Cushioning Performance of Hilbert Fractal Sandwich Packaging Structures under Quasi-Static Compressions

The sandwich structure of cushioning packaging has an important influence on the cushioning performance.Mathematical fractal theory is an important graphic expression.Based on Hilbert fractal theory,a new sandwich structure was designed.The generation mechanism and recurrence formula of theHilbert fractal were expressed by Lin’s language,and the second-orderHilbert sandwich structure was constructed fromthermoplastic polyurethane.The constitutive model of the hyperelastic body was established by using the finite element method.With the unit mass energy absorption as the optimization goal,the fractal sandwich structure was optimized,and the best result was obtained when the order was 2.5 and the unit layer thickness was 0.75 mm.TheHilbert sandwich structure was compared with the rice-shaped sandwich structure commonly used in industry,and the Hilbert fractal structure had better energy absorption.This has practical significance for the development and application of newcushioning packaging structures.

The Failure Mechanisms of Ultra-high Molecular Weight Polyethylene Fibre Composites Under In-plane Compression

The in-plane compressive characteristics of the ultra-high molecular weight polyethylene(UHMWPE)fibre(Dyneema█)reinforced composites,both in 0/90°and±45°fibre orientations with respect to the loading direction,have been investigated.The composite made from unidirectional high modulus fibres(volume fraction 83%)and low strength polyurethane matrix(volume fraction 17%)is layered in an orthogonally alternating manner.The different failure mechanisms for the composites with 0/90°and±45°fibre orientations have been detected with the methods of experimental measurement,SEM observation and theoretical analysis.The composites specimens of 0/90°fibre orientation failed with macro-buckling of the high-modulus UHMWEP fibre layers with the matrix damage,whereas the specimens of±45°fibre orientation failed with the shearing of the soft matrix.Hence,the composite specimens in 0/90°fibre orientation had higher stiffness as well as compressive strength than those in±45°fibre orientation.The failure criteria of the composites under in-plane compression was employed to characterize the failure mechanism.Compared with the traditional thermoset matrix,the soft thermoplastic matrix leads to lower strength and higher failure strain of fibre reinforced composites under in-plane compression.In addition,the composite specimens cut by waterjet machine exhibited higher stress levels than those cut by bandsaw that introduced more initial imperfections with the temperature rising and tensile shocking.The comparison between the methodologies for cutting the tough composites can provide a valuable suggestion to obtain required composite structures without reducing the mechanical properties.

Direct Generation of Intense Compression Waves in Molten Metals by Using a High Static Magnetic Field and Their Application

Compression waves propagating through molten metals are contributed to degassing, accelerating reaction rate,removing exclusions from molten metals and refining solidification structures during metallurgical processing of materials. In the present study, two electromagnetic methods are proposed to generate intense compression wavesdirectly in liquid metals. One is the simultaneous imposition of a high frequency electrical current field and a staticmagnetic field; the other is that of a high frequency magnetic field and a static magnetic field. A mathematical modelbased on compressible fluid dynamics and electromagnetic fields theory has been developed to derive pressure distributions of the generated waves in a metal. It shows that the intensity of compression waves is proportional to thatof the high frequency electromagnetic force. And the frequency is the same as that of the imposed electromagneticforce. On the basis of theoretical analyses, pressure change in liquid gallium was examined by a pressure transducerunder various conditions. The observed results approximately agreed with the predictions derived from the theoreticalanalyses and calculations. Moreover, the effect of the generated waves on improvement of solidification structureswas also examined. It shows that the generated compression waves can refine solidification structures when they wereapplied to solidification process of Sn-Pb alloy. This study indicates a new method to generate compression wavesby imposing high frequency electromagnetic force locally on molten metals and this kind of compression waves canprobably overcome the difficulties when waves are excited by mechanical vibration in high temperature environments.

Theoretical analysis on quasi-static lateral compression of elliptical tube between two rigid plates

Based on the rigid plastic theory; the load-deflection functions with and without considering the effect of strain hardening are respectively derived for an elliptical tube under quasi-static compression by two parallel rigid plates. The non-dimensional load-deflection responses predicted by the present theory and the finite element simula- tions are compared, and the favorable agreement is found. The results show that strain hardening may have a noticeable influence on the load-deflection curves of an elliptical tube under quasi-static compression. Compared with the circular counterpart, the ellip- tical tube exhibits different energy absorption behavior due to the difference between the major axis and the minor axis. When loaded along the major axis of a slightly oval tube, a relative even and long plateau region of the load-deflection curve is achieved, which is especially desirable for the design of energy absorbers.

