[±θ/902]碳/环氧层板横向裂缝与分层的测试及分析

本工作对[±θ/90₂]_S和[0_n/90₂]_S两系列碳/环氧层板在拉仲载荷下的横向裂缝与分层损伤,进行了实验研究和有限元分析.采用声发射技术跟踪配合显微观测多向层板损伤过程,分析了θ角变化与力学性能、初始损伤、累积等的关系.表明实测横向开裂与分层结果和采用能量判据有限元计算预测比较,符合良好.同时在扫描电镜内进行各类层板压缩试验,动态观测破坏形貌,讨论了不同θ铺层角的微观破坏机理.     

复合材料层合板准静态压痕损伤研究

本文对复合材料层合板准静态横向压缩特性损伤进行了研究.在损伤模拟过程中采用机体开裂和分层扩展判据,分类考虑了不同的损伤形式,通过修正损伤层的材料常数来模拟层板损伤所造成的局部刚度下降对横向压痕过程的影响.损伤模拟结果与超声C扫描的结果吻合较好.     

含单分层损伤复合材料层合梁的动力屈曲研究

采用有限差分（FDM）方法求解了含初始缺陷和单个分层损伤复合材料层合梁的轴向刚性质量块撞击的脉冲动力屈曲问题。基于Hamilton原理导出了考虑所有惯性影响以及一阶横向剪切变形（FSDT）影响时单个分层损伤复合材料梁的非线性动力屈曲控制方程；采用B—R准则判断梁动力屈曲时刻，同时确定刚性质量块的临界冲击速度。重点研究脱层、冲击速度、初始几何缺陷等因素对复合材料层合梁脉冲动力屈曲的影响。     

界面脱粘对正交铺设SiC/CAS复合材料基体开裂的影响

采用细观力学方法研究了正交铺设SiC/CAS复合材料在单轴拉伸载荷作用下界面脱粘对基体开裂的影响。采用断裂力学界面脱粘准则确定了0°铺层纤维/基体界面脱粘长度, 结合能量平衡法得到了主裂纹且纤维/基体界面发生脱粘(即模式3)和次裂纹且纤维/基体界面发生脱粘(即模式5)的临界开裂应力, 讨论了纤维/基体界面剪应力、 界面脱粘能对基体开裂应力的影响。结果表明, 模式3和模式5的基体开裂应力随纤维/基体界面剪应力、 界面脱粘能的增加而增加。将这一结果与Chiang考虑界面脱粘对单向纤维增强陶瓷基复合材料初始基体开裂影响的试验研究结果进行对比表明, 该变化趋势与单向SiC增强玻璃陶瓷基复合材料的试验研究结果一致。     

复合材料加筋板低速冲击损伤的数值模拟

建立了复合材料加筋板在横向低速冲击载荷作用下的渐进损伤有限元模型。该模型考虑了复合材料加筋板受低速冲击时的纤维断裂、基体开裂及分层脱粘等五种典型的损伤形式, 在层内采用应变描述的失效判据, 结合相应的材料性能退化方案, 通过编写VUMAT用户自定义子程序以实现相应损伤类型的判断和演化。在层间以及筋条与层板间加入界面元, 模拟层间区域的情况, 结合传统的应力失效判据和断裂力学中的能量释放率准则来判断分层损伤的起始和演化规律。通过对数值模拟结果与实验数据的比较, 验证了模型的合理性和有效性。同时探讨了不同位置、不同冲击能量以及含初始损伤(脱粘)等因素对复合材料加筋板低速冲击性能的影响。     

