flow, consolidation and compression functions, compression and preshear works are explained. As an example, the flow properties of an ultrafine limestone powder are shown. These constitutive models are used to evaluate shear cell test results for apparatus design to ensure reliable powder flow. Finally, conclusions are drawn concerning particle stressing, powder handling behavior and product quality assessment in processing industries.     

High-Temperature Behavior of Indent and Creep-Nucleated Cracks in Vitreous-Bonded Alumina

Crack behavior was studied at elevated temperatures in a commercial vitreous-bonded alumina for two types of cracks: one introduced by indentation at room temperature and the other by the creep process. Indentation cracks with relatively small initial size grew progressively longer during creep before they became blunt and arrested; however, they continued to widen throughout the creep process. Larger indentation cracks under high stress condition continued to grow until failure. The evolution of creep-nucleated cracks was so fast that they were observed only in their arrested state. Once observed, their length remained essentially constant, but they did grow in width. The crack-opening displacement rates of both types of cracks were linearly related to the creep rate as predicted by fracture mechanics for stationary cracks. All but the specimens with the largest indentation crack exhibited flaw tolerance in that they failed by the coalescence of creep-nucleated cracks instead of the growth of a single crack. The results illustrate the crack behavior in the brittle-to-ductile transition regime for ceramics that deform by grain boundary sliding.     

High-Temperature Mechanical Behavior of Stoichiometric Magnesium Spinel

The elastic and mechanical behavior, from room temperature up to 1300^deg;C, of Stoichiometric polycrystalline magnesium aluminum spinel is studied. Elastic modulus, fracture toughness, and modulus of rupture measurements and observations of polished and fracture surfaces have been performed. Two well-differentiated regions of fracture behavior as a function of temperature have been found. In the low-temperature region, this material behaves elastically, whereas in the high-temperature (>800^deg;C) region, plastic phenomena take place.     

High-Temperature Creep Deformation of Alumina — SiC-Whisker Composites

The creep resistance at temperatures between 1200° and 1300°C in air of alumina—SiC-whisker composites was investigated via four-point flexure to examine (1) the effect of whisker content and (2) the influence of densification additives (i.e., Y₂O₃ (plus MgO)). The creep resistance of polycrystalline alumina is greatly improved with the addition of ≤ 20 vol% SiC whiskers. The interlocking/pinning of grains by whiskers which limits grain-boundary sliding contributes to the improvement in creep resistance. However, the creep rates of alumina composites in air increase at whisker contents ≥ 30 vol%. Electron microscopy observations suggested that the degradation in creep resistance for whisker content ≥ 30 vol% originated from (1) the promotion of creep cavitation and subsequent microcrack generation from the higher number density of nucleation sites and (2) more extensive formation of grain-boundary amorphous phase(s) associated with an observed increased oxidation rate. Along this one, the excellent creep resistance of alumina composites containing 20 vol% SiC whiskers was significantly degraded by the presence of the intergranular amorphous phases introduced by the addition of the Y₂O₃ densification additive.