Guided wave field calculation in anisotropic layered structures using normal mode expansion method

Electromechanical impedance (EMI) method is an effective and powerful technique in structural health monitoring (SHM) which couples the mechanical impedance of host structure with the electrical impedance measured at the piezoelectric wafer active sensor (PWAS) transducer terminals. Due to the electromechanical coupling in piezoelectric materials, changes in structural mechanical impedance are reflected in the electrical impedance measured at the PWAS. Therefore, the structural mechanical resonances are reflected in a virtually identical spectrum of peaks and valleys in the real part of the measured EMI. Multi-physics based finite element method (MP-FEM) has been widely used for the analysis of piezoelectric materials and structures. It uses finite elements taking both electrical and mechanical DOF’s into consideration, which allows good differentiation of complicated structural geometries and damaged areas. In this paper, MP-FEM was then used to simulate PWAS EMI for the goal of SHM. EMI of free PWAS was first simulated and compared with experimental result. Then the constrained PWAS was studied. EMI of both metallic and glass fiber composite materials were simulated. The first case is the constrained PWAS on aluminum beam with various dimensions. The second case studies the sensitivity range of the EMI approach for damage detection on aluminum beam using a set of specimens with cracks at different locations. In the third case, structural damping effects were also studied in this paper.. Our results have also shown that the imaginary part of the impedance and admittance can be used for sensor self-diagnosis.

Influence of temperature on concrete beams strengthened in flexure with CFRP

This paper presents a set of numerical and experimental results on the use of guided waves for structural health monitoring (SHM) of crack growth during a fatigue test in a thick steel plate used for civil engineering application. The capability of embedded piezoelectric wafer active sensors (PWAS) to perform in situ nondestructive evaluation (NDE) is explored. Numerical simulation and experimental tests are used to prove that PWAS can perform active SHM using guided wave pitch-catch method and passive SHM using acoustic emission (AE). Multi-physics finite element (MPFEM) codes are used to simulate the transmission and reception of guided waves in a 1-mm plate and their diffraction by a through hole. The MP-FEM approach permitted that the input and output variables be expressed directly in electric terms while the two-ways electromechanical conversion was done internally in the MP-FEM formulation. The analysis was repeated for several hole sizes and a damage index performances was tested. AE simulation was performed with the MP-FEM approach in a 13-mm plate in the shape of the compact tension (CT) fracture mechanics specimen. The AE event was simulated as a pulse of defined duration and amplitude. The electrical signal measured at a receiver PWAS was simulated. Daubechies wavelet transform was used to process the signal and identify its Lamb modes and FFT frequency contents. Experimental tests were performed with PWAS transducers acting as passive receivers of AE signals. The 8-mm thick flange of an I beam was instrumented on one side with PWAS transducers and on the other side with conventional AE transducers (PAC R15I) acting as comparison witnesses. An AE source was simulated using 0.5- mm pencil lead breaks; the PWAS transducers were able to pick up AE signal with good strength. Subsequently, PWAS transducers and R15I sensors were applied to a 13-mm CT specimen subjected to accelerated fatigue testing. The PWAS and R15I transducers signals were collected with PAC data acquisition system using the AE-win software. Comparative results of AE hits and source localization from the PWAS and R15I sensors are given. Active sensing in pitch catch mode was applied between the PWAS transducers installed on the CT specimen and damage indexes were calculated and correlated with physical crack growth as measured optically. The paper finishes with summary, conclusion, and suggestions for further work.