正交铺层PIP-SiC_(f)/SiC复合材料的水淬失效行为EI北大核心CSCD

铺层方式是影响预浸料铺贴工艺制备SiC_(f)/SiC复合材料抗热震性能的关键因素之一。以先驱体浸渍裂解(PIP)工艺制备的SiC_(f)/SiC复合材料为研究对象,基于内聚力模型,对不同正交铺层方式的方形试样开展1200℃高温水淬实验及有限元仿真研究。结果表明:水淬过程中试样表面温度急剧下降,芯部与表面存在较大温差,顶角与芯部温差最高可达1077℃,导致复合材料的主要失效模式为层间开裂和基体开裂,且试样层间开裂位置与铺层方式有关。由模拟结果可知,由于正交铺层方式的不同,试样的层间开裂位移也存在显著差异:[0/90]与[0/90/0]两种铺层方式层间主裂纹开裂位移约为0.61 mm,而[0/0/90/90]铺层主开裂位移仅为0.04 mm。此外,随着水淬时间的增加,层间主裂纹开裂位移逐渐增大,而次裂纹在开裂后逐渐发生闭合现象。     

复合材料正交层板基体裂纹预测

将断裂力学的能量释放率概念及A.S.D.Wang在90°层横向裂纹情况的方法推广到层板后期复杂基体裂纹情况,对[O₂/90n]s层板内的0°层纵向劈裂、0°—90°界面的两种脱层等裂纹形式进行三维有限元分析并得出其应变能释放率与裂纹尺寸的关系曲线,对每种基体开裂给出载荷的理论预测值,与实验结果比较,吻合较好。      

桥面铺装层弯曲正应力分析

基于梁体结构的线弹性理论,对活载作用下桥面铺装层内的弯曲正应力进行了分析,得到了铺装层内弯曲正应力的计算公式,为分析桥面铺装层横向开裂问题提供了理论依据。     

复合材料加筋板低速冲击损伤的数值模拟

建立了复合材料加筋板在横向低速冲击载荷作用下的渐进损伤有限元模型.该模型考虑了复合材料加筋板受低速冲击时的纤维断裂、基体开裂及分层脱粘等五种典型的损伤形式,在层内采用应变描述的失效判据,结合相应的材料性能退化方案,通过编写VUMAT用户自定义子程序以实现相应损伤类型的判断和演化.在层间以及筋条与层板间加入界面元,模拟层间区域的情况,结合传统的应力失效判据和断裂力学中的能量释放率准则来判断分层损伤的起始和演化规律.通过对数值模拟结果与实验数据的比较,验证了模型的合理性和有效性.同时探讨了不同位置、不同冲击能量以及含初始损伤(脱粘)等因素对复合材料加筋板低速冲击性能的影响.     

双向等轴拉伸载荷下二维编织陶瓷基复合材料的应力-应变行为预测

将二维编织结构简化为(0°/90°)s正交铺层结构。采用含损伤变量的剪滞分析理论,解得双向等轴拉伸载荷下,0°层和90°层开裂后各层的应力分布;基于随机基体裂纹演化理论,随机纤维损伤和最终失效理论,确定了0°层和90°层沿纤维方向的应力-应变关系,以及切线拉伸模量与施加载荷之间的关系;然后,将切线拉伸模量代入正交铺层结构的剪滞分析中,进而预测出二维编织陶瓷基复合材料在双向等轴拉伸载荷下的应力-应变关系。预测结果表明:在双向等轴拉伸载荷下,二维编织陶瓷基复合材料的横向和纵向应力-应变曲线基本相同,与单向加载时的应力-应变曲线相近。     

非均匀损伤介质中利用波传播理论对损伤参数的反演

介绍了波在非均匀损伤介质中的传播的反问题,根据波的响应来反演介质的损伤。首先应用Newton迭代法把非线性问题化为线性问题,得到一组不适定的线性方程式组,采用Tikhonov正则化求解,对于正则参数选取采用L线曲线准则,求出了原问题的稳定的、有效的解,获得了与真实损伤度较为吻合的反演参数。工程实例的模拟分析计算证明了上述算法的有效性。     

实现电功率双路测量的方法及电路的非线性度分析

提出了一种实现有功功率和真无功功率双栖测量的方法,对测量方法的原理进行了数学证明,并根据数学公式提出了实现其测量方法的各部分电路原理及其实现方法。对影响测量准确度的关键电路,即功率测量电路的测量非线性度进行了动态分析,提出了电路测量线性度的计算公式,并就影响测量误差线性度的电路关键参数进行了计算机仿真研究。     

