AE Events Research Articles

Combined Acoustic Emission and Ultrasonic Measurements in Cross Laminated Timber-Steel Composite Beam

Combined acoustic emission and ultrasonic measurements were carried out before and during a four-point bending test on a composite floor beam made of cross-laminated timber with length of 11.6 m, width of 1.5 m, and thickness of 0.28 m. One key aspect of the composite design is the formation of the shear-resistant connection between the panel and steel girder. For the measurements a 16-channel system with combined acoustic emission and transmission measurements was used. The signals were digitized with a very high frequency of 10 MHz with amplitude resolution of 16 bits. To monitor acoustic emission activity a network of 16 AE sensors were attached to the surface of the specimen in the central part of the specimen where the highest deflection and load were expected. For this purpose, broadband AE sensors with measurement frequency of up to 200 kHz were used. The ultimate failure of the specimen occurred at a maximum test load of approximately 200 kN. The deflection at this force was about 200 mm. During the tests which lasted about 50 minutes approximately 8,500 AE events were detected. More than 2,600 events were recorded by at least six channels which makes them well suited for three-dimensional localization. Because of the inhomogeneous cross-laminated timber conventional localization using an unique velocity model are not possible. Therefore, a velocity model was experimentally determined by measure the velocity of the elastic waves in all directions. The greatest value was measured in the longitudinal direction along the beam axis. The average value is approximately 4000 m/s. While in the transverse direction the velocity drops down to approximately 2800 m/s. The lowest value was measured in thickness direction of about 1800 m/s.

Experimental study on the effects of complex discrete joints on the mechanical behavior of rock‐like material

AbstractThe strength and failure of rock masses are ambiguous due to their complex structure. Investigating the strength and failure of jointed rock mass remains a persistent concern in mining engineering. This study aims to investigate the effects of joint dip angles of complex discrete joints on the mechanical properties and fracture evolution of rock‐like material. Rock‐like specimens with complex joints were prepared using 3D printing technology and then tested under uniaxial compression loading. The experimental results reveal the following findings: (1) The anisotropy of rock‐like material with complex joints is lower compared to the rock‐like material with simple joints. (2) The ratio of long‐term strength to uniaxial compressive strength of rock‐like material remains consistent despite changes in the dip angle of the joints. (3) Differing from intact rocks, AE events of the rock‐like material with complex joints are obvious in the initial loading stage and elastic deformation stage. (4) When the dip angle of the joint sets is 0 and 90°, fractures progressively propagate, and the failure mode of the rock‐like material demonstrates tensile failure along the pre‐existing joints. Conversely, when the dip angle of the joint sets is 45° and 135°, fractures are simultaneously initiated at various locations within the rock‐like specimen, resulting in a failure mode of rotational failure by the newly generated block.

Squamous metaplasia is an indicator of acute exacerbation in patients with usual interstitial pneumonia / idiopathic pulmonary fibrosis

BackgroundAcute exacerbation (AE) is a potentially lethal event in patients with usual interstitial pneumonia/idiopathic pulmonary fibrosis (UIP/IPF). However, to date, no pathological predictors of AE have been identified. This retrospective study aimed to elucidate the pathological features that could predict AE in patients with UIP. MethodsWe reviewed the pathological findings of 91 patients with UIP/IPF and correlated these findings with AE events. Thirteen histological variables related to acute lung injury were evaluated by three independent observers and classified as positive or negative. The patients’ clinical data during follow-up were collected and reviewed for AE. A recursive partition using the Gini index for the prediction of AE was performed, with each pathological finding as a candidate for branching. ResultsTwenty patients (22%) developed AE during the median follow-up duration of 40 months. Thirty-eight patients died (15 due to AE and 23 for other reasons). The median time interval from surgical lung biopsy to AE onset was 497 (interquartile range: 901–1657) days. Histologically, squamous metaplasia was positively associated with AE (odds ratio: 4.7, P = 0.015) and worse event-free survival in patients with UIP (P = 0.04). Leaf scoring based on the Gini index for recursive partition, including five positive findings (squamous metaplasia, neutrophilic infiltration, septal widening, Kuhn's hyaline, and fibrin), showed a sensitivity of 90% with a specificity of 74.7% (area under curve: 0.89). ConclusionsWe found that squamous metaplasia is an important histopathological finding that predicts AE events and tends to unfavorable outcome in patients with UIP/IPF.

