编织复合材料的一种数值模型

提出编织复合材料的一种均匀化的数值模型,为工程应用提供了简便而有较高精度的数值计算方法。首先,对于编织复合材料的代表性单胞建立了精细有限元的胞元模型;其次,将单胞有限元分析结果引入基于三类变量广义变分原理的三维体罚单元,从而将一类有细观编织结构的复合材料等效为可用于宏观尺度计算的复合材料单元。通过数值算例将此方法的计算结果与实验和其他数值计算结果进行了比较,验证了该方法的有效性与实用性。     

三维四向大编织角复合材料的细观强度分析

编织角是影响三维编织复合材料力学性能的最重要因素.实验数据表明:大编织角复合材料在单向拉伸作用下的破坏形式较为复杂,其应力-应变曲线呈现非线性特性.本文建立了细观应力场的均匀化列式和有限元求解方法,运用该方法对三维大编织角复合材料的细观应力分布进行了数值模拟,结合相关的强度理论对材料进行失效分析,并进一步对材料的拉伸强度进行预测.强度计算结果与实验结果较为吻合.     

三维四向编织复合材料有效性能的预报

根据三维四向编织复合材料中纤维束的空间几何结构特征, 建立了比较合理的三胞模型。模型中考虑了3种单胞各自纤维束的空间结构和弯曲, 同时引入纤维束填充因子来描述各类单胞中纤维束的不同截面形状对材料弹性常数的影响。基于刚度体平均方法, 建立了相应的刚度预报模型, 得到了三维四向编织复合材料的工程弹性常数。用细观力学方法分析了工艺参数和尺寸效应对材料有效性能的影响规律。不同尺寸试件的数值预报结果和实验结果吻合较好。      

三维编织复合材料制件的细观单胞元分析

讨论了实现三维编织复合材料细观结构与宏观响应直接相关连的计算方法——细观单胞元法。该方法采用细观结构单胞作为离散单元对三维编织复合材料制件进行宏观网格剖分,在细观结构分析的基础上,将微结构上的物理量转换为细观单胞元的宏观节点变量,建立了细观单胞元刚度矩阵的求解方法。数值结果表明,该方法能反映细观结构参数的变化对材料宏观物理量的影响。     

基于均匀化理论的三维编织复合材料细观应力数值模拟

从基于小参数渐近展开和摄动方法的均匀化理论出发,给出了求解细观应力的数学表达式。通过有限元方法对三维编织复合材料的细观应力场进行数值模拟,并结合适当的强度准则对拉伸极限强度下单元的失效情况进行判断,得出材料强度的一种细观失效判据。通过该方法得到的应力计算结果与实验结论基本相符。      

三维多向编织复合材料非线性本构行为的细观数值模拟

针对三维多向编织复合材料, 在已建立的单胞几何模型及材料力学性能细观计算力学分析方法的基础上,引入M urakam i 的几何损伤理论模拟纤维束的细观损伤行为, 建立了预报该类材料非线性本构行为数值模拟及细观损伤机理的有限元分析方法。结合实例预报了碳/环氧四向编织复合材料本构的非线性行为, 并与实验结果进行了对比。      

三维六向编织复合材料渐进损伤模拟及强度预测

基于三维六向编织复合材料的细观结构,假设第六向纱线的截面形状为菱形,建立了三维六向编织复合材料的渐进损伤有限元模型。采用Linde等提出的失效准则,引入周期性位移边界条件,对三维六向编织复合材料的纵向拉伸应力-应变行为进行了渐进损伤数值模拟,讨论了单胞模型在纵向拉伸载荷作用下的细观损伤起始、扩展和最终失效的演化过程,并预测了材料的拉伸强度。在此基础上,进一步研究了编织角、纤维体积分数和编织纱水平取向角等参数对材料纵向拉伸力学性能的影响规律。研究结果表明,三维六向编织复合材料的轴向纱线拉伸断裂是导致其破坏的最主要因素。所得数值结果与现有试验值吻合较好,验证了该模型的有效性,为更深入研究此类材料的力学性能奠定了基础。     

三维四向编织复合材料剪切性能的数值预报

应用三维四向编织复合材料的参数化建模技术, 在“米”字型枝状体胞计算模型基础上, 讨论了该材料在剪切变形中体胞的边界条件, 较为真实地模拟了该材料的细观结构, 进一步对该材料的剪切模量进行了数值预报;最后通过结果的分析, 总结了剪切模量随不同编织角及纤维体积含量的变化关系。     

