可降解高聚物PLGA降解溶蚀的仿真模型

针对可降解高聚物材料聚乳酸和聚乙醇酸的共聚物(PLGA)降解溶蚀的计算机仿真问题,采用蒙特卡罗方法确定聚合物的随机降解寿命的计算表达式,依此建立了PLGA材料降解溶蚀过程的数学模型,并开展溶蚀模型的验证实验,数值仿真与验证实验结果表明,文中的聚合物随机计算寿命值表达式具有更明确的物理意义,所建立的降解溶蚀模型可以直观地反映材料的降解溶蚀过程,材料半衰期之前仿真结果与实验结果一致性较好。该仿真模型能为可降解高聚物材料的功能器件提供一种设计方法。     

Review Processes and influencing parameters of the solid particle erosion of polymers and their composites

The solid particle erosion behaviour of polymers and polymeric composites has been reviewed. Attention was paid to the effects of testing variables (e.g., erodent type, size and flux, impact angle) and target material characteristics (e.g., crystallinity, crosslink density, reinforcement content and arrangement). The occurring failure mechanisms were classified and discussed. Various predictions and models proposed to describe the erosion rate (ER) were listed and their suitability was checked. Recommendations were given how to solve some open questions related to the structure—erosion resistance relationships for polymers and polymeric composites.     

Microstructure and solid particle erosion of carbon-based materials used for the protection of highly porous carbon–carbon composite thermal insulation

Multiparticle erosion tests were performed on candidate coating (colloidal graphite paints) and cladding (dense carbon–carbon composites and graphite foil) materials employed to protect porous carbon–carbon composite thermal insulation in vacuum and inert-gas furnaces that utilize inert gas quenching. The dependence of the erosion rate on the angle of incidence of the erodent was examined and related to the microstructure and the mechanisms of material removal as observed by SEM. In addition, the effect of a thin chemical vapour deposited (CVD) carbon layer on top of a colloidal graphite paint coating and a graphite foil clad was investigated. The coating and cladding materials displayed a greater erosion resistance at all angles of incidence compared to the porous carbon–carbon composite. In general, the greatest erosion rate was found at an angle of incidence of 90°, where the erodent stream is perpendicular to the erosion surface, and brittle fracture was the predominant mechanism of material removal. The exception was the graphite foil material which displayed maximum erosion at an angle of incidence of 60°. For this material, two mechanisms were effective: disruption of the graphite flakes, which are mainly held together by mechanical locking, and a ploughing-like mechanism. The addition of a thin CVD carbon layer to colloidal graphite paint improved performance, whereas the erosion resistance of the graphite foil was slightly degraded as the CVD layer was too thin to prevent the ploughing-like mechanism. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effects of fiber orientation angles of fiber-reinforced plastic on sand solid particle erosion behaviors

Solid particle erosion in industrial applications has been a serious problem in many engineering fields. Earlier studies on fiber-reinforced plastic (FRP) composites were mainly focusing on the erosive wear behavior at several different impact angles. However, the effect of fiber orientation on FRP composites has not been thoroughly investigated. Since fiber orientation is one of the important factors in which causing erosive wear damages to FRP composites, in order to understand the virtue of this problem, it is important to investigate the effect of fiber orientation at different impact angles. In this research, the effect of fiber orientation of unidirectional fiber-reinforced plastic composites on erosive wear behavior was studied. Sandblasting-type erosion tests were conducted on the FRP composites with fiber orientation ranging at three impact angles to clarify the relation between fiber orientation and erosive wear behavior. The Dyneema fiber (ductile material) and the carbon fiber (brittle material) were used for the reinforcement fiber in FRP. From the result, it is confirmed that CFRP composites with higher fiber orientation angle erode faster than the composites with lower fiber orientation angle. But the erosion characteristic of DFRP was almost the same regardless of the fiber orientation angle. The damaged surfaces of the FRP composites were then analyzed using scanning electron microscopy and the possible erosion wear mechanisms were investigated.     