An Efficient Reliability-based Optimization Method for Uncertain Structures Based on Non-probability Interval Model

In this paper, an efficient interval optimization method based on a reliability-based possibility degree of interval (RPDI) is suggested for the design of uncertain structures. A general nonlinear interval optimization problem is studied in which the objective function and constraints are both nonlinear and uncertain. Through an interval order relation and a reliability-based possibility degree of interval, the uncertain optimization problem is transformed into a deterministic one. A sequence of approximate optimization problems are constructed based on the linear approximation technique. Each approximate optimization problem can be changed to a traditional linear programming problem, which can be easily solved by the simplex method. An iterative framework is also created, in which the design space is updated adaptively and a fine optimum can be well reached. Two numerical examples are investigated to demonstrate the effectiveness of the present method. Finally, it is employed to perform the optimization design of a practical automobile frame.     

Vibrations of orthotropic square plates having variable thickness (linear variation)

The effect of fibre orientation on the natural frequencies of thin square orthotropic plates having variable thickness is studied using the Rayleigh-Ritz method for various boundary conditions. In the present study plates having linear variation in thickness in one direction (either X or Y) are studied.     

Polarization of skylight in the O(2)A band: effects of aerosol properties

Motivated by several observations of the degree of linear polarization of skylight in the oxygen A (O(2)A) band that do not yet have a quantitative explanation, we analyze the influence of aerosol altitude, microphysics, and optical thickness on the degree of linear polarization of the zenith skylight in the spectral region of the O(2)A band, between 755 to 775 nm. It is shown that the degree of linear polarization inside the O(2)A band is particularly sensitive to aerosol altitude. The sensitivity is strongest for aerosols within the troposphere and depends also on their microphysical properties and optical thickness. The polarization of the O(2)A band can be larger than the polarization of the continuum, which typically occurs for strongly polarizing aerosols in an elevated layer, or smaller, which typically occurs for depolarizing aerosols or cirrus clouds in an elevated layer. We show that in the case of a single aerosol layer in the atmosphere a determination of the aerosol layer altitude may be obtained. Furthermore, we show limitations of the aerosol layer altitude determination in case of multiple aerosol layers. To perform these simulations we developed a fast method for multiple scattering radiative transfer calculations in gaseous absorption bands including polarization. The method is a combination of doubling-adding and k-binning methods. We present an error estimation of this method by comparing with accurate line-by-line radiative transfer simulations. For the Motivated by several observations of the degree of linear polarization of skylight in the oxygen A (O(2)A) band that do not yet have a quantitative explanation, we analyze the influence of aerosol altitude, microphysics, and optical thickness on the degree of linear polarization of the zenith skylight in the spectral region of the O(2)A band, between 755 to 775 nm. It is shown that the degree of linear polarization inside the O(2)A band is particularly sensitive to aerosol altitude. The sensitivity is strongest for aerosols within the troposphere and depends also on their microphysical properties and optical thickness. The polarization of the O(2)A band can be larger than the polarization of the continuum, which typically occurs for strongly polarizing aerosols in an elevated layer, or smaller, which typically occurs for depolarizing aerosols or cirrus clouds in an elevated layer. We show that in the case of a single aerosol layer in the atmosphere a determination of the aerosol layer altitude may be obtained. Furthermore, we show limitations of the aerosol layer altitude determination in case of multiple aerosol layers. To perform these simulations we developed a fast method for multiple scattering radiative transfer calculations in gaseous absorption bands including polarization. The method is a combination of doubling-adding and k-binning methods. We present an error estimation of this method by comparing with accurate line-by-line radiative transfer simulations. For the O(2)A band, the errors in the degree of linear polarization are less than 0.11% for transmitted light, and less than 0.31% for reflected light. band, the errors in the degree of linear polarization are less than 0.11% for transmitted light, and less than 0.31% for reflected light.     

