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Organisationseinheit der BAM
Basics in acoustic emission
(2024)
A lecture on the basics of Acoustic Emission testing. The lecture provides a general overview of the method, its history, and its applications, followed by an explanation of instruments, measurement processes, and signal characteristics. In the second part, the presentation focuses on different types of data analysis - parameter- and signal-based approaches. At the end of the lecture, a brief overview of the source localization methods is given, focusing on Geiger's method.
A lecture on the basics of Acoustic Emission (AE) testing in research and practice. The lecture provides a general overview of the basics of AE: what is it? What are the AE sources? How and using which equipment it can be detected? The second part of the presentation focuses on different types of data analysis: parameter- and signal-based approaches and gives a brief overview of the source localization methods, focusing on Geiger's method. The last part of the presentation shows application examples in research and in real-structures monitoring.
Acoustic emission (AE) monitoring in concrete structures typically relies on bulky and expensive piezoelectric (PZT) sensors. In this study, we present an initial characterization of compact, low-cost MEMS AE sensors, comparing them with commercially available AE sensors and custom-built high-sensitivity low-cost (HSLC) resonant PZT disc sensors. This study evaluates whether MEMS sensors, traditionally used in metal applications, can be effectively adapted for concrete monitoring.
Initial testing involves the evaluation of MEMS sensors mounted on a concrete specimen, using AE events simulated by pencil lead breaks (PLB) on the surface and by actuators embedded within the concrete medium. Despite their lower sensitivity, the MEMS sensors successfully detect AE signals even at frequencies away from their resonance, demonstrating potential for use beyond their originally intended applications in metals. Their narrowband, repeatable response resembles that of resonant PZT sensors commonly used for detecting damage initiation and locating sources.
These preliminary results highlight the potential of MEMS sensors in AE monitoring of concrete, especially in applications where size, cost, and sensor-to-sensor consistency are important, and where signal characteristics fall within their tunable frequency range.
The uniaxial compression of an initially isotropic concrete specimen induces a damage pattern that is anisotropic: in the direction of loading, the concrete undergoes compaction, while in the directions orthogonal to the loading direction, concrete cracking can be observed. In order to quantify such damage patterns, one possibility is to observe the changes in the stiffness tensor of a compressed specimen with respect to the stiffness tensor of the corresponding virgin state specimen. The eigensolutions of the Kelvin–Christoffel matrix for anisotropic media provide the relation between ultrasound wave velocities in given wavefront and polarization directions and the stiffness matrix components. In this study, it is shown how using the through transmission method for the determination of first-time arrival, one can evaluate the sound wave velocities in different directions and compute the stiffness tensor components for a damaged concrete specimen. Plots of stiffness tensor components as a function of non-recoverable strain give a qualitative measure of the anisotropic strain degradation process.
The crucial part of nuclear waste storage is the construction of sealing structures made of reliable, safe and well–understood materials. We present an extended analysis of long-term multi–sensory monitoring and non–destructive testing (NDT) inspection of two laboratory specimens aiming at potential materials for sealing structures for nuclear waste repositories. Specimens with a volume of 340 litres made from newly developed alkali–activated materials (AAM) and established salt concrete (SC) were analysed using embedded acoustic emission and wireless radio-frequency identification (RFID) sensors, ultrasonic echo imaging, active thermography, and X–ray computed tomography. The monitoring analysis showed lower heat of reaction and 50% less acoustic emission events in AAM compared to SC. However, due to the surface effects of the AAM material, the number of acoustic emission events increased significantly after approximately two months of monitoring. Subsequently performed NDT inspections reliably located embedded sensors and confirmed the absence of major cracks or impurities. The presented laboratory results show the feasibility and potential of comprehensive NDT monitoring and inspection to characterise cementitious and alternative materials as well as the need for multi–parameter long–term monitoring. Thus, our study demonstrates that tailored NDT investigations will help to develop safe sealing structures for nuclear waste repositories.
To safely dispose of nuclear waste in underground facilities, engineered barrier systems are needed to seal shafts and galleries. The material used in these barriers must be adapted to the host rock parameters. Shrinking and cracking must be avoided to provide a barrier with almost zero permeability. For repositories in salt rock environments, several types of salt concrete (SC) are possible construction materials. Within the project SealWasteSafe, we compared the behavior of an innovative alkali-activated material (AAM) with standard SC in their hydration and hardening phase. To monitor the microstructural changes within the two materials SC and AAM, acoustic emission (AE) signals have been recorded for up to ~250 days on 340-litercubic specimens. The phenomenon of AE is defined as the emission of elastic waves in materials due to the release of localized internal energy. Such energy release can be caused by the nucleation of micro-fracture, e.g., in concrete while curing or when exposed to load. The occurrence of AE events gives first rough indications of microstructural changes and potentially occurring cracking and thus, provides insights for structural health monitoring (SHM). The results show, that for the first 28 days after casting, less AE activity was detected in the AAM compared to SC. After 61 days, in the AAM material, the number of AE events exceeded those observed in the SC. However, the majority of the AE detected and located in AAM was related to surface effects, and not to microstructural changes or occurring cracks within the bulk volume. Additionally, the source location analysis indicated, that despite lower activity in SC, we observed some clustering of the events. In contrast, in AAM, the activity inside the specimen is randomly distributed over the whole volume. The monitoring results help to estimate the material’s sealing properties which are crucial to assess their applicability as sealing material for engineered barriers.
