微小型飞行器航空图像拼接算法

针对微小型飞行器所拍摄图像的特点,提出了一种稳健的航空图像拼接算法.此算法首先使用相位相关法估计平移参数并初步确定重叠区域,在重叠区域采用Harris角点检测算法进行特征点提取得到初步的伪匹配集合,并运用稳健的RANSAC算法将伪匹配点集合划分为内点和外点.在内点域内运用透视变换模型精确地估计出图像间的点变换关系.最后采用双线性插值对交接处进行颜色过渡.实验结果表明,该拼接结果比较理想,能有效去除错误配准点     

汽车结构件弯曲变形开裂分析

采用有限元模拟方法对汽车结构件的弯曲变形进行了分析,讨论了弯曲半径、材料参数和板厚尺寸对弯曲成形的影响.结果表明:弯曲模具的半径较小,零件外表面的拉应力增大是造成弯曲开裂的原因之一;制造零件的Q345普碳钢的性能不能满足汽车结构件弯曲成形的要求.     

大型车车身覆盖件冲压成形特征分析及选材研究

为满足大型车身覆盖件冲压成形要求并实现钢板与零件的对路供应,必须在深入解析零件成形特征的基础上,确定高应变部位的成形敏感材料参数.在材料力学性能测试和成形极限分析的基础上,对侧围外板的冲压成形特征和成形敏感参数进行了分析.结果表明:侧围外板零件高应变部位的变形方式为胀形-深拉延变形,n值和r值为影响侧围外板零件成形的主要材料敏感参数.     

基于改进分水岭算法的苹果识别方法

目的 针对自然生长环境下重叠苹果的识别问题,提出一种基于距离变换机制的改进分水岭算法.方法 首先,通过对比实验优选出苹果在RGB、HSI和Lab颜色空间的图像处理模型;然后,采用距离变换和归一化处理方法对苹果的位置特征进行内部标记;进而基于锐化方法提高原图像的边缘强度,并结合内部标记符改进分水岭算法,实现苹果轮廓的分离;最后,基于最小外接圆法提取苹果的单连通区域,实现重叠苹果的识别.结果 简化了目标与背景的分割过程,通过内部标记法改进了分水岭算法,实现重叠目标的轮廓重建,解决重叠苹果的过度分割问题.结论 实验结果表明,该算法对重叠苹果的准确识别率高于92％,验证了所提方法的有效性.     

功能梯度Levinson圆板弯曲解的均匀化和经典化表示

基于Levinson三阶剪切变形理论,研究了材料性质沿厚度任意连续变化的功能梯度材料圆板的轴对称弯曲问题。首先,建立了功能梯度材料圆板在Levinson板理论下轴对称弯曲问题位移形式的控制微分方程,其中考虑了拉-弯耦合和三阶剪切变形效应。然后,利用载荷等效关系以及均匀板的经典理论控制微分方程,导出功能梯度圆板在Levinson剪切变形理论下弯曲解与经典理论下均匀圆板的挠度之间的解析转换关系,给出了转换系数的计算公式。由此,可将功能梯度材料圆板在Levinson三阶剪切理论下的弯曲问题转化为相应均匀薄圆板在经典理论下的弯曲问题求解,以及转换系数的计算问题。     

轻量化拼焊板构件塑性成形研究进展

拼焊板构件因容易实现轻量化、低成本、短周期成形制造而被广泛应用于飞机、汽车工业中,但在这类构件的塑性成形中,焊缝引入的材料、几何、边界条件非线性显著增加了成形复杂性,导致成形质量及成形极限下降,尤其是在具有强烈不均匀变形特征的弯曲、旋压等局部加载塑性成形中表现得更加明显,极大制约了轻量化拼焊板构件的精确塑性成形。为此,国内外学者在拼焊板构件弯曲及旋压变形机理与成形规律方面开展了大量研究。从焊接材料不均匀力学行为表征、拼焊板构件弯曲及旋压成形有限元建模、焊缝特征对变形行为影响及工艺设计等方面综述了相关研究进展,最后提出了拼焊板构件塑性成形仍面临的关键难题与挑战,对认识和发展轻量化拼焊板构件塑性成形具有重要指导意义和参考价值。     

任意梯度分布简支功能梯度板的三维弯曲分析

研究了正交各向异性功能梯度板的三维弯曲问题。假设材料参数沿板厚方向按同一函数规律变化,基于状态空间法,在板的上下表面作用机械载荷的情况下,获得了简支功能梯度平板弯曲问题的Peano-Baker 级数解。通过算例,验证了 Peano-Baker级数解的正确性,同时也分析了材料参数沿板厚方向为余弦函数分布时,不同梯度参数对平板响应的影响。结果表明Peano-Baker 级数解具有很好的收敛性。     