An incremental procedure for deformation analysis of elastic-plastic frames

In the present paper an incremental procedure is formulated for first-order elastoplastic analysis of plane frame structures discretized into a finite number of beam elements and described by piecewise linear elastic-perfectly plastic constitutive laws, under the assumption of both reversible and irreversible behaviour of material and using piecewise linear yield conditions at any desired degree of discretization in the space of the active stress resultants (axial force, shear force and bending moment). The proposed method, by using the independent elastic-plastic kinematical compatibility equations, restrains the problem sizes within not more than twice the number of the redundant unknowns in the complete elastic frame, regardless of the degree of discretization of the piecewise linear yield conditions, still maintaining the advantage exhibited by the Mathematical Programming methods of requiring only one factorization of the matrix governing the problem when no local unloading occurs. Furthermore, the outlined algorithm allows the additional computational effort to be restrained in the case of local unloading, inasmuch as it requires a new factorization to be performed of a part of the matrix governing the problem, whose size is small with respect to the total size of the matrix.     

考虑平方阻尼及分段线性刚度铰接塔-油轮系统的分岔与混沌特性

研究了铰接塔-油轮系统在规则激励下的分岔和混沌特性。将该系统简化为单自由度分段线性恢复刚度,含平方阻尼的动力学分析模型,建立了铰接装载塔的分段非线性运动方程。使用增量谐波平衡法(IHB)获得系统周期解,结合Floquet理论判断系统周期解的稳定性,使用增量弧长法进行路径跟踪,获得了系统响应曲线和通向混沌的道路,发现两种由倍周期分岔导致的混沌运动。并且,为了验证系统的混沌运动,计算得到了两种混沌运动从产生到消失过程的最大Lyapunov指数图。     

土-结构三维动力分析的线性-非线性混合子结构法

为提高土-结构三维非线性动力分析问题的计算效率,根据结构存在局部塑性区域的特点,提出了适合于非线性子结构与多个线性子结构存在边界耦合情况的线性-非线性混合约束模态综合法。该方法将整体体系划分为线性和非线性两类子结构,对线性子结构依照势能判据截断准则进行自由度的缩减,并最终与非线性子结构进行综合,进而获得非线性体系的动态响应。进一步将该方法应用到土-高层建筑相互作用体系的三维动力分析算例中,针对两种不同的土体边界条件分别进行了多组地震动激励作用下的动力时程分析,验证了该方法的精确性与有效性。最后,针对非线性土体区域范围的多种不同取值进行了多组数值试验,结果表明:对于该文算例,当非线性土体区域的范围取为上部结构水平尺寸的3倍时即可获得满意的计算结果,进一步说明了该方法的有效性。     

A widely linear temperature to frequency converter using athermistor in a pulse generator

A modification of an existing technique for thermistor linearization is presented. A temperature-measuring circuit using a thermistor in a specially designed pulse generator is used to obtain a highly linear relation between absolute temperature and frequency. The use of a general-purpose operational amplifier and two switching transistors in the design of the pulse generator provides a temperature scale that is linear over a wide range of values on a high degree of sensitivity. The result shows that an extremely high degree of linearity is achieved by this method     

X‐FEM in isogeometric analysis for linear fracture mechanics

The extended finite element method (X‐FEM) has proven to be an accurate, robust method for solving problems in fracture mechanics. X‐FEM has typically been used with elements using linear basis functions, although some work has been performed using quadratics. In the current work, the X‐FEM formulation is incorporated into isogeometric analysis to obtain solutions with higher order convergence rates for problems in linear fracture mechanics. In comparison with X‐FEM with conventional finite elements of equal degree, the NURBS‐based isogeometric analysis gives equal asymptotic convergence rates and equal accuracy with fewer degrees of freedom (DOF). Results for linear through quartic NURBS basis functions are presented for a multiplicity of one or a multiplicity equal the degree. Copyright © 2011 John Wiley & Sons, Ltd.