纯Al纳米晶体材料的力学性能研究

以纯铝粉(99.7%)为原料,采用不同的球磨参数(球料比、球磨时间),利用高能球磨法在室温下制备了具有纳米晶粒的纯Al体材料.通过X射线衍射(XRD)及显微硬度等手段,对不同球磨工艺参数下制备的纯Al体材料的晶粒尺寸及力学性能进行了测试和分析,并对样品的晶粒尺寸与硬度之间的关系进行了深入的探讨.研究发现,所制备的纯Al纳米晶体材料的平均晶粒尺寸在47～66nm范围内;随着球磨时间的延长,样品的显微硬度呈现出先升高后降低再升高的趋势,并在球磨时间为5h时,球磨样品的硬度达到最高值;屈服强度与晶粒尺寸之间符合正的Hall-Petch关系.     

蠕变/疲劳共同作用下寿命估算方法

通过载荷谱转换，将带保载时间的蠕变／疲劳循环用不带保载时间的纯疲劳循环代替，提出蠕变／疲劳共同作用时的寿命估算方法。对１２Ｃｒ１ＭｏＶ钢母材和焊材的蠕变／疲劳交互作用试验数据的分析结果表明，本方法方便、实用。提出一个表征蠕变／疲劳连续加载时交互作用行为的参数，蠕变／疲劳寿命比。分析认为，材料的蠕变疲劳交互作用行为与该比值的大小腾。材料在蠕变／疲劳共同作用下呈这是呈负交经作用，并非材料固有的特性，还取决于载荷条件。     

四边简支FGM矩形板非线性振动中的内共振

基于Reddy高阶剪切变形板理论、Galerkin法与Hamilton原理,在同时考虑几何非线性、材料物性参数随温度变化且材料组分沿厚度方向按幂律分布的情况下,通过理论推导建立起四边简支功能梯度材料矩形板受面内与横向简谐载荷共同作用下的非线性运动控制微分方程;在此基础上建立了受热载荷作用下的具有二阶模态的非线性运动控制微分方程。分析了面内静态载荷对于给定参数下系统不同内共振的影响,发现随着面内静态载荷的不同系统会出现1:1、1:2 与1:3的内共振关系。计算结果发现由于不同内共振关系对应的面内静态载荷不同,不同内共振关系下所求得的一阶频率大小各不相同,所对应的瞬态响应也会不同。     

碳/碳复合材料的激光烧蚀行为与机制

碳/碳(C/C)复合材料作为性能优良的耐烧蚀材料得到了广泛的应用,其作为抗激光烧蚀材料的潜力待被发掘。本文制备了不同密度的C/C复合材料,在无氧环境下以CO₂激光器为光源,探究了高能激光与C/C复合材料之间的作用机制,系统地分析了材料的特性和激光参数不同对烧蚀表现的影响。采用三维轮廓仪对线烧蚀率进行表征。结果表明,随着烧蚀时间或激光功率的变化,C/C复合材料的烧蚀表现均为非线性变化。C/C复合材料的本征特性决定了其热量载荷。密度越高的C/C复合材料,其热量载荷越高,烧蚀性能越好。热量载荷与激光热流密度之间的关系则决定了材料的烧蚀表现,当激光的热流密度大于材料的热流载荷时,烧蚀速率会呈阶跃式攀升。     

真空橡胶的动静态力学性能及本构关系的研究

针对真空橡胶的特性展开了MTS实验和SHPB实验,通过实验获得了材料的基本力学参数和应力-应变曲线,并对材料的变形行为、应变率相关效应和破坏特性作了必要的分析,在此基础上给出了材料的本构关系。结果表明,真空橡胶材料在准静态加载下,应力3MPa时,应变率效应不明显;在冲击载荷作用下,随着加载应变率的提高,应变率效应比较明显。     

弹性地基上复合材料夹层梁的热过屈曲

通过考虑轴线可伸长变化及剪切变形等因素影响,建立了热载荷作用下复合材料夹层梁受弹性地基约束时的几何非线性控制方程。利用打靶法数值求解所得强非线性边值问题,并获得了一边不可移铰支一边固定的夹层梁在横向均匀升温作用下的静态热过屈曲和热弯曲变形数值解。给出了临界升温与地基弹性参数之间的关系曲线,得到了模态跃迁性质,分析了载荷热与地基刚度参数对一阶模态的过屈曲平衡构形的影响。     

微压入法在纸张微观力学性能研究中的应用

将微压入测试方法应用于对纸张纤维强度及纤维之间结合强度的测试是一种行之有效的方法,通过对被测纤维及纤维之间结合部位的载荷-位移关系曲线的分析,可以计算出纤维及结合点处的弹性模量和弹性接触刚度,这种测试方法为进一步研究纤维性能与纸张性能之间的关系打下基础.     