Reissner矩形板的不连续弯曲问题研究

本文用分区加权残值法研究Ｒｅｉｓｓｎｅｒ矩形板在几何形状、边界条件、作用载荷等不连续时的弯曲问题。将研究对象按结构和载荷的具体情况划分为若干连续的区域,在每个区域内用不同的试函数代入该域内的控制方程,得到内部残值,并代入边界条件和各区域的协调条件得到边界残值和连续性残值,然后用最小二乘法消除残值,求得试函数并据以求出板的内力。数值算例表明,该方法收敛性好,精度较高,可适性强。     

基于支持向量机与改进分水岭的红细胞识别算法研究

针对因红细胞重叠以及细胞试剂里含有杂质而造成红细胞识别不准确的问题,提出了一种基于支持向量机(Support Vector Machines,SVM)与改进分水岭算法的红细胞识别算法.算法首先提取原始图像的二值化图像中每个连通区域的形状特征;利用SVM识别单个连通区域与重叠连通区域;利用改进分水岭算法将重叠区域分割为单...     

磨具弯曲成形法非球面研磨的研磨模型

磨具弯曲成形法是加工光学非球面零件的新方法,是线接触研磨过程.给出了弯曲成形法研磨的原理,基于Preston方程,限定磨具弯曲成形法特定研磨条件,分析了加工面上的任意一点的压力函数和相对速度函数,推导了磨具弯曲成形法研磨材料去除量函数表达式,推导了研磨作用时间函数.该研磨模型为磨具弯曲成形法研磨实践提供了理论依据.     

尾减输入齿轮疲劳失效分析

对某发动机尾减系统尾减输入齿轮疲劳失效的原因进行了系统的分析。通过宏观检查、断口分析、金相检验以及受力分析等,确认了该齿轮的失效过程为：在大弯曲载荷作用下疲劳裂纹于锥齿中部齿根部位起始,裂纹初期沿径向呈15°左右的小角度疲劳扩展;齿部出现裂纹后,齿轮振动频率发生变化,进而导致齿轮出现节径型振动,使得裂纹逐渐沿径向疲劳扩展;出现径向裂纹后剩余齿由于截面减少以及起始部位已基本无渗碳层其强度较低等原因,导致剩余齿出现弯曲疲劳断裂。     

A meshless local Petrov-Galerkin scaled boundary method

The scaled boundary finite-element method is a new semi-analytical approach to computational mechanics developed by Wolf and Song. The method weakens the governing differential equations by introducing shape functions along the circumferential coordinate direction(s). The weakened set of ordinary differential equations is then solved analytically in the radial direction. The resulting approximation satisfies the governing differential equations very closely in the radial direction, and in a finite-element sense in the circumferential direction. This paper develops a meshless method for determining the shape functions in the circumferential direction based on the local Petrov-Galerkin approach. Increased smoothness and continuity of the shape functions is obtained, and the solution is shown to converge significantly faster than conventional scaled boundary finite elements when a comparable number of degrees of freedom are used. No stress recovery process is necessary, as sufficiently accurate stresses are obtained directly from the derivatives of the displacement field.     

Modeling and optimization of spring-back in bending process using multiple regression analysis and neural computation

The current work involves both modeling and optimization approaches to achieve minimum spring-back in V-die bending process of heat treated CK67 sheets. Number of 36 experimental tests have been conducted with various levels of sheet orientation, punch tip radius and sheet thickness. Firstly, various predictive models based on statistical analysis, back-propagation neural network (BPNN), counter propagation neural network (CPNN) and radial basis function network (RBFNN) have been developed using experimental observations. Then the accuracy of the developed models has been compared based on values of mean absolute error (MAE), and root mean square error (RMSE). Secondly, the model with lowest values of MAE, and RMSE has been applied as objective function for optimization of process using imperialist competitive algorithm (ICA). After selection of optimal bending parameters, a confirmation test has been conducted to prove the optimal solutions. Results indicated that the radial basis network fulfills precise prediction of process rather than the other developed models. Also, confirmation tests proved that both RBFNN and ICA could predict and optimize the process vigorously.     

功能梯度材料圆板的三维弹性分析

基于线弹性理论的基本方程,选用两个位移分量和两个应力分量作为状态变量,利用状态空间法建立了功能梯度材料轴对称圆板的三维状态方程.根据微分求积法,将状态方程在径向进行离散,考虑周边固支的边界条件,采用打靶法数值求解了材料常数沿板厚按幂率变化的轴对称弯曲问题和自由振动问题,为求解功能梯度材料三维弹性响应提供了一种方法.并且给出了功能梯度材料三维圆板的静动态响应受组分材料分布以及板厚径比变化的影响规律.     

Static and free vibration analysis of sandwich cylindrical shell based on theory of elasticity and using DQM

In this paper, static and free vibration analysis of a sandwich cylindrical shell is performed using theory of elasticity formulation. The core layer is made of functionally graded material with material properties varying along the thickness direction according to a simple power law. For the case of simply supported boundary conditions, equations of motion and equilibrium equations are solved analytically by applying a state-space technique along the radial direction and Fourier series expansion along the axial and circumferential direction. When boundary conditions are not simply supported, a semi-analytically solution is performed by using the differential quadrature method along the axial direction. The present approach is validated by comparing the obtained numerical results with those published in the available literature. Moreover, effects of boundary conditions, graded direction, mid-radius to thickness and length to mid-radius ratios on bending and vibration behavior are considered.     

