In this work we present the results of a measurement campaign performed on high strength concrete, to investigate fatigue behavior under cyclic loading in terms of a Structural Health Monitoring (SHM) system. The specimens, small concrete cylinders, were equipped with acoustic emission (AE) and ultrasonic (US) sensors which recorded signals from the onset of beginning fatigue processes in the material until complete damage of the specimen. The parameters monitored have been the acoustic emission activity and the ultrasonic signals time-of-flight respectively travel velocity. The results are in accordance to tests on different concrete material and demonstrate the capability of the proposed methods to trace the fatigue process of concrete. As roundup, results obtained with similar sensing technologies on a large scale structure are presented.

The successful design of Structural Health Monitoring Systems requires efficient simulation tools. Especially for implementing ultrasonic monitoring methods the insight into sound propagation problems inside the structure is essential when using guided waves. Although the sound propagation of guided waves in plate like structures and pipes is well understood, for more complex geometries or anisotropic materials simulations are necessary to predict the received signal depending on the type of excitation. In this contribution an innovative approach is presented for the simulation of propagation of guided waves using Elastodynamic Finite Integration Technique (EFIT). Starting with simple plate like geometries the dispersive behavior is illustrated by analyzing the propagation of different modes. Furthermore, selective excitation is introduced into the model and mode-flaw interactions are studied on different flaw types. The investigation is extended by modeling the wave propagation in structures with more complex geometries. The validity of the simulation results is verified by comparing with experimental data.

Acoustic Emission (AE) generation in 2014 aluminium sheets is investigated in this study. Fatigue tests were conducted with samples under constant amplitude loading, monitoring the rates of AE signals generated during fatigue crack propagation. The load output from the test machine was correlated with recorded AE signals as a means for AE source characterisation. The results showed 3 stages in AE generation with the vast majority of AE signals recorded occurring around or below the mean cyclic load from the emergence of crack to the period just before sample failure where they appeared across the entire loading range.

The aim of the work presented in this paper is to provide guidelines for extending the modeling capacities and improve quality and reliability of 2-D guided wave propagation models using commercially available finite element method (FEM) packages. Predictive simulation of ultrasonic nondestructive evaluation (NDE) and structural health monitoring (SHM) in realistic structures is challenging. Analytical methods can perform efficiently modeling of wave propagation are limited to simple geometries. Realistic structures with complicated geometries are usually modeled with the finite element method (FEM). Commercial FEM codes offer convenient built-in resources for automated meshing, frequency analysis, as well as time integration of dynamic events. We propose to develop FEM guidelines for 2-D Lamb wave propagation with a high level of accuracy. The proposed 2-D guided wave problem will be the pitch-catch arrangement in a full 3-D geometry plate involving guided waves between a transmitter piezoelectric wafer active sensor (PWAS) and receiver PWAS. In addition, corrosion damage is added to this problem to simulate the detection of damage, and assess the detectability threshold. The general approach is to run a series of FEM models. These FEM models will be compared with the experimental data and with our 1-D analytical homemade software.

The majority of today’s damage detection techniques rely on linear macroscopic changes in global vibration signatures or local wave scattering phenomena. However, damage in real-world structures often initiates from fatigue cracks at microscopic levels, presenting highly nonlinear characteristics which may not be well evidenced in linear macroscopic changes. By exploring the nonlinearities of higher-order acoustoultrasonic (AU) waves, an active approach for characterizing fatigue cracks was established. Nonlinearities of higher-order AU waves, subjected to the existence and accumulation of fatigue cracks, were explored. Fundamental investigation was carried out to link the nonlinearities of AU waves to the relative distance between a sensing path and the fatigue crack. Results from simulation and experiment match well in between, which can be used to quantitatively evaluate fatigue cracks. Compared with existing detection approaches based on nonlinear AU waves, this method embodies uniqueness including utilization of a permanently attached active sensor network comprising miniaturized sensors, well accommodating the purpose of structural health monitoring.

Guided waves can travel over a long distance, they are highly sensitive to any discontinuity they might encounter along their propagation path, and they can be generated and sensed by piezo electric transducers easily bonded to the external surface of the structure under investigation. When guided waves are generated and sensed by an array of phased sensors, it’s possible to steer the wave-front in a specific direction (beamforming technique, widely used in electromagnetic radar applications). The improvements respect to the omnidirectional reception/transmission is known as receive and transmit gain, respectively. In this work a linear array of sensors is used to generate an ultrasonic wavefront steered in a specific direction, like structural radar. This effect is achieved by the combination of constructive and destructive interference of signals generated by the linear array of sensors, sequentially fired with appropriate time shifts. Numerical simulations are carried out with the LS-DYNA, an explicit Finite Element (FE) code, on an aluminum panel 6061-T6 (0.7m × 0.8m × 1mm). The damage to be identified is a 5mm diameter hole with 20mm edge cracks, located at a distance of 250 mm away from the center of the array along a direction at 60°. The array of sensors consists of 9 disk-shaped piezo patches (10mm in diameter and 0.2 mm in thickness) with a pitch of 12mm. A five-cycles sine signal with 225 kHz center frequency and in a Hanning window is used as the ex.....

