一维快速拉伸下无氧铜的动态断裂与破碎

利用电磁膨胀环技术对无氧铜环进行了不同加载电压下的动态拉伸实验。对无氧铜在破碎前出现塑性失稳多重颈缩进行了初步分析,利用能量平衡的破碎理论给出了无氧铜环的破碎分布。三维数值模拟结果表明,颈缩区温升比非颈缩区高,且多重颈缩在环周上的分布服从泊松分布。     

Controlling Necking in Extrusion Film Casting Using Polymer Nanocomposites

The research described was concerned with the effect of layered-silicate-based organically modified nanoclay fillers on controlling the extent of necking in a polymer melt extrusion film casting (EFC) process. We show that a linear polythylene resin (such as a linear low-density polyethylene—LLDPE) filled with a very low percentage of well-dispersed (or intercalated) nanoclay displays an enhanced resistance to the necking phenomenon. In general, melt-compounded nanoclay-filled LLDPE resin formulations displayed a higher final film width (less necking), thus a lower final film thickness (greater draw down for the same draw ratio), and cooled down faster when compared to the base LLDPE resin. Incorporation of nanoclay filler in the mainly linear chain LLDPE resin led to significant modification of the melt rheological properties that, in turn, affected the melt processability of these formulations. Primarily, the intercalated nanoclay-filled LLDPE formulations displayed the presence of strain-hardening in unaxial extensional rheology. Additionally, the presence of well-dispersed nanoclay in the LLDPE resin led to a display of prominent extrudate swell indicating the presence of melt elasticity in such formulations. The presence of melt elasticity, as shown by shear rheology and strain-hardening, observed by uniaxial extensional rheology, contributed to the LLDPE nanoclay formulations displaying an enhanced resistance to necking for these films. It can be concluded that linear chain polymers susceptible to necking in an EFC process can be made more resistant to such necking by using nanoclay fillers at very low levels of loading.     

无氧铜环颈缩区孔洞长大与局域化温升效应研究

 利用金相显微镜,对电磁加载下无氧铜（M态TU1）环碎片的颈缩部位进行了金相分析,观察到了颈缩区孔洞长大、汇通及孔洞壁熔化现象。利用球形孔洞模型,分析了颈缩区孔洞长大条件,发现周向偏应力更适合于描述膨胀环颈缩区的孔洞长大。据此,讨论了颈缩区的局域化温升效应。研究结果为延性金属环膨胀颈缩失稳的细观机理研究提供参考。     

Effects of Deformation Rate on the Necking of an Amorphous Copolyester Studied by Modulated DSC

The tensile loading-induced necking in notched specimens of an amorphous copolyester (aCOP) was studied by modulated differential scanning calorimetry (MDSC). It was shown that necking occurred by cold drawing since the enthalpy of cold crystallization and that of the subsequent melting agreed fairly with each other. Increasing deformation in the necking zone and increasing deformation rate of the specimens shifted the onset of cold crystallization toward lower temperatures and yielded a slightly higher glass transition temperature (T_g). This was attributed to the molecular orientation caused by mechanical loading. The finding that the melting contained a non-reversing part was considered as appearance of possible microcrystallinity. The T_g range was strongly influenced by the deformation rate and reflects the thermomechanical history of the samples accordingly. This revised version was published online in August 2006 with corrections to the Cover Date.     

The application of a Gauss-Eyring model to predict the behavior of thermoplastics in tensile experiments

The Gaussian expression for the isothermal tensile deformation of thermoplastics including the proposed strain hardening constant G_p, has been combined with the Eyring flow equation to provide a new relation describing the rate of strain of a thermoplastic in terms of the true stress and the extension ratio under isothermal conditions. In conventional mechanical tests this model can be used to quantify the tendency to strain localization, to predict the natural draw ratio and the inversion point where the true engineering stress passes through a minimum. The latter is expected to correlate with the value of the extension ratio where crazes do not propagate under tension. The equation is most easily demonstrated in constant load experiments where they agree well with published work. However, for a more precise evaluation of the theory the constant G_p should be measured separately and the calculated results compared with other tests on the same material. Where necking occurs it is possible to use a simplified plug flow model to calculate neck profiles. These show that no special assumptions are required to account for necking which results directly from the interaction of geometric thinning and strain hardening, even where true strain softening is absent. The procedure makes it possible to illustrate the way in which the form of the neck can be affected by the rate of extension or in a constant load experiment by the applied load. ©1995 John Wiley & Sons, Inc.     

