Creep Behavior of Uranium Carbide-Based Alloys

Compression creep tests were performed on fully dense specimens of UC_1.01, UC_1.05, UC_1.01.+ 4 wt% W, and U_0.9Zr_0.1C_1.01+ 4 wt% W. Steady-state creep rates were measured from 1400° to 1800°C in a vacuum of 1.33 × 10^-3 N/m² (1 × 10^-5 torr) at stresses of 4.55 to 69.0 MN/m² (660 to 10,000 psi). The data for UC_1.01 could best be fit by an expression of the form ɛ= 1773σ^6.024 exp (106.5/RT) , where σ is the steady-state creep rate (h^-l), σ is the applied stress (MN/m²), and the creep activation energy is given in kcal/mol. The stress dependence for creep of UC_1.05 decreased with decreasing temperature because of second-phase precipitation; therefore, a unique creep activation energy could not be established for this U/C ratio. At all temperatures, the creep strength of UC_1.05 exceeded that of UC_1.01. For example, at 1700 ° C steady-state creep rates for UC_1.05 are ∼1/4 those for UC_1.01, but at 1400°C the creep rates are ∼ 3 orders of magnitude less. At 1700°C, creep rates for UC alloys are ∼4 orders of magnitude lower than those for unalloyed UC_1.01.     

Tensile Creep and Creep Rupture Behavior of Monolithic and SiC-Whisker-Reinforced Silicon Nitride Ceramics

The tensile creep and creep rupture behavior of silicon nitride was investigated at 1200° to 1350°C using hotpressed materials with and without SiC whiskers. Stable steady-state creep was observed under low applied stresses at 1200°C. Accelerated creep regimes, which were absent below 1300°C, were identified above that temperature. The appearance of accelerated creep at the higher temperatures is attributable to formation of microcracks throughout a specimen. The whisker-reinforced material exhibited better creep resistance than the monolith at 1200°C; however, the superiority disappeared above 1300°C. Considerably high values, 3 to 5, were obtained for the creep exponent in the overall temperature range. The exponent tended to decrease with decreasing applied stress at 1200°C. The primary creep mechanism was considered cavitationenhanced creep. Specimen lifetimes followed the Monkman–Grant relationship except for fractures with large accelerated creep regimes. The creep rupture behavior is discussed in association with cavity formation and crack coalescence.     

Tensile Creep Behavior of a Vitreous-Bonded Aluminum Oxide under Static and Cyclic Loading

Creep deformation and rupture behavior of a vitreousbonded aluminum oxide was investigated under uniaxial static and cyclic tensile loadings at 1000°, 1100°, and 1175°C. The material was more creep resistant, i.e., having lower creep strain rates, under cyclic loading compared to that under static loading. For the same maximum applied stress, the ratio of steady-state creep rate under static loading to that under cyclic loading at 1100°C was approximately 100. However, the value of this ratio decreased to about 10 when the testing temperature was raised to 1175°C or lowered to 1000°C. Under static loading the material had more propensity to develop creep damage in the form of micro- and macrocracks, leading to early failure, whereas under cyclic loading the creep damage was more uniformly distributed in the form of cavities confined to the multigrain junctions. Viscous bridging by the grain boundary second phase may be the primary contributor to the lower creep deformation rate and improved lifetime under cyclic loading.     

Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride

The creep behavior of a commercial grade of Si₃N₄ was studied at 1350° and 1400°C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 ± 22.9 kJ/mol, and compression, 489.2 ± 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple-junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ∼18% of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution–precipitation of Si₃N₄. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution–precipitation creep of Si₃N₄ did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.     

基于十字交叉法的陶瓷胶粘剂剪切蠕变性能研究

本研究设计了“十字交叉法”陶瓷胶粘剂剪切蠕变试验装置,选取刚性环氧树脂及柔性硅酮结构胶进行剪切蠕变试验,研究了环境温度、剪切应力、粘结面积等因素对胶粘剂剪切蠕变的影响,通过模型拟合对胶粘剂的剪切蠕变行为进行了分析和预测,探究了两种胶粘剂的蠕变破坏模式。结果表明:采用十字交叉法能够准确便捷地测试陶瓷胶粘剂的蠕变性能。增大胶粘层柔性、提高环境温度、增大剪切应力都会加速蠕变的发展,但粘结面积对蠕变速率无明显影响。刚性环氧树脂胶粘剂试样的蠕变失效形式为粘结层内聚破坏及界面脱粘,符合时间硬化模型;柔性硅酮结构胶试样失效形式为粘结层内聚破坏,符合Burgers模型。     

