Kaiser Effect Research Articles

Stress triggering and stress memory observed from acoustic emission records in a salt mine

SUMMARY Some of the most interesting questions in geosciences are whether results from laboratory experiments can be applied to processes in the earth crust and whether in situ studies with high spatio-temporal resolution can bridge the gap between laboratory work and seismology. In this study, acoustic emission (AE) activity caused by stress changes due to the backfilling of a cavity in an abandoned salt mine is studied to answer questions regarding (1) the dependence of AE event rates, event distribution and b-value on the stress state, (2) the stress memory effect of rock (Kaiser effect), (3) the possibility to detect significant changes in the system like the initiation of macrocracks and (4) the possibility to estimate future activity from previous AE records. The large number of events studied (>3 × 10 5 ) allows a spatial resolution of the order of 1 m and a temporal one on the order of 1 hr. Stress changes are created due to the thermal expansion and contraction of the rock mass in response to the temperature changes caused by the backfilling. A roughly 20 × 50 × 50 m section of the mining complex just above the backfilled cavity is well covered by a network of 24 piezo-electric receivers and poses an optimal volume for the study. Results of a 2-D finite element thermoelastic stress model are in agreement with the spatio-temporal AE event distribution. In addition to the initial upward migration of the AE event front, which correlates with the calculated stress field, the rock salt exhibits a pronounced Kaiser effect for the first few thermal loading cycles throughout the whole study region. The deviation from the Kaiser effect during later loading cycles seems to be caused by the initiation of a planar macroscopic crack, which is subsequently reactivated. AE activity tends to concentrate along this macrocrack. Calculated b-values decrease before and increase after the supposed initiation of the macrocrack supporting this explanation. In intact rock volumes not subjected to macrocracking a linear relation between the maximum event rate and the calculated absolute Coulomb stress increase is observed. This indicates that future maximum AE event rates can be estimated from expected loading. AE activity during stress loading cycles is most prominent in regions with Coulomb stress maxima indicating possible shear cracking that has the potential to create macrocracks. Strong bursts of AE activity observed during thermal unloading phases are concentrated in regions for which the minimum principal stress becomes tensile. These regions exhibit significantly higher b-values than those active during thermal loading. We interpret these weak events with tensile microcracking.

Effectiveness of nonlinear ultrasonic and acoustic emission evaluation of concrete with distributed damages

Nonlinear ultrasonic (NLU) and acoustic emission (AE) techniques are used for nondestructive evaluation of concrete, damaged under compression loading. Experiments were carried out in 18 cubic specimens (150 × 150 × 150 mm) cast with three different w/c (six specimens for each w/c). Three specimens at each w/c were used for AE monitoring and three others for NLU evaluation. The NLU evaluation is based on measuring the change in fundamental amplitude with increasing damage and output power level. In acoustic emission testing technique four sensors were used to listen to the wide range of events under various loading and unloading cycles. An increase in AE hits was observed with increasing damage. Each loading and unloading stage was carefully examined for Kaiser and Felicity effects in order to assess the concrete deterioration. It was proposed to measure Felicity ratio at three different loading levels, corresponding to AE hits at 3%, 5%, and 10% of the AE hits at the previous maximum load, respectively. Normalized values of Felicity ratio were plotted and compared with the NLU test data. Correlation between acoustic emission and nonlinear ultrasonic techniques in assessing damage growth in concrete was investigated.

Manifestations of point and extensive defects of bulk-metallic glasses

In spite of the fact that the one-parameter free-volume model is inadequate to provide a reasonable description of structural defects of bulk-metallic glasses (BMGs), this model is still in use due to its simplicity and conformity with naive ideas on the BMG structure. Meanwhile, the polycluster model of the metallic-glass structure takes into account the occurrence of locally preferred short-range and medium-range ordering, stable point defects (vacancies, interstitials, semi-vacancies, semi-interstitials, and impurities), and extensive defects (dislocations, cluster boundaries, and triple joints). Some of the defects, such as cluster boundaries and triple joints, are accessible for the direct observation by means of the field-emission microscopy (FEM). Others manifest themselves in macroscopic kinetic and mechanical phenomena. In this report, we briefly describe the point and extensive defects of metallic polyclusters and present some results of direct and indirect observations of their manifestations. The cluster boundaries, triple joints, and sub-clusters are identified by means of FEM. Vacancies and short-range ordering are revealed through the recovery annealing kinetics of BMGs irradiated with 2.5 MeV electrons. Since the vacancy production by irradiation is accompanied by the generation of interstitials (due to the Frenkel-pair formation), one can consider these results as an evidence of the interstitial formation in BMGs. However, the stability of interstitials in metallic glasses remains to be investigated. A pronounced manifestation of dislocations (first of all, cluster-boundary dislocations) is the Kaiser effect observed in a set of BMGs. A discussion of the observed phenomena is given.

