有机玻璃断裂韧性断面形态组份结构关系的研究

测定了有机玻璃的抗裂纹增长因子K_(1C)与材料组份、拉力机夹头速度的关系。发现增塑剂和交联剂对抗裂纹性能的影响,可从聚合物分子运动能力对裂纹端点塑性屈服过程的影响说明。在对断面形态、断面层厚度、折光指数以及K_(1C)值与拉力机夹头速度关系研究之后,认为用“撕布”模式和次级断裂模式分别解释慢裂纹、快裂纹扩展过程中的实验现象是合适的。     

Approaching zero: using fractured crystals in metrology for replica molding

This report presents a simple and convenient method to generate nanoscale fractures (cracks) in smooth, single-crystalline Si substrates. The cracks propagated as approximately straight lines along the {100} crystal planes with controllable length defined by a stabilizing backlayer. Close to its tip, the crack presented a vertical offset of the two planes as step of smoothly decreasing height, ranging from the microscale to the atomic scale. The edges of a crack were in close contact at the tip of the crack but were separated at the edge where the crack was initiated. These steps served as ideal test features for probing the limits of the replication of soft lithography. Analysis of topography of original and replicated features (in "hard" poly(dimethylsiloxane and polyurethane) by atomic force microscopy demonstrated that steps down to 0.4 nm could be reproduced; these features approach the dimensions of atoms.     

Kinetic aspects in the drying dissipative crack patterns of colloidal crystals

Kinetics of the dissipative structure formation in the course of drying the colloidal crystals of silica spheres (103 nm in diameter) in aqueous deionized suspension on a rinsed cover glass has been studied by the close-up video observation. The patterns of the broad ring of the hill accumulated with the spheres coexisted with the many spoke-like cracks. The characteristic convection flow of the spheres and the interactions between the spheres and substrate were important for the pattern formation. Cracks formed suddenly in the course of drying along the outside edge first, then toward the center, and stopped around the middle point between the outside edges and the frontier of suspension area. The further growth of the cracks took place at the adjacent place of the previous crack side by side and cooperatively. After the fast formation of these cooperative spoke-like cracks was completed, then all the crack lines further developed very slowly and simultaneously toward the center with the similar rate as that of the movement of the drying frontier of the suspension area toward center. Rates of the fast and slow modes of crack formation were 6.2 mm/s and 0.0098 mm/s, respectively, at the sphere concentration of 0.033 in volume fraction.     

A model for the drying process during film formation in waterborne acrylic coatings

Establishing drying mechanisms during film formation in waterborne acrylic coatings is a technologically important problem, however complex, and still poorly understood. A model for the prediction of evaporation kinetics is proposed in this paper, where films are supposed to dry normally with respect to the film surface, and a drying front separates a top dry region from a bottom wet region. The model accounts for the competition between water evaporation and particle diffusion that determines the degree of vertical homogeneity, but also for the competition between water evaporation and particle deformation that ultimately establishes the rate-determining step in film formation processes. The model was validated by performing gravimetric water-loss experiments on latexes of acrylic polymers of various composition, various particle size and stabilizing systems, under different environmental temperatures and humidity, and various initial film thicknesses in order to evaluate the effect of the different factors that can in principle influence the film formation process.     

应变速率和电位对应力腐蚀裂纹扩展的影响

以滑移-溶解-再钝化模型为基础,推导出应力腐蚀裂纹扩展速率与裂尖应变速率和电位之间的理论公式.计算表明,在裂纹扩展速率与裂尖应变速率的关系曲线中有两个特征区域.裂纹扩展速率在区域I随裂尖应变速率增加而增大,而在区域II不随裂尖应变速率的改变而变化.用慢应变速率拉伸技术（SSRT）测量了304L不锈钢的裂纹增长速率.当电位控制在区域II的阳极区时,理论计算的裂纹扩展速率与实验得到的结果比较吻合.     

Crack propagation behavior in rubber materials

Within the linear viscoelastic theory, crack tip fields are calculated at various crack tip velocities. A transition from rubbery to glassy material behavior in the vicinity of the crack tip can be observed. Shear and bulk behavior is analyzed separately. Whereas the increase of tearing energy at higher crack tip velocities can be ascribed to the shear behavior, bulk behavior influences the fracture mechanism. The results support experimental investigations that the instability separating stable from unstable crack propagation is related to a change in the fracture mechanism. At low crack tip velocities, material separation is the result of formation, growth, and coalescence of cavities. At high crack tip velocities, cavitation is suppressed and fracture is driven by a rather brittle mechanism resulting in a decreased amount of energy to propagate the fracture process zone. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 5, pp. 882–891. This article was submitted by the authors in English.     

Do directional primary and secondary crack patterns in thin films of maghemite nanocrystals follow a universal scaling law?

