Nikolaos A. Chrysochoidis

Assessing the effects of delamination on the damped dynamic response of composite beams with piezoelectric actuators and sensors

Experimental and analytical studies are presented investigating the effect of delamination cracks on the low-frequency dynamic response of composite beam specimens with surface attached piezoelectric actuators and sensors. Frequency response functions of laminated beams with piezoceramic actuators and piezoelectric sensors having single delamination cracks of various sizes are measured. Comparisons with mechanical actuation–accelerometer sensor configurations illustrate the sufficiency and advantages of piezoelectric actuator–sensor pairs in self-monitoring the effects of delamination on the dynamic response. Modal frequencies and damping values of the beams are also measured and their dependence on delamination size is studied. Comparisons with analytically predicted results are shown. Finally, the sensitivity of damage indices based on changes in modal frequency, modal damping, and modal peak density to the crack size is quantified.

Delamination detection in composites using wave modulation spectroscopy with a novel active nonlinear acousto-ultrasonic piezoelectric sensor

A novel structural health monitoring (SHM) methodology, based on nonlinear wave modulation spectroscopy, is presented for the detection of delamination cracks in composites. The basic element is a novel active nonlinear acousto-ultrasonic piezoelectric sensor enabling low-cost and wide-frequency operational bandwidth. The active sensor configuration involves two piezoceramic wafer actuators, each one excited with a low- and high-frequency signal respectively, and a piezoceramic sensor, all permanently bonded on the tested structure. Experiments are conducted on two sets of composite strips containing delamination cracks of different sizes. Measured results illustrate first the efficiency of the nonlinear ultrasonics methodology to detect delamination cracks, as well as, the potential and benefits of the new active sensor. The sensitivity of the active sensor response to the crack size and the applied high-frequency carrier signals at the actuators, vary at various frequency and voltage levels indicating the appropriate testing setup. Additionally repeatability of proposed SHM methodology is studied.

High-frequency Dispersion Characteristics of Smart Delaminated Composite Beams

Many promising techniques for structural health monitoring rely on the usage of in situ piezoelectric actuators and sensors, which provide the ability of self-excitation and monitoring of the damage effect on the structural vibration and wave propagation. This article presents layerwise mechanics and a finite element capable of describing the response of delaminated composite beams with piezoelectric actuators and sensors. The layerwise beam theory approximates the through-thickness, in-plane displacements, and electrical field as a continuous assembly of piecewise linear layerwise fields through the thickness. This theory further describes the field discontinuities across the delamination as additional degrees of freedom. The introduction of a variable transverse displacement field provides the capability to capture important symmetric and antisymmetric high-frequency modes, guided waves, and other complex phenomena. Pseudo Wigner-Ville distributions provide the frequency dispersion characteristics of predicted and measured time responses of beams with single delaminations and an active piezoelectric sensor pair. Time—frequency plots obtained from the experimental and numerical data are correlated and their ability to reveal the presence and size of delamination is investigated.

On the delamination detection in composite beams with active piezoelectric sensors using non-linear ultrasonics

This paper investigates the potential of a novel SHM method for the detection of delamination cracks in composites which exploits the nonlinear ultrasonic response with in-situ d31 piezoceramic actuators and sensors. Composite beam specimens with artificially created delamination cracks are tested, entailing two piezoceramic actuator patches, the first to generate a low frequency, high power modal excitation and the second a high frequency acoustical wave, as well as a piezoceramic sensor. Nonlinearities induced at the high-frequency signal, such as sidebands at the spectral components as long as modulations at the measured sensory voltage are evaluated as damage indicators. Experimental results quantify the potential of the method in detecting small delamination cracks through spectral sideband components. The influence of high-frequency on the effectiveness of the method is shown. Additionally, the effect of the magnitude of applied voltage on the low frequency actuator on the formation of spectral components is investigated. Finally, the obtained results of the present method are compared with a guided wave based pitch and catch SHM method using the same actuator-sensor pair to excite and monitor the propagation of the first symmetric and asymmetric Lamb waves.

