Mechanisms of interactive crazing in PC/SAN microlayer composites

Mechanisms are proposed for the two types of interactive crazing that have been observed in PC/SAN microlayer composites when the PC layer thickness is on the micron-size scale. It is demonstrated that when the PC layer is thin enough the deformation zone that forms at a craze tip can interact with the next-neighboring SAN layer. By measuring the dimensions of the craze tip in scanning electron micrographs, it was found that the craze-tip opening does not depend on the SAN layer thickness, i.e., the length of the SAN craze. Consequently, the size and shape of the resulting plastic zone in the PC layer are also independent of layer thickness. The zone that forms in the PC layer consists of a colinear plastic zone together with a pair of micro-shearbands that grow at an angle of about 45°. When the PC layer is less than 6 μm, the elastic stress concentration from the colinear plastic zone increases the probability of crazing in the neighboring SAN layer with the formation of craze doublets that consist of two aligned crazes in neighboring SAN layers. By taking into consideration the Weibull distribution of crazing, a craze doublet fraction comparable to the 30% observed experimentally was predicted with a stress intensification factor in the range of 1.03–1.05. When the PC layer thickness is less than 1.3 μm, the length of the colinear plastic zone is comparable to the PC layer thickness. Formation of a craze at the point of impingement of the plastic zone on the neighboring SAN layer leads to craze arrays with many aligned crazes in neighboring SAN layers. At higher strains, the micro-shearbands grow through the PC layers and extend into several adjacent SAN and PC layers. This produces a change in deformation mechanism in the SAN layers at the yield instability, from craze opening to shear yielding. © 1994 John Wiley & Sons, Inc.     

Interdiffusion in microlayered polymer composites of polycarbonate and a copolyester

The interdiffusion of two miscible polymers, polycarbonate (PC) and a copolyester (KODAR), was studied at temperatures from 200 to 230°C. The two polymers were coextruded as microlayer composites with up to 3713 alternating layers. The microlayer structure provided a large area of intimate contact between the two polymers with minimal mixing. Initially, two glass transition temperatures were observed by DSC that were intermediate between the glass transition temperatures of the pure components. Upon annealing, the glass transition temperatures shifted closer together, reflecting the extent to which inter-diffusion had occurred. After no more than 2 h of annealing, a single glass transition temperature was observed. A model was formulated based on Fick's law of diffusion that related the mutual diffusion coefficient, D, to the change in the glass transition temperatures. The model also incorporated an “equivalent” residence time to account for diffusion that occurred during the coextrusion process. It was not necessary to consider the concentration dependence of D to satisfactorily describe the data with this model. For the temperature range from 200 to 230°C, the value of D varied from 4.0 × 10^?16 to 1.6 × 10^?15 m²/s. The activation energy of interdiffusion was determined to be 95 kJ/mol. © 1994 John Wiley & Sons, Inc.     

Notch tip damage zone in biaxially oriented polypropylene at low temperature

It is well established that biaxial orientation produces large improvements in the mechanical properties of polypropylene; this study further shows that the large improvements in mechanical behavior are magnified especially below the glass transition temperature. In this study, the irreversible deformation behavior of polypropylene during sharp single-edge notch tension testing has been studied at two levels of biadial orientation at ?40°C. Unoriented polypropylene formed a narrow wedge-shaped damage zone that grows with increasing stress until catastrophic fracture occurs in a brittle manor. The damage zone consisted of many crazes that mainly grew perpendicular to the loading direction. The 50% oriented material initially developed a wedge-shaped damage zone that grew wider as loading increased. This resulted in a drop of the length-to-width ratio at high sample extensions. The specimen fractured with stable crack growth in a ductile manner, showing a large resistance to crack growth. The 80% oriented material had a circular damage zone that consisted of many delamination crases. These crases grew by splitting the specimen in the thickness direction. Stable crack growth dominated the final failure process with the 80% oriented material showing nearly three times the toughness of the unoriented material. © 1994 John Wiley & Sons, Inc.     