On the plastic deformation of bulk syndiotactic polypropylene

 The plastic deformation of syndiotactic polypropylene (sPP) bulk samples has been investigated. A structural comparison of the initial states before and after plastic deformation by necking is carried out by X-ray diffraction observations. Independent of the initial states (amorphous, semi-crystalline with different crystal phases), only the planar all-trans crystal form of sPP is present in the deformed samples after necking. Form these results, molecular mechanisms of the plastic deformation in the neck zone of semi-crystalline polymers will be discussed. Received: 11 February 1997 Accepted: 15 August 1997     

Basal slip localization in zinc single crystals. The Considère analyses

The tensile and compression tests were performed on zinc single crystals oriented for slip in the basal slip system. During the first stage of the stress–strain curve, the localized necking was typical of strain localization in the tensile specimens. Single or multiple necks were formed along the specimen length. The range of temperatures and the strain rates for single necking of the sample was determined. The formation of such necking depends on strain hardening characteristics and can be predicted by the Considère criterion. On the other hand, propagation of the necked area along the sample length was not predictable by this criterion. Localized sliding and specimen kinking was indicative of the strain localization observed for different specimens compressed under the same conditions, i.e. temperature and strain rate. A decrease in the compression force and in the cross-sectional area with anvil displacements produced localized sliding. On the other hand, a continuous increase in the compression force was representative of tests leading to specimen kinking.     

A comparative study of jet formation in nozzle‐ and nozzle‐less centrifugal spinning systems

Centrifugal spinning, a recently developed approach for ultra‐fine fiber production, has attracted much attention as compared with the electrospinning, due to its high yield, no solution polarity and high‐voltage electrostatic field requirements, etc. In this study, the jet formation process and spinning parameters on jet path are explored and compared in nozzle‐ and nozzle‐less centrifugal spinning systems. For nozzle‐less centrifugal spinning, fingers are formed at the front of thin liquid film due to the theory of Rayleigh–Taylor instability. We find that the lower solution concentration and higher rotational speed favor the formation of thinner and longer fingers. Then, the critical angular velocity and initial jet velocity for nozzle‐/nozzle‐less centrifugal spinning are obtained in accordance with the balance of centrifugal force, viscous force, and surface tension. When jet leaves the spinneret, it will undergo a series of motions including necking and whipping processes, and then, a steady spiral jet path is formed with its radius getting tighter. Finally, we experimentally study the effect of rotational speed and solution concentration on jet path, which shows that the higher rotational speed results in a larger radius of jet path while the solution concentration has little effect on it. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1547–1559     

Wide‐angle X‐ray scattering study of the lamellar/fibrillar transition for a semi‐crystalline polymer deformed in tension in relation with the evolution of volume strain

This work is devoted to the study of the deformation mechanisms of a high‐density polyethylene deformed in tension. Specific treatments were applied to synchrotron wide‐angle X‐ray scattering patterns obtained in situ with the aim of quantifying: (i) the evolution of the apparent crystal sizes during the deformation process, (ii) the reorientation dynamics of the fragmented crystals while aligning their chains along the drawing axis during the establishment of the fibrillar morphology, and (iii) the reorientation dynamics of the amorphous chains. In addition, the volume strain evolution was measured using 3D digital image correlation. The cavitation phenomenon was found to mainly occur during the lamellae fragmentation phase. At the end of the deformation process, when the lamellar structure is destroyed, the fragmented crystals have new degrees of freedom and become free to rotate to align their chains along the drawing axis. Crystal fragmentation is then no longer needed to allow material deformation, and there is no further volume strain increase. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1470–1480     

Damage of semicrystalline polyamide 6 assessed by 3D X-ray tomography: From microstructural evolution to constitutive modeling

In this study an attempt is made to link the damage and microstructural evolution of semicrystalline polymers, in particular polyamide 6, to the macroscopic material behavior during tensile and creep tests. Tensile specimens, removed before failure were seen to have undergone striction. They were examined using synchrotron radiation tomography. These samples showed elongated axisymmetric columns of voids separated by thin ligaments of material. These observations were confirmed and refined through a cryofractography experiment of a different tensile sample, stopped before failure. An attempt was made to obtain quantitative data about void volume fraction and morphology through image analysis. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1516–1525, 2010