Characterization of Tensile Damage for 3D-C/SiC Composites by Acoustic Emission

The various damage mechanisms in 3D-C/SiC composites are identified using acoustic emission (AE) signal parameters, and the Felicity effect is studied on different unloading stress. As a result, the damage mechanisms in 3D-C/SiC composites can be identified successfully by the amplitude, average frequency and relative energy, and there are several damage modes together during uniaxial tensile process. The Kaiser effect is almost absent and the Felicity ratio fluctuates at 0.95 on lower stress stage and drops when the relative stress is above 65%.

A numerical model for fluid injection induced seismicity at Soultz-sous-Forêts

During fluid injection experiments at the geothermal site of Soultz-sous-Forêts (France), more than 114,000 induced seismic events with magnitudes between −2.0 and +2.9 were detected by a local downhole monitoring network. Of these, 35,039 events are sufficiently constrained to be located. Hypocenters align along a sub-vertical, planar structure with the apparent width being dominated by data scattering indicating that seismic activity predominantly occurs along a (pre-existing) larger scale fault structure. For this scenario, we present a numerical model to simulate hydraulic overpressures and induced seismicity during hydraulic injection. The numerical model is based on the physical processes of fluid pressure and stress diffusion with triggering of the induced seismicity being controlled by Coulomb friction. Even in its simplest form of a fault zone without any structural heterogeneity, the numerical model reproduces typical observations at Soultz-sous-Forêts, such as number and magnitude of induced events, hypocenter locations (including the Kaiser effect), occurrence of post-injection seismicity, and the largest magnitude event occurring several days after shut-in.

AE기법에 의한 압축력을 받는 고인성 섬유보강 시멘트 복합체의 손상 평가

고인성 섬유보강 시멘트 복합체는 시멘트 매트릭스 내 보강된 섬유의 계면부착응력에 의해 다수의 미세균열분산 및 손상저항성능을 갖게 되나, 이를 구조물에 적용하기 위해서는, 고인성 섬유보강 시멘트 복합체의 파괴거동을 규명함과 동시에 보강섬유에 따른 시멘트 매트릭스의 마이크로 파괴메커니즘에 대한 이해가 요구된다. 이 연구에서는 단조 및 반복가력시 고인성 섬유보강 시멘트 복합체의 파괴특성 및 음향방출신호특성을 규명하기 위하여 총 4 시리즈의 시험체가 사용되었으며, 주요 실험변수는 섬유의 종류(PE, PVA, SC), 혼입률, 하이브리드 타입, 가력방법(단조, 반복) 이다. 실험결과, 고인성 섬유보강 시멘트 복합체의 압축거동에 따른 손상진전은 섬유의 혼입률 및 하이브리드에 따라 상이하게 나타났다. 또한 음향방출신호로부터, 각 하중단계의 2, 3번째 사이클에서의 진폭 감소 특성이 나타났으며, 이는 각 사이클별 변형률 증가와의 관련성을 보여 이를 이용한 강도 예측이 가능할 것으로 판단된다. 또한 최대강도의 80%까지 펠리시티 효과 및 카이저 효과가 나타났으며, 하이브리드 섬유 혼입시 매크로 균열 제어로 인해 손상의 복원 및 분산능력이 뛰어난 것으로 나타났다.

