Effect of electrical field on crazing test of enameled wire in water

We investigated the tendency to crazing to enameled wire under applying DC voltage in an electrolyte in the different conductivity. Crazing occurred in the enameled wire film in the case of connecting the wire to the negative pole, but did not occur in the film in the case of connecting the wire to the positive pole. It was found that the penetration of H₃O⁺ ion, due to electroosmosis, is the major factor for crazing.     

Periodic crazing on polymethylmethacrylate film by localized bending

Crazing was induced on a polymethylmethacrylate film using an original, low‐cost, simple process in which the film was bent using a wedge‐shaped plate (referred to herein as a crazing edge, or an edge). In this process, stress concentration and stress opening occur spontaneously by feeding the film past the edge, and crazing is periodically induced (periodic craze). The craze interval, width, and depth depend on the localized bending conditions at the tip of the edge. Two different types of crazing mechanism were suggested. It was assumed that the films were either crazed by bending along the tip of the edge or by bending according to the curvature induced by the three‐point bending system. The calculated maximum stress determined by the three‐point bending process and the experimental values of the craze interval and width showed similar tendencies. These results suggested that the craze depth could be determined using this bending process and known mechanical properties and crazing conditions. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Measurement of the residual mechanical properties of crazed polycarbonate. I: Qualitative analysis

A new technique to quantify the bulk craze density of transparent plates was used to characterize the craze growth behavior of polycarbonate at various stress levels. The craze growth rates were found to exponentially increase with an increase in stress, obeying the Eyring equation for thermally activated processes in the presence of an applied stress. The residual mechanical properties of crazed polycarbonate were then correlated to the crazing stress, relative craze density and strain rate. The results show that increasing the bulk craze density does not affect the yield stress but decreases both the failure stress and ductility of polycarbonate. Also, a crazing stress of 40 MPa was found to cause a much larger degree of degradation of failure properties than a crazing stress of 45 MPa. Correlating the crazing stress to the craze microstructure revealed that fewer, larger crazes form at the lower crazing stress. Therefore, flaw size has a greater effect on the failure properties of polycarbonate than flaw quantity.     

Craze initiation in glassy polymers: Quantifying the influence of molecular orientation

A study has been made of crazing stress in oriented glassy polystyrene. The aim was to develop a methodology for prediction of crazing stress in glassy polymers with frozen-in molecular orientation. Oriented specimens of two grades of monodisperse polystyrene (PS) and one grade of polydisperse PS were produced by uniaxial melt-drawing and subsequent quenching of compression-moulded bars. Birefringence and crazing stress parallel to the draw direction (in the presence of diethylene glycol) were measured on miniature beam specimens cut from them. The crazing stress increased substantially with orientation, and the magnitude of the increase relaxed approximately on a timescale associated with the longest Rouse time. Specifically, a linear correlation was found, to within experimental scatter, between the increase in crazing stress and the orientation expressed in terms of frozen-in conformational stress, as predicted by the theory of Maestrini and Kramer [13]. The inverse gradient (constant β in the theory) was found to be 0.059 ± 0.002, when inferring the conformational stress from the measured birefringence. Crazing was found to be suppressed in favour of yielding in the most highly oriented specimens, and this could be explained in terms of the differing sensitivities of crazing and yield to molecular orientation.     

Formation of thermally induced craze and prevention by using infrared radiation annealing method

It is important to study the residual stress distribution and craze formation of transparent overmolded polymeric parts, because the overmolding can cause stress concentration near the interface region and the craze can be generated by the concentrated stress. Thermally induced crazing of an electric appliances product overmolded by using a transparent polymer is discussed in this study. Craze criterion was determined to be 20 MPa at the temperature higher than 60°C by using mechanical characteristic analysis of the polymeric material. Crazes were observed in the product annealed at over 60°C due to thermal residual stresses generated by the temperature difference between the surface and inside of the part upon surface heating. However, they were not observed in the product annealed by infrared radiation (IR) at over 60°C due to simultaneous heating throughout the thickness of the product. The numerical residual stress results were in good agreement with the experimental results, indicating that overmolding process, thermoviscoelastic stress development of the polymeric part, and IR annealing process were considered properly in the three-dimensional numerical analysis. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Characterisation of CVD grown diamond and its residual stress state

