Dynamic crack growth mechanism and lifetime assessment in plasma sprayed thermal barrier system upon temperature cycling

Failure of plasma-sprayed thermal barrier coatings (TBCs) is very complicated upon temperature cycling, therefore, to ascertain the crack propagation behavior is beneficial to understand the failure mechanism and life prediction of TBCs. In this paper, a finite element model is developed by coupling the dynamic growth of thermally grown oxide and dynamic crack propagation to explore the failure of TBCs induced by the instability of the interface between top coat (TC) and bond coat (BC). The thermal cyclic lifetime is deduced by obtaining the thermal cycles corresponding to the occurrence of complete delamination. The influence of the non-uniformity of the interface on thermal cyclic lifetime is quantitatively evaluated. Sensitivity studies including the effects of constituent properties and crack distance to the interface on the thermal cyclic lifetime are further examined. The results show that the incipient cracks usually nucleate above the valley due to the large tensile stress, and the shear stress near the peak plays a very crucial role. The crack growth involves three stages with different fracture dominated-mode. The crack propagation behavior obtained by simulation is in line with that observed by experiments. The TBCs system with a uniform interface exhibits a longer thermal cyclic lifetime compared to the non-uniform interface. Coating optimization methods proposed in this work may provide an alternative option for developing a TBCs system with longer service lifetime.     

粒径及声场对SiO2超细颗粒流化行为的影响

采用内径为56 mm的玻璃管流化床,考察了平均粒径分别为5~10 nm(1#), 0.5 mm(2#)及10 mm(3#)的SiO2超细颗粒在无声场及声场存在下的流化行为. 无声场时,1#和2#颗粒可在较高的气速下形成稳定聚团,单位质量颗粒团间作用力与原生颗粒相比显著下降,因而可实现稳定的聚团流化,3#颗粒因颗粒间粘性力较大,无法实现稳定流化. 40~60 Hz的声场对3种超细颗粒的流化行为均可起到一定的改善作用,在此频率范围外,声场的作用不明显. 提高声压级,可以使1#和2#颗粒团发生一定程度的破碎,聚团尺寸减小,最小流化速度降低. 在实验范围内,添加声场无法使3#颗粒实现稳定流化.     

Environmental effects of silicone oil on tensile and fatigue properties of polystyrene

A series of polished polystyrene specimens, coated with silicone oil, were tested in tension and in tension-compression fatigue cycling at 21 Hz. Oils of four different viscosities, ranging from 5 cSt to 1000 cSt, were utilized. The craze initiation stress and the tensile fracture stress both increased with the increase of oil viscosity. For the 1000 cSt oil, the stress-strain curve was essentially the same as that of an uncoated sample. The average lifetime to fracture in the fatigue tests depends upon the stress amplitude and on the oil viscosity. For the 1000 cSt oil, the fatigue behavior is similar to that of uncoated samples, except at high imposed stresses, where lifetime is somewhat lower. For the low viscosity oil, the average lifetime is from one to two decades less than for uncoated samples. Discussion is given of the influence of stress amplitude and oil viscosity on the test results and on the morphology of the fracture surfaces.     

A Die Pressing Test for the Estimation of Agglomerate Strength

A die pressing test was developed for quick and inexpensive estimation of the agglomerate strength of ceramic powders. The critical nominal pressure ( p _c) at which contact areas between agglomerates start to increase rapidly was found from the relationship between change in sample height and applied pressure in uniaxial single-ended die pressing. A quantitative microscopic method was used for measuring the area fraction (Ψ) of agglomerates which transmits the force through the assembly. A die pressing agglomerate strength, σ_d, is defined as σ_d= 0.7 p _c/Ψ. This strength was compared with the agglomerate tensile strength obtained from single agglomerate diametral compression tests and found to be 50% higher than the latter because of multipoint loading. A suggested guideline is that the mean agglomerate tensile strength is approximately 52% of p _c determined in a die pressing test for spherical agglomerates. In addition to agglomerate tensile strength, the mean agglomerate size, the interior macropore structure of agglomerates, as well as the packing efficiencies between and inside agglomerates can be estimated by the procedure.     

Influence of strain on the thermal behaviour of poly(chlorotrifluoroethylene)

The effects of uniaxial drawing on the free volume of poly(chlorotrifluoroethylene) have been investigated using the positron annihilation lifetime technique. Annealing measurements were made both in unstrained and maximum strained conditions of the polymer to understand the influence of residual stress on the thermal behaviour. The results indicate uncoiling of the molecular chains upon stretching accompanied by orientation of the polymeric chains. The results further indicate an increase of the glass transition temperature of the drawn film and a negative thermal expansion beyond the glass transition temperature, supported by an increase in activation energy for the strained polymer. © 1999 Society of Chemical Industry     

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.     