Influences of clean fracturing fluid viscosity and horizontal in-situ stress difference on hydraulic fracture propagation and morphology in coal seam

The viscosity of fracturing fluid and in-situ stress difference are the two important factors that affect the hydraulic fracturing pressure and propagation morphology. In this study, raw coal was used to prepare coal samples for experiments, and clean fracturing fluid samples were prepared using CTAB surfactant. A series of hydraulic fracturing tests were conducted with an in-house developed triaxial hydraulic fracturing simulator and the fracturing process was monitored with an acoustic emission instrument to analyze the influences of fracturing fluid viscosity and horizontal in-situ stress difference on coal fracture propagation. The results show that the number of branched fractures decreased, the fracture pattern became simpler, the fractures width increased obviously, and the distribution of AE event points was concentrated with the increase of the fracturing fluid viscosity or the horizontal in-situ stress difference. The acoustic emission energy decreases with the increase of fracturing fluid viscosity and increases with the increase of horizontal in situ stress difference. The low viscosity clean fracturing fluid has strong elasticity and is easy to be compressed into the tip of fractures, resulting in complex fractures. The high viscosity clean fracturing fluids are the opposite. Our experimental results provide a reference and scientific basis for the design and optimization of field hydraulic fracturing parameters.

Experimental study on I/II/III mixed mode fracture characteristics of a combined rock mass under creep loading

I/II/III mixed mode fractures of intersecting joint fissures often occur in natural rock masses, and jointed rock masses are prone to rockbursts in deep underground engineering when subjected to long-term crustal stresses. However, most studies of the mechanical mechanisms of these intersected joints have been conducted by simplifying two-dimensional joint model tests. Furthermore, the fracture mechanisms of two-dimensional intersected joints under tension and compression are completely different from those of three-dimensional joints. This paper presents a novel prefabricated specimen with combinations of intersecting joints capable of detecting the failure behaviours of rock I/II/III mixed mode fractures under creep loading. Uniaxial compression and multistage creep tests are performed on prefabricated sandstone specimens with intersecting joints of 0°/0°, 0°/30°, 0°/60°, and 0°/90°. The experimental results show that with the increase in the number of prefabricated intersecting joints, the uniaxial compressive strength and elastic modulus values of the sandstone specimens gradually decrease. In addition, the sandstone specimens experience relatively few AE events and minor axial strain variations in the first creep stage and the second creep stage of the multistage creep test. The axial strain increases sharply due to the sharp increase in the number of AE events in the third creep stage. The 0°/60° sandstone specimen undergoes accelerated creep failure, resulting in mixed X-shaped tensile‒shear rupture. The RA value is high based on the quantification of the creeping cracks using the acoustic emission parameters of the rise angle (RA) and average frequency (AF). The AF values of the 0°/0°, 0°/30°, and 0°/90° sandstone specimens are high. The experimental results show that a larger joint intersection angle leads to greater mutual restraints and greater effects of prefabricated crack propagation in the rock specimens, thus increasing the final failure strength. Finally, based on the acoustic emission count, a characteristic variable D suitable for characterizing the creep damage evolution of a joint rock mass is established. The findings of this paper can facilitate an effective understanding of the creep effect of I/II/III mixed mode fracture and its micromechanism. The research results will have a certain reference value for the detection and risk mitigation of instantaneous and time-delayed rockbursts.

Experimental Study on the Butterfly Shape of the Plastic Zone around a Hole near Rock Failure

The precursor of rock failure around a hole has always been one of the research hotspots in the field of rock mechanics, and the distribution of the plastic zone is often adopted to reflect the location and form of rock failure. The shape of the plastic zone around a hole before rock failure can guide the mechanism of and early warning methods for geotechnical engineering disasters, while previous theoretical research and numerical simulation results show that the shape of the plastic zone around the hole is butterfly shaped under specific stress, which is referred to as butterfly failure theory. Studies also indicate that the butterfly shape of the plastic zone around a hole is considered to be the main cause of many disasters, which signifies the importance of studying the morphology of the plastic zone near rock failure. Therefore, this study is committed to finding the specific shape of the plastic zone near rock failure through relatively accurate and a high number of AE event location results, and the final experimental results show that the plastic zone around the hole is basically a butterfly shape near rock failure. This study verifies the correctness of the butterfly failure theory and provides an important reference for the study of geotechnical engineering disaster mechanisms and monitoring methods. The fact that the plastic zone in the early stage of rock failure in this study tends to be butterfly shaped preliminarily indicates the fractal law of rock failure. In the moment before rock failure, the distribution of AE events is more regular, which leads to large-scale collapse type failure.

