The effects of the biaxial stretching of leather on fibre orientation and tensile modulus

Leather has been subjected to different degrees of equal biaxial strain (up to 20%) during drying and its tensile modulus has been measured when dry. The collagen fibre orientation distribution in the dried leather has been assessed using wide angle X-ray diffraction. It was found that drying under biaxial strain caused the tensile modulus to increase markedly (by up to 400% at 20% biaxial strains) but with a dependence on the angle of test axis in relation to the principal axes of biaxial strain. The fibre orientation distribution in planes parallel to the surface was affected less by biaxial strain than in planes perpendicular to the surface and it is concluded that the latter type of fibre reorientation is the main factor responsible for the observed increases in tensile modulus.     

自密实轻集料混凝土双轴受力力学性能

为探究自密实轻集料混凝土双轴力学性能,采用三轴试验机对其进行双轴压-压和双轴拉-压试验,得到不同工况下的应力-应变曲线及其破坏形态,通过提取应力-应变曲线峰值应力和峰值应变,并与相关文献普通混凝土与轻集料混凝土研究成果对比,分析自密实轻集料混凝土双轴力学性能。研究结果表明:双轴压-压工况下,当侧向压应力较低时,试件主要呈现剪切破坏形态;当侧向应力较高时,试件呈劈裂破坏形态。双轴拉-压工况下,试件主要呈劈拉破坏形态,与侧向应力无关。随着侧向压应力的提高,自密实轻集料混凝土主压应力相对比无侧向应力工况明显提高,峰值应力最大提高均值幅度为68.08%,主拉应力随侧向压应力的提高逐步降低,最大降低幅度为62.35%。应用Kupfer双轴受力破坏准则验证自密实轻集料混凝土受侧向应力影响变化规律较为保守,同时基于Kupfer提出自密实轻集料混凝土双轴力学性能破坏准则,所得到的破坏准则方程具有良好的适用性。      

Mechanical properties of biaxial weft-knitted flax composites

There has been an increasing interest of using natural fibers to replace glass fibers for composite reinforcement nowadays. However, most of the developments have focused on the random discontinuous fiber composite system. In this study, low cost flax continuous yarn was used to make non-crimp fabric for composite reinforcement based on a biaxial weft-knitted structure. A modified flat knitting machine was developed to produce this kind of reinforcement. Both NaOH treated and untreated reinforcements were used for fabricating the composite samples through the vacuum assisted resin transfer molding process (VARTM). The mechanical properties of the flax yarn, reinforcement and composites were tested and the effect of the NaOH treatment was discussed. The results revealed that although the NaOH treatment resulted in the reduction of the mechanical properties of both yarn and reinforcement, the mechanical properties of the composites could be considerably improved by the NaOH treatment of the reinforcement. The study has provided a simpler way of using low cost natural fiber yarns made from flax long-fiber bundles to produce high performance composites.     

The set and mechanical behaviour of partially processed leather dried under strain

The stress-strain behaviour of partially processed leather which has been dried under a range of uni-axial strains has been investigated. It has been found that the dependence of the tensile modulus on the strain applied during drying is non-linear, increasing slowly at first then more rapidly later on. A two-dimensional microstructural model based on an idealised fibre network can describe this non-linear relationship and account for differences between samples. High values of set are produced by drying under strain and some of this set is retained even after soaking in water. It is suggested that this is due to the formation of stable crosslinks between the chemically modified collagen fibres that comprise leather.     

Experimental study of the behaviour of leather under vacuum conditions

A characterization of the device-pumping behaviour of our vacuum reactor for plasma treatment of materials at low pressure is presented. The analysis of the pumpdown curves measured both in the viscous and in the molecular regimes was made to study the outgassing flowrates of several leather materials. Estimates of the relevant flowrates in different pressure ranges and measurements of the flow composition have been obtained. Such data can be interpreted in the framework of simplified desorption models and could be used in order to design the pumping system needed for vacuum treatment of such materials. We have also measured other related quantities such as the moisture uptake and the weight loss.     