In steel members strengthened by carbon-fiber reinforced polymer (CFRP) plates, the thermal stresses are introduced in the steel members, the CFRP plates and the adhesive layers when temperature changes because the linear thermal expansion coefficients of steel and CFRP are mismatched. As so far, the authors proposed a technique to reduce the thermal stress in steel members strengthened by CFRP plates, which involves bonding aluminum alloy plates with CFRP plates. In the proposed method, the thermal stress in steel member can be reduced so that there are negligible levels of stress in steel member when the cross sectional areas of CFRP and aluminum plates are designed to correspond to the coefficient of thermal expansion of steel, even though the thermal stresses are introduced in the CFRP and aluminum plates. In this study, to confirm the maintaining the thermal stress reduction in steel member by proposed method, thermal stress measurement in steel plate strengthened by CFRP and aluminum plates was carried out about 21 months. In this research, the thermal stress introduced in the steel plate strengthened by CFRP plates was also measured. Furthermore, to assume the thermal shear and normal (peel) stresses in adhesive layers, FE analysis with plane stress element was employed. As the result, it was shown the thermal stresses in steel plate with CFRP plate were able to calculate by using composite theory and measured temperature. Furthermore, in steel plate strengthened by CFRP and aluminum plates, the thermal stress introduced in steel plate was negligible-small through the all-season. It was found the thermal stresses in steel plate with CFRP plates as well as CFRP and aluminum plates were also estimated by using composite theory and measured temperature. In the steel plate strengthened by CFRP and aluminum plates, the thermal shear and normal stresses in adhesive layer glued to steel plate become smaller than that in the conventional CFRP bonded specimen. However, the shear stress in adhesive layers between CFRP and aluminum plates in proposed method was higher than the thermal stress in adhesive layers between CFRP plates in conventional method.

Embedded-ultrasonics structural radar (EUSR) is a new concept and methodology for in-situ nondestructive evaluation (NDE) and structural health monitoring (SHM) of thin-wall structures. EUSR consists (a) an array of piezoelectric wafer active sensors (PWAS) embedded into the structure; and (b) electronic modules for signal transmission/reception, processing, and interpretation. EUSR utilizes guided elastic waves (Lamb waves) generated omnidirectionally into the thin-wall structure by surface-mounted permanently-attached PWAS. The paper starts with the general concepts of the EUSR algorithm: transmission beamforming, reception beamforming, and time of fight determination. Next, details of the Lamb wave generation with PWAS, verification of group-velocity dispersion curves, identification of optimal excitation frequency, and confirmation of wave front omnidirectionality are discussed. In the third part of the paper, the actual implementation of the EUSR method in a proof-of-concept demonstration is presented. The construction of the PWAS phased array is described, and detection of cracks located broadside and offside of the PWAS array is illustrated. The method is shown to be easy to use through a visually interactive LabView™ interface. Very good detection accuracy is observed. Though the proof-of-concept experiments presented in this paper were illustrated on metallic structures, the EUSR concept works just as well on composite and hybrid structures.

Ultrasonic guided waves have been successfully applied in Non-Destructive Evaluation (NDE) and Structural Health Monitoring (SHM) of pipelines of various industries. Guided waves are complicated because of their dispersive nature and infinite number of propagating modes. Mode control is of great significance for application of guided waves. The classical Normal Mode Expansion (NME) method is adopted to analyze guided wave mode control in hollow cylinders by using time delay axial array transducers and time delay circular array transducers. Guided wave mode control can be implemented when the axial array transducers and circular array transducers are specially designed. The theoretical prediction is verified by finite element numerical evaluation.

The use of piezoelectric wafer active sensors (PWAS) for structural health monitoring (SHM) purposes is state of the art for acousto-ultrasonic-based methods. For system reliability, detailed information about the PWAS itself is necessary. This paper gives an overview on frequent PWAS faults and presents the effects of these faults on the wave propagation, used for active acousto-ultrasonics-based SHM. The analysis of the wave field is based on velocity measurements using a laser Doppler vibrometer (LDV). New and established methods of PWAS inspection are explained in detail, listing advantages and disadvantages. The electro-mechanical impedance spectrum as basis for these methods is discussed for different sensor faults. This way this contribution focuses on a detailed analysis of PWAS and the need of their inspection for an increased reliability of SHM systems.