碳纤维/环氧树脂脉管自润滑复合材料的制备及摩擦性能

以聚乳酸（PLA）为前驱体,碳纤维（CF）为增强体,通过经纬编织成PLA-CF复合纤维织物;以环氧树脂（EP）为基体,甲基硅油为润滑介质,采用前驱体蒸发（VaSC）技术和真空浸油方式分别制备了脉管自润滑环氧树脂（VAEP）材料和CF/VAEP复合材料。利用SEM对CF/VAEP复合材料的断面、摩擦面和对偶面形貌进行表征,以万能材料试验机和高速环块摩擦磨损试验机对CF/VAEP复合材料的力学和摩擦学性能进行了测试。结果表明:脉管结构和CF提升了EP的润滑性能和力学性能。当径向脉管密度为8孔/10 mm时,VAEP的摩擦系数和磨损率较纯EP分别降低了67.95%和85.71%;当径向脉管密度为8孔/10 mm,CF经纬丝束比为8/4时,CF/VAEP复合材料的拉伸强度、拉伸模量、弯曲强度、弯曲模量比相同径向脉管密度的VAEP分别提高了203.33%、44.16%、325.78%、311.37%。      

5A06铝合金中厚板反拉深变形行为

中厚板变形过程中的弯曲和反弯曲效应影响其拉深变形能力。本工作以航空航天常用的5A06铝合金筒形件为研究对象,采用厚度为4.5mm的中厚板进行反拉深数值模拟和实验研究,分析了变形过程中应力、应变分布特点,讨论了3种凹模截面结构形式下的变形方式,以及应变路径随着凹模圆角变化规律。结果表明:凹模内圆角与直壁区过渡区在壁厚方向上存在应力和应变梯度,并在该处外侧产生最大径向拉应力,导致了拉深破裂的发生。采用半圆形凹模截面结构时,极限拉深深度达到203mm,相对于平面凹模结构增加了40%。半圆形凹模结构能减小弯曲效应,有效降低过渡区的应力梯度和最大应力数值,有利于提高5A06铝合金中厚板拉深变形能力。     

Dynamical modeling and experimental evidence on the swelling/deswelling behaviors of pH sensitive hydrogels

A chemo-electro-mechanical fields coupling model was developed based on the Nernst-Planck equation, Donnan theory and elastic mechanics to investigate the swelling/deswelling dynamics of pH sensitive hydrogels. To test the model, chitosan hydrogels with pH sensitivity have been successfully prepared through a radiation crosslinking approach induced by Co-60 gamma ray. The deformation ratio of the hydrogels at radial direction is more than 100% when pH changes from 7 to 9. A finite element simulation was carried out to investigate the stress formation and diffusion inside the hydrogel in its deswelling process. Results showed that the data of simulation fit the experimental results well. Moreover, a stress-diffusion mechanism was proposed to explain the deswelling process.     

弹性地基扇形权弯曲问题的复Bessel函数级数解

本文对于在Winkler型弹性地基上的扇形板弯曲问题给出了一种级数解法。对于沿径向简单支撑的扇形板,在周向采用正弦三角函数展开后,将关于径向待定函数的Bessel方程用复Bessel函数表示其解。然后,将其变换成实解形式,从而得到原问题齐次方程的实通解。在此基础上,对于扇形板受集中力或沿周向线分布载荷作用下的弯曲解给出了一般计算方法以及任意横向载荷作用下挠曲函数的表达式,并给出了沿径向和周向都为简单支撑情形的算例。     

Additional bending moment analysis of the trigeminal universal joint installed by single or twin radial bearing北大核心CSCD

To find out the causes of vibration and noise of the trigeminal universal joint which appeared in the installation and use process, the additional moment distribution with the varies of the input shaft angular and the deflection angle was analyzed. Kinematics diagrams of the trigeminal universal joint when the output shaft installed by single radial bearing or twin radial bearing were established. The system coordinates were established under the two installation ways, and the motion equations were established by the direction cosine matrix tools. It indicated that the trigeminal universal joint installed by the single radial bearing was an approximately isometric speed transmission, and was a constant velocity transmission with twin radial bearing installed. Furthermore, the additional moment component on the input shaft and the output shaft was analyzed under the two installation ways. According to the virtual displacement principle, the additional moment on the tripod universal joint when the output shaft installed by twin radial bearing and single radial bearing were determined. When installed by single radial bearing, the deflection moment exists and the vibration frequency of additional moment was three times of the input shaft as a sine curve. The variation trend of the additional moment was straight up with the increase of deflection angle. When installed by twin radial bearing, the deflection moment was zero and the additional moment increased gradually with the increase of deflection angle but did not fluctuate. The analysis revealed that additional moment existed in the trigeminal universal joint system under both installation ways would produce bending vibration. This study is of great significance to study the causes of vibration and determine the nonlinear dynamics of the system.