A Model for the Diffusion of Moisture in Adhesive Joints. Part II: Experimental

The concentration dependence and temperature dependence of the diffusion coefficient, as discussed in Part I, are validated with literature data on poly(styrene) and on poly(vinylacetate). The effect on diffusivity, of a uniaxial tensile stress state and of a biaxial tensile stress state, is measured with permeation tests on stretched poly(ethyleneterephthalate) (PET) films. The influence of semi-crystallinity is briefly discussed. Further, diffusivity measurements under a tensile stress state, under a compressive stress state, and under a pure shear stress state are performed on Ultem^® polyimide films, using a modified sorption technique. Good agreement between theoretical predictions and experiment is found. Finally, predictions by the solubility model discussed in Part I are compared with data on low density polyethylene and on Ultem polyimide.     

EAA/NBR TPV压缩应力松弛可逆回复机制及模型构建

采用动态硫化法制备了乙烯-丙烯酸共聚物/丁腈橡胶热塑性硫化胶(EAA/NBR TPV),系统研究了其力学性能、微观结构、压缩应力松弛及可逆回复行为,探讨了可逆回复机制并构建了数学模型,还研究了橡塑比对可逆回复不同阶段应力松弛时间的影响。结果表明,当EAA/NBR质量比为40/60时,TPV综合性能最佳且应力松弛行为主要由EAA连续相提供,此时NBR分散相对应力松弛行为的影响较小;升高热处理温度或延长热处理时间均可显著加快TPV压缩应力松弛可逆回复程度;当热处理温度接近EAA树脂相熔点时,TPV可逆回复程度接近100%。     

Variation in In-Plane Stress during Densification and Crystallization of Titania Gel Coatings Deposited on Different Substrate Surfaces

Titania gel films were prepared from a coating solution with a starting mole ratio of Ti(OC₃H₇ⁱ)₄:H₂O:HNO₃:C₂H₅OH=1:1:0.2:30 on Si(100) wafers by spin coating, with and without sol–gel-derived, fired silica, and titania underlayers. In situ measurement was conducted on the in-plane stress in the titania gel films during heating up to 500°C at 5°C/min. Basically, for all the gel films, the in-plane, tensile stress increases once with increasing temperature, and then decreases around the temperature of crystallization. The increase in stress was thought to result from the capillary pressure due to solvent evaporation and from the densification of the coatings. The reduction in stress around the temperature of crystallization was ascribed to the accelerated atomic diffusion in the coatings and to the difference in thermal expansion coefficient between the coatings and the Si wafer. However, when the stress–temperature curves were examined more in detail, differences were found in the variation in stress, depending on the underlayers. Then, the question of how the underlayers could induce the crystallization and relax the strain in the coatings was discussed.     

Necking of high-density polyethylene

Necking draw of high-density polyethylene is studied at draw rates of 2.5 × 10^?2 to 25 mm/min and at temperatures of ?40° to 80°C. Effect of temperature and draw rate on necking stress is interpreted in terms of viscoelastic flow of amorphous phase accompanying orientation of crystallites. It is proved that reducibility of draw rate and temperature holds and that the reduction factor obeys approximately the Williams-Landel-Ferry equation. Necking stress at an extremely low draw rate, critical necking stress, is discussed in terms of the phase equilibrium under stress between two states before and after microfracture of crystallites. The theory, with some approximations, leads to the equation by Iida in which the critical necking stress is expressed by fusion parameters. The thermodynamic behavior of isothermal necking is discussed and a phenomenologic criterion for necking is presented.     

碳/碳化硅复合材料刹车盘/片热应力场分析

采用有限元方法分析二维正交碳纤维增强碳化硅(C/SiC)复合材料制成的汽车刹车盘/片在刹车过程中引起的非线性热力耦合行为,主要研究在强制对流和热辐射作用下刹车结构的温度变化,讨论不同材料属性对刹车温度场的影响以及在温度场和膨胀系数耦合下C/SiC刹车盘/片中热应力和形变情况。数值结果表明:在双重散热条件下需要更多时间用于降温,而垂直于刹车面的热导率分量对温度传导或者降温影响较大;对于C/SiC刹车盘/片每一次刹车行为等效于一次热应力的加载和卸载,而每次产生的热应力可能突破C/SiC的极限弹性强度引起的残余塑性形变,而这种不断累积的残余效果继而引起C/SiC刹车盘/片失效。     

温度和激发剂浓度对碱激发矿渣净浆流变特性的影响

为了获得温度和激发剂浓度对碱激发矿渣净浆流变特性的影响,以NaOH为激发剂,S95粒化高炉矿渣为被激发组分,探讨了不同激发剂浓度(1 mol/L、2 mol/L和3 mol/L)和温度(20 ℃、30 ℃和40 ℃)下矿渣净浆在凝结硬化过程中屈服应力和表观黏度的变化特征。结果表明:以NaOH为激发剂的矿渣净浆表现出非牛顿体特征,屈服应力的发展符合Bingham流体模型。激发剂浓度和温度越高,碱激发矿渣净浆的屈服应力发展越快,并且屈服应力平稳期和增长期时长均随激发剂浓度和的提高而缩短;表观黏度时变模型可通过η=η₀+e^-btⁿ表示,且初始表观黏度η₀、b和n值均随温度和激发剂浓度的提高而提高。     