拉弯联合载荷下弹塑性J积分估算方法研究

基于计算J积分的等效原场应力方法,利用等效原场应力σeff只有外加载荷意义而不再具有拉伸或者弯曲等载荷类型方面的属性,提出了利用现有含裂纹结构的纯拉伸以及纯弯曲J积分全塑性解直接计算拉弯联合载荷下的J积分简化估算方法。该方法可以直接利用已经存在的J积分纯拉伸和纯弯曲全塑性解来计算拉弯联合载荷下的J积分,简化了拉弯联合载荷下J积分全塑性解的计算;并且可以应用于任意应力-应变材料,包括Ramborg-Osgood关系的材料和任意单调加载非R-O关系材料。计算过程简便。并通过与有限元计算结果对比对之进行验证,说明其工程实用性,为对含裂纹结构进行弹塑性断裂评定奠定基础。     

Si3N4结合SiC耐火陶瓷裂纹萌生细观数值分析

将复合球模型导入损伤力学模型中,模拟了材料在高温环境下受拉和受压损伤时的细观演化过程;推导了材料损伤参数与界面相参数之间的函数关系。研究表明,当外加载荷小于材料损伤临界值、介于材料损伤临界值和最大承载值之间时,不论材料承受拉载还是压载,都存在无损伤演化的线弹性阶段和微裂纹扩展区逐渐增大的损伤强化阶段;若应力达到最大承载值后仍继续增大,材料损伤将过渡到损伤局部化阶段,损伤局部化的连续也就是材料宏观裂纹萌生的开始。     

非均匀载荷作用下偏心磨损套管稳定性准则

采用小参数摄动法建立了非均匀载荷作用下具有初始椭圆度的磨损套管稳定性准则,根据计算得到的磨损套管相对应变及其临界值之间的关系确定磨损套管是否发生挤毁失效。分析了磨损套管相对应变及其临界值与套管外径、壁厚、磨损深度、载荷非均匀度、初始椭圆度之间的关系。套管外径增加导致失稳临界应变减少,磨损套管保持其稳定的壁厚也随之增加。非均匀载荷作用下磨损套管相对应变与初始椭圆度之间是线性关系,套管初始椭圆度越大,相对应变越大。相对应变与载荷非均匀性之间是抛物线关系。磨损套管载荷非均匀性越大,相对应变越大。非均匀载荷作用下磨损套管的相对应变随磨损深度线性增加。该准则能够用于评价非均匀载荷作用下含磨损缺陷套管的井筒的完整性。     

Poisson's ratios in glass fibre reinforced plastics

The characterisation of the mechanical properties of an orthotropic composite material generally requires nine interdependent elastic constants: three Young's moduli, three shear moduli and three Poisson's ratios. In most papers it is the practice to quote only two orthogonal axial moduli, a shear modulus and a Poisson's ratio in the plane of the laminate. However, the value of Poisson's ratio is a function of the orientation of the loading axis relative to the principal axis of the reinforcement fibres, both in and through the plane of the laminate. In an earlier paper, the correlation of experimental and theoretically predicted Poisson's ratios was reported around the angles in the plane of the laminate. Both unidirectional and woven roving fibreglass panels were tested. Accurate prediction of Poisson's ratio was shown to be critically dependent on the value of shear modulus used. This paper reports an extension of the previous work to consider the through-plane properties and will examine the results in the context of the Lempriere constraints.     