One of the most important quality factors in the judgement of thin diamond layers is the adhesion between the substrate and the layer which is limited by the residual stress state. The main reason for residual stress in coatings is the misfit in various properties of the layer and also the substrate, e.g. thermal expansion and crystal lattice type. The most common method of residual stress determination, based on X-ray diffraction, has to date found little application in the study of diamond films. In this paper, the method is applied to determine the residual stress in a CVD diamond film grown on a polycrystalline Al substrate. The results are interpreted with regard to the crystal structure and orientation of the layer, determined by X-ray diffraction and scanning electron microscopy.     

Measurement of the residual mechanical properties of crazed polycarbonate. II: Design of experiments analysis

The Design of Experiments (DOE) approach was used to build quantitative empirical models of the residual mechanical properties of crazed polycarbonate as functions of relative craze density, crazing stress, and strain rate. Crazing did not affect the yielding behavior of polycarbonate, but increasing the strain rate increased the yield stress according to the Eyring theory. The Eyring activation volume for yielding of crazed polycarbonate was measured to fall between reported values for conformational changes of a dilute solution of polycarbonate chains and yielding of uncrazed polycarbonate. Also, with as little as 1% relative craze density, the failure stress was approximately 10% lower and the ductility was, on the average, 50% lower than for uncrazed polycarbonate properties. It was also found that increasing the crazing stress from 40 to 45 MPa increased the failure stress and ductility for a given magnitude of relative craze density.     

The effect of water on the tensile yield of polystyrene

When PS is tested in tension but immersed in water the yield stress is unchanged and the crazing stress is increased, as compared with tests conducted in air. The crazing pattern is otherwise unchanged. If the PS is first equilibrated with water and then tested under water, both crazing stress and yield stress are reduced. A very large number of small crazes are generated which result in a 23% increase in the fracture strain.     

Crazing studies of polystyrene. I. A new phenomenological observation

This paper reports a new phenomenological observation regarding the stress crazing of glassy polystyrene. It was found that the applied stress to initiate a craze, often called the critical crazing stress, is independent of molecular weight. Further, the gross structure of the craze does depend on molecular weight, and other phenomenological aspects previously reported have been reaffirmed. These observations are interpreted in the light of knowledge from the literature and provide a better understanding of the crazing process.     

Heat treatment of thin carbon films and the effect on residual stress, modulus, thermal expansion and microstructure

Thin carbon films are used to hermetically seal and improve the performance of devices exposed to extreme conditions. Such films, which are deposited by chemical vapor deposition, develop residual thermal stresses due to a mismatch in the coefficient of thermal expansion between the film and substrate. Residual stresses reduce the adhesion of the film, and are a common cause of coating failure. This work investigates heat treatment as a potential technique to reduce residual stresses in thin carbon films. The magnitude of the residual stress has been challenging to measure due to the associated size scales and mechanical properties. In this study, experimental measurements of mechanical properties and residual stresses in thin carbon films are performed using nanoindentation and Raman spectroscopy. The results relate surface residual stresses to film thickness and heat treatment temperature. The approach presented in this study is a nondestructive and non-intrusive method for measuring residual surface stress and properties in thin films, and is ideal for small or curved-surface specimens such as optical fibers and other photonic devices.     