Investigation on acoustic emission characteristics of fault stick-slip under different lateral pressures

To explore the effect of different stress environments on fault-slip rockbursts. Bidirectional shear friction experiments with different lateral pressures were conducted on precracked syenogranites buried at 800 m. The macroscopic statistical parameters (cumulative number of AE events, magnitude and b value) and local characteristic parameters (amplitude and dominant frequency) of acoustic emission during the stick-slip process under different lateral pressures were investigated. In addition, based on fractal theory, the nonlinear characteristics of AE spectrum were analyzed. On this basis, the microscopic mechanism of fault stick-slip was discussed. The results show that the lateral pressure influences the friction strength of the fault and stick-slip motion characteristics. With increasing lateral pressure, the proportion of transgranular shear fractures increases, which leads to an increase of cumulative number of AE events and magnitude. The periodic decrease in the b value is more significant at high lateral pressure. There is a good correlation between a high-magnitude AE event and a stress drop. The AE frequency with phased response characteristics can be used to effectively identify the evolution of fault stick-slip instability at the laboratory scale. A sharp increase in the amplitude of the dominant frequency can be regarded as one of the precursory features of fault stick-slip instability. The AE frequency spectra have multifractal characteristics, that differ among the different stages. The maximum multifractal dimension and spectral width can reflect the difference in energy released during fault stick-slip motion.

Longitudinal evaluation of adverse events due to steroid use in primary immune thrombocytopenia: A population-based study.

This study aimed to investigate the association between the steroid use patterns and the risk of AEs in patients with primary immune thrombocytopenia (ITP). A total of 2691 newly diagnosed adults with ITP between 2011 and 2018 were identified from the National Health Insurance Research Database in Taiwan, and the date of first steroid use was defined as the index date. Post-index steroid use was calculated on a 90-day basis as a time-dependent variable and categorized by the average prednisolone-equivalent daily dose (<10 mg vs. ≥10 mg) and intensity (medication possession ratio <80% vs. ≥80%). Patients were followed up for 1 year from the index date for acute AE events, while chronic AEs were assessed until death, or end of 2019. Compared to patients with low-dose+low-intensity steroid use, those with high-dose+high-intensity steroid use were associated with a higher risk of acute AE (adjusted incident rate ratio [aIRR]: 1.57, 95% confidence interval [CI]: 1.38-1.78, p < 0.01) and chronic AE (aIRR: 1.26, 95% CI: 1.08-1.47, p < 0.01). Metabolic/endocrine and ophthalmologic disorders demonstrated the strongest correlation with a high dose and intensity. The joint effect of steroid dose and intensity was observed in patients with ITP, and the findings suggest that steroids should be used carefully.

Long-term acoustic emission monitoring of a new alkali-activated material for sealing structures in nuclear waste repositories

The crucial part of nuclear waste storage is the construction of sealing structures made of reliable, well-understood, and safe materials. Within the SealWasteSafe project, we compared the performance of an innovative alkali-activated material (AAM) and standard salt concrete (SC), as potential materials for sealing structures for nuclear waste repositories. Two 340-liter-cubic specimens were studied for up to ∼250 days by a multisensory monitoring setup. Specifically, the long-term acoustic emission monitoring aimed to analyze the development of microstructural changes within materials. The monitoring analysis showed fewer acoustic emission events in AAM compared to SC in the first 61 days. After approximately two months of monitoring, the number of AE events in AAM significantly exceeded the number of events in SC. The analysis showed, however, that the increased AE activity was mainly caused by the surface effects of the AAM material and not by the formation of cracks within the material. This contribution presents the use of acoustic emission analysis, both in the time and frequency domains, for monitoring and characterization of materials with potential use as engineering barriers for nuclear waste repositories.