Shear coupling effects on stress and strain distributions in wood subjected to transverse compression

The mechanical behaviour of a wood board subjected to transverse compression is relevant to the performance of glulam beams and solid wood structures. The wood material can be described as polar orthotropic, due to the annual ring structure and to the differences in moduli in different directions in the radial–tangential plane. Strain measurements are performed on single wood boards using a whole-field digital speckle photography technique. Finite element analysis is performed and compared with experimental data. Good agreement in terms of strain fields and apparent moduli is observed between predictions and data. The experimental data show strong variations in local strain due to the polar orthotropic behaviour of wood in this plane, and the extremely low value for shear modulus G_rt as compared with the other moduli. This leads to shear coupling effects resulting in large local shear deformation and correspondingly low effective stiffness under transverse global loading.     

Shear Modulus Determination of Cuboid Metallic Open‐Lattice Cellular Structures by Analytical,Numerical and Homogenisation Methods

Abstract: The shear response of irregular open‐lattice cellular cores made of interconnected metallic struts is analysed, and the core shear modulus in the three material principal directions is derived. The analytical approach is based on the technical beam theory, which is used for the determination of the unit‐cell response under complex loading. The influence of the strut geometrical parameters and unit‐cell shape and size on the cellular core shear stiffness is determined. The analytical determination of the unit‐cell properties is successfully validated by a reference numerical model of the unit cell, which is developed for comparison purposes. Furthermore, the homogenisation principles are applied to the prediction of the shear response of a core block structure comprising a high number of unit cells, for which experimental results were available; the comparison revealed that the experimental results coincide well with the results obtained from the homogenised model.     

Influence of Compression and Shear on the Strength of Composite Laminates with Z-Pinned Reinforcement

The influence of compression and shear loads on the strength of composite laminates with z-pins is evaluated parametrically using a 2D Finite Element Code (FLASH) based on Cosserat couple stress theory. Meshes were generated for three unique combinations of z-pin diameter and density. A laminated plate theory analysis was performed on several layups to determine the bi-axial stresses in the zero degree plies. These stresses, in turn, were used to determine the magnitude of the relative load steps prescribed in the FLASH analyses. Results indicated that increasing pin density was more detrimental to in-plane compression strength than increasing pin diameter. Compression strengths of lamina without z-pins agreed well with a closed form expression derived by Budiansky and Fleck. FLASH results for lamina with z-pins were consistent with the closed form results, and FLASH results without z-pins, if the initial fiber waviness due to z-pin insertion was added to the fiber waviness in the material to yield a total misalignment. Addition of 10% shear to the compression loading significantly reduced the lamina strength compared to pure compression loading. Addition of 50% shear to the compression indicated shear yielding rather than kink band formation as the likely failure mode. Two different stiffener reinforced skin configurations with z-pins, one quasi-isotropic and one orthotropic, were also analyzed. Six unique loading cases ranging from pure compression to compression plus 50% shear were analyzed assuming material fiber waviness misalignment angles of 0, 1, and 2°. Compression strength decreased with increased shear loading for both configurations, with the quasi-isotropic configuration yielding lower strengths than the orthotropic configuration.     

On the effect of orthotropy in a cracked cylindrical shell

A pressurized cylindrical shell containing a longitudinal crack is considered. It is assumed that the material has a special orthotropy, namely that the shear modulus of the sheet may be evaluated from the measured Young's moduli and the Poisson's ratios rather than being an independent material constant. Two examples, one for a mildly orthotropic (titanium) and the other for a strongly orthotropic (graphite) material approximately satisfying the condition of special orthotropy are given. The results show that the stress intensity factors are rather strongly dependent on the degree of orthotropy.     

Effect of on-axis tensile loading on shear properties of an orthogonal 3D woven SiC/SiC composite

The present study examines in-plane and out-of-plane shear properties of an orthogonal 3D woven SiC fiber/SiC matrix composite. A composite beam with rectangular cross-section was subjected to a small torsional moment, and the torsional rigidities were measured using an optical lever. Based on the Lekhnitskii’s equation (Saint–Venant torsion theory) for a orthotropic material, the in-plane and out-of-plane shear moduli were simultaneously calculated. The estimated in-plane shear modulus agreed with the modulus measured from ±45° off-axis tensile testing. The effect of on-axis (0°/90°) tensile stress on the shear stiffness properties was also investigated by the repeated torsional tests after step-wise tensile loading. Both in-plane and out-of-plane shear moduli decreased by about 50% with increasing the on-axis tensile stress, and it is mainly due to the transverse crack propagation in 90° fiber bundles and matrix cracking in 0° fiber bundles. It was demonstrated that the torsional test is an effective method to estimate out-of-plane shear modulus of ceramic matrix composites, because a thick specimen is not required.     