Controllably crazed polystyrene: Morphology and permeability

The effects of n-heptane and heat treatment on the structural and transport properties of polystyrene films (biaxially oriented and unoriented) were studied to determine whether these treatments improve the film as selective barriers for separation of molecules differing only slightly in size and shape. n-Heptane treatment of biaxially oriented polystyrene produces a sandwich structure composed of expanded, crazed, surface layers surrounding an apparently unaffected central core. The crazed layers contain a continuous network of interconnected channels. The core provides the total resistance to gas permeation, hence, the overall effect of n-heptane treatment is fabrication of a thinner more permeable membrane. By restricting the stress-cracking treatment to one face of the film, it should be possible to make high flux, anisotropic membranes—a type of membrane which is required for successful development of membrane separation processes. n-Heptane treatment of cast, annealed polystyrene results also in a crazed polymer, but the crazing is in the form of spherical voids, and the films, even with a residual uncrazed core, are too weak to be useful as separation membranes. The crazing process in both types of polymer specimens is characteristic of case II non-Fickian diffusion in which the kinetics are apparently controlled by polymer relaxation processes. Sorption of isopentane into cast, annealed polystyrene does not cause visible crazing but the kinetics are again non-Fickian. Desorption of isopentane into n-heptane-treated polystyrene releases the appreciable residual n-heptane in the film which could not be removed by long-term exposure to vacuum. Analysis of D(0) values for isopentane in n-heptane treated films indicates that the polymer surrounding the visible voids in the film is essentially unaltered polystyrene with only a small fraction of the voids being interconnected by open channels.     

Softening temperature of glassy polymers as affected by residual stresses

Heat deflection temperature and Vicat softening point of polystyrene and polymethylmethacrylate with various thermal histories were measured. Surface residual stresses were characterized by liquid crazing. Residual stress profiles across the sample thickness were also measured. It is suggested that the softening behavior of glassy polymers is affected by the state of the residual stresses in addition to the traditionally accepted free volume approach.     

Zur spannungsrißbildung von hochpolymeren

Crazing is a phenomenon, which is often observed in polymers, when mechanical stresses are applied. In this paper we report about investigations of crazing of polymethylmethacrylate as an example of an amorphous polymeric material. We deal with parameters which influence the formation of crazes. A method is described which allows one to avoid crazing, thus an improvement of the material concerning the formation of crazes is achieved. We further attempt to interprete the formation of crazes. The anology between crazing and fracture and flow behavior (cold drawing) of polymers is stressed. We suppose that the primary molecular processes underlying these phenomena are basically the same, and that they consist of an orientation of the macromecules in the direction of the applied stress.     

Influence of hydrogen on the residual stress in nanocrystalline diamond films

Nanocrystalline films were deposited by microwave-plasma CVD at a pressure of 200 mbar from an Ar/H₂/CH₄ plasma where the hydrogen fraction in the process gas was varied between 2 and 7%.Residual stress is a critical parameter in thin film deposition and especially important for technical applications of nanocrystalline diamond because high residual stress can lead to cracking or even to delamination of the film from the substrate. An ex-situ optical device was used to measure the residual stress of the substrate.It is shown that by controlling the process parameters the residual stress in the NCD films can be adjusted in a wide range even from compressive to tensile.The films were characterized by two wavelength scanning micro Raman spectroscopy and SEM.In this work a correlation is made between the intrinsic stress measurements and the Transpolyacetylene peaks (around 1120 cm^− 1 and 1450 cm^− 1) in the Raman spectra of NCD films. It is shown that the intensity and the FWHM of the peaks correlate with the tensile stress in the films. A model correlating the Raman spectra to the grain size and thus to the intrinsic stress measurements is given in this paper.     

粘弹性流体在微孔道中流动的受力分析

针对亲油岩石水驱后残余油膜的存在形式及聚合物溶液的粘弹特性,选取粘弹性流体中的上随体Maxwell本构方程并使用计算流体软件Polyflow对模型进行数值分析。对比研究了在不同的松弛时间,不同的孔道宽度下,聚合物溶液作用于残余油膜上的应力T11的变化趋势,并且将水与聚合物溶液的驱替结果进行了比较。结果表明:随着聚合物溶液弹性的增强,对油膜的作用力T11就越强;孔道宽度与油膜高度越接近,对油膜的作用力就越强;聚合物溶液对油膜的作用力较水溶液强。     