三维五向编织复合材料的力学性能分析Ⅱ: 细观应力数值模拟

在实验观察的基础上, 考虑轴纱的挤压变形及纱线填充因子变化, 修正了现有的细观结构模型, 建立了更加真实反映三维五向编织复合材料内部编织结构的单胞模型。利用该模型采用3D 有限元法计算了材料的轴向弹性模量, 数值预测与实验数据吻合较好。分析了单胞在拉伸载荷作用下的细观应力分布, 为材料进一步的强度预测奠定了基础。      

圆管状立体机织复合材料的多尺度分析

采用多尺度耦合的数值模型研究了圆管状立体机织复合材料的力学性能。建立了反映纤维束中纤维/基体二相材料的微观尺度单胞和反映周期性编织结构的细观尺度扇形单胞,并重点讨论了扇形单胞的周期性边界条件。通过逐级计算微观单胞、细观单胞的平均弹性常数,得到了圆管状立体机织复合材料的刚度参数,实现了由组分材料性能及编织参数预测圆管的宏观弹性性能,模型预测刚度与试验结果吻合。另一方面,研究了从大到小各尺度耦合的应力分析,对于圆管环向应力非周期分布的情况,建立了嵌入细观单胞的环状模型,进行了复杂荷载下从宏观圆管结构、到细观纤维束尺度、再到微观纤维尺度之间的逐级应力分析。     

三维机织复合材料的一种梁单元细观力学模型

根据三维机织复合材料中纤维束排列和变形的周期性特点,推导了一种细观梁单元模型。该模型考虑了纤维束的拉 (压) 弯耦合效应和纤维束之间的相互作用,可以描述纤维束和基体中的细观应力分布,并得到宏观的力学性能。针对一种典型的三维机织复合材料,首先根据编织参数,确定其细观几何结构,取最小周期的一段纤维束作为分析胞元,用上述细观梁单元分析了该段纤维束在面内拉伸荷载下的细观应力分布,计算出平均模量, 并用材料试验和细观实体有限元对本模型进行了检验,结果与本文的预测吻合良好。研究表明,拉、弯耦合效应引起的纤维束中的细观弯曲应力同平均轴向应力相比,不可忽略。      

Analysis of micro fracture in human Haversian cortical bone under transverse tension using extended physical imaging

We propose a procedure to investigate local stress intensity factors at the scale of the osteons in human Haversian cortical bone. The method combines a specific experimental setting for a three‐point bending millimetric specimen and a numerical method using the eXtended Finite Element Method (X‐FEM). The interface between the experimental setting and the numerical method is ensured through an imaging technique that analyses the light microscopy observations to import the geometrical heterogeneity of the Haversian microstructures, the boundary conditions and appearing crack discontinuities into the numerical model. The local mechanical elastic Young's moduli are measured by nano‐indentation, and the Poisson ratios are determined by an imaging technique of the stress–strain fields. The model is able to access three scales of measurement: the macro scale of the material level (mm), the micro scale inside the Haversian material for stress–strain fields (10–100µm), and the sub‐micro scale for the crack opening profiles (1–10µm ) and fracture parameters (stress intensity factors). The model is applied to several patients at different aging stages. Copyright © 2009 John Wiley & Sons, Ltd.     

矫直辊辊型曲线对中厚板残余应力的影响

为深入研究矫直过程中矫直辊辊型曲线对中厚板残余应力的变化规律,分析了新一代矫直机的典型弯辊结构,针对其中一种弯辊结构运用接触有限元软件计算了其弯辊效果,得到了从负弯到正弯等多种矫直辊辊形曲线.建立了矫直辊辊形曲线对残余应力影响的解析模型和流程,运用数值方法求解该模型.研究结果表明:弯辊量对较宽的轧件的残余应力影响不显著,即使弯辊手段用反了,残余压应力仍然可以得到一部分消减;弯辊量对较窄的轧件的残余应力影响很大,如果弯辊手段用反了,残余拉、压应力都得不到有效消减.     

M5-36-11 No.20.5风机轴断裂分析

采用宏微观检验、化学分析和显微硬度检测等方法对风机轴的断裂原因进行了分析。结果表明，该轴断裂是由于轴表面存在焊接缺陷，在旋转弯曲应力作用下产生应力疲劳开裂。     

Experimental Investigations of Fracture Process in Concrete by Means of X‐ray Micro‐computed Tomography

The paper describes investigation results on fracture in notched concrete beams under quasi‐static three‐point bending by the X‐ray micro‐computed tomography. The two‐dimensional (2D) and three‐dimensional image procedures were used. Attention was paid to width, length, height and shape of cracks along beam depth. In addition, the displacements on the surface of concrete beams during the deformation process were measured with the 2D digital image correlation technique in order to detect strain localisation before a discrete crack occurred. The 2D fracture patterns in beams were numerically simulated with the finite‐element method using an isotropic damage constitutive model enhanced by a characteristic length of micro‐structure. Concrete was modelled as a random heterogeneous four‐phase material composed of aggregate, cement matrix, interfacial transitional zones and air voids. The advantages of the X‐ray micro‐computed tomography were outlined.     