Effect of Creep Damage on the Tensile Creep Behavior of a Siliconized Silicon Carbide

The tensile creep behavior of a siliconized silicon carbide was investigated in air, under applied stresses of 103 to 172 MPa for the temperature range of 1100° to 1200°C. At 1100°C, the steady-state stress exponent for creep was approximately 4 under applied stresses less than the threshold for creep damage (132 MPa). At applied stresses greater than the threshold stress for creep damage, the stress exponent increased to approximately 10. The activation energy for steady-state creep at 103 MPa was approximately 175 kJ/mol for the temperature range of 1100° to 1200°C. Under applied stresses of 137 and 172 MPa, the activation energy for creep increased to 210 and 350 kJ/mol, respectively, for the same temperature range. Creep deformation in the siliconized silicon carbide below the threshold stress for creep damage was determined to be controlled by dislocation processes in the silicon phase. At applied stresses above the threshold stress for creep damage, creep damage enhanced the rate of deformation, resulting in an increased stress exponent and activation energy for creep. The contribution of creep damage to the deformation process was shown to increase the stress exponent from 4 to 10.     

High‐Temperature Creep Behavior of Dense SiOC‐Based Ceramic Nanocomposites: Microstructural and Phase Composition Effects

In this work, dense monolithic polymer‐derived ceramic nanocomposites (SiOC, SiZrOC, and SiHfOC) were synthesized via hot‐pressing techniques and were evaluated with respect to their compression creep behavior at temperatures beyond 1000°C. The creep rates, stress exponents as well as activation energies were determined. The high‐temperature creep in all materials has been shown to rely on viscous flow. In the quaternary materials (i.e., SiZrOC and SiHfOC), higher creep rates and activation energies were determined as compared to those of monolithic SiOC. The increase in the creep rates upon modification of SiOC with Zr/Hf relies on the significant decrease in the volume fraction of segregated carbon; whereas the increase of the activation energies corresponds to an increase of the size of the silica nanodomains upon Zr/Hf modification. Within this context, a model is proposed, which correlates the phase composition as well as network architecture of the investigated samples with their creep behavior and agrees well with the experimentally determined data.     

Creep Behavior of a Sintered Silicon Nitride

A commercial sintered silicon nitride has been crept in bending and compression at temperatures of 1100°C to 1400°C. In the as-sintered condition the material contains an amorphous intergranular phase. This phase undergoes partial devitrification as a result of high temperature exposure. Preannealing the material to a stable microstructure has very little effect on the creep properties. Deformation behavior compares well with that predicted from a model for creep due to viscous flow of a non-Newtonian grain boundary phase. In bending, the model predicts an initial constant strain rate at low strains as the intergranular phase is squeezed out from between grains under compression. Samples crept in compression are not expected to have this same initial constant strain rate regime. The model also predicts a strong initial strain rate dependence (in bending) on the initial thickness of the amorphous grain boundary layer. Experimentally this strain rate is not affected by partial grain boundary crystallization, suggesting that partial devitrification does not alter the intergranular film thickness or viscosity. This is supported by transmission electron microscopy, which has shown that crystallization of the intergranular phase occurs largely in the pockets between grains, leaving amorphous films between grains.     

HDPE蠕变行为的研究

研究HDPE的蠕变行为不仅有助于从分子运动机理上揭示聚合物的黏弹性行为,还能够预测材料使用过程中的尺寸稳定性及长期承载能力,减小工程设计误差,确保材料使用的安全性,因而具有重要的理论意义和实用价值。回顾了蠕变理论的发展,综合了近年来有关HDPE蠕变行为及形变机理的研究进展,并以HDPE单向拉伸格栅为例简要分析了此项研究的发展及应用前景。     

Tensile Creep Behavior of a Gas-Pressure-Sintered Silicon Nitride Containing Silicon Carbide

