Acoustic Emission Monitoring of Progressive Damage of Reinforced Concrete T-Beams under Four-Point Bending

Acoustic Emission (AE) is revealed to be highly adapted to monitor materials and structures in materials research and for site monitoring. AE-features can be either analyzed by means of physical considerations (geophysics/seismology) or through their time/frequency waveform characteristics. However, the multitude of definitions related to the different parameters as well as the processing methods makes it necessary to develop a comparative analysis in the case of a heterogeneous material such as civil engineering concrete. This paper aimed to study the micro-cracking behavior of steel fiber-reinforced reinforced concrete T-beams subjected to mechanical tests. For this purpose, four-points bending tests, carried out at different displacement velocities, were performed in the presence of an acoustic emission sensors network. Besides, a comparison between the sensitivity to damage of three definitions corresponding to the b-value parameter was performed and completed by the evolution of the RA-value and average frequency (AF) as a function of loading time. This work also discussed the use of the support-vector machine (SVM) approach to define different damage zones in the load-displacement curve. This work shows the limits of this approach and proposes the use of an unsupervised learning approach to cluster AE data according to physical and time/frequency parameters. The paper ends with a conclusion on the advantages and limitations of the different methods and parameters used in connection with the micro/macro tensile and shear mechanisms involved in concrete cracking for the purpose of in situ monitoring of concrete structures.     

The Correlation between Shrinkage and Acoustic Emission Signals in Early Age Concrete

This study analysed the processes of damage formation and development in early age unloaded concrete using the acoustic emission method (IADP). These are of great importance in the context of the durability and reliability of a structure, as they contribute to reducing its failure-free operation time. Concrete made with basalt aggregate and Portland or metallurgical cement cured under different conditions after demoulding was the test material. The obtained damage values were compared with the measured concrete shrinkage, and a shrinkage strain–acoustic emission signal (resulting from damage) correlation was found. The correlation allows easy measurement of damage level in the early period of concrete hardening, and consequently can be the basis of a non-destructive method.     

Nonlinear Energy Evolution Characteristics of Diorite Examined by Triaxial Loading–Unloading and Acoustic Emission Tests

Acoustic emission (AE) is often accompanied by the propagation of internal microcracks in loaded rock samples, and it essentially reflects microinstability phenomena driven by energy redistribution under stress. In this paper, loading and unloading tests were carried out to investigate the internal nonlinear damage evolution characteristics of diorite samples under different unloading confining-pressure rates. The nonlinear mechanical characteristics of the strain energy sequence of diorite were studied by applying nonlinear dynamics and basic chaos theory and MATLAB software. Moreover, the evolution characteristics of AE counts and AE energy of rock samples were investigated, and their microcrack-propagation modes were analyzed based on the RA–AF scatter distribution of AE and a two-dimensional Gaussian mixture model. Finally, according to the evolution characteristics of energy and AE, the nonlinear damage evolution mechanism of diorite under loading and unloading conditions was revealed. The results show that, before the loading and unloading peak strength, when the strain-energy-promotion coefficient, r, is equal to 1 or changes in the ranges of 1–3, 3–3.57, and ≥3.57, the strain-energy evolution of diorite presents the characteristics of supercritical stability, nonlinear stability, period-doubling stability, and chaos, respectively. Meanwhile, the greater the rate of the unloading confining pressure, the earlier the period-doubling bifurcation and chaotic mechanical behavior will occur. After loading and unloading peak strength, the sudden decrease of high-density AE counts and AE energy or the sudden transition of the strain-energy-promotion coefficient from >0 to <0 can be used as an important criterion for the complete failure of rock samples.     

Acoustic Emission Behavior of Early Age Concrete Monitored by Embedded Sensors

Acoustic emission (AE) is capable of monitoring the cracking activities inside materials. In this study, embedded sensors were employed to monitor the AE behavior of early age concrete. Type 1–3 cement-based piezoelectric composites, which had lower mechanical quality factor and acoustic impedance, were fabricated and used to make sensors. Sensors made of the composites illustrated broadband frequency response. In a laboratory, the cracking of early age concrete was monitored to recognize different hydration stages. The sensors were also embedded in a mass concrete foundation to localize the temperature gradient cracks.     

