基于压电阻抗技术监测混凝土强度发展的实验研究

本文基于压电阻抗（EMI）监测技术,对混凝土立方体标准试块的强度发展进行了监测。制作3种不同配合比的标准试块,将压电片粘贴于标准试块表面和埋置于体内,用精密阻抗分析仪HP4294A提取压电片在不同养护龄期时的电导纳信号,并测试相应标准试块的抗压强度。试验结果表明,随着混凝土养护龄期的增长,表面粘贴和体内埋置两种压电片的电导频谱均发生了变化,但是变化形式不同。对表面压电片,得到了其共振频率的变化量与标准试块抗压强度之间的定量对应关系;对内置压电片,通过引入指标δ建立了电导频谱与标准试块抗压强度的定量关系。本文工作表明采用EMI技术监测早龄期混凝土在硬化过程中强度发展是一种可行且可靠的无损检测方法。     

Cymbal压电发电换能器有限元分析

通过建立Cymbal压电发电换能器的机电耦合有限元分析模型,计算分析了换能器结构参数对输出电压和谐振频率的影响以及外接负载对Cymbal换能器输出电压和输出功率的影响。研究表明,为了降低换能器的工作频率和提高换能器的输出电压,应增大换能器的空腔底部直径和减小换能器的空腔高度;在选择金属端冒和压电陶瓷厚度等参数时,应综合考虑换能器系统的刚度和外界振动源的频率特性和加速度特性;在任意一个频率点上,Cymbal换能器均存在一个最佳的外接负载,使得换能器的输出功率最大,而这个最佳的负载阻抗就等于Cymbal换能器在这个工作频率点上的输出阻抗。文中还提出并分析了基于外加预应力的多振子级联方式Cymbal压电发电换能器系统的结构。     

防振锤非线性阻抗的实验研究及参数识别

防振锤被广泛用于减轻架空输电线微风振动,阻抗是评价其减振性能的重要指标。针对防振锤所具有的非线性特性,设计了一套利用正弦扫频测试阻抗的方案,通过测量防振锤在不同的激振速度下的阻抗谱,识别防振锤在不同振动速度下的的固有频率和阻抗,并计算出每一阶固有频率所对应的阻尼比。分析了振动速度对防振锤振动参数的影响。最后,通过对比防振锤阻抗谱的实验结果和计算结果,指出了现有防振锤线性力学模型的局限性。     

基于压电阻抗方法的油气管道裂纹损伤定量研究

为了解基于压电阻抗方法识别管道裂纹损伤程度的可行性,通过人工切割方法模拟管道裂纹损伤,采用压电阻抗方法对管道裂纹损伤进行监测试验。实测各种损伤工况下压电片的阻抗信号,分析试验数据,提取均方根差(RMSD)作为损伤指标。基于RMSD 损伤指标与损伤面积的变化规律,建立了管道损伤程度量化表征的数学模型。试验结果表明:当管道出现裂纹损伤后,PZT 阻抗频谱曲线明显向左偏移,峰值对应频率下降;随着管道裂纹损伤程度不断加大,RMSD 值逐渐增长,RMSD 损伤指标能定性地判断管道损伤状况。所建立的数学模型在一定程度上实现了管道裂纹损伤的定量分析。     

含金属芯压电纤维的纵向振动研究

建立了悬臂杆结构含金属性芯电纤维的纵向振动模型和动态测试模型。基于第一类压电方程,推导了外加简谐激励电压时,悬臂杆结构含金属芯压电纤维的等效纵向外力;基于纵向振动理论,推导出纵向振动模型;并由电位移,得到表面电荷、电流、导纳,建立了动态测试模型,通过测量共振频率fr、反共振频fa率和低频电容,计算出压电纤维的几个主要参数：弹性柔顺系数 、机电耦合系数 、介电常数 和压电常数 。详细叙述了纵向振动模型和动态测试模型的建立过程,并给出了具体的测试方法,提供了一个实际试样的测试结果。测试结果表明,根据纵向振动模型所建立的动态测试方法可以快速、准确地测量含金属芯压电纤维的主要参数。     