Microstructure comparison of ZK60 alloy under casting,twin roll casting and hot compression

The microstructures of ZK60 alloy under conventional direct as-casting (DC),twin roll casting (TRC) and twin roll casting followed by hot compression (TRC-HC) were analyzed by optical morphology (OM),electron backscatter diffraction (EBSD) and X-ray diffraction (XRD).The deformation condition of hot compression is 350 ℃,0.1 s?1.The microstructural evolution under TRC-HC deformation followed by annealing at different temperatures and time was discussed.The results show that TRC provides more modified microstructure compared with DC.Twins are found in TRC processing;dynamic recrystallization (DRX),shear bands and twins are found in TRC-HC.A short annealing time has little effect on hardness,while during a long time annealing,it is found that low annealing temperatures increase the micro-hardness and high temperature decreases it.

Low-Velocity Impact and Compression after Impact(CAI)Behaviors of Carbon-Aramid/Epoxy Hybrid Braided Composite Laminates

Low-velocity impact and in-plane axial compression after impact(CAI)behaviors of carbon-aramid/epoxy hybrid braided composite laminates were investigated experimentally.The following three different types of carbon-aramid/epoxy hybrid braided composite laminates were produced and tested:(a)inter-hybrid laminates,(b)sandwich-like inter-hybrid laminates,and(c)unsymmetric-hybrid laminates.At the same time,carbon/epoxy braided composite laminates were used for comparisons.Impact properties and impact resistance were studied.Internal damages and damage mechanisms of laminates were detected by ultrasonic C-scan and B-scan methods.The results show that the ductility index(DI)values of three kinds of hybrid laminates aforementioned are 37%,4%and 120%higher than those of carbon/epoxy laminates,respectively.The peak load of inter-hybrid laminates is higher than that of sandwich-like inter-hybrid laminates and unsymmetric-hybrid laminates.The average damage area and dent depths of inter-hybrid laminates are 64%and 69%smaller than those of carbon/epoxy laminates.Those results show that carbon-aramid/epoxy hybrid braided composite laminates could significantly improve the impact properties and toughness of non-hybrid braided composite laminates.

A New Method Which Combines Arithmetic Coding with RLE for Lossless Image Compression

This paper presents a new method of lossless image compression. An image is characterized by homogeneous parts. The bit planes, which are of high weight are characterized by sequences of 0 and 1 are successive encoded with RLE, whereas the other bit planes are encoded by the arithmetic coding (AC) (static or adaptive model). By combining an AC (adaptive or static) with the RLE, a high degree of adaptation and compression efficiency is achieved. The proposed method is compared to both static and adaptive model. Experimental results, based on a set of 12 gray-level images, demonstrate that the proposed scheme gives mean compression ratio that are higher those compared to the conventional arithmetic encoders.

双层圆钢管混凝土长柱压扭滞回性能试验

为研究双层圆钢管混凝土长柱在压、扭荷载作用下的力学性能,利用研制的Stewart六自由度加载平台,进行了两个普通圆钢管混凝土长柱和两个双层圆钢管混凝土长柱试件在纯扭、压扭作用下的低周往复试验。对比分析了各试件的承载力、扭转变形、耗能、滞回性能,进行了有限元参数分析。研究表明:普通圆钢管混凝土长柱和双层圆钢管混凝土长柱均具有较好的抗扭能力;与普通圆钢管混凝土长柱相比,双层圆钢管混凝土长柱的初始刚度和承载力略有提升,滞回曲线更饱满,耗能能力和延性大幅提升;参数分析表明含钢率一定时,内层钢管径厚比越大,对抗扭越有利;一定范围内的轴向荷载,可提高钢管混凝土柱的抗扭能力。

