The Effect of Cooling Mode on Slow Crack Growth Resistance of Polyethylene Pipe

High-density polyethylene (HDPE) pipes have been widely used as gas or water transport pipes owing to their comprehensive advantages. One of the principal failure modes determining pipe service lifetime is slow crack growth (SCG) with the crack occurring first at the inner surface due to the slow cooling rate of the pipe's inner wall during polyethylene (PE) pipe extrusion. In order to change the conventional cooling mode and increase the cooling rate in the inner wall of PE pipe during extrusion, a novel extrusion equipment was designed and manufactured by our research team. For this paper, compressed air as a cooling medium was introduced through the interior of the hot extruded pipe during its extrusion to realize the quick inner wall cooling, and the effects of the inner wall's cooling rate on the microstructure and mechanical properties of the PE pipe were investigated. The experimental results showed that simultaneously cooling of both the outer and inner walls could decrease the difference in the solidification rate across the pipe and reduce the residual internal stresses in PE pipe. The quick cooling of the inner wall of the extruded pipe could also decrease the PE crystal thickness, and increase the number of tie molecules in the inner wall, which is a key parameter determining the resistance to SCG. As a result, compared to the PE pipe produced by the conventional extrusion, the crack initiation time of the PE pipe manufactured by the novel method increased from 27 h to 45 h and the crack growth rate was slower.     

Effect of Die Temperature on Morphology and Performance of Polyethylene Pipe Prepared via Mandrel Rotation Extrusion

The effect of die temperature on the mechanical performance and morphology of polyethylene (PE) pipe prepared via mandrel rotation extrusion is described. The experimental results showed that during the rotation extrusion of PE pipe, the hoop flow caused by the mandrel rotation was superimposed on the axial flow to deviate the formed shish-kebab from the axial direction which was favorable to improve the hoop strength of the PE pipe. However, high die temperature caused relaxation of most of the oriented molecular chains and the consequent formation of isotropic crystals in the PE pipe, whereas too low a die temperature led to imperfections in the PE pipe. As a result, there was an optimum temperature range for the enhancement of the hoop strength. When the mandrel rotated at 6 r/min, the hoop strength of the PE pipes prepared at the die temperature of 170°C reached the maximum value, 31.8 MPa, 22% and 29% higher than that at 150°C and 210°C, respectively.     

导电导磁管道外任意放置线圈激励下脉冲涡流场时域解析解

利用二阶矢量位和洛伦兹互易定理,解析求解了导电、导磁金属管道外任意放置线圈激励下非轴对称涡流场的频域解.利用求频域式极点处留数的方法求解拉普拉斯反变换,得到了脉冲电流激励下检测线圈两端感应电压以及管壁内脉冲涡流分布的时域解析式.分析比较了不同线圈放置方式下管壁内脉冲涡流的分布和扩散过程,以及感应电压对管壁的灵敏度.研究结果表明:当线圈轴线沿管道径向法线方向放置时,得到的感应电压时域信号最强,对壁厚的检测灵敏度最高.     

Oriented Crystallization of Polyethylene in Compatible Blends with Polyamide 11

The oriented crystallization of polyethylene (PE) in uniaxially oriented films of compatible blends with polyamide 11 (PA11) was studied. The PE sample used was a random copolymer of PE with methacrylic acid (MAA), poly(ethylene-co-methacrylic acid) (PEMAA), with 4wt% MAA units. Oriented films of PA11/PEMAA blends were prepared by uniaxial drawing of the melt-mixed blends. The drawn films with fixed lengths were heat-treated at 120°C for 3min to melt the PE component, followed by cooling the sample to room temperature at a rate of 2°C/min to recrystallize the PE (designated slowly crystallized sample). The PE component crystallized in elongated domains of PEMAA with diameters of 0.5–2 μ m and lengths of 5–10 μ m for the PA11/PEMAA = 80/20 blend, resulting in the oriented crystallization of PE crystals. The crystal b-axis of PE was highly oriented in the direction perpendicular to drawing, while the crystal a-axis was weakly oriented in the drawing direction in the slowly crystallized sample of the PA11/PEMAA = 80/20 blend. The a-axis orientation of PE crystals in the PA11/PEMAA = 80/20 blend contributes to the improvement of mechanical properties in the direction perpendicular to drawing.     