Synergistic effects of stochastic loading stress and time on stress-rupture damage evolution and lifetime of fiber-reinforced ceramic-matrix composites at intermediate temperatures

In this paper, the synergistic effects of the stochastic loading stress and time interval on the stress-rupture damage evolution and lifetime of fiber-reinforced ceramic-matrix composites (CMCs) at intermediate temperatures are investigated. The relationships between the stochastic loading stress and stochastic time interval, internal damage evolution, macro strain, broken fiber fraction, and stress-rupture lifetime of fiber-reinforced CMCs are established. The effects of the stochastic loading stress and stochastic time interval, constituent properties, damage condition, and environment temperature on the stress-rupture time, strain evolution, and internal damage of SiC/SiC composite under stress-rupture loading are discussed. The experimental stress-rupture behavior of SiC/SiC composite under constant and stochastic loading condition are predicted. The stress-rupture lifetime decreases with increase of stochastic stress level, stochastic time interval, and environmental temperature, and increases with the fiber volume and interfacial shear stress in the slip region.     

分散染料易染海岛复合聚酯纤维的热应力分析

以易水解聚酯(EHDPET)和新型分散染料常压可染聚酯(NEDDP)分别作为海与岛组分,通过熔融纺丝方法,制备了不同复合比的海岛复合聚酯的预取向丝(POY)。采用动态热应力仪模拟纤维加弹工艺,对EHDPET-NEDDP POY进行了不同拉伸温度、拉伸速度、拉伸倍数下的热应力测试。结果表明:随着温度从140℃提高到180℃,POY的热应力呈减小趋势;而当拉伸速度从60 m/s提高至100 m/s,或拉伸倍数从1.4增大到1.8时,POY的热应力均呈增大的趋势。     

Stress Rupture in Tension and Flexure of a SiC-Whisker-Reinforced Silicon Nitride Composite at Elevated Temperatures

Stress rupture of a 20 vol% SiC whisker-reinforced Si₃N₄ composite processed by gas pressure sintering was investigated by both tension and flexure methods. The stress exponents for the stress rupture decrease with increasing temperature. The fracture surfaces of both tensile and flexural stress rupture at 1000°C consist of mirror, mist, and hackle regions. The size of the mirror region increases with decreasing stress. Crack propagation is a mixture of intergranular and transgranular modes at 1000°C. Both tensile and flexural fracture surfaces under constant stress at 1200°C were characterized by a rough zone and a mirror zone; the size of the rough zone increased with decreasing stress. Creep crack growth occurred at 1200°C, which is a process of cavity nucleation, growth, and interlinkage in front of a crack. The transition of fracture mechanisms with temperature is discussed.     

Thermal/Strain History Effects on Creep of Refractory Concrete

A temperature and stress cycling technique was developed to examine the effects of thermal/strain history on creep of refractory concrete. Creep of a 90+% Al₂O₃ refractory concrete and a high-purity calcium aluminate cement was investigated under stresses of 3.4 to 20.7 MPa at 500° to 1200°C. It was found that on initial heat-up of a cured specimen of either the concrete or cement, the ΔH_c∼130 to 170 kJ/mol. After subsequent cycling of the temperature, ΔH_C ∼620 to 720 kJ/mol. The stress exponent for the initial application of stress was <1. For subsequent cycling of the stress n >2.5. It was determined from these and other results that multiple "deformation" processes are acting simultaneously on initial heat-up of the form:
These processes include crystallization, phase changes, sintering, stress-aided sintering, and steady-state creep. Mechanisms of creep and failure are discussed.     

定向碳纳米管/高密度聚乙烯复合材料体系流变行为的研究

对定向碳纳米管进行酸化处理以后，采用机械共混法制备了定向碳纳米管／高密度聚乙烯复合材料，利用毛细管流变仪研究了高密度聚乙烯和定向碳纳米管／高密度聚乙烯复合材料的流变行为．讨论了剪切应力、剪切速率、温度以及定向碳纳米管的加入对体系流变行为的影响。结果表明：高密度聚乙烯和定向碳纳米管／高密度聚乙烯复合材料都属于假塑性流体．高密度聚乙烯的非牛顿性要大于定向碳纳米管／高密度聚乙烯复合材料；随剪切速率和剪切应力的增大及温度的升高．熔体表现粘度均减小；随着定向碳纳米管含量的增加，定向碳纳米管／高密度聚乙烯复合材料的表观粘度先减小后增大。     

Plastic Deformation of ZrB2 Single Crystals

The active slip planes, the Burgers vector, and the critical resolved shear stress to induce macroscopic deformation in ZrB₂ single crystals were determined. Room-temperature microhardness indentation and high-temperature uniaxial compression loadings were used to induce deformation. Slip occurred in a close-packed α direction on prismatic planes at room temperature and on the basal plane at high temperatures. The high-temperature yield stress and yield drop are discussed in terms of a Widmanstaetten-type precipitate observed in the crystals.     