음향방출 기법을 이용한 섬유보강 시멘트 복합체의 정량적 손상평가

Dept. of Civil System Engineering, Konyang University, Chungnam 320-711, KoreaABSTRACT Fiber is an important ingredient in fiber-reinforced cement composite (FRCC) which can control fracture of cementcomposite by bridging action. In compliance with the action of the fiber and the aggregate size, it also showed a different failure mech-anism. For practical application, it is needed to investigate the fracture behavior of the FRCC and to understand the micro-mechanismof cement matrix with reinforcing fiber. In order to evaluate a characteristics of fracture process in the FRCC, acoustic emission (AE)technique was used for the analysis and evaluation of FRCC damage by acoustic emission under flexural and cyclic compressive load-ings. The AE signals were monitored by AMSY4 AE instrument during the entire loading period. The specimens are reinforced with0, 1.0, 1.5 and 2.0% (by volume) Polyvinyl alcohol (PVA) fiber. The test results showed that the damage progress of the FRCC wascharacteristic for the fiber replacement ratio. As a result of analyzing the felicity ratio (FR) values, it is shown that this values can beused for evaluating the degree of FRCC damage. On the whole the felicity ratio values of FRCC are shown between 0.4 and 1.1. And,the AE kaiser effect was shown in the all FRCC specimen. In addition, the damage behavior and the microscopic fracture process ofthe FRCC are evaluated using the AE parameters, such as calm ratio, b-value and felicity ratio. The purpose of this reserch was toadvance the state of knowledge regarding the applicability of acoustic emission as an evaluation method for FRCC.Keywords : fiber-reinforced cement composite, bridging action, Kaiser effect, calm ratio, b-value

Numerical simulation on the acoustic emission activities of concrete

Acoustic emission (AE) is a very important tool to study the cracking process and to evaluate the integrity of structures. The numerical simulation on AE activities is helpful for us to explain the AE data observed during the AE test of concrete and to therefore clarify the associated failure mechanism. Based on the knowledge that the AE events of concrete under external loading are closely related to its internal damage, it becomes possible for us to simulate these AE activities of concrete at meso-level with a numerical tool. In this paper, a numerical model is used to simulate the AE activities of mortar and concrete beams under three-point bending. Based on numerical simulation, the load–displacement curves and corresponding AE activities of mortar and concrete specimens are obtained and are calibrated against the corresponding experimental results. Subsequently, this numerical model is utilized to simulate the AE activities of mortar specimen when it is subjected to cyclic loading, and the Kaiser effect of AE is reproduced.

Time-sensitivity of the Kaiser effect of acoustic emission in limestone and its application to measurements of in-situ stress

Measuring in-situ stress by using the Kaiser effect in rocks has such advantages as time-efficiency, low cost and little limitation, but the precision of the method is dependent on rock properties and delay time of the measurement. In this paper, experiments on the Kaiser effect in limestones were performed, and it was found that the limestones had good ability to retain a memory of their recent stress history and high time-sensitivity. The longer the experiment was delayed from the extraction of the stone, the larger the Felicity ratio was. As the Felicity ratio approached 1, significant Kaiser effect was observed. In-situ stress should be determined by the limestone measurements when the delay time was 40–120 days. Finally, the in-situ stress in a limestone formation could be successfully measured in practice.

Investigation of Fault Mechanisms during Geothermal Reservoir Stimulation Experiments in the Cooper Basin, Australia

Abstract Subsequent to an initial reservoir stimulation in 2003, a second long-term fluid-injection experiment was conducted in the Cooper Basin, Australia, in 2005 to further enhance and enlarge the existent geothermal reservoir. Approximately 25,000 m 3 of water were injected into the granitic crust at 4250 m depth, thereby inducing about 16,000 seismic events, which were recorded by a local eight-station network. Event magnitudes calibrated by recordings of the permanent network of Geosciences Australia are in the range between M L -1.2 and 2.9 with seismic moments between 10 9 and 10 13 N m. Hypocenter locations could be determined for 8886 events. The spatial hypocenter distribution indicates that the seismicity occurred on the same subhorizontal layer structure identified for the initial reservoir stimulation, which is further extended in the course of restimulation. Early seismicity is located near the outer rim of the previous zone of seismic activity and subsequently migrates both toward and away from the injection well. The immediate vicinity of the injection well remains seismically quiet during restimulation. These effects can be explained by a simple Kaiser effect model, where the induced seismicity is controlled by the in situ fluid pressure increase. Compound fault-plane solutions based on P -phase polarities indicate a similar fault mechanism for nearly all events. The dominating fault mechanism is consistent with the regional stress field acting on a larger scale fracture zone with an orientation as outlined by the hypocenter distribution. A small number of events with reverse mechanisms might indicate the existence of conjugated fractures locally intersecting the main fracture zone. Cumulative slip determined by mapping the slip contribution of individual events onto the fracture zone indicates that most fracture patches in the reservoir slipped repeatedly, accumulating up to several centimeters of shear displacement.

