Molecular weight distribution effects on craze micromechanics

Quantitative transmission electron microscopy has been used to investigate the micromechanics of crazes in monodisperse and polydisperse polystyrene (PS). In the monodisperse PS, a large stress is observed at the craze tip, but this stress falls rapidly with distance behind the tip. In the polydisperse PS (or in blends in which a small percentage of monodisperse low molecular weight PS has been added to monodisperse high molecular weight PS) the stress at the craze tip is lower and falls much less rapidly with distance behind the craze tip. These changes are attributed to the effects of diluting the network of entangled high molecular weight chains with molecules that are too short to entangle. This dilution increases the molecular stress on the load bearing chains of the network, thus increasing the ease of craze fibrillation.     

Mechanism of craze thickening during craze growth in polycarbonate

It has been shown previously that dry craze growth kinetics in bisphenol-A polycarbonate (PC) subjected to creep are compatible with a model in which craze growth occurs at constant aspect ratio, the rate-controlling process being craze thickening due to homogeneous creep of craze material. In the present work, craze thickening during growth was investigated by electron microscopy and by an optical interference method in order to check this hypothesis. Results indicate that thickening occurs by drawing in fresh material from the craze—matrix interface. The previous model is modified in order to take these findings into account.     

Stress analysis around a three-dimensional craze in glassy polymers

Stress analysis around a three-dimensional craze and a three-dimensional craze containing a penny-shaped crack has been made. The craze is treated as a transversely isotropic, oblate spheroid embedded in an isotropic glassy polymer. The craze is assumed to consist of primary fibrils and cross-tie fibrils, such that a penny-shaped crack may form at the central regime of the craze. The craze surface stress, the stress field near the craze tip outside the craze region, and the stress intensity factor in the crack tip are determined by using Eshelby's equivalent inclusion method. Numerical values of the fracture toughness and the stress needed to sever a craze fibril at the crack tip are calculated and the results appear to be in good agreement with the data given in the literature.     

无梁式抽油机塔架的有限元分析

本文利用ANSYS Workbench软件对新研发的8吨无梁式抽油机的塔架进行有限元分析,在满足新型抽油机使用性能要求的前提下,获取性价比较高的塔架设计方案,以降低生产成本和提高经济效益.     

Stress-strain behavior of the craze

The constitution and properties of crazes in glassy polymers and their relation to crack propagation are reviewed. New evidence is discussed which shows the craze to be much softer than the parent polymer but capable of sustaining large stresses and strains up to the point of failure. Craze failure is much more dependent on polymer molecular weight than is craze formation, and this difference is reflected in changes in both fracture surface morphology and crack toughness with molecular weight. Finally craze mechanical properties are thought to be integrally related to the mechanical behavior of high impact plastics.     

Diffraction studies of craze structure

Recent work using small angle scattering techniques to study craze structure is reviewed. Three different radiations, electrons, X-rays, and neutrons were used to study four problems in the area. These were (1) the structure of crazes in thin films, (2) the structure of a single craze/crack in a bulk material, (3) the effect of organic environments, and (4), the effect of mechanical fatigue on craze structures. It is shown that the high intensity of a synchrotron source permits the examination of the process of growth and breakdown of a single craze in polystyrene and also the study in real time of the short- and long-term changes during fatigue of crazes. The H/D neutron contrast effects for neutrons were used to study environmental crazes with the environment still present.     

Some molecular aspects of craze formation

Summary The changes in molecular orientation occurring during craze initiation and growth are discussed. Craze initiation is considered as a stress-induced, rotational, collective reorganisation of stiff chain sections in volume elements having a particularly large compliance. An energy balance is established for the transformation of a partially oriented matrix into craze fibrils which relates the fibril diameter to fibrillation stress and surface energy parameters. The predicted dependence of fibril diameter on temperature and preorientation is confirmed by experiments on polycarbonate.Dedicated to C. C. Hsiao and J. A. Sauer on the occasion of the 30th anniversary of the publication of their paper: On Crazing of Linear High Polymers     