Stability characteristic of hypersonic flow over a blunt wedge under freestream pulse wave

To investigate the stability characteristic of hypersonic flow under the action of a freestream pulse wave, a high-order finite difference method was employed to do direction numerical simulation (DNS) of hypersonic unsteady flow over an 8° half-wedge-angle blunt wedge with freestream slow acoustic wave. The evolution of disturbance wave modes in the boundary layer under a pulse wave and a continuous wave are compared, and the wall temperature effect on the hypersonic boundary layer stability for a pulse wave disturbance is discussed. Results show that, both for a pulse wave and a continuous wave in freestream, the disturbance waves inside the nose boundary layer are mainly a fundamental mode; the Fourier amplitude of pressure disturbance mode in the boundary layer for a pulse wave is far less than that for a continuous wave, and the band frequency of the former is wider than that of the latter. All disturbance modes decay rapidly along the streamwise in the nose boundary layer. In the non-nose boundary layer, the dominant mode is transferred from fundamental mode into second harmonic. The transformation of dominant mode for a pulse wave appears much earlier than that for a continuous wave. Different frequency disturbance modes present different changes along streamline in the boundary layer, and the frequency band narrows around the second harmonic mode along the streamwise. Keen competition and the transformation of energy exist among different modes in the boundary layer. Wall temperature modifies the stability characteristic of the hypersonic boundary layer, which presents little effect on the development of fundamental modes and cooling wall could accelerates the growth of the high frequency mode as well as the dominant mode transformation.     

Experimental Validation of a Two-Dimensional Model of the Transient and Steady-State Characteristics of a Wicked Heat Pipe

Abstract

The transient behavior of a copper-water wicked (80 mesh SS-304 screen) heat pipe during start-up is studied. The transient axial temperature distributions are measured and successfully compared for all the three sections of the heat pipe with the numerical solution of a developed two-dimensional model that utilizes the concept of a growing thermal layer in the wall and the wick region and takes into account axial conduction during transient operation. Steady-state temperature profiles are obtained as a limiting case. The experimental and theoretically predicted time required to attain steady state are in close agreement as well.     

Vibration analysis and sound field characteristics of a tubular ultrasonic radiator

A sort of tubular ultrasonic radiator used in ultrasonic liquid processing is studied. The frequency equation of the tubular radiator is derived, and its radiated sound field in cylindrical reactor is calculated using finite element method and recorded by means of aluminum foil erosion. The results indicate that sound field of tubular ultrasonic radiator in cylindrical reactor appears standing waves along both its radial direction and axial direction, and amplitudes of standing waves decrease gradually along its radial direction, and the numbers of standing waves along its axial direction are equal to the axial wave numbers of tubular radiator. The experimental results are in good agreement with calculated results.     

Propagation and reflection of lower hybrid waves launched by slow waveguiding circuits

Using finite length, modified slow waveguiding circuits, cold lower hybrid waves can be made to propagate in one or two ways along the axial direction, corresponding to the π/2- or π-modes characteristic for these periodic structures. On reflection at a metal wall, phase reversal is demonstrated.     

磁流体动力学磁控进气道流场分布数值模拟

采用磁流体控制的办法能提高超燃冲压发动机的推进性能。结合了Maxwell方程组和描述流体力学规律的Navier-Stokes方程组,并对这些方程进行了简化,建立了适用于计算磁流体动力学(MHD)超燃冲压发动机磁控进气道流场分布的数值模拟模型。研究了在特定工况下,流场特征、电势、电流以及提取功率等参数的变化。磁流体发生器能够降低管道出口马赫数和流向速度,降低出口处的总焓,但是出口处的温度有所提高。在电极上,电势保持常数,而绝缘壁面的电势较高,电场在电极端点出现周期性的极性。y方向电流在电极板附近很高,而在绝缘板上几乎为零。电流主要从正电极流向负电极,而且沿着x方向略有减小。y方向电流最大值出现在绝缘壁面上,而绝缘板上的z方向值几乎为零。z方向电流最大值出现在管道的边角处,而在绝缘壁面上几乎为零,电流在绝缘壁面的法线分量为零。     

Thermal optics distortion of face pump slab lasers with two pump arrangements

Two pump arrangements, which are the slow axis and the fast axis of LD stacks parallel to the length direction of the slab, are simulated. From the results the optical intensity along the width of the slab is Gaussian and super-Gaussian distribution, respectively. Then based on heat conduction equation and conductively cooling boundary condition, the calculation of the distribution of absorbed pump power, temperature and thermal stress with the tool of finite element analyses is performed. From these results we numerically calculate the thermal focusing in zigzag and perpendicular zigzag planes using the ray tracing method running at low and high repetition rates. It is found that the temperature distribution along width has a significant impact on the thermal stress and thermal optics distortion, especially running at high repetition rate or high average power. Finally, the conclusion was that the pump arrangement with the fast axis of LD stacks parallel to the length direction of the slab is superior to the slow axis of LD stacks parallel to the length direction of the slab in decreasing thermal lens effect and obtaining high beam quality.     