基于修正单层梁理论的夹层梁最大弯曲正应力计算

目的计算夹层梁横截面的最大弯曲正应力。方法将夹层梁等效成等截面均质单层梁,进而推导出了理论计算公式,并在此基础上进行了三点弯曲试验的算例研究。结果当破坏载荷与夹层梁横截面的尺寸一定时,随着芯层与总厚度比的增加,修正单层梁理论计算的最大正应力值逐渐增加,而单层梁理论计算的结果为恒定值。对于同样结构的夹层梁,随着芯层弹性模量与表层模量比的增加,修正单层梁理论计算的最大正应力与单层梁理论的差异值越来越小。结论修正单层梁理论与层合梁理论计算的结果是一致的,该方法可有效进行最大弯曲正应力的预测与计算。     

低雷诺数下微型四旋翼飞行器气动和振动特性分析

振动噪声环境预示技术是高速飞行器总体设计、结构设计的重要内容,对高速飞行器振动噪声预示问题的国内外研究进展进行简单回顾,再结合作者的工程与研究实践,从振动噪声源、传递特性和响应分析技术等几个方面,提出若干需要深入研究的关键技术,并指出这些技术的完善,将有助于高速飞行器全弹道、全频段振动噪声响应预示和控制问题的解决。     

Procedure to Perform a Validated Incremental Hole Drilling Measurement: Application to Shot Peening Residual Stresses

Abstract: The paper describes a test rig designed to check and assess the accuracy of the incremental hole drilling (IHD) method. An external load produces a controlled linearly through thickness variable uniaxial stress field (reference bending stress), known with good accuracy, that can be applied and removed at each hole increment. After the separation between the bending relaxed strain from the residual stress relaxed strain, it is possible to reproduce the bending stress distribution in order to have complete confidence of the residual stress measurement. The bending verification of the IHD method was already proposed by other investigators before, but residual stress measurements were then performed on independent configurations. The proposed testing procedure gives a ‘real time’ verification of the residual stress measurement. Any experimental malfunctioning due to the operator inexperience, or any error during the stress calculation from the relaxed strain would produce an evident difference between the expected reference bending stress and the IHD bending stress output. Moreover, the reference bending stress helped for understanding that the not perfect hole cylindrical shape causes an underestimate of the predicted stress near the surface. A zero depth offset correction was proposed. This correction was tested on the reference bending stress, and then applied to the residual stress prediction. Three shot peening residual stresses IHD measurements were successfully validated by means of the bending stress; moreover, they were in good agreement with independent X‐ray diffraction measures also proposed in the paper.     

Bottom up fabrication of a nickel-lead zirconate titanate piezoelectric microcantilevers

Proof of concept for the novel bottom up manufacture of a piezoelectric microelectromechanical system cantilever demonstrator is presented. Cantilever structures, consisting of 10 µm lead zirconate titanate (PZT) and 21 µm nickel, were fabricated by electroforming Ni structures on top of PZT films, subsequently patterning the PZT and removing the temporary sacrificial silicon support structure. Following release the cantilevers exhibited minimal bending indicating low levels of internal stress. The resonant frequency was calculated to be 33.2 +/− 2 kHz, which is in agreement with that of 33.8 kHz measured using an impedance analyzer and 33.9 kHz measured using the mechanical vibration method.     

Fatigue crack growth through a residual stress field introduced by plastic beam bending

We present predictions and measurements of fatigue crack growth rates in plastically bent aluminium 2024‐T351 beams. Beam bending and fatigue were carefully controlled to minimize factors other than residual stress that could affect the fatigue crack growth rate, such as large plastic strains or residual stress relaxation. The residual stress introduced by bending was characterized by a bending method and by the slitting method, with excellent agreement between the two methods. Crack growth rates were predicted by three linear elastic fracture mechanics (LEFM) superposition based methods and compared to experimental measurements. The prediction that included the effects of partial crack closure correlated with experimental data to within the variability normally observed in fatigue crack growth rate testing of nominally residual stress free material. Therefore, we conclude that crack growth through residual stress fields may be predicted using the concept of superposition as accurately as crack growth through residual stress free material, provided that the residual stress is accurately known, the residual stress remains stable during fatigue, the material properties are not changed by the introduction of residual stress, and that the effect, if any, of partial crack closure is taken into account.