Piezoelectric wafer active sensors (PWAS) used in structural health monitoring (SHM)applications are able to detect structural damage using Lamb waves. PWAS are small,lightweight, unobtrusive and inexpensive. They achieve direct transduction between electricand elastic wave energies. PWAS are charge mode sensors and can be used as bothtransmitters and receivers. The focus of this paper is to find a suitable in situ piezoelectricactive sensor for sending and receiving Lamb waves to be used in the SHM of structureswith a curved surface. Current SHM technology uses brittle piezoceramic (PZT) waferactive sensors. Since piezoceramics are brittle, this approach could only be used on flatsurfaces. The motivation of our research was to explore the use of flexible piezoelectricmaterials, e.g. piezoelastic polymers such as PVDF. However, PVDF stiffnessis orders of magnitude lower than the PZT stiffness, and hence PVDF Lambwave transmitters are much weaker than PZT transmitters. Thus, our researchproceeded in two main directions: (a) to model and understand how piezoelectricmaterial properties affect the behaviour of piezoelectric wafer active sensors; and(b) to perform experiments to test the capabilities of the flexible PVDF PWAS incomparison with those of stiffer but brittle PZT PWAS. We have shown that, withappropriate signal amplification, PVDF PWAS can perform the same Lamb wavetransmission and reception functions currently performed by PZT PWAS. Theexperimental results of PZT-PWAS and PVDF-PWAS have been compared with aconventional strain gauge. The theoretical and experimental results in this study gave abasic demonstration of the piezoelectricity of PZT-PWAS and PVDF-PWAS.

Structural health monitoring (SHM) of carbon fibre reinforced polymers (CFRP) provides a valuable way to assess the condition of these materials, which possess desirable traits, but exhibit little elastic failure. Further complications arise due to their multi component nature, which means they can fail in a variety of ways. High-fidelity magnetostrictive SHM technology exhibits higher cost when utilising top-end materials that contain rare-earth elements. At the expense of fidelity, cost can be reduced by using Fe-based ribbons and cheap transducers. This reduction in accuracy can be overcome by coupling a detection array with the development of specific detection schemes for each failure type to reduce signal complexity. In this work, we investigate the failure of CFRP coupons in Mode 1 and 2 failure, using FeSiB ribbons as the actuator either bonded between the CFRP ply or co-cured on the surface. During the measurement, a set of 4 pancake coils are used to measure the change in inductance, as the CFRP fails, thus demonstrating that this magnetostrictive-coil set-up can be used to measure cracks forming and propagating within the CFRP, hence it is a promising SHM system. For the first time, it is demonstrated that there is a correlation between the induction signal and the elastic energy stored within the CFRP coupon.

Bars are significant load-carrying components in engineering structures. In particular, L-bars are typical structural components commonly used in truss structures and have typical irregular asymmetric cross-sections. To ensure the safety of load-carrying bars, much research has been done for non-destructive testing (NDT). Ultrasonic guided waves have been widely applied in various NDT techniques for bars as a result of the long-range propagation, low attenuation, and high sensitivity to damages. Though good for inspection of ultrasonic guided waves in symmetric cross-section bar-like structures, the application in asymmetric ones lacks further research. Moreover, traditional damage detection in bars using ultrasonic guided waves usually depends on a single-mode at a lower frequency with lower sensitivity and accuracy. To make full use of all frequencies and modes, a multi-mode characteristic-based damage detection method is presented with the sum of multiple signals (SoM) strategy for L-bars with asymmetric cross-section. To control the desired mode in multi-mode ultrasonic guided waves, excitation optimization and weighted gathering are carried out by the analysis of the semi-analytical finite element (SAFE) method and the normal mode expansion (NME) method. An L-bar example with the asymmetric cross-section of 35 mm × 20 mm × 3 mm is used to specialize the proposed method, and some finite element (FE) models have been simulated to validate the mode control. In addition, one PZT is applied as a contrast in order to validate the multielement mode control. Then, more FE simulations experiments for damage detection have been performed to validate the damage detection method and verify the improvement in detection accuracy and damage sensitivity.