Cracks due to a shrinking film restricted by adhesion to a surface are observed in nature at various length scales ranging from tiny crack segments in nanoparticle films to enormous domains observed in the earth's crust. Here, we study the formation of cracks in magnetic films made of maghemite (gamma-Fe2O3) nanocrystals. The cracks are oriented by an external magnetic field applied during the drying process which presents a new method to produce directional crack patterns. It is shown that directional and isotropic crack patterns follow the same universal scaling law with the film height varying from micrometer to centimeter scales. Former experimental studies of scaling laws were limited to small variations in height (1 order of magnitude). The large variation in height in our experiments becomes possible due to the combined use of nanocrystals and electron microscopy. A simple two-dimensional computer model for elastic fracture leads to structural and scaling behaviors, which match those observed in the experiments.     

MECHANICAL ANALYSIS OF THE FORMATION AND PROPAGATION PROCESS OF FRACTURE SYSTEMS AND ITS APPLICATION

In this paper, the mechanical mechanism and propagation process of crustal fracture system are studied by using the principle and method of fracture mechanics. The trendings of two pairs of the X-shaped shearing fracture networks existing widely on the crust are calculated and the main causes to form them from the earth's rotation and thermal activity are considered. After the formation of X-shaped shearing fracture system, parts of them became a closed one but parts of them became more active and propagated further or interacted on each other since the stress field changed in the whole or regionally. Based on the propagating processes of single fracture and fracture systems, the formation process of the zigzag fractures of shear-tension and shear-compression are studied. Some examples showing that the propagating processes of fracture system can be applied to the research of earthquake prediction, are also indicated in this paper.     

金属应变电化学研究进展

从宏观应变电化学现象，到微观层次的局部裂纹环境测量与模拟、裂纹扩展预测模型化、再钝化法确立裂纹扩展速率以及应变电化学过程控制等方面，综述了金属应变电化学的发展历程和主要的研究进展。     

Measurement of rapid crack propagation in pressure pipes: A static S4 approach

A method for creating rapid crack propagation in pressurized pipes under slow static loading using modified S4 apparatus is described. In the development of the method a complexity involved with dynamic loading in the S4 test (ISO 13477) is eliminated by the use of a displacement controlled static loading machine. The experimental system consisted of an universal testing machine, a low compliance wedge loading device, notch tip quenching apparatus and a pipe specimen where a through thickness hole is drilled to accommodate the wedge loading device. The pipe specimen is made in such a way that a section containing a hole is free from the internal pressure, while the rest of the specimen is made to carry the internal pressure which would eventually drive the unstable crack along the pipe axis. The idea of such rapid crack initiation under static loading was derived from the concept of time-temperature equivalence, where impact loading may in part be simulated by lowering the temperature at the site of rapid crack initiation. The details of the method for rapid crack propagation under static loading are described and the correlation of the results to rapid crack propagation obtained by ISO 13477 is illustrated. The two methods were shown to compare quite well in terms of critical pressure determination and the details regarding normalized rapid crack length versus the internal pressure curve as well as the crack propagation pattern.     

2D spatially periodic architectures via the drying of 1D holographically photopatterned polymer solutions

We demonstrate an efficient and versatile method for selectively generating 1D and 2D periodic polymer structures in the submicron to tens of micron range by the directed drying of 1D photopatterned polymer solutions. Ultraviolet (UV) holographic lithography (Xia et al. Chem. Rev. 1999, 99, 1823-1848) is initially used to create 1D periodic cross-link and density variations in the polymer/volatile solvent solutions. These variations act as anisotropic barriers (walls) that direct the subsequent solvent evaporation process. Somewhat akin to directional drying (Allain and Limat Phys. Rev. Lett. 1995, 74, 2981-2984) and directional solidification (Pelc, P. Dynamics of Curved Fronts; Academic Press: San Diego, CA, 1988) experiments, the drying exhibits channel-like interface propagation behavior. The combination of the instabilities and minimization of the interface area during drying can be effectively used to produce larger scale 2D pseudohexagonal polymer-strut structures, or by the addition of a monomeric component to the polymer solution, the instabilities can be suppressed, resulting in the formation of 1D linear gratings.     

Photopatternable hybrid sol–gel films: A liquid Si NMR investigation of the inorganic network formation

Methacryloxypropyltrimethoxysilane precursor has been involved in the realization of optical elements in crack free thick films (ranging from 15 to 100 μm), through spatially controlled photopolymerization. First, alkoxysilane functions were partially hydrolyzed and condensed. Then, using a photoinitiator, free radical photopolymerization was proceeded by irradiating the sample under UV or visible light. Since an organic network is formed in the matrix of the primarily formed inorganic network, understanding the formation of the silicate backbone was of first importance to ensure the creation of crack free thick films through efficient polymerization.