For years, soil-structure interaction (SSI) has been a subject of interdisciplinary studies, although full-scale SSI experiments and the use of real data are rare. To study SSI in a well-known and controlled environment, a full-scale experiment, EuroMASS, was conducted at the Piana di Toppo test site in Northeast Italy. For the needs of the experiment, a simple structure consisting of a lumped mass overtopping a steel column on a concrete base was designed and assembled at the test site. In April 2022, a three-component instrumentation network was installed to record the seismic noise, weak earthquake motions, and active source signals to study the structure, the foundation, and the soil seismic responses. The experiment was designed to collect data that can be used for accurate dynamic characterization of the structure and analysis of wave propagation in the soil-structure system.
Active-source ultrasonic monitoring using ultrasonic pulse velocity measurements and coda wave interferometry (CWI) can provide information about the evolution elastic properties of concrete with increasing damage. The acoustic emission (AE), a passive method based on "listening” to the radiation of elastic waves in solids caused by irreversible changes within the material structure, gives the first indications of microstructural changes and microcracking. The question is: how the registered microcracking activity alters the elastic properties of concrete?
Our study aims to quantify the connection between the evolution of AE events and the overall elastic properties of concrete, assessed via active-source ultrasonic testing. We analyze the spatiotemporal evolution of AE events, changes in wave propagation speed, and nonlinear effects during stepwise quasi-static loading/unloading of concrete specimens. By integrating CWI with AE analysis, we achieve a comprehensive assessment of the evolution of the sample's damage throughout the entire loading process. Our findings underscore the complementary nature of these methods, revealing that a singular approach may not provide a complete picture of the extent of damage.
In this study, the wavefield radiated from a building to its surroundings is identified and extracted from M4.6 earthquake recordings collected by sensors installed in a building and on the nearby athletic field in Matera (Italy) using a new approach for soil-structure interaction assessment. The proposed approach for earthquake data analysis combines in an innovative way two methods already used in seismology and engineering seismology: deconvolution and polarization analysis. The approach enables the identification, reconstruction, and characterization of the wavefield radiated from a vibrating building into its surroundings, and the estimation of the amount of energy associated with it. The approach consists of four steps: estimation of the resonant frequencies of the building, deconvolution of the earthquake recordings from a building and its surroundings, identification of the seismic phases, reconstruction of the signal transmitted from the building to its surroundings, and estimation of its energy, and polarization analysis. Analysis of recordings of the M4.6 event highlighted that the motion related to the wavefield radiated from the building to the ground was mostly linearly polarized in the radial and transverse planes, while a clear ellipticity was observed only in the horizontal plane. The wavefield radiated from the building might be dominated by unconventionally polarized surface waves, i.e., quasi-Rayleigh waves or a combination of quasi-Rayleigh and quasi-Love waves. The results indicated that the energy transmitted from the analyzed vibrating building to its surroundings was significant and decreased the ground motion shaking due to the out-of-phase motion.
To safely dispose of nuclear waste in underground facilities, engineered barrier systems are needed to seal shafts and galleries. The material used in these barriers must be adapted to the host rock parameters. Shrinking and cracking must be avoided to provide a barrier with almost zero permeability. For repositories in salt rock environments, several types of salt concrete (SC) are possible construction materials.
Within the project SealWasteSafe, we compared the behavior of an innovative alkali-activated material (AAM) with standard SC in their hydration and hardening phase. To monitor the microstructural changes within the two materials SC and AAM, acoustic emission (AE) signals have been recorded for up to ~250 days on 340-liter-cubic specimens.
The phenomenon of AE is defined as the emission of elastic waves in materials due to the release of localized internal energy. Such energy release can be caused by the nucleation of micro-fracture, e.g., in concrete while curing or when exposed to load. The occurrence of AE events gives first rough indications of microstructural changes and potentially occurring cracking and thus, provides insights for structural health monitoring (SHM).
The results show, that for the first 28 days after casting, less AE activity was detected in the AAM compared to SC. After 61 days, in the AAM material, the number of AE events exceeded those observed in the SC. However, the majority of the AE detected and located in AAM was related to surface effects, and not to microstructural changes or occurring cracks within the bulk volume. Additionally, the source location analysis indicated, that despite lower activity in SC, we observed some clustering of the events. In contrast, in AAM, the activity inside the specimen is randomly distributed over the whole volume. The monitoring results help to estimate the material’s sealing properties which are crucial to assess their applicability as sealing material for engineered barriers.
