玻璃态高聚物细观损伤断裂统计力学

对玻璃态高聚物内部细观损伤断裂判据、机理、动力学及统计模型的最新进展，进行了较为全面、系统的介绍和总结，并简要概述了细观损伤统计描述在金属材料及玻璃态高聚物领域里的应用。     

高聚物细观损伤演化的研究进展

聚合物的银纹化损伤与断裂是一个复杂和重要的研究课题。简述了银纹引发的(热)力学条件和银纹成核的微观机理。结合最新的研究进展,对银纹向前扩展的弯月面不稳定机理、银纹增厚的蠕变机理和界面转入机理作了较详细的分析与综合。考虑银纹细观结构中横系的作用,对银纹结构模型、银纹微纤断裂判据、微纤断裂行为的分子量和缠结密度相关性以及银纹与裂纹相互作用等问题进行了较详细的综述。指出银纹生长和断裂的深入研究可望建立材料宏观断裂韧性和材料细观结构以及微观参数之间的关联,为进行材料韧性的微观设计提供一条可行的途径。并对今后这一领域的研究方向和重点进行了展望。      

松孔镀铬层的高效超声珩磨

作者使用自行研制的立式超声珩磨装置对钢质薄壁气缸套松孔镀铬层进行了超声珩磨试验。试验结果使用人造金刚石和ＣＢＮ油石时，与普通珩磨相比，超声珩磨镀铬层可提高珩磨效率１４－５９倍，表面粗度Ｒａ可达０．２－１．２７μｍ，加工精度可达ＩＴ６－ＩＴ７。     

The Relationship between Craze Structure and Molecular Weight in Polystyrene as Revealed by µSAXS Experiments

Summary: The phenomenon of crazing in polymers has received considerable attention in the past as it is thought to play a pivotal role in determining the performance of polymers under load. One aspect of particular interest has been the interconnection between molecular structure, craze characteristics, and macromechanical properties. In the present study, three different grades of polystyrene (PS) with different molecular weights have been systematically investigated in situ with synchrotron radiation microfocus small‐angle X‐ray scattering (µSAXS). The results suggest that there are different mechanisms operating in PS samples with low and very high molecular weights, compared to those of medium‐to‐high molecular weight. Previously it was thought that, above the critical molecular weight of entanglement, the effect of molecular weight on PS's mechanical behaviour at room temperature was negligible.

Craze evolution as a function of strain in PS.     

Effect of triaxial stress constraint on the deformation and fracture of polymers

One purpose of this paper is to give a brief overview on the research status of deformation, fracture and toughening mechanisms of polymers, including experimental, theoretical and numerical studies. Emphasis is on the more recent progresses of micromechanics of rubber particle cavitation and crazing, and the development of fracture criteria for ductile polymers. The other purpose is to study the effect of triaxial stress constraint on the deformation and fracture behavior of polymers. Polycarbonate (PC), acrylonitrile-butadienestyrene (ABS) and PC/ABS alloy are considered in this investigation. A series of circumferentially blunt-notched bars are used to experimentally generate different triaxial stress fields. The fracture surfaces of specimens with different notch radius are examined by scanning electron microscope (SEM) to study the fracture and toughening mechanisms of polymer alloy. It is shown that the triaxial stress constraint has a significant effect on the deformation, fracture and toughening of PC, ABS and PC/ABS alloy. We will also discuss the extent to which a micromechanics criterion proposed by the first author can serve as a fracture criterion for ductile polymers. A new ductile fracture parameter is emphasized, which can be employed to evaluate the fracture ductility of polymers. Stress state independence of the parameter for the PC, ABS and PC/ABS alloy has been experimentally verified. The project supported by the National Natural Science Foundation of China (10125212), the Trans-Century Training Program Foundation and the Key Research Fund of the Education Ministry of China (01159)     

Theoretical development and experimental validation of a thermally dissipative cohesive zone model for dynamic fracture of amorphous polymers