Wavelet entropy-based damage characterization and material phase differentiation in concrete using acoustic emissions

Damage evolution in concrete is a complex phenomenon involving micro-cracks that originate from different material phases (mortar matrix, ITZ and aggregate), and culminate into a major crack. The information about the individual damaged material phases can provide major insights into the global failure behaviour, cracking path, and fracture processes. The evolution of damage, damage mechanisms and the material phase of origin of micro-cracks is highly influenced by the type of concrete (Normal and high strength). In order to investigate the evolution of damage in concrete of varying strength, notched beam specimens with different water-to-cement ratios have been prepared and tested under CMOD control. The evolution and growth of damage have been continuously monitored during testing through acoustic emission techniques. Based on acoustic emission results, it has been demonstrated that the damage progression in concrete can be better described as a function of the wavelet entropy of the AE events occurring during the entire loading duration, and therefore, an analytical model for the damage evolution has been proposed. The study reveals that, under the high-strength category, the level of deterioration reduces as the w/c ratio rises, while in normal-strength concrete, a reverse trend is observed. Furthermore, a novel approach has been proposed that utilizes wavelet entropy and amplitude to differentiate amongst various damaged material phases with the help of an unsupervised clustering algorithm. Normal-strength concrete with a w/c ratio of 0.4 has been used to demonstrate the proposed methodology and has been validated with the help of experiments performed on the mortar and the parent stone of aggregate. A higher occurrence of mode-I cracks compared to mode-II cracks across all material phases has been observed for the specimen. Additionally, ITZ and aggregate exhibit a greater proportion of tensile and shear cracking events than the matrix phase. In the end, a deep learning convolutional neural network (CNN) model is devised utilizing labelled scalogram images of the AE waveforms to discriminate between the various damaged material phases. A 10-fold training cross-validation of the dataset has been carried out to achieve a stable performance. The deep learning model performs well in discriminating the damaged material phases with high training, validation, and testing accuracy.

Failure process and characteristics of deep concrete-supported arch tunnel under true-triaxial stress

In deep underground rock engineering, tunnel structures after concrete support are often subjected to failure because of stress variation. In this study, to understand the failure process and characteristics of deep concrete-supported tunnel, true-triaxial compression tests were conducted on concrete-supported and unsupported tunnel model specimens using a true-triaxial test system. An AE system and a microcamera were used to capture the AE events and failure evolution process of the hole sidewalls in rock specimens, respectively. The test results show that after concrete support, the failure characteristics of the hole sidewalls are converted from dynamic rockburst failure to static spalling failure, and the initial vertical failure stress of the hole sidewalls is significantly increased. The failure of the hole sidewalls in the concrete-supported tunnel model specimen experiences the calm stage, the cracking and buckling of concrete-support layer stage, the spalling of concrete-support layer stage, and the spalling of concrete-support layer and rock layer stage. The cumulative AE counts of the concrete-supported tunnel model specimen are less than those of the unsupported one at any same time during loading. Comparing the failure processes and characteristics of hole sidewalls in the two types of specimens, the curbing effect of concrete support on the failure of tunnel is confirmed. The results of this study contribute to the understanding of the failure behavior of deep rock tunnels with concrete support.

Natural joint effect on mechanical characteristics and fracture evolution of In-Site rocks under uniaxial compression

Understanding the strength features and failure characteristics of in-site rocks with various natural joints is an important fundament for studying the stability of underground engineering. The rock specimens with different joint angles, joint lengths, and joint distribution types were prepared and tested in uniaxial compressive test UCT combined with the acoustic emission AE detection. The natural joint categories of rock specimens were divided into single-joint type, cross-joint type, and mixed-joint type according to the number of joints and the gradual deterioration degree of integrity. The research results showed that the lower integrity could degrade the mechanical properties. The AE parameters as well as three types of energies all exhibited a firstly increasing and then decreasing trend with the increase of dominant joint angle αA. Rock specimens with lower intactness generated AE events with higher intensity and energy, resulting in a higher proportion of energy loss and lower severity of damage. The integrity of the natural jointed rock was verified by fractal dimension. The activity of micro-cracks within natural jointed rocks was characterized by both fractal dimension and AE b-value, and found that their trends were synchronized. A classification criterion that combined clustering techniques, AE parameter features, and genetic algorithms for tensile and shear cracks generated during the rock failure process was proposed. It’s demonstrated that the ratio of shear cracks to the total cracks showed an initial increase followed by a decrease with the increase of αA. Hoek-Brown's empirical formula was improved and the rating scores related to the uniaxial compressive strength Rucs-scores were continuouszed for natural jointed rock.