含孔CFRP正交层合板的双轴拉伸力学行为

采用加载臂开槽的中心开孔等厚度十字形试样,实验研究了正交对称铺层碳纤维增强聚合物基复合材料（CFRP）层合板在双轴拉伸载荷作用下的力学行为,分析了3种双轴加载比对其拉伸强度和破坏行为的影响。研究表明:纤维被切断的铺层部分在拉伸作用下容易与其相邻铺层脱粘,导致层合板承载力下降;等双轴加载时,在孔边的被切断纤维与连续纤维间基体在横向拉伸和纵向剪切组合作用下首先开裂;非等双轴加载时,在垂直于快速拉伸方向的铺层中沿孔边应力集中处先出现基体裂纹;随着加载比的增大,快速拉伸方向的细观结构损伤随载荷的增大发展更快,刚度下降更快,破坏时主裂纹的扩展方向更趋于垂直于快速拉伸方向;强度包络线的分析表明快速拉伸方向的拉伸强度随加载比的增大呈缓慢增大的趋势。      

双向拉伸PTFE微孔膜的制备及其孔性能

以聚四氟乙烯(PTFE)细粉料为原料通过一系列机械操作：推挤、滚压和拉伸制得双向拉伸微孔膜．膜的孔性能由GTL—D孔径测定仪和扫描电镜观察膜形态结构来测定．实验结果表明：PTFE粉料和拉伸条件影响相互联系的各项膜孔性能数据，而在不同机械操作阶段的膜形态结构又有显著的差别、双向拉伸微孔膜是呈孔径大小较均匀的纤维网状结构．     

Mechanical properties of composite structures fabricated with the vacuum induced preform relaxation process

A new technique called vacuum induced preform relaxation (VIPR) can be used to improve the processing of composite parts manufactured using vacuum resin infusion methods. The VIPR process is a method for manipulating and guiding the resin filling pattern during a vacuum assisted resin transfer molding (VARTM) manufacturing process with a relatively small external vacuum chamber. This VIPR chamber can be sealed against the flexible molding surface of a VARTM mold and used to create vacuum above the preform. This causes the compressive forces compacting the fabric to decrease allowing the resin to flow faster in the effected region under the chamber. Thus the chamber can influence the resin flow pattern as well as avoid the formation of voids due to merging flow fronts. When the regulated vacuum in the chamber is applied it temporarily decreases the fiber volume fraction of the preform. It is important to investigate if this relaxation has a permanent adverse effect on the mechanical properties of the composite. The results of these tests strongly suggest that the use of the VIPR process does not compromise the mechanical properties of composite structures.     

三维机织碳/碳复合材料双轴压缩载荷下的力学行为

基于三维机织碳/碳复合材料的细观结构特征, 设计平板十字形试样, 在材料双轴力学性能试验机上开展了复合材料单轴、 双轴加载压缩试验, 对比分析了三维机织碳/碳复合材料在双轴压缩载荷下的力学行为。研究表明: 三维机织碳/碳复合材料的压缩行为表现为非线性、 脆性断裂; 双轴载荷作用下非线性特征更为显著, 压缩模量随应力的增加而增大, 强度与模量相较于单轴有较大幅度增加, 双轴压缩载荷作用下材料的强化效应显著; 试样破坏位置并未出现在试样中心区, 而是发生在试样的加载端部或十字形试样的加载分枝根部, 主要表现为基体开裂、 纤维断裂和层间脱粘, 碳布及其层间界面剪切强度的强弱直接影响材料的压缩强度。     

Luminescence properties of potassium chloride activated with Ce under vacuum ultraviolet excitation

采用水热蒸发法制备了KCl∶Ce3＋荧光粉。测量并分析了材料在室温下的真空紫外激发光谱及相应的发射光谱。结果表明激发谱显示6个峰,峰位分别为149、194、206、219、233和251nm。其中149nm的激发峰是基质吸收引起的;194、206、219、233和251nm是Ce3＋离子的4f→5d跃迁引起的。发射峰显示双峰结构,峰位分别是311和326nm。此峰对应于Ce3＋离子的5d→4f（2F5/2,2F7/2）跃迁。     

Crystal twinning in simultaneous biaxial stretching of gelation-crystallized ultra-high molecular weight polyethylene

Structure and orientation development in simultaneous biaxial stretching of gelation-crystallized ultra-high molecular weight polyethylene (UHMWPE) was examined by means of X-ray pole figures. The dry gel film exhibits a stacked lamellar structure similar to single-crystal mats with preferential crystalc-axis orientation normal to the film surface. The biaxially stretched gelationcrystallized films reveal a complex crystal orientation of (1 1 0), (2 0 0) and (0 2 0) planes. The detailed analysis of these pole figures manifests the occurrence of crystal twinning in addition to the orientation of crystallites. Crystal transformation of orthorhombic to monoclinic structure was not observed during biaxial stretching. The orientation distribution functions of crystallites were calculated assuming affine deformation. The orthogonality of crystal 1 1 0-c axes and crystala-c axes was further assumed to be conserved during the crystal twinning and biaxial orientation, respectively. Model simulation with contributions of 30% crystal twinning and 70% crystal orientation yields the best fit with the X-ray pole figures of biaxially stretched UHMWPE films. The affine model appears to be valid up to a biaxial draw ratio of 1.8×1.8, after which the results deviate from ideality with increasing draw ratio.On leave from Department of Materials Science and Engineering, Nagoya Institute of Technology, Nagoya, Japan.     