Research on the Evolution of Shield Segment Cracks Based on Acoustic Emission and CMOD

In order to explore the cracking law and failure characteristics of segments, a model test of shield segment cracking was conducted. The microscopic and macroscopic crack evolution process of the segment is studied by using acoustic emission detection technology and crack opening displacement (CMOD). According to the acoustic emission signal and CMOD, characteristics generated in the process of segment cracking, in the form of numerical value, the evolution characteristics of each stage of segment cracking are directly reflected. Based on acoustic emission energy and CMOD, the segment cracking damage model was established to determine the segment fracture damage degree. The result shows that segment cracking can be divided into three stages, and the acoustic emission detection results and CMOD have different degrees of change in each cracking stage. This proves that both the acoustic emission acquisition results and CMOD can be used as evaluation indicators of damage degree. Acoustic emission can accurately identify the crack evolution process, and the yield strengthening is an important stage of crack damage evolution. The damage data points in this stage account for 76.83% of all the damage data points, the occurrence rate of damage data points is 0.225 s, and the density of data points in the damaged area is 3.219 × 10⁻⁴ mm³, which is larger than the other two stages. The segment cracking damage model can effectively reflect the segment cracking degree and provide a reference for the actual segment cracking assessment.     

Compressive Fatigue Behaviour of High-Strength Concrete and Mortar: Experimental Investigations and Computational Modelling

A high-strength concrete and mortar subjected to compressive fatigue loading were comparatively investigated using experimental and computational techniques. The focus of the investigations was on the influence of the coarse aggregate in high-strength concrete. Accordingly, the fatigue behaviour was analysed experimentally using the macroscopic damage indicators strain, stiffness and acoustic emission hits. The results clearly show differences in the fatigue behaviour between the concrete and the mortar, especially at the lower stress level investigated. The basalt coarse aggregate here improves the fatigue behaviour of the concrete. Indication of a negative effect can be seen at the higher stress level. A finite element approach with a gradient-enhanced equivalent strain-based damage model combined with a fatigue model was used for the computational simulation of the fatigue behaviour. The damage model includes a differentiation between tension and compression. The fatigue model follows the assumption of the reduction in the material strength based on the accumulated gradient-enhanced equivalent strains. A random distribution of spherically shaped basalt aggregates following a given particle size distribution curve is used for the simulation of concrete. The comparison of the experimentally and computationally determined strain developments of the concrete and mortar shows very good agreement.     

Evaluation Approach of Fracture Behavior for Asphalt Concrete with Different Aggregate Gradations and Testing Temperatures Using Acoustic Emission Monitoring

Different aggregate gradations of asphalt concrete possess dissimilar skeleton structures, leading to diverse macroscopic and mechanical characteristics. Acoustic emission (AE) technology can realize real-time monitoring of the whole damage evolution process of materials. The objective of the present investigation was to demonstrate the fracture characteristics of asphalt concrete with three types of aggregate gradations, including dense-graded asphalt concrete (AC), stone mastic asphalt (SMA), and open-graded friction course (OGFC) under indirect tensile load on account of the acoustic emission (AE) technique. The Marshall compaction method was used to prepare specimens, and the indirect tensile test (IDT) and AE monitoring were conducted simultaneously at different temperatures. The corresponding AE parameters containing energy, cumulative energy, count, and cumulative count were adopted to characterize the fracture process of asphalt concrete with different aggregate gradations. The impact of temperature on the damage characteristics of asphalt concrete was also assessed. Test results indicated that the AE parameters could effectively classify the damage stages of asphalt concrete, and specimens with different aggregate gradations exhibited different AE characteristics during failure processes. The combination of AE parameters and cumulative AE parameters can accurately characterize the damage characteristics of asphalt concrete. SMA specimens possessed the best overall performance among these three types of asphalt concrete in terms of the variations in energy and cumulative energy at different temperatures. The findings obtained in this study can provide a practical AE-based evaluation approach for demonstrating the fracture mechanism of asphalt concrete with different aggregate gradations.     