Liquid ²⁹Si NMR spectroscopy was used to investigate the inorganic network formation. Material preparation required evaporation of the volatile solvents released by the sol–gel process and limitation of the condensation degree. Both conditions were achieved by a drying process at room temperature. The structure and the composition of the dried sols were investigated and compared to non-dried sols. NMR peak fitting pointed out to the presence of a large variety of cyclic and linear oligomers in the sol. The structure of the dried sol appeared to depend both on the aging time and on the storage temperature. All these results have to be taken into account when the condensation degree has to be limited for specific optical applications.     

Experimental and theoretical investigation of stress corrosion crack (SCC) growth of polyethylene pipes

Degradation of polymers is usually manifested in a reduction of molecular weight, increase of crystallinity in semicrystalline polymers, increase of material density, a subtle increase in yield strength, and a dramatic reduction in toughness. Stress corrosion cracking (SCC) results from strongly coupled thermo-mechano-chemical processes, and is sensitive to material composition and morphology. The individual crack propagation stage is critical in determining the lifetime of pipe. Based on author's previous works, crack layer (CL) theory model is adopted in this study to describe the individual stress corrosion (SC) crack propagation kinetics and the time interval from crack initiation to instability and break through. The effect of localized chemical degradation at the crack tip on SC crack growth kinetics is addressed. Typical SC crack growth is presented and discussed as a step-wise manner based on the proposed model. In addition, scanning electron microscopy (SEM) observation and Fourier transform Infrared spectroscopy (FTIR) analysis of failed samples obtained by accelerated SCC tests are applied to validate the proposed model. SEM is useful to identify the change of fracture mechanisms from chemically driven crack to mechanically driven crack by the formation of visible striations. FTIR analysis enables tracking of the accumulation of chemical degradation by detecting the amount of carbonyls on the crack surface. Carbonyl index is defined to compare the amount of chemical degradation quantitatively. The purpose of this paper is to continue to develop the technical theory and understanding behind SCC phenomena to facilitate all polymer pipe industries and in particular the polyethylene pipe industry to design better resins and piping systems.     

Microflow and crack formation patterns in drying sessile droplets of liposomes suspended in trehalose solutions

Anhydrobiotic preservation potentially provides a means of long-term storage of mammalian cells in carbohydrate glasses under ambient conditions. During desiccation, sessile droplets of glass-forming carbohydrate solutions exhibit complex phenomena, including fluid flow, droplet deformation, and crack formation, all of which may alter the cell preservation efficacy. Cell-sized liposomes were employed as a model system to explore these phenomena in diffusively dried sessile droplets of trehalose solutions. Two factors were identified that strongly influenced the features of the desiccated droplets: the underlying surface and the liposomes themselves. In particular, the surface altered the droplet shape as well as the microflow pattern and, in turn, the moisture conditions encountered by the liposomes during desiccation. A ring deposit formed when the droplets were dried on polystyrene, as would be expected owing to the capillary flow that generally occurs in pinned droplets. In contrast, when dried on the more hydrophilic glass slide, the resulting droplets were thinner, and the liposomes accumulated near their centers, which was an unexpected result likely owing to the glass-forming nature of trehalose solutions. As might be anticipated given the variations in liposome distribution, the choice of surface also influenced crack formation upon continued drying. In addition to providing a preferential path for drying, such cracks are relevant because they could inflict mechanical damage on cells. The liposomes themselves had an even more profound effect on crack formation; indeed, whereas cracks were found in all droplets containing liposomes, in their absence few of the droplets cracked at all, regardless of the surface type. These complex drying dynamics merit further investigation in the development of anhydrobiotic preservation protocols, particularly with regard to the role therein of surface hydrophobicity and the cells themselves.     

Effects of substrate constraint on crack pattern formation in thin films of colloidal polystyrene particles

Crack formation and the evolution of stress in drying films of colloidal particles were studied using optical microscopy and a modified cantilever deflection technique, respectively. Drying experiments were performed using polystyrene particles with diameters of 47 ± 10 nm, 100 ± 16 nm, and 274 ± 44 nm that were suspended in water. As the films dried, cracks with a well-defined spacing were observed to form. The crack spacing was found to be independent of the particle size used, but to increase with the film thickness. The characteristic crack spacing was found to vary between 20 and 300 μm for films with thickness values in the range 3-70 μm. Cantilever deflection measurements revealed that the stresses that develop in the film increase with decreasing film thickness (increasing surface-to-volume ratio). The latter observation was interpreted in terms of the effects of a substrate constraint which causes the build up of stresses in the films. This interpretation was confirmed by crack formation experiments that were performed on liquid mercury surfaces in which removal of the substrate constraint prevented crack formation. Experiments were also performed on compliant elastomer surfaces in which the level of constraint was varied by changing the substrate modulus. The cracking length scale was found to increase with decreasing substrate modulus. A simple theory was also developed to describe the substrate modulus dependence of the cracking length scale. These combined experiments and theory provide convincing evidence that substrate constraints are an important factor in driving crack formation in thin colloidal films.     

