网格曲面中孔洞的光滑填充算法研究

三角网格模型是几何描述的一种重要形式，有着广泛的应用。但三角网络模型常常会存在孔洞缺陷。这些孔洞的存在一方面影响视觉效果，另一方面会影响许多后续的操作，如快速原型制造、有限元分析等，因此有必要对这些孔洞进行修补。目前绝大多数孔洞填充算法是将网格模型中的孔洞提取成空间多边形，并对孔洞多边形进行三角化。这种处理方法的主要缺陷是没有考虑网格曲面在孔洞附近的几何形态，因而填充部分不能与整个曲面光滑地融为一体。笔者提出了一种三角网格曲面中孔洞的光滑填充算法。该算法根据孔洞周围网格曲面的几何信息来增加孔洞内部的采样点，然后再对增加的采样点进行三角化，较好地解决了填充部分与整体曲面光滑连接的问题。     

基于Trimmed NURBS曲面几何特征的数字化自适应采样

曲面轮廓度误差评价和多边域逼近均要求对理论曲面进行数字化自适应采样。在对当前曲面数字化方法分析的基础上，针对Trimmed NURBS曲面，研究了在给定精度下的曲面数字化自适应采样方法，给出了基于曲率特征函数的曲线自适应采样和NURBS曲面多边域离散算法，提出了一种对大规模曲面网格点检测路戏优化问题很有效的进化算法一蚁群算法，并进行了计算机仿真实验验证。仿真结果表明，所提出的规划方法可显著提高数字化采样效率，具有很大的实用价值。     

基于回溯双向波前法虚拟修补点云孔洞

针对虚拟修补边界点凹凸不平且分布不均的点云孔洞效果欠佳的问题,提出了回溯双向波前法虚拟修补包含复杂边界的孔洞.以具有复杂边界孔洞的青铜器模型为例:首先,三角网格化青铜器点云数据并依据网格化结果提取出孔洞边界,通过比较边界点集的曲率波动幅度以去除伪孔洞边界;其次,以孔洞边界点集的回溯结果为初始点集,并结合向量叉积对初始点集进行凹凸性分类,再对凹、凸点分别采用正、反向波前法逐圈新增点集直至补全;最后,利用最小二乘法拟合曲面平滑新增点集,获得最终修补结果.实验结果表明该方法与划分子洞波前法、曲线流收缩法相比,结构相似性的平均值分别提高了81.14%和93.8%,且曲率差异性更低,修补网格的顶点密度与原始网格更相近且过渡自然,能有效修补复杂边界孔洞.     

基于GA-BP神经网络的散乱点云孔洞自动修补

点云孔洞修补作为点云数据处理中的关键技术,直接影响点云的质量和完整性。利用遗传算法（genetic algorithm,GA）优化的BP（back propagation,反向传播）神经网络（简称GA-BP神经网络）是一种修补效果较好的散乱点云孔洞修补方法。但基于GA-BP神经网络的散乱点云孔洞传统修补方法的多个步骤需借助逆向软件通过人机交互的方式完成,导致修补过程繁琐且耗时较长。为此,提出了一种基于GA-BP神经网络的散乱点云孔洞自动修补方法。通过计算机编程将孔洞识别、孔洞区域插值和孔洞修补相结合,实现从残缺点云模型直接到完整点云模型的自动修补,无须进行复杂的人机交互和数据转换。实验结果表明,所提出的方法可有效避免因数据转换而造成的数据失真,减少了人机交互工作量,方便而高效地修补了散乱点云的孔洞,且得到的修补点云密度均匀,这对提高点云孔洞修补效率和质量具有重要意义。     

自由曲面上曲线物理空间和参数空间互相转换算法

根据曲面曲线上的点与曲面参数域之间的对应关系,提出了一种实用的物理空间与参数空间的转换算法.曲面和曲线离散后,用迭代法求得曲线离散点列对应的曲面参数值,以求得的参数值为型值点在曲面参数平面上构造插值曲线,然后根据插值曲线细分曲面曲线可得到曲面曲线在曲面参数平面上更加精确的投影曲线.计算实例表明投影曲线映射到三维空间与原曲面曲线高度一致.     