An Economic Model to Estimate Costs of Cytokine Release Syndrome and Neurological Events Among Patients Treated with CAR T Cell Therapies for Relapsed or Refractory Follicular Lymphoma

Background: CAR T cell therapies have exhibited promising clinical responses in populations that are traditionally challenging to treat. Axicabtagene ciloleucel (axi-cel) and tisagenlecleucel (tisa-cel) are 2 CAR T cell therapies now available in the United States (US) for the treatment of R/R follicular lymphoma (FL) after 2 or more lines of systemic therapy. Lisocabtagene maraleucel (liso-cel) is currently being investigated in clinical trials for the treatment of patients with R/R second-line high-risk and third line or later FL. As observed in other approved indications, the improved response and survival rates from the introduction of CAR T cell therapies are typically accompanied by the potential for AEs, such as cytokine release syndrome (CRS) and neurological events (NEs). However, the rates and severity of these AEs tend to vary across CAR T cell therapies. Understanding the economic implications of these 3 CAR T cell therapies and their associated AEs is crucial for informed decision-making and to underscore key economic and safety differences across the 3 CAR T cell therapies. The primary objective of this analysis was to estimate the average per-patient weighted costs of CRS and NEs for a patient with R/R FL treated with a CAR T cell therapy (liso-cel, axi-cel, or tisa-cel) based on clinical trial rates. Secondary objectives included estimates for (1) the opportunity cost of treating with axi-cel or tisa-cel versus liso-cel and (2) the additional number of patients eligible for treatment with liso-cel based on a hypothetical sample of 100 treated patients. Methods: An economic model was developed using data from the TRANSCEND FL (NCT04245839; n = 130), ZUMA-5 (NCT03105336; n = 124), and ELARA (NCT03568461; n = 97) studies. CRS and NE rates reported in the FL studies were incorporated in a decision analytic economic model to estimate costs from a health care system perspective in 2023 US dollars. Microcosting analysis data from the TRANSCEND FL study were used as CRS and NE cost inputs; the model assumed that CRS and NE management costs across CAR T cell therapies would not differ. The analysis used a Monte Carlo simulation approach to address uncertainty surrounding the clinical and economic data. The model output was a weighted average that represented cost for the average treated patient, including those with and without CRS and/or NEs. The average per-patient total cost difference between CAR T cell therapies was used to estimate the number of additional patients who could be treated with liso-cel versus axi-cel or tisa-cel. Results: Rates of CRS grade 1‒2/≥ 3 were 56.9/0.8% for liso-cel, 71.8%/6.5% for axi-cel, and 48.5%/0.0% for tisa-cel. Rates of NE grade 1‒2/≥ 3 were 13.1%/2.3% for liso-cel, 41.1%/ 15.3% for axi-cel, and 34.0%/ 3.1% for tisa-cel. The overall per-patient weighted average costs for CRS and NEs were estimated to be $20,626 for liso-cel, $44,096 for axi-cel, and $26,460 for tisa-cel (Figure). The per-patient weighted average cost per CRS event was $14,631 for liso-cel, $21,299 for axi-cel, and $12,456 for tisa-cel. The per-patient weighted average cost per NE was $5994, $22,797, and $14,004 for liso-cel, axi-cel, and tisa-cel, respectively. Total per-patient cost differences for liso-cel compared with axi-cel and tisa-cel were −$23,470 and −$5835, respectively. The resulting opportunity costs from treating 100 patients with liso-cel instead of axi-cel and tisa-cel would allow for the treatment of an additional 5.2 and 1.3 patients, respectively. Conclusions: This economic analysis revealed lower overall per-patient weighted average costs associated with liso-cel compared with axi-cel and tisa-cel, primarily due to lower CRS and NE rates versus axi-cel and lower NE rates versus tisa-cel. In comparison with axi-cel and tisa-cel, liso-cel demonstrated robust economic and clinical value, with lower rates of CRS and/or NEs associated with substantial cost savings per AE event.