Effect of strain history on craze microstructure

Crazes formed under a constant tensile strain in polystyrene (PS) have a dense network of fibrils with an extension ratio $\text{λ ? 4}$, but a midrib of higher λ forms by drawing fibrils from the craze-matrix interface in the high stress region just behind the craze tip. Stepwise increases in tensile strain during craze growth should thus produce layers of fibrils of different λ, which can be revealed by transmission electron microscopy (TEM) of crazes in stepwise strained PS films. When the time interval between strain increments of 0.5–1% is one minute, TEM images show ‘ridges’ of lower λ fibrils, corresponding to the position of the craze-matrix interface at the time of the strain increments. The ridges appear to be the analogue of the bulge remaining on a macroscopic fibre which has been allowed to stress age by stress relaxation before resuming drawing and imply that rapid stress ageing must occur near rthe craze-matrix interface so that more material is drawn into the craze in preference to increasing the λ of the existing fibrils.     

Temperature dependence of craze shape and fracture in polycarbonate

The shape of the craze at the tip of a loaded crack has been determined by optical microscopy for polycarbonate. The effect of temperature was examined, and measurements were made on samples of different molecular weight. In all cases the craze shape can be described to a good approximation by the Dugdale model for the plastic zone at a crack tip. The crack opening displacement depended on sample molecular weight, but was independent of temperature. Fracture toughness values deduced from the craze shape were in good agreement with plane strain fracture toughness obtained from direct cleavage fracture measurements, on the assumption that failure occurs by combined plane strain and plane stress fracture modes.     

Kinetics of crack tip craze zone before and during fracture

Irreversible deformations form the main energy consumption routes for absorbing supplied energy and thus determine the resistance of materials to fracture. In many polymeric materials under mode-I loading conditions three regions of irreversible deformation behavior can be distinguished as a function of the stress intensity factor K_I or strain energy release rate G_I, respectively:

• no crack growth for K_i < K_Io
• slow crack growth for K_Io < K_I < K_Ic
• rapid crack propagation for K_I < K_Ic

In the three regions the amount and geometrical extent of irreversible deformations may be different and the influence of time, crack speed, and temperature may generate different results. In this paper the influences of time and crack speed on the kinetics of the craze zone at the crack tip will be considered in some detail, with respect to results mainly obtained for poly(methyl methacrylate), PMMA, of high molecular weight under quasi-static loading conditions.     

Kinetics of alcohol assisted craze growth in polycarbonate

The kinetics of alcohol assisted, relaxation controlled craze growth in polycarbonate (PC) were studied between 280 to 340K in methanol, ethanol, propanol, butanol, and pentanol. Viscous deformation of plasticized craze material controlled the growth rate of a single craze during stress relaxation in single edge notched (SEN) specimens. The activation energy increased with craze driving force in all alcohols, except methanol, because of diffusion limited plasticization. The craze stress increased with the alcohol's ability to plasticize; this is not understood, but may be due to solvent Induced crystallization.     

基于细观力学的延性构件的设计

本文阐述了依据细观力学原理设计高性能纤维增强水泥基复合材料(HPFRCC)的原则。普通钢筋或FRP筋混凝土构件需设置大量的箍筋保证延性。HPFRCC强度较高，受拉和受剪时延性较好，作为基体配置钢／FRP筋后，与相同配筋条件的混凝土梁相比，承载力和延性均提高。     

The micromechanics and microstructure of CO2 crazes in polystyrene

Although CO₂ at 1 atmosphere pressure is not a crazing and/or cracking agent for polystyrene (PS), we have established that it becomes one at higher pressure. Crazes grown from cracks in PS thin films in high pressure CO₂ are investigated using transmission electron microscopy (TEM). The fact that broken craze fibrils retract strongly upon exposure to high pressure CO₂ gas suggests that the primary effect of the CO₂ is plasticization, not surface energy reduction. Quantitative analyses of TEM micrographs of crazes grown at CO₂ pressures in the range 5 to 100 MPa at 34°C and 45°C have been carried out to find the craze fibril volume fractions v_f(x) and the surface displacements w(x) along each craze. From the fibril volume fraction profile along the craze, the dominant craze thickening mechanism of CO₂ crazes is shown to be the same as that for air crazes, i.e. the surface drawing mechanism, and not the fibril creep mechanism. The craze surface stress profile is computed from the craze surface displacements using a distributed dislocation analysis. These profiles all show a stress concentration at the craze tip which falls to a roughly constant value σ_b, over the rest of the craze. The fracture toughness G_Ic (and critical stress intensity factor K_Ic) for propagation of a crack in PS at these CO₂ pressures can also be computed. All these quantities (V_f, σ_b, G_Ic and K_Ic) show pronounced minima as a function of CO₂ pressure at 20 MPa, the same CO₂ pressure at which T_g of the polymer also reaches a minimum. These minima are more pronounced at 45°C than at 34°C. The G_Ic's and K_Ic's are depressed by orders of magnitude at the minimum, which corresponds to the qualitative observation that CO₂ becomes a severe cracking agent at these pressures. These observations provide additional confirmation that the major mechanism for the environmental crazing and cracking of PS by CO₂ is plasticization of the craze fibrils and surfaces.     