高强型钢超高性能混凝土短柱轴压性能试验及其有限元分析

为研究高强型钢超高性能混凝土短柱的轴压性能,对试件进行了轴心受压试验,设计参数包括配钢率、箍筋间距和箍筋形式。观察了试件的轴压破坏过程,获得了破坏形态及轴向荷载-位移曲线,分析了承载能力、变形能力、刚度等轴压性能指标,以及超高性能混凝土、纵筋、箍筋和型钢的应变发展规律。在试验研究基础上,采用ABAQUS建立了高强型钢超高性能混凝土短柱轴压性能有限元分析模型,计算结果与试验结果吻合较好,进而进行了参数分析。结果表明:试件发生的是典型轴压破坏,中部表面出现“锯齿形”裂缝;轴向荷载-位移曲线会出现2次荷载峰值;箍筋间距减小时,试件的承载能力和变形能力均提高,但刚度退化速率没有显著变化;配钢率增大时,试件的承载能力提高,变形能力先增大后减小,刚度退化变缓;型钢强度增大时,试件的承载能力和变形能力均提高,提高速度先快后慢;超高性能混凝土抗压强度增大时,试件的承载能力提高,变形能力降低;超高性能混凝土受拉能够达到峰值应变;纵筋在极限点之前发生受压屈服;箍筋在极限点时应变不足屈服应变的1/3,但在破坏点前达到受拉屈服;型钢翼缘在极限点前后发生屈服,腹板屈服晚于翼缘。

考虑尺寸效应的再生混凝土受压破坏细观模拟研究

再生粗骨料取代率、粗骨料粒径及截面尺寸对再生混凝土单轴压缩破坏行为产生影响。文章采用细观尺度数值模拟方法,建立了再生混凝土二维细观模型,模拟分析了再生混凝土单轴压缩破坏的应力-应变曲线、损伤拓展以及破坏的全过程。结果表明:静态加载下,再生混凝土的损伤由内向外拓展,先出现在最薄弱的界面过渡区,再发展到砂浆基质;截面尺寸、粗骨料粒径和再生粗骨料取代率的改变会影响到再生混凝土的破坏模式和力学性能;再生混凝土存在明显的尺寸效应,不同截面尺寸下,再生粗骨料的粒径和取代率的增大都会在一定程度上削弱尺寸效应行为,而当再生粗骨料取代率为100%时,尺寸效应得到有效抑制。

周向约束对木材压缩吸能特性的影响

通过准静态压缩试验,考察轻木、泡桐、杉木、毛白杨4种木材在周向约束条件时,顺纹和横纹的压缩力学行为、破坏形态、吸能特性,并利用t检验分析其差异显著性。结果表明:①周向约束木材的顺纹压缩应力-应变曲线仍表现为“三阶段”特征,周向约束可抑制木材剪切扩展、大幅屈曲变形等非稳态破坏,曲线平台阶段应力基本与破坏强度持平。②泡桐、杉木、毛白杨顺纹压缩的吸能能力、平台阶段平稳性显著提高,其单位体积吸能值分别提高了15.51%、53.02%、48.15%,理想吸能率增大了35.93%、24.24%、86.00%。③周向约束木材的径向与弦向压缩力学行为、吸能特性相似,木材压头侧边纤维受试样中部拉伸和剪切作用发生竖向剥离,并沿压缩方向产生裂纹。④与无约束压缩相比,周向约束轻木、泡桐横纹压缩应力-应变曲线形状无明显变化,径向压缩单位体积吸能值分别提高了325.51%、182.60%,弦向压缩单位体积吸能值分别提高了430.57%、255.66%;杉木、毛白杨周向约束时横纹压缩应力-应变曲线变为无平台区的“两阶段”形式,理想吸能率均显著降低。

挤压AM80-(0.2Sr)镁合金准静态载荷拉-压变形力学响应行为及其不对称性研究

通过室温条件下的准静态拉伸和压缩试验,研究了挤压AM80-(0.2Sr)镁合金拉伸和压缩变形的力学响应行为及不对称性,并探讨了Sr元素对力学响应行为的影响。结果表明:挤压AM80和AM80-0.2Sr镁合金均具有显著的拉-压变形不对称性。挤压AM80和AM80-0.2Sr的拉伸与压缩的屈服强度比b分别为2.06和2.28,拉伸与压缩的极限强度比c分别为0.78和0.81,且拉伸应变硬化率大幅低于压缩变形。此外,在拉伸变形过程中,两镁合金试样均出现了明显的动态应变时效,其锯齿波动由C型逐渐演变为B型。相较于压缩变形,两种镁合金试样在拉伸变形过程中的动态应变时效更明显。适量添加Sr元素有利于提升拉伸的屈服强度和极限强度,但会降低断裂应变以及压缩的屈服强度和极限强度,并抑制拉伸变形过程中的动态应变时效。两种镁合金的断裂失效机制均为微裂纹的形核和穿晶扩展,微裂纹优先在变形较为严重且出现孪晶交叉的晶界处形核,然后沿与拉伸载荷夹角约为45°的方向扩展,直至贯穿多个晶粒。

