空心玻璃微珠填充环氧树脂复合材料力学性能

对不同填充质量比的改性空心玻璃微珠(HGB)/环氧树脂复合材料进行了拉伸、压缩准静态实验。研究了改性空心玻璃微珠不同填充量对复合材料密度、弹性模量、拉伸强度和压缩强度的影响, 并分析其应力松弛。实验发现, 材料的各项数据随填充比增加均有所降低。空心玻璃微珠的填入使材料表现出脆性破坏, 但破坏前有较大的变形, 破坏后回弹率大, 说明玻璃微珠的填充增强了材料的弹性。HGB/环氧树脂复合材料具有明显的应力松弛现象, 且填充比越高, 应力松弛速率越大, 可见HGB/环氧树脂复合材料具有明显的黏弹性。      

“离位”增韧预成型体压缩特性

针对"离位"增韧预成型体的液态成型工艺性,研究了两种不同结构形式增韧层"离位"增韧预成型体的厚度压缩特性。分别采用多孔薄膜结构增韧层、高孔隙率无纺布结构增韧层与碳纤维织物交替铺层制备"离位"增韧预成型体,采用力学试验机测试其厚度压缩特性。实验结果表明,预成型体压缩过程中,在相同压力水平下,增韧层的引入会降低预成型体的纤维体积分数;不同压力水平下,预成型体的压缩行为与增韧层结构形式有关。此外,采用织物预成型体厚度压缩本构模型,对"离位"增韧预成型体的压缩厚度进行了预测,通过模型预测值与实验值的比较,确定了模型中的经验指数k=2时,两者吻合较好。     

经编多轴向织物的压缩性能研究

真空袋工艺由于采用了柔性的上模具,使得增强织物的厚度很难控制,从而最终的纤维体积含量也就难预测和控制。研究了多轴向织物在真空袋工艺的压缩行为,在干态压缩实验中,讨论了时间,层数,纤维束细度,织物结构以及铺层角度对于最终纤维体积含量的影响。在湿态压缩中讨论了树脂的润湿作用以及润湿后一些织物的反弹行为。对不同纤维体积含量的复合材料横截面进行分析,讨论了不同压力下导致织物压缩的因素。     

1Cr18Ni9Ti 不锈钢管滞后回弹的黏弹塑性建模和有限元模拟

目的研究1Cr18Ni9Ti不锈钢管在单轴拉伸和回转牵引弯曲卸载后的滞后回弹现象,利用有限元软件进行滞后回弹结果的预测。方法通过系列单轴拉伸和应力松弛试验得到1Cr18Ni9Ti不锈钢管黏弹塑性材料参数,采用双耗散黏弹塑性材料模型在有限元ABAQUS软件进行拉伸和回转弯曲卸载后滞后回弹的模拟预测,并与试验结果对比。结果利用该黏弹塑性材料模型预测得到的滞后回弹变化趋势与试验值吻合较好,数值上接近试验值。结论基于双耗散黏弹塑性材料模型进行滞后回弹的建模,在瞬时回弹后,弹塑性组合单元和黏弹性组合单元中存在相反的残余应力,从而驱动材料持续发生黏弹性变形,可能是产生滞后回弹的原因。有限元分析中考虑了加载过程中的黏性响应,其预测结果更加接近实际。     

准静态压缩下的蜂窝纸板承载性能实验研究

对蜂窝纸芯和纸板准静态压缩实验,测定分析面纸、孔径比、厚度对蜂窝承载性能的影响.结果表明:蜂窝纸板压缩呈非线性特征,其压缩变形过程经历黏弹性、黏弹塑性、塑性坍塌3个阶段;面纸、厚度、孔径比不同程度影响蜂窝纸板的承载性能.随厚度、孔径比增大,承载力降低,面纸对承载力的影响减小;原纸定量增大,抗弯性能增强,纸板纵向弯曲强度大于横向弯曲强度.     

