机器视觉疲劳裂纹扩展试验摄像头自动聚焦方法

在机器视觉疲劳裂纹扩展试验中,为了能够满足对裂纹宏观观察裂纹和准确定位裂纹尖端的要求,需要采用变焦镜头放大和缩小采集图像范围,针对于摄像头变焦后的聚焦问题,提出了摄像头的自动聚焦方法。首先建立图像采集系统,采集裂纹图像至计算机,通过中值滤波对图像预处理,去除噪声干扰,选取裂纹区域作为聚焦窗口,采用Laplace算子法作为清晰度评价函数,并提出一种变步长穷举法进行聚焦搜索。最后设计了以ARM7为核心的摄像头运动控制器。实验表明,所提出的方法能够实现疲劳裂纹扩展试验中摄像头在各种情况下的准确自动聚焦,为下一步精确测量疲劳裂纹扩展参数奠定基础。     

采用散斑测量的复杂曲面叶片修复自适应定位

针对于航空发动机叶片修复过程中传统夹具定位无法满足复杂曲面叶片损坏形式多样的要求,提出了采用散斑测量的复杂曲面叶片修复自适应定位方法。首先,构建视觉测量系统,通过对叶片表面散斑点的匹配与空间解算得到叶片立体点云数据和参考测量点空间坐标,完成叶片三维构型。然后,建立自适应定位模型,通过对各定位坐标系进行位姿矩阵解算,实现欠定位装夹下曲面叶片在机床坐标系下的自适应定位。最后,基于德玛吉五轴数控加工中心进行自适应定位实验,验证了定位方法的有效性与实用性。实验结果表明,定位各方向平移向量偏差小于0.25mm,旋转矩阵欧拉角偏差小于0.2°,且通过一次装夹固定及视觉测量提取叶片形貌与位置信息,减少了定位工序,可为后续对损坏复杂曲面叶片加工复形的自适应定位修复提供实验依据。     

基于幅度调制的非线性超声相控阵成像方法*

超声相控阵是超声检测领域常用的材料缺陷定位和成像技术,可便捷快速地对裂纹、孔洞等缺陷进行成像。但是,传统超声相控阵方法对较小缺陷如闭合裂纹不太敏感。非线性超声信号因对材料性能退化以及微小缺陷敏感而广受关注。本文针对疲劳闭合裂纹检测,提出一种基于幅度调制的非线性超声相控阵成像方法,通过测量物理聚焦和虚拟聚焦两种聚焦模式下超声扩散场中的声能差,并将其作为非线性参量,实现疲劳裂纹闭合部分的定位成像和定量表征。将该法应用于7075铝合金试样疲劳裂纹的实验测量,并研究了扩散场信号延迟时间对非线性超声相控阵成像结果的影响。结果表明：相较于传统超声相控阵全聚焦法,基于幅度调制的非线性超声相控阵成像方法能够更准确地定位和成像疲劳裂纹闭合部分;延迟时间的选择对疲劳裂纹长度的表征精度影响较大,本文研究了该延迟时间的选择方法并实现了检测结果的优化。     

振动激励条件下复合材料损伤生热的研究

为实现对复合材料表面及浅表面损伤的快速检测,采用振动热成像技术,对复合材料在疲劳、冲击等不同作用下产生的损伤进行了检测研究。分析了瞬时性和周期性两种内生热源形式下平板构件的表面温度场分布。基于有限元法计算了表面裂纹、浅表面分层及内部脱粘等不同类型损伤在振动激励条件下的生热情况。在设计制作复合材料疲劳裂纹试件和冲击损伤试件的基础上,利用超声波发生器和红外热像仪开展了试验研究。结果表明:振动热成像技术能够快速准确地检测到复合材料表面及浅表面的紧贴型损伤;通过红外序列图中热斑形状可判断损伤的类型。     

基于测量不确定度的视觉惯性自适应融合算法

针对环境复杂的工业现场定位精度低、适应性差、鲁棒性低等问题,提出一种基于测量不确定度的视觉惯性自适应融合算法,分析基于隐函数模型的视觉定位测量不确定度,并依据视觉定位测量不确定度自适应调整卡尔曼滤波模型中的参数,校正视觉观测偏差,增强视觉惯性融合定位算法在不同观测条件下的鲁棒性。利用精密三轴转台及激光跟踪仪T-mac位姿测量系统对所提融合定位算法的定位精度进行实验验证。实验结果表明,相比传统扩展卡尔曼滤波方法,所提方法能满足视觉观测较差条件下的准确定位需求。     