Microstructure and residual stress of laser rapid formed Inconel 718 nickel-base superalloy

The microstructure and residual stress of laser rapid formed (LRFed) nickel-base superalloy Inconel 718 was investigated. The as-deposited microstructure of an LRFed Inconel 718 alloy is composed of columnar dendrites growing epitaxially along the deposition direction, and the columnar dendrites transformed to unevenly distributed equiaxed grains after annealing treatment at high temperature. Residual stress evaluation in microstructure scale by Vickers micro-indentation method indicates that the residual thermal stress is unevenly distributed in the LRFed sample, and it has a significant effect on the recrystallization during solution annealing treatment. The residual stress is introduced by rapid heating and cooling during laser rapid forming. There is an alternative distribution between high residual stress regions and low residual stress regions, within a single deposited layer, resulting in a similar distribution of recrystallized grain size.     

Investigation of scaling laws in a turbulent boundary layer flow with adverse pressure gradient using PIV

We present an experimental investigation and data analysis of a turbulent boundary layer flow at a significant adverse pressure gradient at Reynolds number up to Re_θ = 10, 000. We combine large-scale particle image velocimetry (PIV) with microscopic PIV for measuring the near wall region including the viscous sublayer. We investigate scaling laws for the mean velocity and for the total shear stress in the inner part of the boundary layer. In the inner part the mean velocity can be fitted by a log-law. In the outer part of the inner layer the log-law ceases to be valid. Instead, a modified log-law provides a good fit, which is given in terms of the pressure gradient parameter and a parameter for the mean inertial effects. Finally we describe and assess a simple quantitative model for the total shear stress distribution which is local in wall-normal direction without streamwise history effects.     

外应力对Y波导器件尾纤消光比的影响

本文研究了外应力对Y波导器件尾纤消光比及其温度稳定性的影响机理.在对保偏光纤固定部分进行应力分析的基础上,研究了外应力的大小、方向对于光纤应力双折射及尾纤消光比的影响,并分析了消光比的全温稳定性.结果表明,外应力是影响消光比常温性能及全温稳定性的主要因素.当外应力的方向与光纤固有应力双折射的主轴不重合时,尾纤最大消光比会低于保偏光纤的固有消光比,并且耦合后波导芯片与光纤慢轴之间对轴失准.当温度变化时,外应力的变化将导致消光比随温度的波动.当外应力沿着保偏光纤固有双折射主轴方向时,能够确保尾纤在耦合过程中实现最大的消光比,并保证波导芯片偏振方向与光纤慢轴的严格对准,此时尾纤消光比具有最佳的温度稳定性.对V型槽和U型槽光纤固定块进行了尾纤消光比性能及其温度稳定性的比较分析,试验验证表明,V型槽方案相对于难以保证外应力方向与光纤慢轴重合的U型槽固定块设计方案而言,可以有效改善消光比性能及其全温温度稳定性.     

Ultrasonic guided wave parameters for detection of axial cracks in feeder pipes of PHWR nuclear power plants

The dispersion curves for the feeder pipes in PHWR nuclear power plants were determined. The wave modes used for the detection of notches in the feeder pipe were confirmed as F(m,2) and/or L(0,1) by an analysis of short time Fourier transformation (STFT). The axial notches in the straight pipe were not detectable, but an axial notch in a bent pipe was detected with the mode at the frequency of 500 kHz. Initial F(m,2) and/or L(0,1) modes contains a circumferential displacement and might be converted to certain complicated modes in the bent region, which is sensitive to the axial notch. The circumferential guided wave technique was also applied for quantitative evaluation of the axial notches. The waves generated by a rocking motion of the transducer along the circumferential direction were estimated as the circumferential guided waves after a review of the acquired data and the dispersion curves.     