多孔硅泡沫材料力学特性的影响因素

通过"两段式"的发泡方法制备了多孔硅泡沫材料,通过材料压缩和应力松弛实验等方法研究了环境温度、初始压缩量、材料厚度和密度等因素对材料力学性能的影响。通过不同密度、不同厚度多孔硅泡沫的静态压缩试验以及使用扫描电镜观察不同压缩阶段泡孔结构的变化,分析了硅泡沫材料的压缩变形机制及吸能特性。研究表明,硅泡沫材料的松弛性能与环境温度、初始压缩量以及材料的厚度、密度密切相关。环境温度越高,材料的松弛率越大,应力达到稳定状态的时间也越长。而相同环境条件下初始应力和应力松弛极限随着压缩率的增加而增大,但松弛率却因压缩率的增加而减小。同时,材料厚度对泡沫材料的压缩性能和应力松弛有较大的影响,是材料制备工艺中不容忽略的重要影响因素。     

发泡聚丙烯的静态压缩性能

目的 以发泡聚丙烯为研究对象,研究厚度和密度对发泡聚丙烯静态压缩性能的影响规律。方法 通过静态压缩试验,得出不同密度和厚度下的力-位移曲线,进一步处理得到应力-应变曲线、能量吸收效率曲线以及比吸能、总能量吸收图和抗压强度。通过这些曲线分析密度和厚度对发泡聚丙烯材料静态压缩性能的影响。结果 密度、厚度不同的发泡聚丙烯材料,其应力-应变曲线的形态基本相同。当厚度一定时,密度越大,总能量吸收、比吸能及抗压强度也越高。当密度一定时,材料越厚,其总能量吸收越高、比吸能越低,厚度对密实化应变和抗压强度的影响可忽略。结论 在对缓冲包装进行优化设计时,为了防止出现过度包装导致资源浪费或欠包装导致被包装物出现损毁等情况,应充分比较泡沫材料的厚度和密度对缓冲和吸能性能的影响,并根据试验对比结果选择最优方案。     

黏弹性热流固耦合作用诱导聚合物变形的机理EI北大核心CSCD

模内微装配成型有望成为高效低成本产业化聚合物微型机械制造技术,而如何准确预测和精密控制其二次成型过程的黏弹性热流固耦合变形仍是其工业化的技术瓶颈。基于考虑微装配界面周围高温黏弹性熔体流动环境边界约束作用,建立了描述模内微装配成型黏弹性热流固耦合作用诱导聚合物变形的理论模型。研究表明,黏弹性与纯黏性熔体热流固耦合的本质区别在于熔体的弹性特性能有效抑制微装配界面的热流固耦合作用,使得预成型微型轴的变形和装配界面的热流固耦合载荷均随二次成型熔体松弛时间的延长而减小,且其变形受控于微装配界面所承受的黏弹性热流固耦合压力、黏弹性支撑正应力和黏性拖曳剪切应力,选用高弹性二次成型熔体有利于抑制黏弹性热流固耦合变形,可提高微装配加工精度。     

黏弹性热流固耦合作用诱导聚合物变形的机理

模内微装配成型有望成为高效低成本产业化聚合物微型机械制造技术,而如何准确预测和精密控制其二次成型过程的黏弹性热流固耦合变形仍是其工业化的技术瓶颈。基于考虑微装配界面周围高温黏弹性熔体流动环境边界约束作用,建立了描述模内微装配成型黏弹性热流固耦合作用诱导聚合物变形的理论模型。研究表明,黏弹性与纯黏性熔体热流固耦合的本质区别在于熔体的弹性特性能有效抑制微装配界面的热流固耦合作用,使得预成型微型轴的变形和装配界面的热流固耦合载荷均随二次成型熔体松弛时间的延长而减小,且其变形受控于微装配界面所承受的黏弹性热流固耦合压力、黏弹性支撑正应力和黏性拖曳剪切应力,选用高弹性二次成型熔体有利于抑制黏弹性热流固耦合变形,可提高微装配加工精度。     