纤维增强复合材料层板高速倾斜冲击损伤数值模拟

根据复合材料三维黏弹性本构关系, 建立了纤维增强复合材料层板高速倾斜冲击损伤的数值分析模型。该模型在复合材料层间引入界面单元模拟层间分层, 结合三维Hashin失效准则进行单层板面内损伤识别, 引入材料刚度折减方案, 采用非线性有限元方法, 研究高速倾斜冲击下复合材料层板的破坏过程和损伤特性。研究结果表明: 层板的主要损伤形式是层间分层、 基体微裂纹和纤维断裂; 冲击速度不变而入射角度增大时, 剩余速度减小, 层板损伤面积在一定入射角度范围内有明显变化; 入射角度不变而冲击速度增大时, 剩余速度增大, 层板损伤面积在一定速度范围内也有明显变化。     

Abrasive Blasting of Metal Matrix Composites

This paper investigates the feasibility of dry erosion by blasting alumina erodent on aluminum composite reinforced with silicon carbide particles. The erosion rate is dependent on the erodent velocity and the matrix hardness. Two different material removal mechanisms are observed due to different attack angles. Blasting between 30-60° is recommended for optimum erosion rate and surface quality. Material removal mechanisms and surface quality of blasted and ground composites are compared. Although its erosion rate is two orders of magnitude less than that for rough grinding, dry blasting is suitable as a finishing process of composites with irregular contours.     

Erosion–corrosion behavior of plastic mold steel in solid/aqueous slurry

Erosion–corrosion behavior of a precipitation hardenable plastic mold steel (NAK80) has been investigated by using a rotated slurry erosion rig containing a slurry comprising 20 wt% Al₂O₃ particle and 3.5% NaCl solution. The erosion–corrosion rate and the synergism between erosion and corrosion have been determined under various conditions. The major environmental parameters considered are impact angle, impact velocity, and particle size. Post-test examination was conducted to identify the material degradation mechanism involved. The erosion–corrosion mechanisms of NAK80 mold steel at high-impact angles are dominated by the formation of impact pits, dissolution of metallic matrix, and plastic deformation fatigue spalling, whereas at low-impact angles, the mechanisms are dominated by the formation of impact pits, dissolution of metallic matrix, fatigue cracks, and cutting. The observed synergism between these mechanisms is much more accentuated at an oblique impact angle than that at a normal impact angle. At a given impact angle, the erosion–corrosion rate is found to increase with the impact velocity and the size of solid particles. The maximum peak of the erosion rates lies at oblique angles between 30° and 45°, whereas the maximum peak of the erosion–corrosion rates appears at 45°, and the erosion–corrosion rate is higher than the erosion rate alone at all angles examined. There is a positive synergism between erosion and corrosion for NAK80 mold steel in solid/aqueous slurry. The synergistic effect is 40–60% of the total weight loss. The contribution of synergism to the total weight loss depends upon the impact velocity; however, it is almost independent of the impact angle and particle size.     

PROCESSING ASPECTS OF SHAPING ADVANCED MATERIALS BY ELECTRICAL DISCHARGE MACHINING

The principles of applying electrical discharge in die-sinking and wire-cutting machines are reviewed. This technique causes material to be eroded by switching electrical current on and off between the tool and the work-piece through a dielectric fluid. For the case of metals and many ceramics and ceramic composites, the mechanism for this erosion is melting or perhaps evaporation/condensation. For refractory materials, a new mechanism for erosion, thermal spalling, was observed. The role of the dielectric fluid, its purity, and the proper nitration system are discussed. The effect of various operating conditions, including the current, pulse time duration, and wire feed rate, on the material removal rate and the surface quality is also reviewed. It was seen that EDM can be applied to machining advanced materials, including single phases and composites of ceramic/ceramic and ceramic/metal, if a minimum electrical conductivity is met.     

Evolution and calibration of a numerical model for modelling of hybrid-fibre ECC panels under high-velocity impact

A material model for hybrid-fibre engineered cementitious composites (ECC) under impact loading is developed and calibrated in this paper, and size effect, appropriate erosion criteria and strain rate effect are investigated and accounted for in the model. Employing the new material model, a numerical model and modelling technique are developed to model the impact behaviour and impact process of hybrid-fibre ECC panels using LS-DYNA commercial software. The material model and the numerical model developed in this paper are validated against the experimental results.     

考虑攻角的长杆弹斜穿透中厚铝靶机理

攻角对长杆弹斜侵彻有重要影响,该文通过大量数值模拟研究了攻角对长杆弹斜穿透中厚铝板的影响机理。基于实验验证的有限元模型,开展了变速度和攻角的多工况数值模拟,得到了侵彻过程中弹体的减加速度大小、速度方向以及整体弯曲的变化规律,分析了侵彻速度、倾角和攻角对侵彻阻力、弹体弯曲和弹道偏转的影响。结果表明:带攻角斜侵彻时,负攻角对弹体弯曲的影响明显大于正攻角,且弹体弯曲随着侵彻速度的增大而减小;随着斜侵彻速度的增大,攻角引起弹体甩尾和弹道偏转越明显,此时带攻角的斜侵彻过程的能量损耗机理明显不同于正侵彻和无攻角的斜侵彻。     