HDPE/OMMT纳米复合材料挤出胀大比的影响因素研究

利用流变分析仪研究了高密度聚乙烯(HDPE)/有机蒙脱土(OMMT)纳米复合材料在挤出过程中的挤出胀大行为及其影响因素,深入讨论了剪切应力、剪切速率、口模温度、口模长径比及OMMT用量对HDPE/OMMT纳米复合材料挤出胀大比B的影响.结果表明,B值随着剪切应力或剪切速率的增加而增大,并且与剪切应力近似呈线性关系;随着口模温度的升高或长径比的增加而减小,当口模长径比较小时,B值受剪切应力或剪切速率的影响尤为显著;同时B值随OMMT用量的增加而逐渐减小.     

SGF/HDPE体系双向增强管材结构与性能研究

利用自行研制的剪切拉伸双向应力场挤管机头装置挤出短玻纤增强高密度聚乙烯（HDPE）管材，研究了在不同剪切应力场段温度下，周向剪切应力场的剪切套转速对管材周向和轴向强度的影响，确定出最佳工艺条件，并通过DSC测试和SEM测试对管材力学性能提高的原因进行了探讨。     

Fracture Surface Energy of Glass

Fracture surface energies of six glasses were measured using the double-cantilever cleavage technique. Values ranged from 3.5 to 5.3 J/m² depending on the chemical composition of the glass and the temperature of the test. The fracture surface energy increased with decreasing temperature and increasing Young's modulus; however, exceptions to this behavior were noted. The magnitude of the values obtained is discussed with respect to the theoretical strength of glass and possible irreversible effects at the crack tip such as stress corrosion and plastic deformation are considered.     

Shear flow behaviors of poly(p‐phenylene benzobisoxazole) spinning dope

In this study, the shear flow properties of Poly(p‐phenylene benzobisoxazole) (PBO)/poly(phosphoric acid) (PPA) spinning dope were studied by means of capillary rheometer. The effect of shear stress, temperature, PBO concentration, and PBO molecular weight on the apparent viscosity of PBO/PPA dope was discussed. The results showed that the apparent viscosity of the dope decreased with the increase of the shear stress and the temperature. The flow behavior index increased with the increase of temperature, which indicated that the non‐Newtonian behavior of the dope became weaker at high temperature. Moreover, it was also found that at high shear stress, the apparent viscosity of the dope was insensitive to the temperature, PBO molecular weight, and PBO concentration. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Transient Stress Intensity Factors for Edge and Corner Cracks in Quench-Test Specimens

The transient temperature and stress fields in a quenched rectangular bar were computed and used to determine the stress intensity factors for midside and corner-edge cracks. The stress intensity factors, K ₁, are presented as a function of time, flaw size, and aspect ratio. The variations of AT, along the crack edge are given and the self-limiting growth of the crack depth is discussed in relation to the nonlimited surface growth.     

电铸镍钴合金中内应力的影响因素

讨论了硫酸盐体系中影响镍钴合金电铸层内应力的因素。结果表明,内应力随钴离子质量浓度的增加而升高;随电流密度的增大先升高后降低,在6A/dm2附近时应力最大;随温度的升高而降低;pH在3.0~4.5时应力较小。加入0.2~0.5g/L添加剂可有效消除内应力。     

Fracture initiation in bi-material joints subject to combined tension and shear loading

Abstract

Linear elastic solution of the stress field near an interface corner of bi-material joints is of the form Hr^λ?1, where r is the radial distance from the corner, H is the stress intensity factor and λ–1 is the order of the singularity. Finite element analysis is used to determine the magnitude of H for a butt joint subject to remote shear; the obtained solution complements existing solution for remote tension and uniform change in temperature. The theoretical solution of the singular shear stress is shown to be in good agreement with the corresponding finite element solution. The effect of combined remote tension, remote shear and uniform change in temperature on the failure loads and failure mechanisms is experimentally determined for brass/araldite/brass butt joint. It is shown that the failure envelope in tensile stress – shear stress space is elliptical and the failure loads decrease with increasing cure temperature due to thermal residual stress associated with the curing process. The application of the results to the assessment of onset of failure in composite patch repair is discussed.     

Experimental Observations of Frictional Heating in Fiber-Reinforced Ceramics

The influence of fatigue loading history and microstructural damage on the magnitude of frictional heating and interfacial shear stress in a unidirectional SiC fiber/calcium aluminosilicate matrix composite was investigated. The extent of frictional heating was found to depend upon loading frequency, stress range, and average matrix crack spacing. The temperature rise attained during fatigue can be significant. For example, the temperature rise exceeded 100 K during fatigue at 75 Hz between stress limits of 220 and 10 MPa. Analysis of the frictional heating data indicates that the interfacial shear stress undergoes an initially rapid decrease during the initial stages of fatigue loading: from an initial value over 20 MPa, to approximately 5 MPa after 25 000 cycles. Over the range of 5 to 25 Hz, the interfacial shear stress was not significantly influenced by loading frequency. The implications of frictional heating in fiber-reinforced ceramics are also discussed.