一种大孔径静态干涉高光谱成像数据压缩方法

大孔径静态干涉成像遥感数据的数据量较大,需要寻找一种合适的方法对其压缩。从大孔径静态干涉成像机理出发,分析了干涉数据的空间和干涉维冗余性,并基于现有成熟混合压缩编码方法,提出了基于相似干涉曲线与不同光程差之间冗余去除的算法,对冗余数据进行去除预处理。对干涉数据进行基于曲线表的干涉曲线编码表示,对不同光程差之间的图像进行相关性预测,减少了大孔径静态干涉成像遥感图像的量化深度并降低了图像的信息熵,再结合JPEG2000算法进行无损或有损压缩。实验结果表明,对于大孔径静态干涉成像数据,该算法可实现压缩比为3.1倍的无损压缩,有损压缩的率失真曲线也优于其他对比算法,其复原图像反演出的光谱曲线的光谱角和相对二次误差均优于其他对比算法处理的数据,有效保护了光谱信息。

挤压态Mg-Al-Mn-(0.2Sr)镁合金的准静态压缩变形行为

采用准静态压缩实验研究了挤压态Mg-Al-Mn-(0.2Sr)镁合金在0.0001~0.33 s^(-1)应变速率范围内的变形行为。结果表明:挤压态Mg-Al-Mn-(0.2Sr)镁合金的流变应力响应行为具有一定的应变速率相关性,具体表现为流变应力随加载应变速率的增加呈现先降低,后增加,再降低的演变趋势。在压缩变形过程中,两种镁合金试样均产生了较为明显的动态应变时效(DSA)。产生DSA所需的临界应变随加载应变速率的增加而增加,且应力-应变曲线的锯齿类型具有强的应变速率相关性。添加Sr元素降低了镁合金沿径向压缩的流变应力,但对DSA行为具有一定的促进作用。相同应变速率下,挤压态Mg-Al-Mn-0.2Sr镁合金产生DSA的临界应变明显较小,且锯齿幅值略大。此外,Sr元素对孪生也具有一定的抑制作用,导致挤压态Mg-Al-Mn-0.2Sr镁合金的孪晶界出现不同程度的扭曲。

碳纤维增强聚碳酸酯复合材料在高温中的准静态压缩强度

选择碳纤维平纹织物、碳纤维单向布分别和聚碳酸酯制备热塑性复合材料,包括碳纤维增强聚碳酸酯平纹层合板、单向板和正交层合板,并自制的高温装置与MTS(Material Testing System)搭配使用,研究上述三种复合材料在20℃、120℃、150℃、180℃四种温度下的面外准静态压缩行为,分析结构和温度对性能的影响,探究不同条件下的失效模式。结果表明:碳纤维增强聚碳酸酯平纹层合板的最大应力值和压缩模量与正交层合板接近,均高于碳纤维增强聚碳酸酯单向板;且温度对三种复合材料的面外准静态压缩性能有很大影响,随温度升高,准静态压缩模量和最大应力值都减小。

EPE网套包装皇冠梨的静力学特性及损伤研究

目的模拟皇冠梨在物流运输中受静压情况,研究发泡聚乙烯(Expandable Polyethylene,EPE)网套包装皇冠梨在横向、纵向及斜向上的静压力学特性及损伤。方法主要采用皇冠梨准静态压缩试验和有限元分析法。结果横向、纵向及斜向仿真与试验的结果相对误差分别为1.21%、5.49%和5.55%,验证了有限元模型的准确性。与皇冠梨裸果相比,在横向、纵向及斜向静压下,EPE网套包装皇冠梨出现初始损伤的位移分别提高了3.5倍、2.9倍和2.3倍;压缩位移为15 mm时,EPE网套包装皇冠梨承受的静压力分别减小了76.1%、74.4%和51.0%;梨果压缩至破裂时,EPE网套包装皇冠梨承受的静压力分别提高了17.9%、14.9%和21.5%,抵抗外界变形量分别提高了1.9倍、1.8倍和1.7倍。EPE网套包装皇冠梨损伤面积和体积百分比均保持在5%以下。结论EPE网套包装对静压下皇冠梨的缓冲效果明显,提高了其静压承载力,减小了静压损伤,可为皇冠梨缓冲包装设计及采收减损提供一定的参考。

