A Combined Static/Dynamic Technique for Damage Detection of Laminated Composite Plates

There is a growing demand to develop viable techniques for effective damage detection of composite structures, and the dynamics-based approach has been broadly used in structural health monitoring. A new combined static/dynamic technique for improved damage detection of laminated composite plates is presented. The promise of the technique is that under the sustaining static load, the abnormality of dynamic response due to damage may become more pronounced and easy to be detected. The experimental program consists of testing an E-glass/epoxy composite plate with an embedded delamination under a pre-set static compressive force, and the dynamic response of laminated composite plates is measured using two different actuator–sensor systems: (1) PZT (lead–zirconate–titanate) actuators and scanning laser vibrometer (SLV) sensing system (PZT–SLV), and (2) PZT actuators and Polyvinylidenefluoride (PVDF) sensors (PZT–PVDF). The influence of sustaining static forces to dynamic response of delaminated composite plates is evaluated. The numerical finite element (FE) analysis is also conducted to verify the effectiveness of this technique. The experimental and numerical mode shapes are used to detect the presence, location, and size of the delamination and to study the effect of static load on dynamic response. Two relatively new damage detection algorithms (i.e., Simplified Gapped Smoothing Method (GSM) and Generalized Fractal Dimension (GFD)) are employed to analyze the Uniform Load Surface (ULS) calculated from the experimental and numerical data. From the dynamic response and analysis results using the damage detection algorithms, it is observed that as the sustaining static load increases, the delamination is much easier to be identified through the enlarged damage parameters. The present combined static/dynamic technique is capable of magnifying the effect of damage, thus improving the effectiveness of damage detection.     

A Motion Amplifier Using an Axially Driven Buckling Beam: I. Design and Experiments

For active materials such as piezoelectric stacks, which produce large force and small displacement, motion amplification mechanisms are often necessary – not simply to trade force for displacement, but to increase the output work transferred through a compliant structure. Here, a new concept for obtaining large rotations from small linear displacements produced by a piezoelectric stack is proposed and analyzed. The concept uses elastic (buckling) and dynamic instabilities of an axially driven buckling beam. The optimal design of the buckling beam end conditions was determined from a static analysis of the system using Euler's elastica theory. This analysis was verified experimentally. A stack-driven, buckling beam prototype actuator consisting of a pre-compressed PZT stack (140 mm long, 10 mm diameter) and a thin steel beam (60 mm× 12 mm× 0.508 mm) was constructed. The buckling beam served as the motion amplifier, while the PZT stack provided the actuation. The experimental setup, measuring instrumentation and method, the beam pre-loading condition, and the excitation are fully described in the paper. Frequency responses of the system for three pre-loading levels and three stack driving amplitudes were obtained. A maximum 16^∘ peak-to-peak rotation was measured when the stack was driven at an amplitude of 325 V and frequency of 39 Hz. The effects of beam pre-load were also studied.     

Solution of free vibration equations of semi-rigid connected Reddy–Bickford beams resting on elastic soil using the differential transform method

The literature regarding the free vibration analysis of Bernoulli–Euler and Timoshenko beams under various supporting conditions is plenty, but the free vibration analysis of Reddy–Bickford beams with variable cross-section on elastic soil with/without axial force effect using the Differential Transform Method (DTM) has not been investigated by any of the studies in open literature so far. In this study, the free vibration analysis of axially loaded and semi-rigid connected Reddy–Bickford beam with variable cross-section on elastic soil is carried out by using DTM. The model has six degrees of freedom at the two ends, one transverse displacement and two rotations, and the end forces are a shear force and two end moments in this study. The governing differential equations of motion of the rectangular beam in free vibration are derived using Hamilton’s principle and considering rotatory inertia. Parameters for the relative stiffness, stiffness ratio and nondimensionalized multiplication factor for the axial compressive force are incorporated into the equations of motion in order to investigate their effects on the natural frequencies. At first, the terms are found directly from the analytical solutions of the differential equations that describe the deformations of the cross-section according to the high-order theory. After the analytical solution, an efficient and easy mathematical technique called DTM is used to solve the governing differential equations of the motion. The calculated natural frequencies of semi-rigid connected Reddy–Bickford beam with variable cross-section on elastic soil using DTM are tabulated in several tables and figures and are compared with the results of the analytical solution where a very good agreement is observed.     