Evaluation of Heat-Induced Damage in Concrete Using Machine Learning of Ultrasonic Pulse Waves

This study investigated the applicability of using ultrasonic wave signals in detecting early fire damage in concrete. This study analyzed the reliability of using the linear (wave velocity) and nonlinear (coherence) parameters from ultrasonic pulse measurements and the applicability of machine learning in assessing the thermal damage of concrete cylinders. While machine learning has been used in some damage detections for concrete, its feasibility has not been fully investigated in classifying thermal damage. Data was collected from laboratory experiments using concrete specimens with three different water-to-binder ratios (0.54, 0.46, and 0.35). The specimens were subjected to different target temperatures (100 °C, 200 °C, 300 °C, 400 °C, and 600 °C) and another set of cylinders was subjected to room temperature (20 °C) to represent the normal temperature condition. It was observed that P-wave velocities increased by 0.1% to 10.44% when the concretes were heated to 100 °C, and then decreased continuously until 600 °C by 48.46% to 65.80%. Conversely, coherence showed a significant decrease after exposure to 100 °C but had fluctuating values in the range of 0.110 to 0.223 thereafter. In terms of classifying the thermal damage of concrete, machine learning yielded an accuracy of 76.0% while the use of P-wave velocity and coherence yielded accuracies of 30.26% and 32.31%, respectively.     

Deep Learning Approach for Damage Classification Based on Acoustic Emission Data in Composite Materials

Damage detection and the classification of carbon fiber-reinforced composites using non-destructive testing (NDT) techniques are of great importance. This paper applies an acoustic emission (AE) technique to obtain AE data from three tensile damage tests determining fiber breakage, matrix cracking, and delamination. This article proposes a deep learning approach that combines a state-of-the-art deep learning technique for time series classification: the InceptionTime model with acoustic emission data for damage classification in composite materials. Raw AE time series and frequency-domain sequence data are used as the input for the InceptionTime network, and both obtain very high classification performances, achieving high accuracy scores of about 99%. The InceptionTime network produces better training, validation, and test accuracy with the raw AE time series data than it does with the frequency-domain sequence data. Simultaneously, the InceptionTime model network shows its potential in dealing with data imbalances.     

Experimental Study on Basalt Fiber Crack Resistance of Asphalt Concrete Based on Acoustic Emission

In this study, the semicircle three-point bending tests of ordinary asphalt concrete and basalt fiber asphalt concrete were carried out and acoustic emission parameters were collected during the test. The differences of the characteristics of acoustic emission parameters between basalt fiber asphalt concrete and ordinary asphalt concrete were analyzed, and the damage stages were divided based on the variation of acoustic emission parameters; Rise Angle and Average Frequency were introduced to study the cracking mode and crack resistance mechanism of asphalt concrete with basalt fiber. The results show that the acoustic emission parameters can well represent the toughening and crack resistance effect of basalt fiber in asphalt concrete, and the damage stages can be divided into three stages: microcrack initiation stage, fracture stage, and residual stage. The duration of the fracture stage and the load resistance time of the specimen were greatly prolonged. The proportion of shear events in the whole failure process increased greatly after the basalt fibers were added, especially in the fracture stage, which reduced the tensile failure tendency of the specimens, and thus improved the bending and tensile performance of the specimens and played a toughening and crack resistance role in the fracture stage.     

Investigation of Changes in Fatigue Damage Caused by Mean Load under Block Loading Conditions

The literature in the area of material fatigue indicates that the fatigue properties may change with the number of cycles. Researchers recommend taking this into account in fatigue life calculation algorithms. The results of simulation research presented in this paper relate to an algorithm for estimating the fatigue life of specimens subjected to block loading with a nonzero mean value. The problem of block loads using a novel calculation model is presented in this paper. The model takes into account the change in stress–strain curve parameters caused by mean strain. Simulation tests were performed for generated triangular waveforms of strains, where load blocks with changed mean strain values were applied. During the analysis, the degree of fatigue damage was compared. The results of calculations obtained for standard values of stress–strain parameters (for symmetric loads) and those determined, taking into account changes in the curve parameters, are compared and presented in this paper. It is shown that by neglecting the effect of the mean strain value on the K′ and n′ parameters and by considering only the parameters of the cyclic deformation curve for ε_m = 0 (symmetric loads), the ratio of the total degree of fatigue damage varies from 10% for ε_a = 0.2% to 3.5% for ε_a = 0.6%. The largest differences in the calculation for ratios of the partial degrees of fatigue damage were observed in relation to the reference case for the sequence of block n₃, where ε_m = 0.4%. The simulation results show that higher mean strains change the properties of the material, and in such cases, it is necessary to take into account the influence of the mean value on the material response under block loads.     