采用分形技术的二维纹理在曲面上的贴图方法

利用分形理论生成的个性化分形图案在纹理贴图中的应用比较广泛。文章阐述了分形技术中仿射变换的二维不规则纹理生成原理,提出了矢量化离散细分二维纹理图案的方法。该方法把纹理图案表示为一系列离散的点,寻找出离散点与曲面空间中任取一局部小三角形面片中各点的对应关系,根据点对点的转换原理,实现由二维纹理空间到曲面空间的贴图。开发了相应系统对上述方法进行了验证。     

逆向工程中自由曲面的自适应数字化算法研究

分析比较了对自由曲面自适应测量的几种方法。在有界曲率的平面曲线的有界性的基础上，提出圆弧外延法，即后续测量点根据已测前三点求得的圆弧曲率沿切线方向或等曲率外延来进行预测，并根据测点矢量来估测保证数字化几何信息的充分性。这样就可利用待测曲面的几何(曲率)特性，使得测点的分布与曲面的曲率保持一致。最后给出了对这一算法的仿真结果。     

基于径向基函数的点云岛屿孔洞自动修复

研究了利用点云获得的模型的孔洞修复,针对目前主要通过人工修复带有岛屿面片的孔洞耗时较长的问题,提出了一种基于径向基函数(RBF)自动修复岛屿孔洞的方法。该方法首先利用最小权重三角化法修复模型主体上的孔洞,其次计算模型主体上孔洞与岛屿面片的相关性,利用模型主体上孔洞和与其相关岛屿面片周围点来计算径向基函数,最后将粗修复后细分的点调整到径向基函数描述的曲面上。实验表明,与其他方法相比,该方法能快速、准确地修复缺陷模型。     

汽车发动机缸盖燃烧室容积数字化检测

本文针对现有缸盖燃烧室容积滴定法测量中测量精度低、测量速度慢等不足,提出一种使用线激光三维测量系统扫描缸盖燃烧室的发动机缸盖燃烧室容积的数字化测量方法,实现了对缸盖燃烧室容积的快速、精确计量,并应用于工程实际。论文介绍了测量原理和检测方法;详细阐述了获取缸盖燃烧室点云后通过一系列滤波及容积计算程序,获得缸盖燃烧室容积的过程,即点云的滤波处理、孔洞修补、曲面重建、燃烧室有效点云提取及容积计算等;最后,基于样机平台,对上述方法进行了实验验证。     

三维无旋矢量场的一种新的可视化方法

提出了三维无旋矢量场的一种新的可视化方法,即构造空间曲面,使得矢量场在曲面上任意一点处垂直于该曲面。首先找到曲面所满足的偏微分方程组,通过采用类似于经典四阶龙格―库塔方法的数值解法对其求解,得到曲面上的离散点,然后进行三角剖分,从而得到逼近于曲面的空间三角网格。论文的偏微分方程组的求解借鉴了常微分方程求解算法的设计思想,构造出的曲面与传统的点图标和线图标相比,在更大程度上揭示了矢量场本身的连续性。     

Surface Degradation and Cracking in Combustion Turbine Blade Cooling Passages

The depth of internal oxidation and nitridation from the surface of the 16 cooling holes in a first-stage turbine blade was measured by optical microscopy after 32,000 hours of service. Maximum depth of penetration was 15.5 mils (0.4 mm) at the trailing edge hole. An effort was made to predict hole surface metal temperatures based on these measurements using the Arrhenius relationship between time and temperature with depth of oxidation assumed to be parabolic with time. Good correlations were obtained between the finite element analysis results and the predictions. In the thickest part of the airfoil, where metal temperature is minimum, intergranular cracks up to 1.2.6 mils (0.32 mm∥ in depth were found at the surface of the cooling holes. Measurable attack was only one to two mils (0.025-0.050 mm). Based on an approximate elastic-relaxation-local inelastic stress analysis, it was calculated that inelastic local strains of over one percent occur at the points of cracking. No cracking was observed in the hotter holes, but cracking did occur in a trailing edge tip cooling hole when weld repair of the tip squealer was attempted indicating that embrittlement had occurred from the environmental attack.     