The pre-stack migration imaging technique for damages identification in concrete structures

Pre-stack migration imaging (PMI) method, which is used in geophysical exploration by the performance of single side detection and visually display, can be used to identify the location, orientation, and severity of damages in concrete structure. In particular, this letter focuses on the experimental study by using a finite number of sensors for further practical applications. A concrete structure with a surface-mounted linear PZT transducers array is illustrated. Three types of damages, horizontal, dipping and V-shaped crack damage, have been studied. A pre-stack reverse time migration technique is used to back-propagate the scattering waves and to image damages in concrete structure. The migration results from the scattering waves of an artificial damage are presented. It is shown that the existence of the damage in concrete structure is correctly revealed through migration process.     

Generalized-order perturbation with explicit coefficient for damage detection of modular beam

A general method is formulated to estimate damage location and extent from the explicit perturbation terms in specific set of eigenvectors and eigenvalues. At first, perturbed orthonormal equation is generated from the perturbation of eigenvectors and eigenvalues to obtain the k-th explicit perturbation coefficients. At second, perturbed eigenvalue equation is generated from the perturbation of eigenvector and eigenvalue, and first-order expansion of the stiffness matrix to obtain other explicit perturbation coefficients. Stiffness parameters are computed from these equations using an optimization method. The algorithm is iterative and terminates under certain criteria. A fixed–fixed modular beam with various numbers of elements is used as test structure to investigate the applicability of the developed approach. By comparison with the Euler–Bernoulli beam, discretization errors are analyzed. In six elements beam, first-order algorithm converges faster for small percentage damage. Second-order algorithm is more efficient for medium percentage damage. For large percentage damage, the second-order algorithm converges more effectively. Meanwhile, for eight elements large percentage damage and ten elements small percentage damage, second-order algorithm converges faster to the termination criterion.     

阶梯压电层合梁的波动动力学特性

采用行波理论系统地研究了压电阶梯梁的自由振动分析以及强迫响应的分析方法. 基于分布 参数理论研究了压电阶梯梁的波传播特性，忽略柔性梁横向剪切和转动惯量的影响，给出了 梁的轴向和横向的简谐波解. 将压电阶梯梁离散化为单元，考虑压电片的刚度和质量的影响， 建立了节点散射模型. 应用位移连续和力平衡条件，推导了节点的波反射和波传递矩阵，在 此基础上，引入波循环矩阵的概念，给出波循环矩阵、波传递系数矩阵的确定方法. 应用波 循环矩阵可以有效地计算结构的固有频率. 另外，应用波传递系数研究了压电陶瓷作动器位 置对其驱动能力的影响. 得出两个主要结论：1)作动器靠近悬臂梁固定端将有较强的驱动 能力，悬臂梁边界反射行波产生弯曲消失波有利于增大压电波的模态传递系数；2)模态传 递系数与固有频率的灵敏度密切相关，波传递系数越大, 对应该处固有频率变化灵敏度越大. 另外，数值算例表明了行波方法比有限元方法具有更高的计算精度.     

Condition monitoring of bones using piezo-transducers

This paper explores the feasibility of employing piezoelectric ceramic (PZT) patches as bio-medical sensors for monitoring condition of bones, through experimental studies on human and rabbit bones. Conductance signatures of PZT patches bonded to bones are acquired using the electro-mechanical impedance (EMI) technique, while the bones are subjected to varying conditions such as and density changes, occurrence of cracks and fracturing. The changes in the signature correlate fairly well with the changes in the condition of the bones. Finally, the effect of healing process is experimentally simulated on rabbit bones, which showed that the conductance signature of the bones shifted back towards the original state after rejoining. The overall results of the study demonstrate good prospects of using PZT patches as bio-medical sensors.     