The application of vibration-based damage detection methods to a wind turbine model is analyzed in this paper. The target is to develop a system that detects and locates damage on a structure subjected to wind excitation. With the proposed procedure vibration data is first processed by an Operational Modal Analysis. The extracted mode shapes are subsequently evaluated by two damage detection algorithms: the Modal Strain Energy method and the Gapped Smoothing Technique. Different types of damage are investigated, including tower damage and a change of foundation stiffness. First, a numerical prestudy is conducted to give information about suitable measurement quantities and density of measurement positions on the structure. Based on the numerical results an experimental setup is arranged, including the equipment of the tower with strain gauges and accelerometers. The results of the experimental work show that locating damage with the proposed approach is feasible.

This work presents how the nonlinear ultrasonic technique of second harmonic generation can be used to monitor damage typical of nuclear reactor structural steel material. Second harmonic generation occurs when an ultrasonic wave interacts with microstructural features that create a nonlinear medium for the propagating ultrasonic wave. This phenomenon is measured by the acoustic nonlinearity parameter. Radiation damage causes microstructural evolution such as changes in dislocation density and the formation of precipitates, both of which have been shown to give rise to changes in the acoustic nonlinearity parameter. Previous work has shown that nonlinear ultrasonic techniques are sensitive to radiation damage, specifically that increases of radiation dose are detectable by changes in the acoustic nonlinearity parameter. For these measurements to be robust, alignment, clamping, and mounting of ultrasonic transducers to a sample must be simple, accurate, and repeatable. Nonlinear ultrasonic measurements were run on two types of nuclear reactor steel samples that were previously irradiated in the Rheinsberg power reactor to two fluence levels, up to 1020 n/cm2 (E > 1MeV), through a previous study by the IAEA. More extensive experiments were run on unirradiated standard Charpy samples to test repeatability of the measurements using the fixture and to isolate measurement variations such as surface roughness and clamping force effects.

An investigation is performed to develop a methodology for optimizing the layout of piezoelectric transducers based actuator-sensor network that will maximize the detection capability of a given SHM system for a hot spot in aerospace structures. The method utilizes a simulation tool for wave propagation as a basis to integrate preselected diagnostic algorithm with an optimization tool to maximize the probability of detection (POD) for a given damage size in a structure. The proposed method minimizes the number of actuators and sensors while maximizing POD through the selection of optimal location for each sensor and actuator. Fatigue cracks in metallic structures were studied in this investigation. This paper will highlight the method as well as some results for metallic structures.

Recent research efforts have demonstrated the ability to remotely and nondestructively excite broadband, multi-mode guided waves in structures through thermoelasticity using a relatively low-powered Q-switched laser. The excited waves can be sensed either locally using one or more transducers or remotely using a laser Doppler vibrometer. Incorporating high-speed mirrors, such a system has been used to effectively and rapidly scan large areas with surface normals as high as 45 degrees at a spatial resolution as small as 0.5 millimeters. The time and space sampling capability of the laser-excited guided wave system enables the processing of measured signals in the full, three-dimensional Fourier domain: horizontal and vertical in-plane wavenumber and frequency. Operating in the frequency-wavenumber domain, we describe and demonstrate an approach to identify, extract the dispersion curves of, and selectively isolate individual guided wave modes. With the ability to separate individual wave modes, the direction-, frequency-, and mode-dependent wave propagation and scattering behavior can be measured and utilized for imaging defects. We introduce two imaging algorithms for utilizing mode-separate measurements. The first, which we show to be effective for area-spanning defects, makes direct estimates of the mode- and frequency-dependent wavelength at each imaging point in the structure. The second, which is more effective for identifying small defects, images the energy o.....

In this paper, we propose an intrinsic error estimator for the data fusion of NDT techniques used to identify the material and damage properties of concrete structures. This error estimator is chosen based on the global distribution of the data fusion in the space of the identified material properties. The main idea is to evaluate the accuracy of the result in estimating the gap between the most and the worst likely solutions. This error estimator is applied to synthetic data depending on the parameters of the data fusion such as the regression laws that linked the material properties to the NDT measurements. This work is part of the C2D2-ACDC project that aims at methodology transfer from five research laboratories, LMA, LMDC, IFFSTAR, GhyMaC and IEMN to industrial partners, EDF and SETRA

One of the main concerns in structural health monitoring (SHM) for composite materials is to discern between changes that are related to actual damage in the structure and changes that originate from non-damaging alterations of the material or the survey system. For both the layout of a piezo-based system (i.e. mainly the sensor/actuator area density) and the interpretation of the acquired data, the changes of the viscoelastic material properties due to normal environmental factors as temperature or moisture absorption have to be considered. Additionally, the SHM system itself and the coupling adhesive used to connect it to the material can be influenced by this factors. Without a strategy to account for those influences, a high false alarm rate has to be expected. In this presentation, experimental and analytical investigations of a SHM system based on Lamb waves measured with surface applied piezoelectric sensors are presented. The influence of temperature and humidity on the measured velocity and damping coefficients is shown as well as the high influence of both factors on the excitability of Lamb waves. Potential and limitations of analytical methods to describe the measurement system and the temperature/humidity-dependence of the piezoelectric elements properties, the adhesive layer and the material itself are investigated.