Behaviour of composite plates with drilled and moulded hole under tensile load

Drilling of composite material causes damages of different natures. To avoid machining in composite material, it is proposed to include holes while moulding. In the literature, there is few works concern the behaviour analysis of specimens with moulded holes. All the literature works concern a specimens with moulded holes made of woven fabric. Moreover no literature is available on local behaviour of specimens around the hole. In this paper, several mechanical tests are carried out on specimens with quasi-isotropic stacking sequence made from UD prepreg with drilled holes and with moulded holes. To analyse the behaviour of the specimens around the hole, the strain field is measured using three-dimensional Digital Image Correlation technique (DIC). The pictures analysed given by the CCD camera reveal that, the damage mechanisms are different between the plates with drilled holes and those with moulded holes. The SEM observation have shown that, the fibre content and porosity content near the hole is around ≈ 8% higher compared to the values obtained far the hole. Moreover the tensile strength of the plates with moulded holes is 28% higher than with drilled holes.     

The development of fatigue damage around fastener holes in thick graphite/epoxy composite laminates

This paper discusses the mechanisms by which damage develops and grows around countersunk fastener holes in composite laminates under fatigue loading conditions. Experiments have shown that the erosion of material between ply layers nucleates delaminations which then grow through the laminate under the action of the fatigue loads. The damage at the hole bore surface produces sites for the nucleation of delaminations. The morphology of the delaminations and ply cracking was mapped extensively and from these maps it was found that the volume of material around the fastener hole, damaged by the fatigue loading, adopted a characteristic shape; the volume of damaged laminate increased towards the faying surface of the laminate and (metal) fixture. This characteristic damage volume was generated by the fastener rocking under the fatigue loads. Growth of the delaminations has been shown to be preceded by intraply cracking and, as fatigue loading proceeded, more delaminations were generated at the hole bore surface. The interfacial region between the composite laminate and the fixture also provided the nucleation site for fatigue cracks in the fasteners. The effects of initial fastener-hole clearances on damage nucleation and growth did not appear to follow any clear pattern. However, coupons with excessive initial hole clearances did appear to exhibit greater than expected damage growth after only one sequence block.Several techniques for the measurement of damage growth (development) were investigated. Stiffness measurements of the test system were found to show only small changes with hole wear and fastener rocking (using shadow moire techniques) also showed only small changes with hole wear. Ultrasonic C-scanning methods were used to map the extent of damage around the fastener holes with fatigue loading.The experimental work has shown that damage development around fastener holes is a complex process, usually producing several delaminations in the region of the fastener hole which grow and may ultimately lead to the failure of the coupon.     

CFRP管面钻削缺陷形成机制

碳纤维增强复合材料(CFRP)传动轴因性能优越广泛应用于汽车、航空航天、船舶、冷却塔风机等轻量化领域,但其钻削过程中容易出现毛刺、分层等缺陷。为了揭示CFRP管面钻削缺陷形成机制,选择双锋角钻头和三尖二刃钻对CFRP管面钻孔,利用分步钻削盲孔和通孔的方法,分析了损伤部位的受力情况,揭示了入口撕裂、出口毛刺和分层产生原因。根据实验结果,发现双锋角钻头钻孔时入口撕裂损伤较大,损伤位置在钻头与管面接触最低点,且主要是那部分水平缠绕CFRP管的纤维,原因是水平缠绕的纤维屈曲变形最大,对切削力更加敏感。双锋角钻头和三尖二刃钻的横刃对孔最终出口分层没有影响,主切削刃的切削行为决定孔最终出口分层。相同钻头钻孔时,轴向力不是唯一影响分层因子的因素,还需考虑切削热。相比双锋角钻头,三尖二刃钻因锋利的外缘尖角能有效划断纤维,使出口分层较小。     