The tensile creep behavior of a gas-pressure-sintered silicon nitride containing silicon carbide was characterized at temperatures between 1375° and 1450°C with applied stresses between 50 and 250 MPa. Individual specimens were tested at fixed temperatures and applied loads. Each specimen was pin-loaded within the hot zone of a split-tube furnace through silicon carbide rods connected outside the furnace to a pneumatic cylinder. The gauge length was measured by laser extensometry, using gauge markers attached to the specimen. Secondary creep rates ranged from 0.54 to 270 Gs⁻¹, and the creep tests lasted from 6.7 to 1005 h. Exponential functions of stress and temperature were fitted to represent the secondary creep rate and the creep lifetime. This material was found to be more creep resistant than two other silicon nitride ceramics that had been characterized earlier by the same method of measurement as viable candidates for high-temperature service.     

紫外光对PP蠕变行为的影响

通过对聚丙烯(PP)施加紫外光的方法研究了紫外光对其蠕变行为的影响,利用蠕变性能测试、GPC、XPS、PALS、SEM等手段考察了紫外光作用下的PP蠕变行为及其失效机理。实验结果表明:紫外光能加速PP的蠕变失效,其失效机理与辐照强度有关;短时间失效主要是由紫外光引起的分子链段的活性变化所致,长时间作用下的失效,主要是紫外光引起的老化导致材料强度的削弱所致;另外,对不同种类的PP,在相同的外部条件(下温度、应力和紫外光,)其失效速率与结构和分子量无关,只与该温度下材料的屈服强度有关,屈服强度越高抗,蠕变能力越好。     

尼龙1212熔体蠕变与回复行为的研究

通过对石油发酵尼龙1212(PFPA1212)熔体进行蠕变及回复试验，研究了温度对蠕变应变、蠕变柔量、蠕变粘度的影响，并对PA1212熔体的蠕变及回复行为特征机理进行了探讨。结果发现：PFPA1212熔体蠕变行为对温度有很强的依赖性，随温度的升高，蠕变柔量不断增加；蠕变粘度在较低温度下随时间增加较快，而在较高温度区随时间增加较缓。     

Comparison of the Creep and Creep Rupture Performance of Two HIPed Silicon Nitride Ceramics

Measurements of the tensile creep and creep rupture behavior were used to evaluate the long-term mechanical reliability of a commercially available and a developmental hot isostatically pressed (HIPed) silicon nitride. Measurements were conducted at 1260° and 1370°C utilizing button–head tensile specimens. The stress and temperature sensitivities of the secondary creep rates were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The stress and temperature dependencies of creep rupture life were determined by continuing individual creep tests to specimen failure. Creep deformation in both materials was associated with cavitation at multigrain junctions. Two-grain cavitation was also observed in the commercial material. Failure in both materials resulted from the evolution of an extensive damage zone. The failure times were uniquely related to the creep rates, suggesting that the zone growth was constrained by the bulk creep response. The fact that the creep and creep rupture behaviors of the developmental silicon nitride were significantly improved compared to those of the commercial material was attributed to the absence of cavitation along two-grain junctions in the developmental material.     

Modeling of Creep Behavior of Particulate Composites with Focus on Interfacial Adhesion Effect

Evaluation of creep compliance of particulate composites using empirical models always provides parameters depending on initial stress and material composition. The effort spent to connect model parameters with physical properties has not resulted in success yet. Further, during the creep, delamination between matrix and filler may occur depending on time and initial stress, reducing an interface adhesion and load transfer to filler particles. In this paper, the creep compliance curves of glass beads reinforced poly(butylene terephthalate) composites were fitted with Burgers and Findley models providing different sets of time-dependent model parameters for each initial stress. Despite the finding that the Findley model performs well in a primary creep, the Burgers model is more suitable if secondary creep comes into play; they allow only for a qualitative prediction of creep behavior because the interface adhesion and its time dependency is an implicit, hidden parameter. As Young’s modulus is a parameter of these models (and the majority of other creep models), it was selected to be introduced as a filler content-dependent parameter with the help of the cube in cube elementary volume approach of Paul. The analysis led to the time-dependent creep compliance that depends only on the time-dependent creep of the matrix and the normalized particle distance (or the filler volume content), and it allowed accounting for the adhesion effect. Comparison with the experimental data confirmed that the elementary volume-based creep compliance function can be used to predict the realistic creep behavior of particulate composites.     