定型剂对经编织物液态成型复合材料工艺性及其力学性能的影响

研究了双马来酰亚胺树脂定型剂含量对0°/90°双轴向经编织物(Non-crimp fabric,NCF)定型效果、液态成型工艺性、复合材料力学性能的影响。采用厚度回弹、C型回弹的方法表征定型效果;采用厚度压缩与偏轴拉伸实验表征带定型剂织物成型工艺性;并采用弯曲实验与层间剪切实验表征复合材料力学性能。实验结果表明定型剂的添加显著提高了NCF织物的预定型效果;经定型剂处理后NCF织物的面内剪切模量有了较大提高,抗剪切变形能力增强;添加定型剂对0°/90°双轴向NCF织物复合材料的力学性能影响不大,但在改善其工艺性的同时并不会降低其复合材料的力学性能。     

脉冲电流对超薄316L板微流道成形精度的影响

目的 研究在金属双极板脉冲电流辅助冲压工艺中,将脉冲电流单独作用于保压阶段、冲压阶段以及作用于整个冲压过程(冲压阶段+保压阶段)3种通电方式对极板流道深度及壁厚分布的影响。方法 以316L不锈钢薄板作为实验材料,调节流经样品的电流参数,分别为0、10、20、30 A,分析脉冲电流参数和通电方式对极板流道深度及壁厚分布的影响规律。结果 在高电参数下,当采用在冲压阶段通电、在保压阶段断电的方式2时,在保压阶段可促进应力松弛,具有最佳的回弹抑制效果,其成形深度回弹可减小73.88%。在不同电参数下,3种成形方式对双极板流道壁厚分布具有相似的影响。槽顶和槽底的壁厚减薄相对较小,减薄量为原板材厚度的2%~6%;流道直壁壁厚减薄为原板厚度的9%~15%;而外圆角和内圆角区域的壁厚减薄较大,达到27%~30%。结论 在金属双极板脉冲电流辅助冲压工艺中,以不同方式施加脉冲电流,均可以显著提升流道深度,对壁厚影响并无明显差别。特别地,采用在冲压阶段通电、在保压阶段断电的方式2可以显著提升对流道回弹的抑制效果。     

Description of depth-dependent nonlinear viscoelastic behavior for articular cartilage in unconfined compression

An optimized digital image correlation (DIC) technique was applied to investigate the depth-dependent nonlinear viscoelastic properties of articular cartilage and simultaneously the biphasic nonlinear viscoelastic relaxation model of cartilage was proposed and validated. The stress relaxation tests were performed with different strain levels and it is found that the initial stress and relaxed stress at any time increase with increasing strain levels. The depth-dependent strain of cartilage was obtained by analyzing the images acquired using the optimized DIC technique and moreover the inhomogeneous relaxation modulus distributions within the tissues were determined at different relaxation time points under strain of 11.35, 19.35 and 30% respectively. The strain rate dependent nonlinear stress and strain curves were obtained for articular cartilage through uniaxial compression tests. It is noted that the Young's modulus exhibits a slight increase near the cartilage surface, and then increases fast with depth and both the magnitude and the variation of the Young's modulus are affected by increasing strain rates. A biphasic nonlinear viscoelastic relaxation model was proposed to predict the depth-dependent relaxation behavior of cartilage under unconfined compression and the results show that there are good agreements between the experimental data and predictions.     

聚合物多相分层流动黏弹性包围机制的数值模拟

针对黏弹性包围引起的复合共挤制品层厚均匀性控制的技术难题,通过数值模拟,研究了黏弹性流变性能参数对黏弹性包围的影响规律和机制.研究结果表明,黏弹性包围是由多相分层流动的第二法向应力差驱动的二次流动诱发,主要取决于成型流动过程中二次流动的方向与强度.熔体二次流动的方向与第二法向应力差的正负号有关,而熔体二次流动强度则与第二法向应力差大小成正比.黏弹性包围随着熔体松弛时间的增加而增大.消除黏弹性包围的理论前提是消除其二次流动,而通过使其流动的第二法向应力差趋于零方可消除其二次流动.第二法向应力差趋于零的前提条件是使无滑移黏着共挤多相分层剪切流动转化为气垫非黏着完全滑移共挤多相分层柱塞流动,而气辅共挤成型工艺的气垫壁面滑移是实现这一转变的有效技术.     