Thermal reliability testing of functionally gradient materials using thermal shock method

We found a solution of an unsteady two-dimensional heat conduction equation in a functionally gradient material (FGM) which is subjected to a double thermal shock, namely, a local heating of a specimen by a power laser beam and cooling of a heated surface by a water-air spray. We developed an analytical method whereby a coating is described as a laminated plate composed of n layers with the constant material properties within a layer. Temperature distribution in a nonhomogeneous laminated plate is obtained in a form of series using the Laplace–Hankel integral transforms. In order to extend the model of a laminated plate to describe FGM where thermal physical characteristics are continuous functions of spatial coordinate, we considered the limiting case of the obtained temperature distribution when the thickness of the layer iΔ _i → 0, and the number of layers n→∞. This allowed us to obtain the temperature distribution in an easy-to-use analytical form which can be used for determining thermal stresses in FGM. The dependence of the temperature distribution in FGM on the operating parameters of a double thermal shock method, e.g., a duration of heating, laser beam radius, the rate of a spray cooling, is discussed. Received on 3 May 1999     

Development, modeling and deflection analysis of hybrid micro actuator with integrated thermal and piezoelectric actuation

Micro actuators are irreplaceable part of motion control in minimized systems. The current study presents an analytical model for a new Hybrid Thermo Piezoelectric micro actuator based on the combination of piezoelectric and thermal actuation mechanisms. The micro actuator structure is a double PZT cantilever beam consisting of two arms with different lengths. The presented micro actuator uses the structure of electrothermal micro actuator in which polysilicon material is replaced by PZT. Also the voltage and poling directions are considered in the lengthwise of PZT beams. As a result, the piezoelectric actuation mechanism is based on d ₃₃ strain coefficient. The tip deflection of micro actuator is obtained using Timoshenko beam theory. Analytical results are compared with FEM results along with other reported results in the literature. The effects of geometrical parameters and PZT material constants on actuator tip deflection are studied to provide an efficient optimization of HTP micro actuator.     

旋转运动柔性梁的时滞主动控制实验研究

对旋转运动柔性梁的时滞主动控制进行实验研究,验证时滞反馈控制的有效性. 实验中采用交流伺服电机带动柔性梁旋转运动,柔性梁上粘贴有压电作动器,用于控制梁的弹性振动. 实验研究考虑如下3种情况:(1)仅使用电机扭矩进行控制,电机扭矩存在时滞;(2)使用电机扭矩和压电作动器同时控制,仅压电作动器存在时滞;(3)使用电机扭矩和压电作动器同时控制,电机和压电作动器存在不同的时滞量. 重点通过实验验证时滞反馈控制的可行性和有效性.      

An experimental technique to measure projectile deceleration history during normal penetration into plain concrete

The present paper presents a new experimental method to measure the deceleration time history of projectiles penetrating into concrete in full-size test. The experiment can be carried out by using an onboard accelerometer to measure the projectile deceleration history and the data are transmitted to a ground recording system. With this experimental method, a series of tests on hemisphere-nose steel projectiles penetrating normally into plain concrete at the velocity region 150–400 m/s have been executed and the deceleration histories obtained. The high frequency portion in the deceleration data has been investigated and proved to be the structure response of projectile. The characteristics of deceleration history have also been analyzed and discussed.     