压电夹层梁的分岔、混沌及主动控制

研究轴向激励作用下简支压电夹层梁的分岔、混沌振动及其主动控制。基于压电材料本构关系、von Karman型几何非线性应变位移关系,考虑耦合正、逆压电效应的比例微分控制策略,运用Hamilton原理建立了压电夹层梁的非线性横向运动偏微分方程并利用Galerkin方法对其进行离散化处理。通过采用数值模拟方法,研究了压电夹层梁的动态分岔。结果表明,通过比例控制增益和微分控制增益都可控制压电夹层梁的横向振动,阻止系统发生混沌运动,保持系统的稳定性。     

利用压电自传感驱动器进行裂纹钢梁损伤识别的实验研究

压电陶瓷是一种智能材料，可以在结构健康监测系统中同时用作传感器和驱动器。基于压电阻抗的损伤识别技术的基本原理，对裂纹钢梁进行了损伤识别和定位的实验研究。将三片压电陶瓷（PZT）粘贴在钢梁表面的不同部位作为驱动器和传感器，通过测量梁损伤前后压电陶瓷片的电阻抗变化来识别梁中的裂纹损伤。从导纳（阻抗的倒数）幅值谱曲线中提取裂纹梁的反谐振频率，通过比较各压电片位置的反谐振频率变化识别了裂纹位置；同时比较不同损伤工况下的反谐振频率变化定性地识别裂纹梁结构的损伤程度。     

二维压电材料动强度因子的扩展有限元计算

采用扩展有限元求解二维弹性压电材料动断裂问题。扩展有限元的网格独立于裂纹,因此网格生成可大大地简化,且裂纹扩展时不需重构网格。采用相互作用积分技术计算动强度因子。比较了标准的力裂尖加强函数和力-电裂尖加强函数对动强度因子的影响,结果表明标准的力裂尖加强函数能有效地分析压电材料动断裂问题。分析了极化方向对动强度因子的影响。数值分析表明采用扩展有限元获得的动强度因子与其他数值方法解吻合得很好。     

用于结构健康诊断的压电阻抗技术

利用自驱动、自传感特性的压电陶瓷片 ,粘贴在外部结构的表面 ,结合动态阻抗的概念 ,提出了一种机电耦合的在线压电阻抗技术用于结构健康诊断。结构健康状况的改变 ,表现为结构动态阻抗的变化 ,则可以通过压电陶瓷片的导率表现出来。以含不同裂纹尺寸的梁结构为例 ,实验分析该种技术的有效性。结果表明 :随着裂纹尺寸的增加 ,压电陶瓷片导率的峰值不仅发生明显偏移 ,而且幅值下降。根据幅值的变化实验给出了能较好地衡量损伤程度的标尺     

基于压电阻抗技术的疲劳裂纹监测系统计量校准

压电阻抗(EMI)技术是二十世纪末新发展起来的一种结构健康监测方法。通过测量粘贴或埋入结构内部的压电材料电阻抗信号的变化,间接反映机械结构的阻抗变化。基于该原理,特种设备检测机构开发了压电阻抗技术的疲劳裂纹监测系统,为了保障该监测系统计量性能准确,研究并搭建了相应的校准测试平台。     

基于谱元法的复合材料裂纹梁Lamb波传播特性研究

基于谱元法提出了一种弹簧元来模拟复合材料梁由于横向裂纹导致的轴弯耦合效应,分析复合材料裂纹梁中Lamb波的传播特性。由断裂力学的相关知识求得弹簧元的刚度,建立复合材料裂纹梁的损伤谱元模型。通过模拟复合材料裂纹梁内Lamb波传播,并和传统的有限元结果进行比较,验证了所提出模型的可行性和有效性。推导了频域内Lamb波各模态的能量计算公式,裂纹处的能量守恒证明了所提出模型的正确性,同时计算表明复合材料梁中裂纹处反射与透射的Lamb波各模态能量随着裂纹深度的变化规律具有单调性,结论可以为定量识别复合材料梁裂纹提供实用依据。     