Rotation effect on near-wall turbulence statistics and flow structures

Turbulent flow in an axially rotating pipe, involving complicated physical mecha- nism of turbulence, is a typical problem for the study of rotating turbulent flow. The pipe rotation induces two effects on the flow. One is the stabilizing effect due to the centrifu- gal and Coriolis forces, which accounts for the relaminarization of the turbulence[1—3] and the reduction of the friction coefficient at the pipe wall. The behavior is also related to the wall streaks inclining to the azimuthal di…     

Deformation analysis of an incompressible composite cylindrical tube subjected to end axial loads and internal constraint

In this paper,the problem of axially symmetric deformation is examined for a composite cylindrical tube under equal axial loads acting on its two ends,where the tube is composed of two different incompressible neo-Hookean materials.Significantly,the implicit analytical solutions describing the deformation of the tube are proposed.Numerical simulations are given to further illustrate the qualitative properties of the solutions and some meaningful conclusions are obtained.In the tension case,with the increasing axial loads or with the decreasing ratio of shear moduli of the outer and the inner materials,it is proved that the tube will shrink more along the radial direction and will extend more along the axial direction.Under either tension or compression,the deformation along the axial direction is obvious near the two ends of the tube,while in the rest,the change is relatively small.Similarly,for a large domain of the middle part,the axial elongation is almost constant;however,the variation is very fast near the two ends.In addition,the absolute value of the axial displacement increases gradually from the central cross-section of the tube and achieves the maximum at the two endpoints.     

Horizontal continuous casting of metals with mould excited by ultrasonic waves

The thermal and mechanical conditions which exist during continuous casting of metals at the interface between bar and mould determine the cooling of the bar as well as the friction between bar and mould.Ultrasonic excitation of a horizontally-operating, continuous casting mould increased considerably the maximum throughput. The average value of the inner wall temperature of the mould rose during ultrasonic excitation. Periodic temperature fluctuations decrease during exposure to ultrasonic waves and a more even cooling of the bars is achieved. The force of removal was also reduced. In particular, peaks in the force when the bars are stuck together are reduced.The ultrasonic treatment also improved the surface finish. There is a relation between the temperature fluctuations of the inner side of the mould and the peak-to-peak roughness height. Grain refinement occurs as well as a more even grain distribution in the casting direction.     

Crystal Morphology of Water-Assisted Injection Molded High-Density Polyethylene with Two Different Molecular Weights

The crystal morphology of water-assisted injection molded (WAIM) parts of high-density polyethylene (HDPE) with two different molecular weights was investigated. The results showed that for the WAIM parts of HDPE with higher molecular weight, oriented lamellar structures formed in the outer layer, whereas spherulites formed in the core and inner layers, at positions both near the water inlet and near the end of the water channel. However, for the WAIM parts of HDPE with lower molecular weight, spherulites formed in all three layers at a position near the water inlet, whereas oriented lamellar structures formed in the outer layer and banded spherulites were dominant in both core and inner layers at a position near the end of the water channel. The crystal morphology development was interpreted with the aid of stress and temperature fields within the mold cavity under melt filling and high-pressure water penetration during the WAIM.     

Effect of Microbubble Gas Saturation on Near-Wall Turbulence and Drag Reduction

The article presents results of an experimental study of the effect of gravitational orientation of the flow along its lower/upper solid boundaries on reduction of turbulent drag and void fraction profiles with injection of gas through a porous channel wall. The shear stress on the wall was measured in the Reynolds number range Re_x = (0.23–1.1) × 10⁷ by floating element transducers; the void fraction profile was determined using a fiber-optic sensor. The void fraction in the inner (near-wall) region of the boundary layer was shown to be a key parameter for turbulent drag reduction. The size of the inner region depends on the gas flow rate, the fluid velocity, the distance downstream of the gas generator, and the gravitational orientation of the wall.     

The mobility of the amorphous phase in polyethylene as a determining factor for slow crack growth

Polyethylene (PE) pipes generally exhibit a limited lifetime, which is considerably shorter than their chemical degradation period. Slow crack growth failure occurs when pipes are used in long-distance water or gas distribution though being exposed to a pressure lower than the corresponding yield stress. This slow crack growth failure is characterized by localized craze growth and craze fibril rupture. In the literature, the lifetime of PE pipes is often considered as being determined by the density of tie chains connecting adjacent crystalline lamellae. But this consideration cannot explain the excellent durability of the recent bimodal grade PE for pipe application. We show in this paper the importance of the craze fibril length as the determining factor for the pipe lifetime. The conclusions are drawn from stress analysis. It is found that longer craze fibrils sustain lower stress and are deformed to a lesser degree. The mobility of the amorphous phase is found to control the amount of material that can be sucked in by the craze fibrils and thus the length of the craze fibrils. The mobility of the amorphous phase can be monitored by dynamic mechanical analysis measurements. Excellent agreement between the mobility thus derived and lifetimes of PE materials as derived from FNCT (full notch creep test) is given, thus providing an effective means to estimate the lifetime of PE pipes by considering well-defined physical properties.