Effect of an organic dicarboxylic acid salt on fractionated crystallization of polypropylene droplets

The effect of a particulate nucleating agent on fractionated crystallization of polypropylene (PP) was studied. A novel method utilizing breakup of PP nanolayers was used to obtain a dispersion of PP droplets in a polystyrene (PS) matrix. An assembly with hundreds of PP nanolayers alternating with thicker PS layers was fabricated by layer‐multiplying coextusion. The concentration of an organic dicarboxylic acid salt (HPN) nucleating agent in the coextruded PP nanolayers was varied up to 2 wt %. When the assembly was heated into the melt, interfacial driven breakup of the thin PP layers produced a dispersion of PP particles in a PS matrix. Analysis of optical microscope images and atomic force microscope images indicated that layer breakup produced a bimodal particle size distribution of submicron particles and large, micron‐sized particles. Almost entirely submicron particles were obtained from breakup of 12 nm PP layers. The fraction of PP as submicron particles dropped dramatically as the PP nanolayer thickness increased to 40 nm. Only large, micron‐sized particles were obtained from 200 nm PP nanolayers. The crystallization behavior of the particle dispersions was characterized by thermal analysis and wide angle X‐ray diffraction. Only part of the PP was nucleated by HPN. It was found that HPN was not effective in nucleating the population of submicron particles. The particulate HPN was too large to be accommodated in the submicron PP particles. On the other hand, the amount of nucleated crystallization qualitatively paralleled the fraction of PP in the form of large, micron‐sized particles. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Structure and deformation of an elastomeric propylene–ethylene copolymer

The elastic behavior of a propylene–ethylene copolymer was investigated. An initial “conditioning” tensile extension up to 800% strain resulted in an elastomer with low initial modulus, strong strain hardening, and complete recovery over many cycles. Structural changes that occurred in the low crystallinity propylene–ethylene copolymer during conditioning, and that subsequently imparted elastomeric properties to the conditioned material, were investigated. Thermal analysis, wide and small angle X‐ray diffraction, and atomic force microscopy measurements were performed at various strains during the conditioning process. Conditioning transformed crystalline lamellae into shish‐kebab fibers by melting and recrystallization. The fibers, accounting for only 5% of the bulk, were interconnected by a matrix of entangled, amorphous chains that constituted the remaining 95%. It was proposed that the shish‐kebab fibers acted as a scaffold to anchor the amorphous rubbery network. Entanglements of the amorphous chain segments acted as network junctions and provided the elastic response. The stress–strain response of materials conditioned to 400% strain or more was described by the classical rubber theory with strain hardening. The extracted value of M_c, the molecular weight between network junctions, was intermediate between the entanglement molecular weights of polypropylene and polyethylene. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 489–499, 2007     

钢质压力管道模态的有限元分析

主要介绍了ansys软件的功能以及压力管道的模态理论计算方法,最后提出了使用ansys软件进行模态分析方法,并得出用该软件分析模态的主要步骤。并且使得在工作中压力管道的震动控制一定要远离固有频率,以避免发生共振而使管道系统发生破坏失效。     

压力管道未融合缺陷的安全分析

对压力管道焊接接头中未融合等缺陷产生原因及安全进行了分析,并确定压力管道的安全状况等级。     

甲基异丙烯基酮合成工艺的改进

对甲基异丙烯基酮的合成工艺进行了改进,比较了在不同温度下,对醇酮中间体和脱水反应收率的影响。     

船用尾轴腐蚀开裂分析

某船用尾轴局部区段发生凹槽状损伤,采用扫描电镜、金相显微镜等设备对损伤尾轴的化学成分、微观组织及断口形貌进行了分析,结果表明,该尾轴在服役过程中由于没有隔绝液态介质,从而导致电偶腐蚀而开裂。     

关于两起工程机械设备早期疲劳失效的分析及探讨

通过宏观分析、扫描电镜、光学显微镜等手段对装载机车架及液压挖掘机的液压泵驱动轴的早期断裂进行了综合分析,结果表明均为制造工艺不当而诱发的疲劳断裂失效,同时对有关防范措施作了探讨。