聚合物的水声声衰减能力与动态力学性能的关系

以描述高分子材料粘弹行为的三元模型为出发点,以声波在高分子介质中的传播理论为依据,推导出了材料的水声声衰减能力与材料的动态力学性能参数包括损耗因子、松弛前的剪切模量、松弛后的剪切模量以及材料的密度和厚度之间的关系式。为实验验证所推导的关系式,设计合成了一系列阻尼性能不同的聚合物,分别测试了它们的动态力学性能和声衰减能力。用材料的动态力学性能参数计算得到的水声声衰减系数与实验测得的声衰减能力相符合。该数学模型为找出吸声系数与材料的动态力学性能之间的关系、指导水声材料设计奠定了基础。     

Selection function of particles under impact loads: The effect of collision angle

The current work investigates the effect of collision angle on the breakage of particles under impact loads. The experiments were performed using a homemade experimental system that accelerates the particles horizontally toward the target using compressed air. The design of the system allows the angle of the target and the air velocity, both adjustable to check different collision angles at different impact velocities; and the tested material that was blown to the target to be collectible for measuring the percentage of broken particles for the analysis. In this study, six different materials were tested by conducting experiments with different collision angles and impact velocities. As expected, the results showed that the collision angle affects the breakage of the particles. When the collision angle becomes acute, i.e., less than 90° (perpendicular collision with the target), it results in less breakage of the particles for all tested materials and at all tested velocities. Consequently, an empirical model got established. This model can predict the median impact velocity that causes half of the particle population to break, depending upon collision angle and particle size.     

Polyurethane conductive blends and composites: synthesis and applications perspective

The objective of this review is to describe the important progress made in the area of synthesis and applications of conducting composites/blends derived from polyurethanes as host material. Polyurethane is one of the most useful polymeric materials with multiple functional applications. The pristine polymer is an insulator and conductivity can be imparted by blending or compositing with conductive materials such as conducting polymers, carbon black, CNTs, and graphite/graphene. The resulting conductive composites are useful as sensors, corrosion-resistance paints, electrostatic dissipaters, EMI shielders, and many others. The review discusses various methods to obtain such conductive polyurethane composites and their intended or demonstrated applications.     

On lateral cracks in glass

The importance of lateral cracks in solid particle erosion of brittle materials has been confirmed as a result of a large number of previous investigations in this area. Even though the underlying mechanism of steady-state erosion of a brittle material is the formation and growth of lateral cracks, the surface morphology of the eroded material does not readily indicate this aspect. This has precipitated the need for a study of single impact events. This study concentrates on lateral cracks in glass produced by solid particle impacts. Single impacts are studied in terms of lateral crack extensions and their probability of chipping at two angles of impact of 20 and 90°. Comparisons between these two sets of data were made at the same normal component of velocity to clearly bring out behavioural differences at the two angles o of impact. Steady-state erosion results are then interpreted in terms of the above results obtained from a study of single impacts. There appears to be marked agreement between these results and experimental observations. The same trend was observed in strength degradation measurements. Increased chipping and lateral crack extensions in the 20° impact situation has been explained in terms of linear elastic fracture mechanics, as opposed to the plastic deformation mechanism proposed earlier. The importance of single impacts in the study of steady-state erosion of brittle materials by solid particle impact is well demonstrated by this study.[/p]     

A review on polymer heat exchangers for HVAC&R applications

Because of their low thermal conductivity, polymers are not commonly considered as a material to construct heat exchangers, except for specific applications, e.g. heat recovery from solvent laden streams, where exotic alloys are required to prevent corrosion. In this review the material properties of polymers are examined, as well as the current state of the art of polymer matrix composites. It is shown that these materials do hold promise for use in the construction of heat exchangers in HVAC&R applications, but that a considerable amount of research is still required into material properties and life-time behavior. A successful application of polymers or polymer matrix composites is based on careful material selection and modification of the design to fully exploit the material properties, as is demonstrated through a series of examples.     

Waterjet penetration simulation by hybrid code of SPH and FEA

The creativity of this work is combining finite element analysis (FEA) and smoothed particle hydrodynamics (SPH) methods to simulate the waterjet (WJ) penetration process. In WJ penetration, the waterjet undergoing extremely large deformation will introduce the distortion of mesh in FEA. To overcome this difficulty, the coupled method of SPH and FEA was developed, in which the waterjet was modeled by SPH particles and the target material was modeled by finite elements. The two parts interacted by contact algorithm of “nodes-to-surface”. Utilizing this hybrid model, waterjet with high velocity penetrating the target materials was calculated and the mechanism of erosion was depicted. The computation result gives the relationship between the jet velocity and the erosion capacity, including the depth of penetration and mass removal, which was compared with the experimental data.     