Experimental Study and Simulation on Compression Character of Warp Knitted Spacer Fabrics

Based on experimental data, the research work on warp knitted spacer fabrics gives compression laws when structural parameters (such as diameter of spacer yarn, areal density, spacer yarn angle and the thickness of spacer fabrics ) of spacer fabric change. ANSYS calculation models were developed, and simulation results matched with experimental data well. The computer simulation on this area provides a fundamental tool which can help designer to decide structural parameters when working stresses are given.     

2.5维机织物剪切性能实验研究

本文基于2-2带衬经结构的层-层正交角联锁织物结构优势,织造了7种不同规格参数的角联锁织物和一种三向正交结构的织物,利用像框实验分析了纬纱密度、纬纱细度、织物层数和不同种结构织物对剪切性能的影响。研究发现,织物中纱线密度越大,其所需的载荷越大,达到锁紧角的时间就越短;载荷随着织物层数的增加而增加;纱线细的织物,其所需载荷也小;相同参数的角联锁织物与三向正交织物的剪切性能基本没有太大差别。     

Analytical approach of mechanical behavior of carpet yarn by mechanical models

The viscoelastic models for describing mechanical behavior of polypropylene carpet yarn are presented in this paper. Since carpet yarn is subjected to tensile and frictional forces during tufting, the mechanical properties are important material features especially in vibration analysis. In this paper, the standard linear model and the standard nonlinear model are developed to simulate the polypropylene yarn tensile, stress relaxation and creep behaviors. The obtained formulas are fitted with experimental data and the ideal model parameters are determined by applying least squares. The results showed that the standard linear model fits well the tensile, relaxation and creep experimental curves, and will be employed to analyze the vibration characteristics of yarn during the tufting.     

Determining effects of moisture in mastic materials using nanoindentation

Asphalt concrete consists of coarse aggregates coated with asphalt binder, matrix, which is a mixture of binder and fine aggregates, and mastic, which is a mixture of asphalt binder and fines passing number 200 sieve (0.075 mm). In this study, nanoindentation tests were conducted on dry and wet mastic materials to determine the contact creep compliance, which is used to examine the effects of moisture in the mastic materials. Indentation creep data were fitted using viscoelastic mechanical models. Results show that the dry mastic materials exhibits viscoelastic behavior, while the wet mastic materials shows less viscoelastic behavior compared to the dry mastic materials. Moisture reduces retardation time significantly in the wet mastic materials. The dry mastic materials follow the linear Burgers viscoelastic model and the wet mastic materials follow the Maxwell viscoelastic model. Stiffness measured on the surface of the wet mastic materials is higher than that of the dry mastic materials. Due to moisture conditioning, mastic sample surface might have eroded that makes it less viscous or become exposed to mastic aggregate, and therefore exhibits high stiffness. Indentation results reveal that the wet mastic is softer below a certain depth from the surface. This study projects that the indenter needs to penetrate more than 4000 nm to reach softer wet mastic materials. Also indentation creep holding time needs be more than 1200 s to reach that target depth in wet mastic materials.     

Influence of properties at micro- and meso-scopic levels on macroscopic level for weft knitted fabrics

Knitted fabrics and particularly weft knitted fabrics are used as composite material reinforcements due to their ability to be draped and to give three-dimensional shape by molding or by knitting. This paper presents the strong connection of all the scales of the knitted fabric (fiber, yarn and fabric) on the final knitted fabrics and its mechanical and physical properties. For this purpose, only one polymer material is used, made of two different fibers in terms of length and fineness. These fibers are used to make different yarns with two structures then three plain-weft-knitted-fabrics are considered in terms of the loop length. The fibers have not the same bending rigidity because fiber cross-section areas are different. This has an influence on the three-dimensional loop shape and on the roughness, thickness and real area of contact of fabrics. This phenomenon is the same with the two yarn structures. The results presented here bring into light that the loop length does not influence the fabric thickness.     