Study on Delamination Damage of CFRP Laminates Based on Acoustic Emission and Micro Visualization

This study investigated the mechanism of delamination damage in the double cantilever beam (DCB) standard test by the use of the strain energy release rate. The curve of the strain energy release rate was verified by the Rise Angle (RA) method. For this purpose, 24-layer carbon fiber/epoxy multidirectional laminates with interface orientations of 0°, 30°, 45°, and 60° were fabricated according to the standard ASTM D5528(13). In the course of this test, acoustic emission (AE) was used for real-time monitoring, and combined with micro visualization, the damage mechanism of composite multidirectional laminates was studied at multiple scales. Combining the AE detection results with micro visualization, it is found that the AE parameters and the damage to multidirectional laminates could realize a one-to-one correspondence. Through the study of the variation of the RA value, load, and strain energy release rate with the crack length, it is proved that the AE parameters can effectively characterize the initiation of delamination damage.     

Ultrasonic Vibration-Assisted Ball Burnishing Tool for a Lathe Characterized by Acoustic Emission and Vibratory Measurements

This paper focuses on a resonant system used to induce a low-amplitude movement and ultrasonic frequency to complement a ball burnishing process on a lathe. The system was characterized through the combination of different techniques. A full vibratory characterization of this process was undertaken with the purpose of demonstrating that the mechanical system—composed of the tool and the machine—does not present resonance phenomena during the execution of the operation that could lead to eventual failure. This dynamic analysis validates the adequateness of the tool when attached to an NC lathe, which is important to guarantee its future implementation in actual manufacturing contexts. A further aim was to confirm that the system succeeds in transmitting an oscillating signal throughout the material lattice. To this end, different static and dynamic techniques that measure different vibration ranges—including impact tests, acoustic emission measurement, and vibration measurement—were combined. An operational deflection shape model was also constructed. Results demonstrate that the only high frequency appearing in the process originated in the tool. The process was not affected by the presence of vibration assistance, nor by the burnishing preload or feed levels. Furthermore, the frequency of the assisting ultrasonic vibration was characterized and no signal due to possible damage in the material of the specimens was detected. These results demonstrate the suitability of the new tool in the vibration-assisted ball burnishing process.     

A Damage Model of Concrete including Hysteretic Effect under Cyclic Loading

A novel damage model for concrete has been developed, which can reflect the complex hysteresis phenomena of concrete under cyclic loading, as well as other nonlinear behaviors such as stress softening, stiffness degradation, and irreversible deformation. The model cleverly transforms the complex multiaxial stress state into a uniaxial state by equivalent strain, with few computational parameters and simple mathematical expression. The uniaxial tensile and compressive stress–strain curves matching the actual characteristics are used to accommodate the high asymmetry of concrete in tension and compression, respectively. Meanwhile, an unloading path and a reloading path that can reflect the hysteresis effect under cyclic loading of concrete are established, in which the adopted expressions for the loading and unloading characteristic points do not depend on the shape of the curve. The proposed model has a concise form that can be easily implemented and also shows strong generality and flexibility. Finally, the reliability and correctness of the model are verified by comparing the numerical results with the three-point bending beam test, cyclic loading test, and a seismic damage simulation of the Koyna gravity dam.     

Mode II Behavior of High-Strength Concrete under Monotonic,Cyclic and Fatigue Loading

An economically efficient yet safe design of concrete structures under high-cycle fatigue loading is a rather complex task. One of the main reasons is the insufficient understanding of the fatigue damage phenomenology of concrete. A promising hypothesis states that the evolution of fatigue damage in concrete at subcritical load levels is governed by a cumulative measure of shear sliding. To evaluate this hypothesis, an experimental program was developed which systematically investigates the fatigue behavior of high-strength concrete under mode II loading using newly adapted punch through shear tests (PTST). This paper presents the results of monotonic, cyclic, and fatigue shear tests and discusses the effect of shear-compression-interaction and load level with regard to displacement and damage evolution, fracture behavior, and fatigue life. Both, monotonic shear strength and fatigue life under mode II loading strongly depend on the concurrent confinement (compressive) stress in the ligament. However, it appears that the fatigue life is more sensitive to a variation of shear stress range than to a variation of compressive stress in the ligament.     