Thermophysical properties of a multilayer capacitor dielectric under vacuum

Results are presented of an experimental investigation of the coefficient of thermal conductivity in a multilayer cellulose capacitor dielectric under vacuum conditions.     

Mechanical and technological properties of vacuum plasma sprayed and hot isostatically pressed Composite materials

Ti/NiCr20TiAl and NiCoCrALY/IN617 composite materials have been produced by vacuum plasma spraying, Arc-PVD and hot isostatic pressing. Regarding their mechanical and technological properties they are dependent on the heat treatment during the production process. Therefore optimized strength properties can be obtained by using thermal age hardening base materials.     

Mechanical properties of magnetorheological elastomers under shear deformation

In this paper, urethane magnetorheological elastomers (MREs) consisting of carbonyl-iron particles in a polyurethane matrix were studied. The volume fraction of particles was equal to 11.5%. Three types of ferromagnetic particles were used, with average particle size ranging from 1 to 70 μm. The elastic (storage) modulus G′ was measured as a function of angular frequency ω and strength of magnetic field. The measured G′ values were approximated with empirical model. The highest magnetorheological effect has been found for samples with 6–9 μm carbonyl-iron powder. The highest increase in the yield stress is observed for samples with particles aligned at 30° to the magnetic field lines. It has been found that rheological properties strongly depend on the MRE microstructure, in particular on the size/shape of particles and their arrangement. By optimizing the particles size, shape and alignment, the stiffness of MREs has been increased under applied magnetic field.     

Radial stretching of a thin hollow membrane: biaxial tension, tension field and buckling domains

The radial stretching of a hollow thin membrane without compressive strength is considered within the framework of the small strain tension field theory. For each type of the uniform boundary conditions, the loading plane is partitioned into the domains of biaxial tension, tension field and buckling. The extent of these domains critically depends on the value of the Poisson??s coefficient and on the aspect ratio of the membrane. The stress and displacement fields are determined at an arbitrary stage of loading, when the outer biaxially stressed (taut) annulus surrounds the inner (tension field) portion of the membrane, characterized by continuously distributed infinitesimal wrinkles. The growth of the tension field as the loading increases is analyzed. It is shown that, depending on the Poisson??s coefficient and the aspect ratio of the membrane, the tension field may or may not spread throughout the whole membrane. For the fixed outer boundary, and the applied tension or a negative displacement at the inner boundary, and for a particular combination of the material and geometric parameters, the tension field instantly spreads to a specific depth within the membrane, dependent on the Poisson??s ratio and the outer radius of the membrane, remaining constant during further increase of the loading.     

Mechanical and thermal properties of polypropylene reinforced with Alfa fiber under different chemical treatment

The interest in using natural fibers as reinforcement for thermoplastic polymers was attracted several studies covering both material science and green technology. The use of plant fiber requires the issue of compatibility between matrix and fibers. This study treat the effect of chemical modification (alkali treatment, etherification treatment and esterification treatment) on the Alfa fiber surface, and its impact on mechanical and thermal properties of composites. To this end, the percentage of fibers was fixed at (20 wt.%), and to evaluate the effect of each chemical modification in Alfa reinforced polypropylene (PP), based on the mechanical and thermal properties of composites. Composites containing chemically modified Alfa fibers were found to possess improved mechanical and thermal properties when compared to non-treated composite. The highest improvement in Young’s modulus was observed with esterified fibers, with a 35% increase. Thermal stability is best increased using etherification-treated fiber, with gains in the temperature up to 80 °C.     

Nonlocal buckling of double-nanoplate-systems under biaxial compression

This paper reports an analytical study on the buckling of double-nanoplate-system (DNPS) subjected to biaxial compression using nonlocal elasticity theory. The two nanoplates of DNPS are bonded by an elastic medium. Nonlocal plate theory is utilized for deriving the governing equations. An analytical method is used for determining the buckling load of DNPS under biaxial compression. Difference between nonlocal uniaxial and biaxial buckling in DNPS is shown. Both synchronous and asynchronous buckling phenomenon of biaxially compressed DNPS is highlighted. Study shows that the small-scale effects in biaxially compressed DNPS increases with increasing values of nonlocal parameter for the case of synchronous modes of buckling than in the asynchronous modes of buckling. The buckling load decrease with increase of value of nonlocal parameter or scale coefficient. In biaxial compression higher buckling modes are subjected to higher nonlocal effects in DNPS. Further the study shows that the increase of stiffness parameter brings uniaxial and biaxial buckling phenomenon closer while increase of aspect ratio widen uniaxial and biaxial buckling phenomenon.