Investigation of fracture source mechanisms through full-field imaging and acoustic emission

The identification and understanding of fracture processes is a vital component in securing engineering structures. By collecting data through experiments or simulations, the identification of material parameters and the understanding of failure mechanisms can be investigated. In the context of in-laboratory experiments, this is done by provoking fracture process through destructive testing (TD) techniques, while collecting data by non-destructive testing (NDT) techniques and postmortem analysis. While NDT techniques are often used individually, increasing interest is given to collective investigations of source activities. Like this, limitations of one method can be compensated by the next. A multi-modal experimental setup is proposed for holistic understanding of source mechanisms. Combined time resolved AE, DIC and in-volume measurements with post-mortem fracture surface analysis are applied on PMMA tensile cracking experiments for the purpose of AE localization, crack tip detection and determination of crack front complexities and kinematics, respectively. Both high and lower amplitude AE signals were captured and respectively associated to localized dynamic instabilities and stable crack advance. Signals are analyzed individually using localization, waveform analysis, and AE descriptor classification. Crack front velocity heterogeneities are identified as valuable quantity to correlate with fracture induced AE responses. For dynamic instabilities, a linear relationship between AE absolute energy and crack propagation area was found. While identification and analyses of local and global dynamic instabilities has shown to be rather trivial, difficulties arose with respect to quasi-static related AE events.

Fatigue fracture quantification in brittle cementitious materials using acoustic emission testing and digital image correlation

Fatigue loading in brittle materials introduces damage at the micro-scale. These micro-fractures can accumulate and cause a significant reduction in material stiffness or even lead to structural failure. Deformation-based monitoring techniques can be inadequate when detecting damage at the micro-level. Hence, here is where advanced non-destructive testing (NDT) methods such as acoustic emission testing (AET) and digital image correlation (DIC) can provide a significant improvement in quantifying fatigue damage. The paper aims to combine AET and DIC for advanced fatigue damage analysis on brittle cementitious mortars. Cylindrical samples with three different sizes are subjected to fatigue loading in a Brazilian splitting test. The damage advancement quantified from cumulative AE event count and energy analysis showed a good correlation with the tensile deformation measurement of the DIC. It was observed that for low cycle loading, the slowly progressing damage evolution may not always occur, instead abrupt failure similar to monotonic loading can occur even after passing several load cycles. The size of the fracture process zone (FPZ) was measured using several AE-based and DIC-based methods. The comparison of the approaches indicated that the AE energy-based method is the most suitable to quantify the FPZ size during fatigue damage progress. From AE-based FPZ analysis of the experimental results, it was concluded that for low-cycle loadings, the lack of a well-developed FPZ can be a cause of the sudden brittle failure.

Pullout damage analysis of steel fibers under dynamic load based on numerical simulation

FRC is constantly subjected to dynamic loads during its service period, and dynamic pullout behavior of the fibers and the coupling effect between fibers can significantly affect the crack initiation and propagation of FRC. In this paper, based on the single fiber pullout behavior of experimental and numerical results, the failure process of the twin fibers dynamic pullout test was simulated by RFPA3D program with discontinuous and heterogeneous characteristics. The influence of fiber embedding length and fiber spacing on the interfacial bond behavior were analyzed in details. The internal damage degree, crack propagation, and interfacial shear stress of all test series were quantitatively characterized. The results indicate that the specimens with various fiber embedding lengths and fiber spacings present distinct damage evolution patterns. The decrease of the peak of AE events is followed by an increase, when the fiber embedding length is increased. In addition, the fiber spacing has a large influence on the distribution and transmission of interfacial shear stress. With the increase of fiber spacing, the peak of interfacial outside shear stress increases, while the peak value of interfacial inside shear stress gradually decreases. Meanwhile, the AE-accumulative energy presents a trend of decreasing first and then increasing.

Numerical simulation of hydraulic fracturing and associated seismicity in lab-scale coal samples: A new insight into the stress and aperture evolution

Hydraulic fracturing and its associated seismic behaviour are increasingly important in the resource sector. In this paper, hydraulic fracturing and induced seismicity by both the constant flow rate and constant pressure injection was successfully simulated in lab-scale coal samples based on the discrete element method (DEM). Using the self-developed code in particle flow code (PFC), numerical simulations of fluid injection, fluid transport and seismic response are achieved simultaneously. The code has been applied to simulate water injection into an intact coal sample as well as coal samples with one pre-existing fracture. Model results follow observations from field practices and laboratory experiments, which demonstrate the reliability of the self-developed code. Induced seismicity by two injection methods shows that hydraulic fractures on the side of the sample with a pre-existing fracture present different cracking propagation patterns compared to the other side with no pre-existing fracture. Meanwhile, more peaks of extension-to-compression ratios calculated from the moment tensors of AE events are observed, which can be used to imply differential stress as well as reveal the impact of the pre-existing fracture on the aperture evolution of hydraulic fractures.