结构微裂纹混频非线性超声检测方法研究

给出了一种基于混频效应的非线性超声微裂纹检测方法。首先,对结构损伤混频检测机理及信号特征提取方法进行了理论分析,之后,根据试件中差频分量及和频分量幅值分布随激励信号频率变化关系,优化确定出混频检测参数。最后,进行了异侧混频激励下无损检测试验研究,并分析了激励信号频率变化对混频检测效果的影响。结果表明,异侧激励混频检测模式不仅可以实现结构中疲劳微裂纹检测,而且可以实现缺陷的定位。且检测信号频率选择对混频检测信噪比有一定的影响。当检测信号中的混频分量幅值最大时,混频检测效果最佳。因此,在优化检测参数基础上,异侧混频激励检测模式可以很好实现结构微裂纹的检测与定位。      

闭合裂纹非共线混频超声检测试验研究

针对结构中微裂纹检测难题,发展了一种闭合裂纹非共线混频超声检测方法。在对非共线混频超声检测机理分析基础上,进行了结构中疲劳裂纹混频非线性超声检测实验。对有无裂纹试件中检测信号进行了滤波和时频分析,结果表明,可根据信号滤波后时域波形中是否存在明显的混频波包或时频分析中是否存在明显的和频分量,实现有无闭合裂纹的判识;通过移动激励探头的位置,控制两列入射声波在试件中的交汇位置,实现试件中不同深度位置的混频非线性检测。并根据测得的混频非线性系数沿试件深度方向上分布,实现了闭合裂纹沿深度方向上长度的测量。研究工作为结构中微裂纹定量评价做了有益探索。      

基于FBG反射谱特征的修补结构裂纹扩展监测

为了提高飞行器壁板裂纹修补结构的安全性,对基于光纤布拉格光栅(FBG)反射谱特征的航空铝合金修补结构疲劳裂纹扩展长度及角度监测方法进行了研究。修补结构在疲劳载荷的作用下,裂纹发生扩展,其尖端附近会产生不均匀应变场,在其作用下FBG传感器反射谱发生变形,通过检测反射谱变形达到监测裂纹扩展的目的。建立了实验样件的模型,使用有限元方法计算得到不同角度下裂纹扩展到不同长度时的应变分布,使用传输矩阵数值计算方法得到相应的FBG反射谱。提取反射谱的主峰偏移、反射谱面积、次峰峰值和三峰峰值等变形特征,建立并训练人工神经网络(ANN),通过ANN建立反射谱变形特征同裂纹扩展情况的关系,实现了对修补结构裂纹扩展的精确监测,监测裂纹长度误差在0.5 mm之内,角度误差在5°之内。解决了修补结构的裂纹扩展精确监测问题,对提高飞行器安全以及节约维护成本有着重要意义。     

不同应变率下两种岩石的压缩破碎特征试验研究

为了研究不同岩石在不同应变率下压缩时裂纹的产生规律及破坏模式,将石灰岩和红砂岩制成试件,研究其在不同应变率和受力模式下裂纹的形成模式。开展了两种岩石的准静态压缩和动态压缩试验,采用高速摄影机记录了裂纹的产生和破坏模式。对两种岩石试件的裂纹形态进行对比,基于岩石的物理性质、受力状态、能量演化分析,得到了在不同应变率下压缩时产生差异性的原因。结果表明:准静态压缩下岩石试件受压的破坏模式也会因应变率的不同而存在差异,并且破坏模式的差异对岩石试件的抗压强度将产生显著的影响;从能量演化的角度分析,入射能量的大小将会决定岩石试样动态抗压强度曲线是否出现起伏;动态压缩时,裂纹的周向扩展速度与岩石抗压强度呈正相关。     

光学元件亚表层裂纹成核临界条件研究

基于压痕实验和连续刚度测量法得到了熔石英材料硬度和弹性模量随压入深度的变化曲线, 系统分析了材料由延性到脆性转变的过程, 确定了熔石英晶体在静态/准静态印压和动态刻划时产生裂纹的临界载荷和临界深度。渐变载荷刻划实验结果表明, 划痕过程诱发的裂纹对法向载荷有很强的依赖性, 载荷较小时材料去除方式为延性域去除。随着法向载荷的增加, 首先产生垂直于试件表面的中位裂纹和平行于试件表面方向扩展的侧向裂纹, 而在试件表面上并没有产生明显的特征。载荷进一步增加后, 侧向裂纹扩展并形成了明亮区域, 最终诱发了沿垂直于或近似垂直于压头运动方向扩展的径向裂纹, 实现了材料的脆性去除。     

A method for crack sizing using Laser Doppler Vibrometer measurements of Surface Acoustic Waves

The goal of non-destructive testing (NDT) is to determine the position and size of structural defects, in order to measure the quality and evaluate the safety of building materials. Most NDT techniques are rather complex, however, requiring specialized knowledge. In this article, we introduce an experimental method for crack detection that uses Surface Acoustic Waves (SAWs) and optical measurements. The method is tested on a steel beam engraved with slots of known depth. A simple model to determine the cracks size is also proposed. At the end of the article, we describe a possible application: fatigue crack sizing on a damaged slat track. This technique represents a first step toward a better understanding of the crack growth, especially in its early stages (preferably when the cracks can still be repaired) and when it is possible to assume a linear propagation of the crack front. The ultimate goal of this research program is to develop a useful method of monitoring aircraft components during fatigue testing.     