Mechanical properties of magnetite (Fe3O4), hematite (α-Fe2O3) and goethite (α-FeO·OH) by instrumented indentation and molecular dynamics analysis

Hardness and elastic properties of pure (crystal) and complex (product of corrosion) iron oxides, magnetite (Fe₃O₄), hematite (α-Fe₂O₃) and goethite (α-FeO·OH), were determined by means of molecular dynamics analysis (MDA) and instrumented indentation. To determine local mechanical properties by indentation, multicyclic loading is performed by using incremental mode. Moreover to study the influence of visco-elastoplastic behaviour of the material, various load-dwell-times were applied at each loading/unloading cycle. To support the indentation results, molecular dynamics analysis based on shell model potential is performed for pure oxides to determine Young's modulus, bulk modulus, Poisson's ratio and shear modulus. The comparison between experimental and theoretical values both with the literature data allows the evaluation of the mechanical properties of the pure oxides. Subsequently, this allows the validation of the mechanical properties of complex oxides which can only be deduced from indentation experiments.     

On stress concentrations for isotropic/orthotropic plates and cylinders with a circular hole

Scale factors (SFs) are widely used in engineering applications to describe the stress concentration factor (SCF) of a finite width isotropic plate with a circular hole and under uniaxial loading. In this paper, these SFs were also found to be valid in an isotropic plate with biaxial loading and an isotropic cylinder with uniaxial loading or internal pressure, if a suitable hole to structure dimension ratio was chosen. The study was further expanded to consider orthotropic plates and cylinders with a center hole and under uniaxial loading. The applicable range of the SFs was given based on the orthotropic material parameters. The influence of the structural dimension on the SCF was also studied. An empirical calculation method for the stress concentrations for isotropic/orthotropic plates and cylinders with a circular hole was proposed and the results agreed well with the FEM simulations. This research work may provide structure engineers a simple and efficient way to estimate the hole effect on plate structures or pressure vessels made of isotropic or orthotropic materials.     

Differences in shear strengths of orthotropic bodies

Summary The failure strengths of anisotropic bodies and especially those of fiber reinforced materials are generally studied at the principal directions of their strength and loading of the bodies. For the most reliable criteria, based on failure functions having the form of a general tensorial polynomial, the modes of failure of anisotropic (orthotropic, transversely isotropic) bodies were studied primarily on the principal stress (₁, ₂, ₃). The elliptic paraboloid failure surface, (EPFS), being one of them, was based on the phenomenological assumption that even the anisotropic media do not fail under the influence of any hydrostatic pressure superimposed with an elementary loading, taking care of the anisotropy.Modes of failure of orthotropic media under the influence of shear loading, which are very important especially for the transfer of loading between fibers and matrix in a single lamina, as well as in the theory of failure of laminates, was up-to-now neglected.In this paper the effect of shear is studied for transversely isotropic and orthotropic materials. The dependence of the pure shear loading on the sign of shear, as well as on the parameters of anisotropy of the medium, is established. Interesting results are revealed, having a direct application on a well-founded theory of strength of laminates.     

三向碳/碳材料端头帽热应力的非线性有限元分析

用三个正交方向里纤维根数之比为2:2:3所编织成的三向碳/碳材料(2-2-33DC/C)制成的端头帽是双模量非线性轴对称结构.在双线性型、轴对称结构分析的程序SAASⅢ基础上进行了改造.对2-2-3 3D C/C端头帽进行非线性热应力分析.在改进的运序中,用材料主方向的受力状态来确定用拉伸还是压缩模量;考虑剪切非线性行为;增添了2-2-3 3DC/C材料的强度准则以检查结构的安全性;改进了绘图功能,除能画应力等高线外,增加了沿某一曲线,画出应力分布图的功能;改变2-2-3编织为2-2-4和2-2-5编织来讨论不同编织对热应力分布的影响.