Evaluation of residual strength in fiber reinforced composites subjected to impact using acoustic emission test

This paper investigates the relation between acoustic emission events and the corresponding behavior of the specimens after damage under tensile loading. Tensile strengths of carbon/glass fiber epoxy composites subjected to impact damage were evaluated experimentally. The variations of amplitude, energy, and dominant frequencies of the AE events were studied to investigate the post-impact behavior of specimens. Different layers (6, 12-ply) and fiber materials were assessed, and their effects were considered. Waveform patterns and frequency domain analyses were also utilized to evaluate the damage severity. This practical method can be used as an advanced and straightforward approach to estimate either the value of the damage or residual strength subsequent to impact in laminated composites. It was proved that the impact damage led to dropping residual strength, especially in carbon/epoxy specimens, since they are sensitive to impact.

Acoustic emission and DSC investigations of anomalous stress-stain curves and burst like shape recovery of Ni49Fe18Ga27Co6 shape memory single crystals

Simultaneous acoustic emission, AE, and DSC measurements under compression along [110]A direction were carried out on Ni49Fe18Ga27Co6 shape memory single crystals. The compression resulted in anomalous stress-strain loops with stress drops/jumps on it, reflecting a sudden formation/dissolution of a more stable (detwinned) martensite structural modification than the thermally induced (twinned) one. The final detwinned martensite, obtained below a certain temperature, was stable even after downloading: during heating it showed a burst-like recovery at about 35K higher transformation temperature than that of the thermally induced one, with an audible click. It was obtained that the number of acoustic events showed strong asymmetry: e.g. for stress induced transformations the number of hits for uploading was larger. Simultaneous measurements of stress-time curves and AE versus time revealed that in the nucleation/dissolution processes in both (up and down) directions, as an intermediate step, the twinned martensite was first formed and resulted in additional AE events. This was interpreted by the easy as well as difficult nucleation of the twinned and detwinned martensites, respectively. The stress drops on the uploading stress-strain curve were attributed to subsequent nucleation of the detwinned martensite from the twinned modification and were followed by a sharp decrease of the AE activity. This indicated that the formation of detwinned martensite is a sudden, fast process and can take place without significant elastic energy storage. This was also in agreement with the fact that during burst like thermal recovery the width of the transition was very small (Af≅ As). Similarly, during downloading the stress jumps were interpreted as sudden dissolutions of the detwinned phase by retwinning and at all stress jumps there were local maxima on the AE activity curve.

Effects of un-parallel notches on the granite fracture properties and acoustic emission phenomena under biaxial test: PFC computation and experimental verification

To assess connections between these notches throughout fracture formation under stress, the mechanical performance of granite specimens with two non-parallel notches underwent a biaxial compression test. The first notch had a 45° angle, whereas the second had a variable angle (0°, 45°, 90°, 130°, and 180°). Three ligament angles (0°, 45°, and 90°) were measured between the inner tips of two notches and the horizontal axis. The ligament length, distance between the inner tips of the two notches, and both notch lengths were each 2 cm. These samples were examined at 0, 1, and 3 MPa confining pressures. By using the particle flow code (PFC), simulated results of the evaluations were run after the trials. It was discovered that the specimens’ failure strengths were influenced by the notch’s geometric characteristics, which in turn impacted the failure mechanism as well as fracture geometry. The number of tensile fractures had an impact on the specimens’ failure strengths. Whilst also increasing the ligament angle, more induced tensile cracks were produced. In the early stages of loading, just a few AE events were found, but after that, they multiplied quickly before approaching the peak stress. By raising the confining pressure, the AE hits were increased and delay failure was occured. The failure strength and AE hits are significantly impacted by the second notch’s angle. In constant ligament angel, the compressive strength of models containing overlapped joint were more than that with non-overlapped joints. In models containing non-overlapped notches, the model with ligament angles of 0° and 45° had maximum and minimum compressive strength, respectively. In models containing overlapped notches, the model with ligament angles of 90° and 45° had maximum compressive strength and minimum compressive strength, respectively. In all ligament angles, minimum compressive strength was occurred when the upper joint angle was 45°. Both the computational and experimental methodologies showed that the specimens’ failure pattern and compressive strength were very comparable.