Kagome点阵结构参数对其压缩特性的影响及轻质化设计

为研究三杆支撑(Kagome)结构的压缩性能并对其进行轻质化设计,通过改变胞元的结构参数,建立了12种不同胞元相对密度的Kagome多层点阵结构模型,利用压缩试验和模拟仿真对其进行准静态压缩力学性能分析。研究发现在胞元高度一定时,支撑杆倾角和截面直径的增大均会提高Kagome多层点阵结构的抗压缩性能,且倾角大小对结构整体最大等效平压强度的影响要大于杆直径的影响。胞元相对密度从16.6%提高到60.7%,整体等效平压模量提高了5.54倍,最大等效平压强度提高了4.52倍,模拟仿真结果和试验结果具有一致性。此外,对Kagome加固点阵结构进行轻质化设计,整体质量直接减少了16.3%,压缩仿真结果表明,轻质化设计小幅提升了点阵结构在压缩坍塌失效之前的整体等效平压模量、最大等效平压强度、相对比强度、比刚度和等参数。

基于HSCA静力破岩的金属矿山遗留矿柱安全低损回采试验研究

针对采用爆破破岩方式对金属矿山遗留矿柱进行回采存在安全性差与矿石损失率高的局限性,将具有无振动特征的高效静态破碎剂(High Range Static Cracking Agent,HSCA)引入遗留矿柱回采中。通过设计HSCA膨胀压应力测试试验与静力破岩试验,分析了HSCA径向膨胀压应力大小及其分布规律与水灰比的关系。分别从膨胀压应力分布特征、主裂纹定向控制方法与破岩时效性3个方面,论证了通过HSCA静力破岩实现金属矿山遗留矿柱安全低损回采的可行性。研究结果表明:当水灰比大于理论最优水灰比时,膨胀压应力随着水灰比的增大而减小。对于上向倾斜装药孔膨胀压应力沿孔底至孔口方向呈增大趋势,对于下向倾斜装药孔膨胀压应力则呈减小趋势。采用HSCA静力破岩对遗留矿柱进行回采,遗留矿柱主裂纹走向可通过对装药孔进行预切槽、增设诱导片或新增诱导孔等方式进行定向控制,进而有效降低非破岩区矿岩的再损伤。因此,对于不宜采用爆破破岩方式对遗留矿柱进行回采的区域,HSCA静力破岩不仅有利于保障遗留矿柱回采安全,而且可提高遗留矿柱回收率。研究成果可为进一步探索金属矿山遗留矿柱回采破岩新理论与新技术提供参考。

浇注PBX替代材料的准静态压缩力学行为及本构模型标定

为研究高聚物粘结炸药(PBX)的准静态压缩行为,对两种典型配方(含铝粉与否)的浇注PBX替代材料在不同应变率下进行单轴准静态压缩试验,并对其力学性能进行了对比分析。同时基于朱‑王‑唐(ZWT)模型提出一种新模型来描述材料的准静态压缩行为,通过遗传算法获取本构模型参数,并使用Fortran语言在Abaqus有限元软件的用户材料子程序(UMAT)接口中进行本构模型的建立。结果表明:浇注PBX替代材料的准静态压缩过程可分为弹性压缩、应力衰减以及失稳破坏3个阶段;准静态压缩力学行为与应变率有明显的相关性,随着应变率的提高,材料的有效压缩应变基本不变,而压缩模量、屈服强度和压缩强度的对数与应变率对数近似呈现出线性关系;铝粉的加入使浇注PBX替代材料压缩模量、屈服强度和压缩强度均有所提升。新构建的本构模型能较好描述浇注PBX替代材料的准静态压缩行为,使用有限元软件对本构模型普适性进行验证,得到仿真计算结果与试验结果间可决系数R^(2)均大于0.98,吻合程度较高。

泡沫填充碳纤维增强复合材料方形蜂窝夹芯准静态压缩特性

提出了一种碳纤维增强复合材料(CFRP)方形蜂窝中内嵌聚甲基丙烯酰亚胺(PMI)泡沫材料的新型轻质夹芯结构并实现了夹芯结构的制备。通过准静态压缩试验和数值模拟研究了不同几何参数夹芯的准静态压缩性能,结果表明:当夹芯栅格间距较小时,夹芯中方形蜂窝的破坏模式主要为脆性断裂破坏,随着栅格间距的不断增大,夹芯结构稳定性逐渐降低,因此夹芯失效模式转变为屈曲失稳。内嵌PMI泡沫后,方形蜂窝夹芯的压缩特性得到了显著改善,其压缩强度增大,能量吸收效率提高;随着内嵌泡沫密度的增大,夹芯的压缩强度和比能量吸收量均呈现出增大的趋势。研究结果可以为抗冲击类夹层结构夹芯的创新性设计提供思路和参考。