Effect of size ratio on the behaviour of agglomerates embedded in a bed of particles subjected to shearing: DEM analysis

This paper presents the results of analysis of the deformation and breakage of spherical agglomerates embedded in a bed of particles and subjected to shearing, a situation commonly encountered in powder granulation. The study is based on three dimensional distinct element method (DEM), in which the inter-particle interactions are governed by theories of contact mechanics. An agglomerate was first generated in a bed of particles having the same size as the primary particles forming the agglomerate. Different size ratios (i.e., the ratio of the diameter of agglomerate to the diameter of surrounding particles) in the range 3-10 were then simulated by varying the size and number of surrounding particles. The agglomerates were subjected to shearing (shear rate and strain of about and 0.3, respectively) and their breakage characteristics were analysed. The agglomerate with the size ratio 10 does not break but undergoes some structural deformation by re-arrangements of contacts. However, the agglomerates with ratio about 7 or smaller suffer breakage. For the size ratio equal or smaller than 5, the agglomerate breaks significantly leading to full disintegration. The results of stress analysis of the agglomerates suggest that the resistance to breakage for the agglomerate with size ratio of 10 is due to the nature of stresses exerted on the agglomerate. For large size ratios the stress on the agglomerate is predominantly hydrostatic. The ratio of deviatoric stress over hydrostatic pressure increases as the size ratio of the agglomerate is reduced. The nature of stresses experienced by agglomerates with smaller size ratios is predominantly deviatoric, thus causing shear deformation and breakage. The results are compared with physical experiments and a satisfactory agreement is obtained.     

Size analysis of polymer latex powder. Comparison of results obtained by means of the I.C.I.–Joyce,Loebl disk centrifuge,the coulter counter,and the electron microscope

The volume frequency size distribution of PVC latex powder was determined by means of three different methods, which all gave the same frequency maximum at about 0.8–0.9 μ diameter. While the electron microscope analysis, which allows the agglomerates to be resolved into individual primary particles, did not show any particles above 1.5 μ in a sample of 9000, both the Coulter counter and the disk centrifuge showed a considerable amount of larger particles.     

气固流化床中颗粒聚团的流动特性

在一套流化床冷模实验装置中研究了A类颗粒在鼓泡床和湍流床内的微观两相流动结构,测量了床层内不同轴、径向位置的瞬时固含率脉动信号,通过MATLAB软件进行解耦并统计分析求解出稠密相与稀疏相的平均固含率,以此为基础拟合了瞬时固含率信号的概率密度曲线,最后从信号中提取出颗粒聚团的有关信息. 结果表明,瞬时固含率为0.05~0.72,乳化相中颗粒聚团平均固含率为0.552~0.562. 颗粒聚团的体积分率和出现频率随表观气速增加而降低,分别为0.01~0.5和0.02~1.6 Hz,持续时间小于0.12 s.     

Positron annihilation and X-ray diffraction studies of cellulose and some derivatives

Positron lifetime measurements are reported over a temperature range from 290 to 380K on cellulose and certain of its derivatives. The lifetime spectra were resolved into three components each attributed to a different mode of positron annihilation in the polymer. The longest lifetime component is associated with the decay of ortho-positronium and is a function of the average cavity distribution in the sample. It is observed that significant differences exist in the lifetimes of positrons in cellulose and its derivatives which can be interpreted as evidence for changes in the free volume distribution in these materials. Comparison of positron decay in samples of cellulose acetate prepared at both room temperature and elevated temperature indicate that the phenomena of gelation in this material is accompanied by a reduction of free volume. The correlation of such observations with the dynamic properties of these polymers is discussed.     

Mechanical Properties of Mullite

The mechanical properties of alkoxy-derived, high-purity, translucent, theoretically dense mullite (3AI₂O₃.2SiO₂) were investigated over the temperature range room temperature to 1500°C. Large agglomerates were found to contribute to the formation of porosity nests which act as strength-controlling flaws at room temperature as well as at high temperatures. Despite the slow crack growth above 1300°C, a slight increase in fracture stress and a large increase in K_Ic were observed up to 1500°C. These increases are explained by the dominance of energy dissipation through plastic relaxation in the plastic zone over grain-boundary sliding due to the presence of the glassy phase.     