Combined extended and superimposed finite element method for cracks

A combination of the extended finite element method (XFEM) and the mesh superposition method (s‐version FEM) for modelling of stationary and growing cracks is presented. The near‐tip field is modelled by superimposed quarter point elements on an overlaid mesh and the rest of the discontinuity is implicitly described by a step function on partition of unity. The two displacement fields are matched through a transition region. The method can robustly deal with stationary crack and crack growth. It simplifies the numerical integration of the weak form in the Galerkin method as compared to the s‐version FEM. Numerical experiments are provided to demonstrate the effectiveness and robustness of the proposed method. Copyright © 2004 John Wiley & Sons, Ltd.     

A 3D cylindrical finite element model for thick curved beam stress analysis

A finite element model is proposed to perform stress analysis for thick curved beams and panels subjected to various types of loadings. The model has 18 nodes in a three‐dimensional cylindrical co‐ordinates system. Three stress components on radial surface (σ_rr, τ_rθ, and τ_rz) and three displacement components (u_r, u_θ, and u_z) are used as nodal degrees of freedom. Therefore, the continuity condition for both stresses and displacements is achieved in the radial direction. Formulation of nodal shape functions and equilibrium equations are based on three‐dimensional elasticity theory and a minimum potential energy method. The accuracy of the method is verified with the standard test problems and exact solutions from the theory of elasticity. The model shows no locking phenomena. Convergence is investigated and the application to layered composite panel is illustrated. Copyright © 2003 John Wiley & Sons, Ltd.     

Three-dimensional mixed mode analysis of a cracked body by fractal finite element method

A semi-analytical method namely fractal finite element method is presented for the determination of stress intensity factor for the straight three-dimenisonal plane crack. Using the concept of fractal geometry, infinite many of finite elements are generated virtually around the crack border. Based on the analytical global displacement function, numerous DOFs are transformed to a small set of generalised coordinates in an expeditious way. No post-processing and special finite elements are required to develop for extracting the stress intensity factors. Examples are given to illustrate the accuracy and efficiency of the present method. Very good accuracy (with less than 3% errors) is obtained for the maximum value of SIFs for different modes.     

有限元简化模型应用于超声换能器模拟分析

依据有限元方法的基本物理思想,在某些不需要计算辐射声场的准确声学参数和波束特性的工程应用方面,对流体模型进行充分简化,提出了简化模型处理的有效方法,利用该方法对超声换能器进行模拟分析,并进行了样品的制作和测试,实测结果与模型简化分析处理的结果基本一致。可以证明,用该方法进行换能器的优化设计是可行和高效的。     

Bounds for element size in a variable stiffness cohesive finite element model

The cohesive finite element method (CFEM) allows explicit modelling of fracture processes. One form of CFEM models integrates cohesive surfaces along all finite element boundaries, facilitating the explicit resolution of arbitrary fracture paths and fracture patterns. This framework also permits explicit account of arbitrary microstructures with multiple length scales, allowing the effects of material heterogeneity, phase morphology, phase size and phase distribution to be quantified. However, use of this form of CFEM with cohesive traction–separation laws with finite initial stiffness imposes two competing requirements on the finite element size. On one hand, an upper bound is needed to ensure that fields within crack‐tip cohesive zones are accurately described. On the other hand, a lower bound is also required to ensure that the discrete model closely approximates the physical problem at hand. Both issues are analysed in this paper within the context of fracture in multi‐phase composite microstructures and a variable stiffness bilinear cohesive model. The resulting criterion for solution convergence is given for meshes with uniform, cross‐triangle elements. A series of calculations is carried out to illustrate the issues discussed and to verify the criterion given. These simulations concern dynamic crack growth in an Al₂O₃ ceramic and in an Al₂O₃/TiB₂ ceramic composite whose phases are modelled as being hyperelastic in constitutive behaviour. Copyright © 2004 John Wiley & Sons, Ltd.     