A Motion Amplifier Using an Axially Driven Buckling Beam: II. Modeling and Analysis

In this paper, the nonlinear behavior of a motion amplifier used to obtain large rotations from small linear displacements produced by a piezoelectric stack is studied. The motion amplifier uses elastic (buckling) and dynamic instabilities of an axially driven buckling beam. Since the amplifier is driving a large rotary inertia at the pinned end and the operational frequency is low compared to the resonant frequencies of the beam, the mass of the buckling beam and the dynamics of the PZT stack are neglected and the system is modeled as a single-degree-of-freedom, nonlinear system. The beam simply behaves as a nonlinear rotational spring having a prescribed displacement on the input end and a moment produced by the inertial mass acting on the output end. The moment applied to the mass is then a function of the beam end displacement and the mass rotation. The system can, thus, be modeled simply as a base-excited, spring–mass oscillator. Results of the response for an ideal beam using this reduced-order model agree with the experimental data to a high degree. Inclusion of loading and geometric imperfections show that the response is not particularly sensitive to these imperfections. Parameter studies for the ideal buckling beam amplifier were conducted to provide guidance for improving the design of the motion amplifier and finding the optimal operating conditions for different applications. An erratum to this article can be found at     

Deformation and fracture of a spheroplast under low-cycle loading at various temperatures

This paper gives the results of an experimental study of the deformation and fracture of a spheroplast under uniaxial low-cycle loading (compression and unloading) at a temperature T = 25 and 100°C. Various mechanisms of damage accumulation at various temperatures and degrees of damage to the material are studied. The experimental results are compared with the well-known dependences taking into account damage accumulation for metals. It is established that the basic propositions of these theories are suitable for the low-cycle fracture of spheroplast — a ductile material of complex structure. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 1, pp. 197–204, January–February, 2009.     

Residual stiffness assessment of structurally failed reinforced concrete structure by dynamic testing and finite element model updating

When an under-reinforced concrete beam structure has been loaded to the point where reinforcing steel on the tension side has yielded, it is deemed to have structurally failed and the full load capacity and stiffness can no longer be developed. When unloaded from the point of failure, the residual stiffness of the structure is difficult to estimate. There is a need to establish the serviceability of the structure and ultimately establish the levels of further loading that can be sustained before total collapse. In this paper we present a method for assessing residual stiffness of such a “failed” reinforced concrete structure, through the application of dynamic testing and finite element (FE) model updating. In an experimental study, failed zones in a beam structure were simulated in a FE model. Through a procedure of sensitivity-based updating using the measured modal properties, the stiffness distribution along the failed beam structure was identified.     

Active control of flexural waves in a phononic crystal beam with staggered periodic properties

The band gaps of a phononic crystal beam with staggered periodic structure are investigated. The periodic system consists of a pure elastic (i.e. PMMA) matrix beam and some piezoelectric (i.e. PZT) patches with coupling between the mechanical–electrical components. The PZT patches connected by negative capacitance circuits are applied to function as the active control system. Based on the condition at the interface between adjacent unit cells, the transfer matrix and localization factor are derived. The influence of the degree of interlacing and negative capacitance circuits are discussed. The numerical results show that another band gap can be generated by the staggered periodic structure of PZT patches. The widths and locations of the band gaps can be changed by the degree of interlacing.     

The effect of a symmetric discontinuity on adjacent material in a longitudinally vibrating uniform beam

A three-element analytical model for a free-free longitudinally vibrating uniform beam containing a symmetric discontinuity is presented. Based on this model, an analytical expression is developed which relates the length of the section containing the damage to the depth of the discontinuity. It thereby fixes the length of the element used to model this section of the beam. This expression was verified experimentally and shown to be material independent. Paper was presented at the 1982 SEM Spring Conference On Experimental Machanics wich was held in Oahu and Maui, HI on May 23–29.     

Waveform fractal dimension for mode shape-based damage identification of beam-type structures