Characteristic impact resistance model analysis of cellulose nanofibril-filled polypropylene composites

Notched and unnotched Izod impact strength of cellulose nanofibers (CNFs) and microfibrillated cellulose (MFC)-filled impact modified polypropylene (PP) composites were measured and compared with microcrystalline cellulose (MCC)-filled composites. An Izod impact fracture initiation resistance theory was formulated and a characteristic impact resistance model was developed to evaluate the unique impact characteristics of cellulose nanofibril-filled PP composites. As filler loading increased CNF and MFC-filled composites showed higher characteristic impact resistance than MCC-filled ones. Among the cellulose fillers used in this study, CNF were found to be the most resistant of the three materials tested in terms of characteristic impact resistance. Even though impact resistance in not the only evaluation tool, characteristic impact resistance is an evaluation tool used to determine the material’s unique and hidden impact characteristics. The characteristic impact resistance model is useful for analysis of the impact behavior of any polymer composite material. It was also found that impact modified PP used in this study is more fracture resistant, but more crack sensitive, than conventional PP.     

聚合物基复合材料有效蠕变响应与单轴拉伸行为的细观力学模拟

基于变分渐近均匀化理论框架建立表征线性黏弹性聚合物基复合材料有效蠕变响应和宏观应力-应变行为的细观力学模型。从线性黏弹性聚合物基复合材料本构方程中构建能量泛函变分表达式出发,采用变分渐近法求解线性黏弹性聚合物基复合材料的有效蠕变柔度系数,并以此为基础计算聚合物基复合材料的时变和单轴拉伸行为。通过算例验证了构建模型的适用性和准确性。由于所有计算均在时间域内完成,不再需要传统线黏弹性复合材料使用的Laplace转换和反演,计算效率大为提高。      

A Systematic Molecular Dynamics Investigation on the Graphene Polymer Nanocomposites for Bulletproofing

In modern physics and fabrication technology, simulation of projectile and target collision is vital to improve design in some critical applications, like; bulletproofing and medical applications. Graphene, the most prominent member of two dimensional materials presents ultrahigh tensile strength and stiffness. Moreover, polydimethylsiloxane (PDMS) is one of the most important elastomeric materials with a high extensive application area, ranging from medical, fabric, and interface material. In this work we considered graphene/PDMS structures to explore the bullet resistance of resulting nanocomposites. To this aim, extensive molecular dynamic simulations were carried out to identify the penetration of bullet through the graphene and PDMS composite structures. In this paper, we simulate the impact of a diamond bullet with different velocities on the composites made of single- or bi-layer graphene placed in different positions of PDMS polymers. The underlying mechanism concerning how the PDMS improves the resistance of graphene against impact loading is discussed. We discuss that with the same content of graphene, placing the graphene in between the PDMS result in enhanced bullet resistance. This work comparatively examines the enhancement in design of polymer nanocomposites to improve their bulletproofing response and the obtained results may serve as valuable guide for future experimental and theoretical studies.     

Ballistic impact behaviour of woven fabric composites: Formulation

Resistance to high velocity impact is an important requirement for high performance structural materials. Even though, polymer matrix composites are characterized by high specific stiffness and high specific strength, they are susceptible to impact loading. For the effective use of such materials in structural applications, their behaviour under high velocity impact should be clearly understood. In the present study, investigations on the ballistic impact behaviour of two-dimensional woven fabric composites have been presented. Ballistic impact is generally a low-mass high velocity impact caused by a propelling source. The analytical method presented is based on wave theory. Different damage and energy absorbing mechanisms during ballistic impact have been identified. These are: cone formation on the back face of the target, tension in primary yarns, deformation of secondary yarns, delamination, matrix cracking, shear plugging and friction during penetration. Analytical formulation has been presented for each energy absorbing mechanism. Energy absorbed during each time interval and the corresponding reduction in velocity of the projectile has been determined. The solution is based on the target material properties at high strain rate and the geometry and the projectile parameters. Using the analytical formulation, ballistic limit, contact duration at ballistic limit, surface radius of the cone formed and the radius of the damaged zone have been predicted for typical woven fabric composites. The analytical predictions have been compared with the experimental results. A good correlation has been observed.