Nonlinear wave structural health monitoring method using an active nonlinear piezoceramic sensor for matrix cracking detection in composites

A structural health monitoring methodology, based on nonlinear wave modulation spectroscopy, is presented and aims to detect matrix cracks in composites. Experiments were conducted on cross-ply carbon/epoxy strips containing matrix cracks, induced via three-point bending. Damage in all tested specimens was categorized according to the acoustic emission hits recorded during loading. The nonlinear ultrasonics methodology is applied via an active nonlinear acousto-ultrasonic piezoelectric sensor, enabling low-cost and wide-frequency operational bandwidth. This active sensor configuration involves two piezoceramic wafer actuators, one excited with a low- and the other with a high-frequency signal, and a piezoceramic sensor, all permanently bonded on the tested structure. The sensitivity of the nonlinear active sensor response at specific high carrier frequencies is depicted and damage indices are proposed. The experimental results illustrate the effectiveness of the nonlinear ultrasonic wave mixing method in matrix crack detection, as well as the potential of the new active sensor for permanent structural health monitoring.

Impact damage detection in composites using an active nonlinear acousto-ultrasonic piezoceramic sensor

This paper investigates the potential of a non linear wave modulation SHM methodology with piezoelectric wafers as actuators and sensors to reveal impact damage in cross-ply Glass/ Epoxy composite plates. In the experimental procedure an electromechanical shaker and piezoceramic wafers are simultaneously used to provide the low and high frequency wave excitation, respectively, while for the acquisition of the modulated carrier wave a piezoceramic sensor was used. Two sets of piezoceramic actuators-sensor pairs are used to propagate the ultrasonic carrier wave into two directions, one parallel and transversely to the fibers of the outer unidirectional ply. Nonlinearities induced by the damage are detected as sidebands in the spectral components of the carrier signal. Experimental results quantify the potential of the method in detecting damage created by very low energy impacts (4 Joules). Additionally, the modulation factor of the sensor signal is proposed as damage index, and is shown to be a consistent and sensitive damage indicator in the impacted plates, for a broad range of carrier wave frequencies.

Layerwise Dynamic Response Models for Delamination Composite Beams with Active Piezoelectric Sensors

A linear layerwise laminate theory and a beam FE are formulated for analyzing the effect of delamination cracks on the dynamic response of composite laminates with piezoactuators and sensors. The model naturally includes, among other things, the excitation of piezoelectric actuators, their interactions with the composite laminate, and the effect of delamination on the dynamic signals of piezoelectric sensors. The assumed layerwise displacement and electric fields are described by linear variations in every layer, moreover, the discontinuities in displacement fields due to delaminations are considered as additional degrees of freedom. In addition, the contact between the crack interfaces is also considered through a generalized normal stress whose magnitude is eventually related to the relative movement of the two crack faces. The transient response of Gr/Epoxy beams with active piezosensors is predicted. The results show the influence of delamination crack on the sensor response, but also quantify the effect of inclusion of interfaces contact into the analysis.

High Frequency Response of Delaminated Composite Beams with Active Piezoelectric Sensors

The current paper presents coupled linear layerwise formulation and FE for delaminated composite beams with in situ piezoelectric actuators and sensors. The new models assume layerwise variations of the normal displacements field through the thickness. Through this assumption the primary normal piezoelectric coefficient is also introduced into the model. Discontinuities at the delamination interface are described as additional degrees of freedom. The formulation naturally includes the excitation of piezoelectric actuators, their interactions with the composite laminate and the effect of delamination at the sensory voltage. Predictions are correlated with a previously developed model and experimental measurements illustrating the benefits of the newly developed analytical model at the time response of composite beams with delamination cracks. Additionally the sensitivity of sensor signal to the presence of small delaminations is also illustrated.

ANALYSIS OF ADAPTIVE SANDWICH COMPOSITE BEAMS WITH PIEZOELECTRIC ACTUATORS AND SENSORS USING COUPLED HIGH-ORDER LAYERWISE MECHANICS

A high-order discrete layer theoretical framework and a finite element are presented for predicting the electrostatic response of sandwich beams with piezoelectric layers. A new layerwise coupled piezoelectric laminate theory is developed, in which quadratic and cubic fields are added to the in-plane displacement and electric potential approximation in each discrete layer, moreover interlaminar shear stress continuity is imposed through the thickness. The stiffness, piezoelectric and permittivity matrices are formulated from ply to structural level. A finite element method and a beam finite element are further developed and used to predict the electrostatic response of various beam configurations. Numerical results and comparisons with linear layerwise beam finite element predictions illustrate the accuracy and capability of the developed mechanics to efficiently capture the global and local stress response of smart piezoelectric composite beams, particularly those of high thickness sandwich foam cores and compliant layers.