Mode shape-based structural damage identification has been a research focus during the last couple of decades. Most of the existing methods need a numerical or measured baseline mode shape serving as a reference to identify damage, and this requirement extremely limits the practicability of the methods. Recently, waveform fractal dimension such as Katz’s waveform fractal dimension (KWD) has been explored and applied to mode shape for crack identification without a baseline requirement. In this study, different from the popular KWD, an approximate waveform capacity dimension (AWCD) is formulated first, from which an AWCD-based modal abnormality algorithm (AWCD-MAA) is systematically established. Then, the basic characteristics of AWCD-MAA on abnormality detection of mode shapes, e.g., crack localization, crack quantification, noise immunity, etc., are investigated based on an analytical crack model of cantilever beams using linear elastic fracture mechanics. In particular, from the perspective of isomorphism, a mathematical solution on the use of applying waveform fractal dimension to higher mode shapes for crack identification is originally proposed, from which the inherent deficiency of waveform fractal dimension to identify crack when implemented to higher mode shapes is overcome. The applicability and effectiveness of the AWCD-MAA is validated by an experimental program on damage identification of a cracked composite cantilever beam using smart piezoelectric sensors/actuators (i.e., Piezoelectric lead–zirconate–titanate (PZT) and polyvinylidene fluoride (PVDF)). The proposed AWCD-MAA provides a novel, viable method for crack identification of beam-type structures without baseline requirement, and it largely expands the scope of fractal in structural health monitoring applications.     

Experimental and numerical studies of thermo-hydrous transfers in concrete exposed to high temperature

The compressive strength of concrete can be as high as 80 MPa at 28 days. High strength concrete (HSC) can be obtained by decreasing porosity and lowering permeability. Concrete, especially HSC, performs poorly when subjected to fire. This is attributed to high thermal stresses and water vapour pressure. High thermal gradient induces high thermo-mechanical stresses in the concrete system. Low permeability prevents water from escaping and induces high water vapour pressure causing cracking and spalling. The aim of this study is both experimentally and numerically study the coupled heat and mass transfers in concrete exposed to elevated temperature. Five concrete mixtures with various cement contents and water cement ratios of a constant aggregate content were studied before and after heating–cooling cycles. The concrete cylindrical specimens were subjected to several tests: compression and splitting tensile tests, measurement of modulus of elasticity, heating–cooling cycles, thermal field and mass loss during the heating–cooling cycles, and permeability tests. Comparisons between the numerical and experimental results on the thermo-hydrous behaviour were reported. Parametric analyses were carried out in order to underline main parameters involved in concrete behaviour at high temperature. The numerical and experimental results included thermal gradient, water vapour pressure, relative humidity, concrete mass losses due to dehydration, and water content for concrete elements heated from 20 to 600°C. The results show the degrees of damage due to the concrete chemical transformations at high temperature.     

Collisional quenching of theA 2∑+(v′=2)HF+ state by He atomsstate by He atoms

Results of an experimental study of the process of quenching of excited states ofHF ⁺ ions in a hydrofluoride-helium electron-beam plasma are reported. The rate constant of quenching ofA ²∑⁺(v′=2)HF⁺ by helium atoms is measured. The ions were excited by activation of the rarefied gas mixture by an electron beam. Diagnostics of internal states of the ions was performed using the electron-vibration-rotation spectrum of their spontaneous emission. Novosibirsk State University, Novosibirsk 630090. Translated from Prikladnay Makhanika i Tekhnicheskaya Fizika, Vol. 39, No. 6, pp. 16–20, November–December, 1998     

Electrical Response of Carbon Nanotube Reinforced Nanocomposites Under Static and Dynamic Loading

An experimental investigation was conducted to study the effect of quasi-static and dynamic compressive loading on the electrical response of multi-wall carbon nanotube (MWCNT) reinforced epoxy nanocomposites. An in-situ polymerization process using both a shear mixer and an ultrasonic processor were employed to fabricate the nanocomposite material. The fabrication process parameters and the optimum weight fraction of MWCNTs for generating a well-dispersed percolation network were first determined. Absolute resistance values were measured with a high-resolution four-point probe method for both quasi-static and dynamic loading. In addition to measuring the percentage change in electrical resistance, real-time damage was captured using high-speed photography. The real-time damage was correlated to both load and percentage change in resistance profiles. The experimental findings indicate that the bulk electrical resistance of the nanocomposites under both quasi-static and dynamic loading conditions initially decreased between 40%–60% during compression and then increased as damage initiated and propagated.