钢丝缠绕液压胶管设计优化有限元分析

钢丝缠绕胶管的爆破压力、轴向变形和径向变形与钢丝缠绕角度密切相关,目前的研究和工程实践中主要是基于压力容器薄膜应力理论和经验公式对四层排布角度相同或者相近的软管进行分析和设计。但是由于薄膜理论的假设建立在胶管小变形和材料各向同性的假设条件下,而由于各层钢丝在受力后,会发生耦合作用,工程上已发现四层角度相同的设计,无论在受力,还是轴向位移都不是最佳方案,不能满足优化的目的。因此,本文使用有限元的分析方法,对四层缠绕胶管的排布角度进行独立优化,发现四层钢丝缠绕胶管每层角度的变化对其本层和其他三层的应力分布均有影响,并且有一定的规律性。利用这一规律,可以实现对钢丝缠绕胶管排布角度的优化,使爆破压力和变形都达到符合使用要求的结果。本文分析的结果被证明和工程试验结果有很好的相关性。     

Role of surface chain mobility in crazing

Thin film experiments have shown that glassy thermoplastics exhibit enhanced molecular mobility across a 10 nm surface layer. This study examines its relevance to crazing. Surface mobility produces a steep yield stress gradient, which constrains the growth of plastic zones from surface flaws. During tensile tests, stresses in these zones increase until the bulk specimen either yields or fractures. Polycarbonate can sustain rising local stresses and strains without damage because its chains have relatively small cross-sectional areas, but high stresses in polystyrene produce chain scission, accelerated relaxation and cohesive failure. The partially degraded plastic zone breaks down to form craze fibrils, an internal necking process that is repeated during craze propagation and fibril drawing. In combination with the linear elastic fracture mechanics model for craze initiation, constraints on yielding around symmetrical surface flaws account for the strong dependence of critical tensile crazing stress σ_1cz on the second principal stress σ_2cz in biaxial tests.     

含氟聚酰胺酰亚胺自粘电磁线漆的合成与性能研究

合成了一种新型含氟二胺单体2-三氟甲基-4,4'-二氨基二苯醚(3FODA),该单体具有较高的反应活性和溶解性。使用3FODA替代4,4'-二胺基二苯醚(ODA),与偏苯三酸酐(TMA)、二甲苯甲烷二异氰酸酯(MDI)通过两步法合成了含氟聚酰胺酰亚胺自粘电磁线漆。试验证明,含氟基团三氟甲基的引入明显提高了漆膜的柔韧性和附着力。     

Adhesion–Delamination Mechanics of a Prestressed Circular Film Adhered onto a Rigid Substrate

A thin circular film clamped at the periphery is adhered to the planar surface of a rigid cylindrical punch. An external tensile load is applied to the punch, causing the film to delaminate from the substrate and the circular contact edge to contract. The film spontaneously separates from the punch, or pulls off, when the contact radius reduces to a range between 0.1758 and 0.3651 of the film radius, depending on the magnitude of the residual membrane stress. The mechanical delamination process is derived by a thermodynamic energy balance based on a coupled interfacial adhesion and residual membrane stress. The theoretical model has significant implications in nanoforce measurement, microelectromechanical systems (MEMS) comprising active moveable films, and biological cell adhesion.     

Adhesion-Delamination Mechanics of a Prestressed Circular Film Adhered onto a Rigid Substrate

A thin circular film clamped at the periphery is adhered to the planar surface of a rigid cylindrical punch. An external tensile load is applied to the punch, causing the film to delaminate from the substrate and the circular contact edge to contract. The film spontaneously separates from the punch, or pulls off, when the contact radius reduces to a range between 0.1758 and 0.3651 of the film radius, depending on the magnitude of the residual membrane stress. The mechanical delamination process is derived by a thermodynamic energy balance based on a coupled interfacial adhesion and residual membrane stress. The theoretical model has significant implications in nanoforce measurement, microelectromechanical systems (MEMS) comprising active moveable films, and biological cell adhesion.