Effect of fabric compaction and yarn waviness on 3D woven composite compressive properties

3D-woven fabrics incorporate through-thickness reinforcement and can exhibit remarkable inter-laminar properties that aid damage suppression and delay crack propagation. However, distortions in the internal architecture such as yarn waviness can reduce in-plane properties, especially in compression. The degree of yarn waviness present in a 3D woven fabric can be affected by a range of factors including weave parameters and manufacturing-induced distortions such as fabric compaction. This paper presents a thorough analysis of the effect of fabric compaction and yarn waviness on the mechanical properties and failure mechanisms of an angel-interlock fabric in compression. Tests were conducted on coupons moulded to different volume fractions and data compared to previous measurements of local yarn angle. Major findings show the importance of yarn straightness on compressive strength and how this can be affected by optimising moulding thickness. Failure initiation was also found to be heavily influenced by weave style and yarn interlacing.     

A Computer Aided Investigation on Strain Movements in Compacts Under Constant Stress Within the Die

A 2.54cm diameter punch and die set has been used in conjunction with a Mayes Hydraulic Machine to examine strain movements within the die of compacts maintained at constant stress. These were prepared from the bases Avicel PH 101 (AV), Sta-Rx 1500 (ST), Paracetamol DC (PC), Emdex (ED) and Eacompress (EC).

Compression force and punch movements were simultaneously monitored using a CBM nicroconputer with disk storage of data.

All compacts showed time dependent consolidation under constar. stress when held at 'holding time' of up to 60s. PC, AV and ST exhibited comparable movements but that of ED and EC ware less.

Elastic recovery on sudden release of load was followed using machine code logging over a period of 90ms. Large recoveries were recorded for AV, PC and ST. If load release was immediate (zero holding time) then the recovery was greater when compared with recovery after holding tines of 30 and 60s, suggesting that some elastic energy was dissipated during the holding time. Elastic recovery of ED and EC was about half that of the other bases. Although these bases also showed reduced recovery with holding tine, percentage reduction was smaller than for the other bases.

Following elastic recovery, a much slower viscoelastic movement could be demonstrated. It was difficult to clearly demarcate the division between viscoelastic and elastic movement but it appeared that the movement on viscoelastic consolidation was comparable to that en recovery when the load was removed.

There are many references to tine dependent effects during tablet compression to be found in the literature. One of the earliest was due to Rees and Shotton (1) who found that the crushing strength of sodium chloride tablets could double after the lapse of one hour from ejection. This has recently been shown by Rue and Barkworth (2) to be due to the existence of a work hardened outer shell which inhibits relaxation by viscoelastic flow.

Several authors (3,4,5) have investigated the stress decay that occurs when a tablet is held at a nominally constant force. Thus Wells and Langridge (5) monitored the fall from a peak pressure of 332MN/m²on a tablet prepared in a single punch machine turned by hand. Similar techniques have been described by Rees and Rue (3), Kiestand and others (4) and Shlanta and Milosovich (6). However as Rees and Rue have pointed out, these methods measure stress relaxation under constant strain conditions. Their results and conclusions differed from those of David and Augsburger (7) who compressed the same direct compression bases on a rotary tablet machine. In a rotary machine compression takes place under virtually constant, stress conditions because of the buffer effect of the powerful springs fitted to the compression wheels.

This paper describes some techniques and observations on the compression of some direct compression bases using a Mayes Hydraulic Testing Machine (WH Mayes (Windsor) Ltd) which is capable of maintaining constant stress conditions on a compact formed in a punch and die set mounted between the platens.