Thermographic Study of AZ31B Magnesium Alloy under Cyclic Loading: Temperature Evolution Analysis and Fatigue Limit Estimation

In this paper, infrared thermography was employed to study the fatigue process of AZ31B magnesium alloy. In order to eliminate the interference caused by the temperature rise of the fixture, a data processing method was proposed, which is based on a special model to describe the temperature change of the specimen. Based on the temperature data after processing, the temperature evolution indicates that AZ31B magnesium alloy has undergone cyclic hardening during fatigue. Three different temperature indicators were selected to evaluate the fatigue limit based on the evolution curve after processing. In addition, the experimental results showed that the temperature data processed by the proposed method can be used to estimate the fatigue limit of AZ31B magnesium alloy. Experiments were performed for both extrusion and transverse directions in consideration of the anisotropy of the AZ31B.     

Damage Analysis of Composite CFRP Tubes Using Acoustic Emission Monitoring and Pattern Recognition Approach

The acoustic emission method has been adopted for detection of damage mechanisms in carbon-fiber-reinforced polymer composite tubes during the three-point bending test. The damage evolution process of the individual samples has been monitored using the acoustic emission method, which is one of the non-destructive methods. The obtained data were then subjected to a two-step technique, which combines the unsupervised pattern recognition approach utilizing the short-time frequency spectra with the boundary curve enabling the already clustered data to be additionally filtered. The boundary curve identification has been carried out on the basis of preliminary tensile tests of the carbon fiber sheafs, where, by overlapping the force versus time dependency by the acoustic emission activity versus time dependency, it was possible to identify the boundary which will separate the signals originating from the fiber break from unwanted secondary sources. The application of the presented two-step method resulted in the identification of the failure mechanisms such as matrix cracking, fiber break, decohesion, and debonding. Besides the comparison of the results with already published research papers, the study presents the comprehensive parametric acoustic emission signal analysis of the individual clusters.     

Tensile Damage Study of Wind Turbine Tower Material Q345 Based on an Acoustic Emission Method

As essential load-bearing equipment to support the nacelle and blades, the tower is subjected to the whole wind turbine loading. This study proposes a new method of combining acoustic emission and normalized accumulation parameters to characterize wind turbine towers Q345 steel damage. First of all, tendency analysis of the acoustic emission signal parameter was conducted to determine damage degree during the damage stage. Secondly, we normalized the accumulation of amplitude and other parameters to compare the proportion of each accumulation parameter at different stages, while studying the spectra of common acoustic emission signals. Finally, comparing the differences and similarities of the normalized accumulation parameters between three different rates, we analyze the effect of rate on the normalized accumulation parameters. These results indicate that the normalized cumulative duration parameter is suitable for characterizing the yield damage occurrence, the normalized cumulative energy parameter is very sensitive to the fracture stage, the normalized cumulative energy parameter is least influenced by the loading rate, and the energy parameter is a sensitivity factor for normalized expression, which to realizes the stage of damage judgment.     

Fatigue-Induced Damage in High-Strength Concrete Microstructure

A high-strength concrete subjected to compressive fatigue loading with two maximum stress levels was investigated and the behaviour was evaluated using the macroscopic damage indicators, strain and acoustic emission hits (AE-hits), combined with microstructural analyses utilising light microscopy and scanning electron microscopy (SEM). A clustering technique using Gaussian mixture modelling combined with a posterior probability of 0.80 was firstly applied to the AE-hits caused by compressive fatigue loading, leading to two clusters depending on the maximum stress level. Only a few cracks were visible in the microstructure using light microscopy and SEM, even in phase III of the strain development, which is shortly before failure. However, bluish impregnated areas in the mortar matrix of higher porosity or defects, changing due to the fatigue loading, were analysed. Indications were found that the fatigue damage process is continuously ongoing on a micro- or sub-microscale throughout the mortar matrix, which is difficult to observe on a mesoscale by imaging. Furthermore, the results indicate that two different damage mechanisms take place, which are pronounced depending on the maximum stress level. This might be due to diffuse and widespread compressive damage and localised tensile damage, as the findings documented in the literature suggest.