CFRP lamella stay-cable and its force measurement based on microwave radar

Sandwich honeycomb structures (aluminum core bonded to Carbon Fiber Reinforced Polymer (CFRP) sheets on either side) are widely employed in the aerospace industry for their high strength to mass ratio. However, they might be subjected to damages such as delaminations of the composite sheets or debondings between the face sheets and the core due to impacts or thermo-mechanical aging. In order to reduce maintaining costs and extend the service time, Guided Waves (GW) based Structural Health Monitoring (SHM) systems are considered an adequate solution. Indeed, GW propagate over long distances and exhibit low attenuation, thus allowing to monitor wide areas with a limited number of sensors. Defect imaging on CFRP composites and honeycomb composite structures using both Delay-And-Sum [1] and correlation-based algorithm Excitelet [2] is presented in this communication. A machine learning algorithm is finally implemented in order to automatically identify and isolate defects on a given cartography map. The machine learning algorithm is trained on an experimental database of false and positive results obtained on representative specimens.

Two ultra-long-span cable-stayed bridge schemes, identical in deck configuration and main span length of 1500 m but differ in steel and carbon fiber reinforced polymer (CFRP) stay cables, are first designed based on the equivalent strength principle. Finite element analyses are then conducted to investigate wind-resistant performances of both schemes, including static structural behaviors, mean wind-induced deflections, buffeting responses, flutter instability, wind-induced cable resonance, vortex-induced cable vibrations and wind-induced local bending deformations of the stay cables. The results indicate that the using of CFRP cables reduces significantly the wind loads, which account for a major part in the total wind loads developed on the entire structure. As a result, mean wind-induced global deflections of CFRP scheme are notably reduced compared with the steel scheme. In terms of buffeting, results of the CFRP scheme are 12%, 14%, and 28% lower than those of the steel one in vertical, torsional, and lateral directions, respectively. No substantial difference is observed between the two schemes regarding the bridge deck flutter stability. As far as wind-induced cable resonance is concerned, the CFRP scheme is obviously superior to the steel one, exhibiting a much lower likelihood of buffeting-induced resonances due to much higher natural frequencies of stay-cables. As far as the vortex-induced vibration is concerned, however, CFRP stay-cables are less favorable than steel ones. Finally, aiming at the inherent shortcoming of CFRP cables, wind-induced bending deformations at anchorage ends are analyzed. The results show bending angles of the CFRP cables are significantly lower than those of the steel cables. With a wind speed as high as 52.97 m/s considered, CFRP stay cables experience only low-to-moderate bending angles, resulting in no significant strength reductions and posing no substantial threat to the structural safety.

The improvement in the flexural response and cracking resistance of steel-concrete composite beams stiffened with Carbon Fiber Reinforced Polymers (CFRP) sheets at the hogging moment regions is investigated experimentally and numerically in this study. The experimental program consists of two composite beams tested in an inverted position to simulate a portion of a continuous composite beam at the hogging moment region. The concrete flanges of the beams are reinforced externally with corrugated steel deck which used also as a permanent formwork. The experimental findings are used to develop and validate a reliable finite element (FE) model which able to simulate the non-linear performance of the stiffened beams as well as plain beams. In addition, a FE analysis on full-scale continuous composite beams with reinforced concrete flange is carried out to confirm the influence of using CFRP strips as a strengthening technique at the hogging moment regions. The beam deformation, steel flange strain, concrete strain, and CFRP strain confirm good agreement between the FE analysis and the test data. The beam capacity is enhanced only by 11% due to a premature failure in the composite flange. Although the composite connection between the concrete flange and the steel beam is improved by enhancing the cracking resistance of the concrete, the ductility of the strengthened beams is reduced by 20% in comparison to the un-strengthened beam.

Carbon fiber-reinforced polymer (CFRP) sheets have recently become popular for use as repair or rehabilitation material for deteriorated carbon fiber reinforced concrete structures. Carbon fiber reinforced concrete beams were analyzed by finite element software ANASYS. Through the finite element analysis, the results showed that using bonded CFRP to strengthen R. C. beams can significantly increase their load carrying capacity. However, the beams with prestressed CFRP can withstand larger ultimate loads than beams with bonded CFRP. Using bonded CFRP to strengthen R. C. beams can obviously reduce the ultimate deflection.