Three-dimensional finite element simulations of microstructurally small fatigue crack growth in 7075 aluminium alloy

Three‐dimensional finite element simulations were performed to study the growth of microstructurally small fatigue cracks in aluminium alloy 7075‐T651. Fatigue crack propagation through five different crystallographic orientations was simulated using crystal plasticity theory, and plasticity‐induced crack opening stresses were calculated. The computed crack opening stresses were used to construct small crack da/dN‐ΔK diagrams. The generated da/dN‐ΔK curves compared well with experimental small crack data from the literature. The variance observed among the da/dN‐ΔK results, which occurred as a consequence of the different crystallographic orientations employed, was found to be of the same order of magnitude as commonly observed variability in small fatigue crack growth data. This suggests that grain orientation is a major contributor to observed small fatigue crack data scatter.     

A three-dimensional least-squares finite element technique for deformation analysis

The development of a three-dimensional least-squares finite element technique suitable for deformation analysis was presented. By adopting a spatial viewpoint, a consistent rate formulation that treats deformation as a process was established. The technique utilized the least-squares variational principle that minimizes the squares of errors encountered in any attempt to meet the field equations exactly. Both velocity and Cauchy stress rate fields were discretized by the same linear interpolation function. The discretization always yields a sparse, symmetric, and positive-definite coefficient matrix. A conjugate gradient iterative solver with incomplete-Choleski preconditioner was used to solve the resulting linear system of equations. Issues such as finite element formulation, mesh design, code efficiency, and time integration were addressed. A set of linear elastic problems was used for patch-test; both homogeneous and non-homogeneous deformations were considered. Additionally, two finite elastic deformation problems were analysed to gauge the overall performance of the technique. The results demonstrated the computational feasibility of a three-dimensional least-squares finite element technique for deformation analysis. © 1997 John Wiley & Sons, Ltd.     

Three-dimensional, parallel, finite element simulation of fatigue crack growth in a spiral bevel pinion gear

This paper summarizes new results for predicting crack shape and fatigue life for a spiral bevel pinion gear using computational fracture mechanics. The predictions are based on linear elastic fracture mechanics theories combined with the finite element method, and incorporating plasticity-induced fatigue crack closure and moving loads. We show that we can simulate arbitrarily shaped fatigue crack growth in a spiral bevel gear more efficiently and with much higher resolution than with a previous boundary-element-based approach [Spievak LE, Wawrzynek PA, Ingraffea AR, Lewicki DG. Simulating fatigue crack growth in spiral bevel gears. Engng Fract Mech 2001;68(1):53-76] using the finite element method along with a better representation of moving loads. Another very significant improvement is the decrease in solution time of the problem by employing a parallel PC-cluster, an approach that is becoming more common in both research and practice. This reduces the computation time for a complete simulation from days to a few hours. Finally, the effect of change in the flexibility of the cracking tooth on the location and magnitude of the contact loads and also on stress intensity factors and fatigue life is investigated.     

Three-dimensional effects on fatigue crack closure in the small-scale yielding regime – a finite element study

Plasticity induced closure often strongly influences the behaviour of fatigue cracks at engineering scales in metallic materials. Current predictive models generally adopt the effective stress‐intensity factor (ΔΚ_eff = Κ_max–Κ_op) in a Paris law type relationship to quantify crack growth rates. This work describes a 3D finite element study of mode I fatigue crack growth in the small‐scale yielding (SSY) regime under a constant amplitude cyclic loading with zero T‐stress and a ratio Κ_min/Κ_max = 0 . The material behaviour follows a purely kinematic hardening constitutive model with constant hardening modulus. Dimensional analysis suggests, and the computational results confirm, that the normalized remote opening load value, Κ_op/Κ_max, at each location along the crack front remains unchanged when the peak load (Κ_max), thickness (B) and material flow stress (σ₀) all vary to maintain a fixed value of . Through parametric computations at various K levels, the results illustrate the effects of normalized peak loads on the through‐thickness opening–closing behaviour and the effects of σ₀/E, where E denotes material elastic modulus. The examination of deformation fields along the fatigue crack front provides additional insight into the 3D closure process.