Regular patterns generated by self-organization of ammonium-modified polymer nanospheres

Macroscopic regular stripe-crack patterns have been observed in the course of drying the aqueous suspensions of ammonium-modified polymer nanospheres. These forms emerged because the evaporation of dispersed water and self-assembly of nanospheres originates shrinkage during drying the aqueous suspensions. The drying condition plays an important role as well as the nature of the ammonium-modified polymer nanospheres for the stripe-crack pattern formation. By means of the vertical deposition method, directional stripe-crack patterns have been achieved in the macroscopic scale. Surprisingly, we have still noted an interesting secondary stripe pattern occurred spontaneously on the stripes.     

The effect of cold rolling on crack propagation behavior in high-density polyethylene

Crack propagation behavior in HDPE was studied. The preliminary orientation of the polymer, which is deformed in its isotropic state via necking and breaks down at the neck propagation stage, improves the crack resistance and ductility of the material. The critical crack opening in preoriented HDPE samples dramatically increases at relatively low draw ratios of cold rolling while the speed of transverse crack propagation decreases.     

Characterizing fracture energy of proton exchange membranes using a knife slit test

Pinhole formation in proton exchange membranes (PEM) may be caused by a process of flaw formation and crack propagation within membranes exposed to cyclic hygrothermal loading. Fracture mechanics can be used to characterize the propagation process, which is thought to occur in a slow, time‐dependent manner under cyclic loading conditions, and believed to be associated with limited plasticity. The intrinsic fracture energy has been used to characterize the fracture resistance of polymeric material with limited viscoelastic and plastic dissipation, and has been found to be associated with long‐term durability of polymeric materials. Insight into this limiting value of fracture energy may be useful in characterizing the durability of proton exchange membranes, including the formation of pinhole defects. In an effort to collect fracture data with limited plasticity, a knife slit test was adapted to measure fracture energies of PEMs, resulting in fracture energies that were two orders of magnitude smaller than those obtained with other fracture test methods. The presence of a sharp knife blade reduces crack tip plasticity, providing fracture energies that may be more representative of the intrinsic fracture energies of the thin membranes. Three commercial PEMs were tested to evaluate their fracture energies (G_c) at temperatures ranging from 40 to 90 °C and humidity levels varying from dry to 90% relative humidity (RH). Experiments were also conducted with membrane specimens immersed in water at various temperatures. The time temperature moisture superposition principle was applied to generate fracture energy master curves plotted as a function of reduced cutting rate based on the humidity and temperature conditions of the tests. The shift with respect to temperature and humidity suggests that the slitting process is viscoelastic in nature. Also such shifts were found to be consistent with those obtained from constitutive tests such as stress relaxation. The fracture energy is more sensitive to temperature than on humidity. The master curves converge at the lowest reduced cutting rates, suggesting similar intrinsic fracture energies; but diverge at higher reduced cutting rates to significantly different fracture energies. Although the relationship between G_c and ultimate mechanical durability has not been established, the test method may hold promise for investigating and comparing membrane resistance to failure in fuel cell environments. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 333–343, 2010     

Drying dissipative patterns of colloidal crystals of silica spheres in organic solvents

Drying dissipative structural patterns formed in the course of drying colloidal crystals of silica spheres (110 nm in diameter) in water, methyl alcohol, ethyl alcohol, 1-propyl alcohol, diethyl ether, and in the mixtures of ethyl alcohol with the other solvents above have been studied on a cover glass. The macroscopic broad rings were formed in the outside edges of the dried film for all the solvents examined. Furthermore, much distinct broad rings appeared in the inner area when the solvents were ethyl alcohol, methyl alcohol, and their mixtures. Profiles of the thickness of the dried films were sensitive to the organic solvents and explained well with changes in the surface tensions, boiling points, and viscosities of the solvents. The macroscopic and microscopic spoke-like crack patterns formed. The drying area (or the drying time) increased (or decreased) as the surface tension of the solvent decreased. However, the absolute values of these drying parameters are determined also by the boiling points of the solvents. Importance of the fundamental properties of the solvents is supported in addition to the characteristics of colloidal particles in the drying dissipative pattern formation.     

Auto-stratification in drying colloidal dispersions: a diffusive model

The mechanism by which the particles in a drying film come into close packing during solvent evaporation has an important role to play in the final film morphology. During drying the particles can develop non-uniform concentrations across the vertical height of the film, depending on their diffusion rate. By applying the principles of classical diffusion mechanics to a hard sphere system, a theory for this novel method of stratification during drying of a two component film has been derived. The model is dependent on the particle Peclet numbers and when one is above unity and the other below, maximum stratification is observed.