Development of spherical crystal agglomerates of an aspartic acid salt for direct tablet making

Agglomerates of an aspartic acid salt were developed by means of a non-typical spherical crystallization technique. The aspartic acid salt was crystallized by a salting-out method combined with cooling. Traditional mechanical stirring crystallization (samples A and B) and the recirculation process (sample C) were used. The control material was commercial aspartic acid salt with very poor flowability and compressibility. The particle sizes of the samples were measured by sieve analysis. The morphology of the crystals and crystal agglomerates was controlled by SEM. The specific surfaces of the products were determined by the BET method and the micropore volumes were calculated via the BJH theory. The Carr index, rearrangement constant, plasticity and compressibility values were calculated. The samples were controlled by thermoanalytical investigations (TG, DTG and DSC). Both of the crystallization techniques used resulted in spherical agglomerates of the aspartic acid salt with very good flowability and compressibility parameters. Primarily sample B, with a closed “cauliflower-like” structure, can be suggested for tablet making involving a large mass (e.g., chewable tablets) by direct tablet pressing. Faster initial cooling rate and slower stirring rate were very favourable in the building-up of crystal agglomerates of sample B with a closed structure and a large particles. Sample B can be suggested further for capsule filling because of its high poured density, very good flowability and fast rearrangement.     

Deformation behaviour of agglomerates under tensile stress

A new instrument has been developed in which the stress—strain behaviour of agglomerates can be measured in uniaxial tension. The instrument permits tension to be measured under predetermined stresses, and further enables the tensile stress—strain behaviour to be determined in the agglomerate in the unstable region.Agglomerates with different bonding mechanisms were studied. In the case where agglomerates are held together by liquid bridges, a model has been developed which permits the stress—strain behaviour to be predicted. This is confirmed by experiments. For agglomerates with a high degree of liquid saturation, the tensile stress—strain behaviour can be predicted from the capillary pressure relation. The tensile stress—strain curves of dry agglomerates and of agglomerates with solid bridges between particles are shown and compared. From the different stress—strain characteristics, the physical nature of the bonds can be elucidated.     

Influence of ion-irradiation on the free volume controlled diffusion process in polycarbonate—a positron lifetime study

Iodine sorption in un-irradiated and ion-irradiated polycarbonate (PC) has been investigated using positron lifetime spectroscopy. The decrease in positron lifetime parameters of ion-irradiated sample is attributed to the free volume modifications, and the formation of more stable free radicals on cross-linking. The difference UV absorption spectrum shows a strong absorption peak at 384 nm which is due to 2,2′-dihydroxybenzophenone; this depicts the possibility of photo stabilisation of polycarbonate on ion-irradiation. A comparative study of sorption process is explained in terms of iodine diffusion in free volume holes in un-irradiated and ion-irradiated polycarbonates. The experimental results on iodine diffusion shows an early saturation of positron parameters in irradiated PC, which is due to an increase in the rate of diffusion as small number of bigger size free volume holes are formed on irradiation. The diffusion process follows Fick's law and, an exponential type of correlation has been observed between fractional free volume and diffusion coefficient which indicates that Fujita's free volume theory is valid before and after ion-irradiation.     

Compaction behaviour of agglomerated alumina powders

The effects of agglomerate properties, such as the binder type, binder content, moisture level, and agglomerate size, on a model compaction process was investigated by using green density-pressure interrelationships for a range of agglomerated alumina powders. The model compaction process involved single ended nominal uniaxial stress transmission in a cylindrical die. The influences of the sample aspect ratio, die wall lubrication, and compaction rate were also investigated. Two types of water soluble polymeric agents, a poly(vinyl alcohol) (PVA) and a poly(ethylene glycol) (PEG), were used. It was shown that certain agglomerate properties have a strong influence upon the compaction behaviour of these ceramic powders. The extent of the compaction is enhanced by using agglomerates with a low agglomerate yield point. In the PVA system, the agglomerate yield points decreased with increasing moisture content. The compaction behaviour of the agglomerates showed a rate dependency, that is, the compaction is retarded with increased pressing rate. The green densities of the compacts prepared in the unlubricated die were lower than those of the compacts prepared in the lubricated die due to the higher wall frictional forces operating in the unlubricated die.     

Role of the effective electrical conductivity of nanosuspensions in the generation of TiO2 agglomerates with electrospray

Suspensions with varying volume fraction of TiO₂ nanoparticles and ionic strength were electrosprayed to obtain agglomerates of different characteristics, which were then deposited to produce films with tailored morphology, thickness, and porosity. The role of the nanoparticle volume fraction in both the effective electrical conductivity of TiO₂ nanosuspensions and the control of the size of agglomerates produced by electrospray was investigated. A simple modified equation for the effective electrical conductivity of TiO₂ nanoparticle suspensions was derived. The equation, which accounted for nanoparticles' diffuse ionic layer and their agglomeration in a liquid, showed that the effective electrical conductivity is not only a function of the liquid and particle conductivities, and the particle volume fraction but also a function of both the thickness of the adsorbed ionic layer on the particles and the particle size. Gradual increase of particle volume fraction resulted in an increase in the suspension's effective electrical conductivity, when the initial liquid conductivity was in the range of 10^?4–10^?3 S m^?1. When the liquid conductivity was in the range of 10^?3–10^?2 S m^?1; however, addition of particles did not have any significant effect on the effective electrical conductivity. Control over the size of the TiO₂ nanoparticle agglomerates was achieved by electrospraying suspensions with liquid electrical conductivity of the order of 10^?3 S m^?1 and by varying the particle volume fraction. Electrospray deposition of suspensions with TiO₂ volume fraction=0.04% resulted in a more compact film with lower porosity and showed better water-splitting performance.     