金属板疲劳损伤非线性兰姆波混频检测

针对金属板结构安全运行需要,开展了金属板结构疲劳损伤非线性兰姆波混频检测方法研究.通过数值仿真,研究了两列A0兰姆波与材料损伤间的非线性相互作用.结果 表明,两列共线A0兰姆波在结构材料损伤处产生单向传播的和频S0波,且和频波幅值随传播距离具有积累增长效应.对不同疲劳程度金属板试件进行了共线混频兰姆波检测实验,结果表明...     

Fatigue models for glassy polymers

Temperature effects on fatigue crack propagation in glassy polymers can be described by the Chudnovsky and Moet model, which is based on the propagation of microdefects near the crack tip. This was shown by fitting several models to fatigue data for polycarbonate and poly (methyl methacrylate). Statistical regression analysis and a global search method were used in the model fitting.     

Baseline-free estimation of residual fatigue life using a third order acoustic nonlinear parameter

Prediction of crack growth and fatigue life estimation of metals using linear/nonlinear acousto-ultrasound methods is an ongoing issue. It is known that by measuring nonlinear parameters, the relative accumulated fatigue damage can be evaluated. However, there is still a need to measure two crack propagation states to assess the absolute residual fatigue life. A procedure based on the measurement of a third-order acoustic nonlinear parameter is presented to assess the residual fatigue life of a metallic component without the need of a baseline. The analytical evaluation of how the cubic nonlinear-parameter evolves during crack propagation is presented by combining the Paris law to the Nazarov-Sutin crack equation. Unlike other developed models, the proposed model assumes a crack surface topology with variable geometrical parameters. Measurements of the cubic nonlinearity parameter on AA2024-T351 specimens demonstrated high sensitivity to crack propagation and excellent agreement with the predicted theoretical behavior. The advantages of using the cubic nonlinearity parameter for fatigue cracks on metals are discussed by comparing the relevant results of a quadratic nonlinear parameter. Then the methodology to estimate crack size and residual fatigue life without the need of a baseline is presented, and advantages and limitations are discussed.     

Scatter of fatigue data owing to material microscopic effects

A common phenomenon of fatigue test data reported in the open literature such as S-N curves exhibits the scatter of points for a group of same specimens under the same loading condition.The reason is well known that the microstructure is different from specimen to specimen even in the same group.Specifically,a fatigue failure process is a multi-scale problem so that a fatigue failure model should have the ability to take the microscopic effect into account.A physically-based trans-scale crack model is established and the analytical solution is obtained by coupling the micro-and macro-scale.Obtained is the trans-scale stress intensity factor as well as the trans-scale strain energy density(SED)factor.By taking this trans-scale SEDF as a key controlling parameter for the fatigue crack propagation from micro-to macro-scale,a trans-scale fatigue crack growth model is proposed in this work which can reflect the microscopic effect and scale transition in a fatigue process.The fatigue test data of aluminum alloy LY12 plate specimens is chosen to check the model.Two S-N experimental curves for cyclic stress ratio R=0.02 and R=0.6 are selected.The scattering test data points and two S-N curves for both R=0.02 and R=0.6 are exactly re-produced by application of the proposed model.It is demonstrated that the proposed model is able to reflect the multiscaling effect in a fatigue process.The result also shows that the microscopic effect has a pronounced influence on the fatigue life of specimens.     

Low-cycle fatigue small crack initiation and propagation behaviour of cast magnesium alloys based on in-situ SEM observations

In-situ observations on the initiation and propagation behaviour of low-cycle fatigue small cracks in cast magnesium–aluminium alloys (AM50 and AM60B) were carried out with scanning electron microscopy (SEM) to elucidate the resistance to fatigue cracking and to evaluate the fatigue small crack growth rate accurately and quantitatively. The results indicate that the fatigue small cracks formed preferentially on β-phase (Mg₁₇Al₁₂) boundaries at room temperature. In addition, the effects of the parameters of stress levels in low-cycle fatigue and temperatures as well as microstructure on fatigue small crack propagation behaviour are revealed. The variation of crack open displacement (COD) with stress levels and cycles at elevated temperature shows that it is unsuitable to estimate the fatigue small crack growth rate of cast magnesium alloys using conventional measurement methods such as the plastic-replica technique due to the obvious difference between microscopic cracks in the open and closed states. Stabilized crack propagation behaviour is limited to cases where the physical crack length is less than 1?mm in low-cycle fatigue.     

利用强度周期性变化的光信号测量光速与介质的折射率

介绍了一种利用强度周期性变化的光信号来测量光速与介质折射率的方法,可以精确测量出光传播距离改变cm数量级时光传播时间的改变.如果在光路中放入介质,用这种方法可以很精确地测量出由此引入的光程差,并给出了一个测量介质折射率的新方法.