芯棒锥面结构对孔冷挤压强化残余应力场的影响

为了在紧固孔周引入均匀的残余压应力,以延长紧固孔构件的疲劳寿命、提高其抗应力腐蚀性能,利用ANSYS有限元软件,建立了轴对称弹塑性有限元模型,对直接芯棒冷挤压强化过程进行了仿真,特别是对芯棒的前锥段曲线结构形式进行了设计与分析,研究了前锥段曲线形式对残余应力场分布的影响.结果表明:孔壁表面的周向残余应力分布复杂且不均匀,比较而言,外凸型正弦曲线型芯棒所产生的残余压应力沿孔壁深度方向分布更加均匀;几种曲线形式的芯棒在上表面近孔边区域均产生了径向残余拉应力,在孔的挤入段产生了轴向残余拉应力,但外凸型正弦曲线型芯棒在上述区域所产生的残余拉应力较小,且分布区域也较小.     

A new approach to enhancement of fatigue life of rail-end-bolt holes

The object of this paper is bolted joint railroads as the accent is put on the material behaviour around the bolted holes. The fatigue failure around rail-end-bolt holes is particularly dangerous, since it leads to derailment of trains and consequently, to inevitable accidents. Moreover, the cracking at rail-ends, which starts from bolt hole surface, causes premature rails replacement. It is well-known that the presence of residual compressive hoop stresses around the bolted holes closes the existing first mode cracks and impedes the formation of new ones and thereby extends the fatigue life of the holed components. In this article a new approach to enhancement of fatigue life of rail-end-bolt holes has been developed on the basis of a novel method and tool for cold expansion (CE) of holes, patented by the authors. The major advantage of the method is in imparting around the holes of beneficial residual compressive hoop stresses which are symmetric toward the middle longitudinal plane of the rail and thus the axial stress gradient is minimum. The developed approach consists in setting and solving a multi-objective optimization task of the CE of the rail-end-bolt holes. Because of the specificity of the studied problem, the optimal solution has been found by finite element (FE) simulations. For that purpose generalized FE model of the object has been developed. Through this model the critical point around the outside holes has been localized and the cycle of variation of hoop stresses has been determined taking into consideration assembly stresses. On the basis of adapting the generalized FE model of the rail joint to simulating the CE process, a comparison between the cases with and without CE of rail-end-bolt holes has been made. On this basis the optimal degree of CE and the successive hole treatment have been found. After simulating the CE process by the determined optimal degree of CE, the beneficial effect of implementing the new approach has been proved.     

Fundamental-solution-based hybrid FEM for plane elasticity with special elements

The present paper develops a new type of hybrid finite element model with regular and special fundamental solutions (also known as Green’s functions) as internal interpolation functions for analyzing plane elastic problems in structures weakened by circular holes. A variational functional used in the proposed model is first constructed, and then, the assumed intra-element displacement fields satisfying a priori the governing partial differential equations of the problem under consideration is constructed using a linear combination of fundamental solutions at a number of source points outside the element domain, as was done in the method of fundamental solutions. To ensure continuity of fields over inter-element boundaries, conventional shape functions are employed to construct the independent element frame displacement fields defined over the element boundary. The linkage of these two independent fields and the element stiffness equations in terms of nodal displacements are enforced by the minimization of the proposed variational functional. Special-purpose Green’s functions associated with circular holes are used to construct a special circular hole element to effectively handle stress concentration problems without complicated local mesh refinement or mesh regeneration around the hole. The practical efficiency of the proposed element model is assessed via several numerical examples.