聚丙烯/碳纳米管微孔注塑发泡行为及力学性能

研究了不同含量单壁碳纳米管(CNT)的加入对聚丙烯(PP)流变、热稳定性及微孔注塑发泡行为和力学性能的影响。结果表明,随着CNT含量从不足1.0 %(质量分数,下同)逐渐增加,PP熔体黏度显著增大,热稳定性逐渐提升,即使CNT的加入量仅为0.15 %也会产生团聚现象,但团聚体尺寸较小且与基体结合紧密;微孔发泡注塑样品中泡孔分布不均匀,泡孔尺寸范围在10~70 μm,直径随着CNT含量增加呈现先减小后增大;PP/CNT复合材料微孔发泡成型后,弹性模量、屈服应力下降不大,但断裂伸长率平均提升了近400 %,不同组分发泡样品间力学性能指标变化不大。     

Investigation of chemi‐crystallization and free volume changes of high‐density polyethylene weathered in a subtropical humid zone

The chemi‐crystallization and free volume changes of high‐density polyethylene (HDPE) exposed to subtropical humid climate of Guangzhou, China, were investigated using Fourier transform infrared spectroscopy, differential scanning calorimetry, wide‐angle X‐ray diffraction, dynamic mechanical analysis and positron annihilation lifetime spectroscopy. An increase in content of carbonyl groups and significant chemi‐crystallization were observed to occur during natural exposure. Chain scission accounted for the chemi‐crystallization and would lead to greater crystallizability of the molecules. The reheating DSC run indicated that the crystallizability of the degraded HDPE molecules increased initially with exposure time and then decreased. Positron data showed the new crystals induced by chemi‐crystallization indeed had more imperfect crystal structure in comparison with the pre‐existing parent crystals, and the free volume located in amorphous regions decreased involving a shrinking of the free volume holes. The shrinkage of free volume holes was correlated with the loss of mobility of HDPE molecules, which was confirmed by the increase of glass transition temperature. The formation of new imperfect crystals might increase the amount of rigid amorphous fraction of HDPE materials, as well as the occurrence of crosslinking reactions of molecules located in the interior of HDPE materials, consequently decreasing the molecular mobility. © 2016 Society of Chemical Industry     

Additives for improving the strength,stiffness, and ductility of epoxy resins

A range of additives at 7 to 20 wt% loading can increase the stiffness and strength of crosslinked epoxy resins by up to 60%, yet the tensile specimens also fail in a ductile fashion. In additive formulations where little chemical bonding occurs between the additive and the epoxy matrix (e.g., the reaction product of 4-hydroxyacetanilide and 1,2-epoxy-3-phenoxypropane), the increase in modulus is directly related to the decrease in free volume available for segmental mobility. The increase in strength results from a combination of the increased modulus and an increased fracture energy, A ductile mode of failure occurs because the cured plastic exhibits a large increase in free volume on straining (low Poinsson's ratio). The strain-induced increase in free volume effectively takes the sample through the glass to rubber transition. In more highly reactive formulations (e.g., the reaction product of 4-hydroxyacetanilide and vinyl cyclohexene dioxide), chemical bonding effects complicate this simple free volume interpretation, but the occurrence of a ductile failure mode is again related to the free volume increase with strain.     

Macrokinetics of Combustion of Monodisperse Agglomerates in the Flame of a Model Solid Propellant

The paper describes a procedure for studying the macrokinetics of combustion of agglomerates in a solid propellant flame using special samples of a model propellant generating monodisperse agglomerates. Empirical dependences of the incompleteness of aluminum combustion in the combustion products of a propellant based on ammonium perchlorate and HMX on time and pressure were established. The mass fraction of oxide accumulated on a burning agglomerate versus the degree of aluminum conversion was determined. For fine agglomerates (310–350 m), this fraction decreases with increase in the degree of conversion. For large agglomerates (400–540 mum), it increases, and, hence, the mass of large agglomerates increases as aluminum burns out. Because of accumulation of oxide, the agglomerate size does not change markedly in the examined range of parameters.