Time-dependent deformation and craze initiation in PMMA: Volume effects

As part of a study of long-term deformation and failure in glassy polymers, the time-dependent tensile compliance and lateral contraction ratio have been simultaneously determined for PMMA as a function of applied stress at room temperature. In the time range 10² to 10⁶s, measurements of longitudinal and lateral strain were made using extensometers of novel design and a new optical interference technique was developed to serve as a check on lateral displacement data. At low stresses, the time-dependence of the tensile, shear and bulk compliance was derived over an extensive timescale range (10^?8 to 10⁶s) by combining the long-time data with transformed complex modulus and Poisson's ratio results. These data illustrated the large influence of the broad β retardation region on the room temperature creep behaviour and the onset of the glass-rubber or α region at long-times. With increasing stress a relatively large decrease in retardation times associated with the α process was principally responsible for the onset of non-linearity and resulted in an increased merging of the α and β regions. Stress-induced structural changes associated with this effect were indicated by a small initial positive contribution to the volumetric strain attributed to the partial erasing of previous physical ageing in the material. A subsequent decrease in volume might involve the mechanical enhancement of the original ageing. The substantial reduction in α retardation times with increasing stress was paralleled by a decrease in craze incubation times and this result is discussed in relation to craze initiation criteria based on considerations on non-linear viscoelastic response.     

锅炉烟管口开裂原因分析与预防措施

对一台卧式快装锅炉烟管管端出现开裂的事故进行分析,并提出避免类似缺陷发生的预防措施。     

Micromechanical analysis of a model for particulate composite materials with composite particles—survey of craze initiation

Particulate-filled materials consisting of a continuous matrix phase and a discontinuous filler phase made up of discrete heterogeneous particles are simulated by an elementary model consisting of a single spherical particle embedded in an infinite matrix, the particle being constituted by a spherical core within a concentric spherical shell. The specific case studied is one in which the particle core and matrix are of the same glassy polymer, and particle shell is a rubbery material. The distributions of six suggested craze initiation factors in the region surrounding the single particle are calculated when the material is subjected to a uniform uniaxial tension at infinity. Results indicate that the critical regions for craze formation are located either at the pole or at the equator of the particle (the polar axis being parallel to the applied tension), depending on the criterion considered and on the relative size of the glassy core in the particle.     

回转窑球面瓦开裂应急处理方案

通过对回转窑球面瓦（灰口铸铁HT20-40）裂纹的修复,详细介绍了球面瓦应急处理方案。通过对设备构造和材质的相互结合,运用新型特殊材料进行修复,解决了铸铁球面瓦损坏无法修复的难题,为水泥设备维修提供一个可行性方案。     

Effect of molecular weight on craze shape and fracture toughness in polycarbonate

The shape of the craze at the tip of a crack has been studied using optical microscopy on polycarbonates of various molecular weights at ?30°C. For all molecular weights studied the craze shape was well approximated by the Dugdale plastic zone model and this model was used to calculate the craze stress and the release rate in plane strain. It was found that the craze dimensions, the craze stress and the strain energy release rate in plane strain all increased with increasing molecular weight. Fracture of macroscopic specimens showed a mixed mode fracture in all molecular weights. By studying the effect of thickness the strain energy release rate in plane strain was calculated for various molecular weights. Agreement was found between these values and those determined from the craze shape measurements. The overall strain energy release rate, the strain energy release rate in plane strain and the contributions from the plane stress mode increased with increasing molecular weight.