A thermally dissipative cohesive zone model is developed for predicting the temperature increase at the tip of a crack propagating dynamically in a nominally brittle material exhibiting a cohesive-type failure such as crazing. The model assumes that fracture energy supplied to the crack tip region that is in excess of that needed for the creation of new free surfaces during crack advance is converted to heat within the cohesive zone. Bulk dissipation mechanisms, such as plasticity, are not accounted for. Several cohesive traction laws are examined, and the model is then used to make predictions of crack tip heating at various crack propagation speeds in the nominally brittle amorphous polymer PMMA, observed to fail by a crazing-type mechanism. The heating predictions are compared to experimental data where the temperature field surrounding a high speed crack in PMMA was measured. Measurements are made in real time using a multi-point high speed HgCdTe infrared radiation detector array. At the same time as temperature, simultaneous measurement of fracture energy is made by a strain gauge technique, and crack tip speed is monitored through a resistance ladder method. Material strength can be estimated through uniaxial tension tests, thus minimizing the need for parameter fitting in the stress-opening traction law. Excellent agreement between experiments and theory is found for two of the cohesive traction law temperature predictions, but only for the case where a single craze is active during the dynamic fracture of PMMA, i.e. crack tip speed up to approximately 0.2c_R. For higher speed fracture where subsurface damage becomes prominent, the line dissipation model of a cohesive zone is inadequate, and a distributed damage model is needed.     

Mechanical behavior and tensile fractography of compatible vinylchloride–vinylacetate–maleic acid terpolymer and nitrocellulose blends

Vinylchloride–vinylacetate–maleic acid terpolymer (VMCH) and nitrocellulose (NC) were blended at 10% (W/V) concentration is cyclohexanone at different weight fractions. Compatible blends were obtained at all weight fractions. This paper reports the mechanical behavior of solvent cast blend films of VMCH and NC. The films were prepared by solution blending and subsequent casting on a mercury surface. Depending on the composition, the tensile behavior ranged from brittle to ductile. The effect of the blend ratio on the properties shows that within the Hookeian region the modulus and strength have a positive deviation from linearity, whereas the elongation has a negative deviation. The effect of the blend ratio on the ultimate properties of the materials shows a positive deviation in strength up to 63 wt% VMCH composition and a negative deviation in elongation and toughness. The tensile fractography of the pure VMCH and VMCH/NC blends shows the presence of peaks, foldings of fibrils along with cavities or voids, which indicate a ductile mode of failure with craze-initiated fracture. Fractography of the pure NC Indicates a brittle mode of failure wit h craze-initiated fracturing.     

玻璃态高分子材料银纹力学研究进展

银纹是玻璃态高分子材料所特有的一种现象, 它既是高分子材料的塑性变形和增韧机理, 又是高分子材料的损伤机理, 还是连接高分子材料微观损伤与宏观破坏的桥梁. 银纹的萌生、长大与断裂是高分子科学和固体力学所共同关注的难题. 过去几十年在试验、理论和数值模诸方面也取得了显著进展, 但较为系统的理论框架尚未建立, 人们试图把细观力学的基本知识与高分子系统银纹化现象联系起来的设想才刚刚开始,且尚未在指导高分子合金设计方面起到关键作用. 本文试图对近10多年来, 高分子银纹化的研究进展予以介绍和评述. 首先简要介绍银纹的基本形貌、萌生判据与生长规律, 然后分别介绍银纹断裂力学、银纹损伤力学和银纹细观力学的研究进展和成果, 最后概要介绍银纹分子动力学研究的最新进展.      

Effect of cyclic stress on enthalpy relaxation in polycarbonate

Possible effects of cyclic stress on physical aging in polycarbonate were investigated using differential scanning calorimetry (DSC) measurements. When the enthalpy overshoot by DSC of specimens of different previous thermophysical aging histories is measured as a function of the cyclic stress amplitudes, two characteristic regimes are observed. By correlating with optical microscopic observations, these regimes are identified as the incubation and crazing stages (denoted regimes I and II, respectively). The enthalpy relaxation behavior in Regime I is similar to thermophysical aging, indicating that the glassy structure as a whole is initially shifted to one where molecular mobility is retarded by relatively low amplitude cyclic stress. A strong interaction is also seen between the enthalpy overshoot and previous physical aging. That is, the more the material is previously aged, and the shorter the incubation period, the longer the crazing region is. As a result, brittle failure occurs over a wider load range compared with less aged specimens.     

Crazes in amorphous polymers I. Variety of the structure of crazes and classification of different types of crazes

In investigating 0.5- to 5 m-thick deformed samples in a 1000 kV high-voltage electron microscope, the formation and structure of crazes were studied in different amorphous polymers (PS, SAN, PVC, and the highly radiation-sensitive polymers PMMA and PC). Different craze types were found and classified. There are some types of true crazes with a fibrillar structure or rather a homogeneous structure, which have to be distinguished from only craze-like deformation zones. A peculiarity was found in PMMA which produces a craze type different in structure from the well-known fibrillated crazes typical of PS.