Approximate Theory of Thermal Stress Resistance of Brittle Ceramics Involving Creep

The effect of stress relaxation by creep on the thermal stress fracture of brittle ceramics at high temperature under conditions of quasi-static heat flow is discussed. It is shown that, to a good approximation, thermal stress relaxation rates can be calculated on the basis of creep rates which correspond to the minimum temperature of the ceramic workpiece. For materials exhibiting linear stress-creep rate dependence, expressions for the relaxation time and maximum temperature difference or heat flux to which ceramic bodies can be subjected are derived in terms of the material variables affecting thermal stress fracture and stress relaxation by creep. A numerical example shows that high-temperature creep can materially affect the thermal stress behavior of brittle ceramics. Appropriate thermal stress parameters are proposed to form the basis of proper material selection for high-temperature environments involving thermal stress and stress relaxation by creep. Conditions for which thermal stress calculations should be based on an elastic or viscoelastic analysis are outlined.     

Steady-State Creep Behavior of Si–SiC C-Rings

Because of the ease of experimental setup as well as economics in sample preparation, C-ring specimens are sometimes chosen for the evaluation of mechanical behavior. In this paper, the long-term creep of siliconized silicon carbide (Si–SiC) C-rings is investigated. Creep tests on a number of Si–SiC C-rings were carried out under constant compressive loads at 1300°C in air. Load-point displacements were continually monitored as a function of time, thereby establishing the steady-state regime as a function of load and ring geometry. Optical micrography on the postcrept specimens was performed to obtain damage zone sizes. A simple curved beam theory was employed to analyze the stress state developed throughout the body during steady-state creep. Loadpoint displacement rates were numerically calculated using both geometric and energy methods. Observed damage zone sizes and shapes within the specimen agreed with those predicted theoretically. Results obtained on the stress solutions are useful as local loading parameters in the study of high-temperture fracture behavior of a cracked C-ring.     

High-Temperature Steady-State Creep in Rutile

The primary objective of this research was to study the interaction of point defects and dislocations in rutile at elevated temperatures. The steady-state creep rate was obtained for near-stoichiometric and vacuum-reduced compression specimens in the range 1050° to 1325°K and for stresses from 2(10⁸) to 9(10⁸) dynes per cm². For a constant stress, strain, and temperature, the activation energy for creep varies continuously from 67 kcal per mole for the near-stoichiometric condition to about 33 kcal per mole for the highest degree of reduction. The activation energy for creep in hydrogen-reduced rutile is about 80 kcal per mole and this leads to the conclusion that the rate-controlling mechanism and/or defect are not the same for hydrogen- and vacuum-reduced rutile. A model based on the interaction of piled-up dislocations on a single slip system and high-density polygon walls formed during creep is proposed to explain steady-state creep in rutile.     

Effect of Yttrium and Lanthanum on the Tensile Creep Behavior of Aluminum Oxide

The tensile creep behavior of two rare-earth dopant systems, lanthanum- and yttrium-doped alumina, are compared and contrasted in order to better understand the role of oversized, isovalent cation dopants in determining creep behavior. It was found that, despite some microstructural differences, these systems displayed qualitatively a similar improvement in creep resistance, supporting the hypothesis that creep is strongly influenced by segregation. Differences in primary creep behavior and activation energy for steady-state creep were, however, observed for these systems. Given these results, it is expected that creep behavior can be further optimized by adjusting the dopant level and by controlling the microstructure.     

Comparison of Bend Stress Relaxation and Tensile Creep of CVD SiC Fibers

Three different CVD SiC fibers were tested for bend stress relaxation (BSR) and tensile creep over a wide range of temperatures, times, and stresses. Primary creep was always observed, even for creep strains on the order of 2%. The BSR and tensile creep results were compared using simple linear viscoelastic principles. It was found that BSR results could predict the same time and temperature dependence as tensile creep; however, BSR-predicted creep strains usually overestimated the magnitude of tensile creep strain. The time, temperature, and stress dependence were determined for all the fibers for the experimental conditions of this study. Some of the primary creep behavior can be explained by load-sharing effects between the core and the CVD SiC substrate and some microstructural changes; however, the extent of primary creep cannot fully be accounted for from this work.