Using Acoustic Emission to monitor fatigue damage and healing in Asphalt Concrete

This paper presents an Acoustic Emission (AE) characterization of damage accumulation and strength recovery in Asphalt Concrete (AC) mixture. A series of uniaxial tensile cyclic tests with and without rest period has been conducted on a 19 mm nominal maximum size of aggregate (NMSA) AC at 20°C. During the fatigue tests, key AE parameters including emission counts were acquired from two piezo-type sensors attached to the middle of a 150 mm high, 75 mm diameter cylindrical specimen. Test results indicate that accumulative AE energy and AE count may be used not only to assess the initiation and propagation of fatigue damage, but also to quantify the beneficial effect of rest period on the performance of AC. The frequency-amplitude analysis shows that crack formations coincide with the peaks of maximum AE amplitude (Amax) with higher frequencies, while healing is best described by Amax with lower frequencies during the rest periods. In addition, it is demonstrated that the Kaiser effect, the stress dependence of AE generation, does not hold for fatigue in AC.

AE BEHAVIOR OF SMART STRESS MEMORY PATCH AFTER VARIABLE AMPLITUDE LOADING

Recently, the structural health monitoring becomes increasingly great important to assure the ease and safety of our life, and it is required significantly to develop non-destructive evaluation for structures such as bridges and tunnels. Some sacrificed specimens have been developed to evaluate the fatigue damage of structures such as fatigue cycles and residual lifetime, but it can be applied only when the stress history is known beforehand. These fatigue sensors need no cable and can be used at low cost in contrast to strain gage. In previous study, a smart stress memory patch was developed as a new fatigue sensor. The patch can measure simultaneously the maximum stress, stress amplitude and the number of fatigue cycles by crack length measurement and Kaiser effect of Acoustic Emission (AE). The crack growth behavior under constant amplitude (CA) loading has been investigated, and AE behavior also has been evaluated only after CA loading. However, AE characteristics after variable amplitude (VA) loading in service are extremely important. Moreover, it is very important to control AE behavior of the smart patch in order to evaluate the applied stress using Kaiser effect. In this study, fatigue test with single overload was investigated to evaluate its influence. Moreover, effect of crack length and heat treatment on AE behavior was also investigated. Finally, AE behavior of the patch was evaluated after fatigue CA loading with overload or VA loading with log-normal distribution and overload.

Electromagnetic emission memory phenomena related to LiF ionic crystal deformation

During the uniaxial compression of LiF ionic monocrystals, acoustic and electromagnetic emissions (EME) are detected. We observed that when the compression is performed in successive loading, unloading cycles and these emissions are being monitored, no new emissions will occur unless the maximum stress of the previous cycle is exceeded, meaning that the material presents memory characteristics. This is observed not only for the acoustic emission (AE), which is the well known Kaiser effect, but for the EME as well. In other words, the material appears to memorize and reveal the previously maximum stress it suffered while being deformed. The importance of an electromagnetic memory feature of a material can be related to various applications in material science, especially when the detection of AE is not feasible or gives false alert. Such cases may very well be earthquakes’ predictive indications, monitoring of mines’ stability, imminent landslides, etc.

Theoretical and experimental analysis on the mechanism of the Kaiser effect of acoustic emission in brittle rocks

The Kaiser effect is formally described as the absence of detectable acoustic emission (AE) events until the load imposed on the material exceeds the previous applied level and is usually used to estimate geostress. By focusing on the heterogeneity of rock material, the mechanism of the Kaiser effect under cyclic loading is analyzed based on statistic damage mechanics. Two groups of granite specimens have been cyclically loaded with two different loading paths to verify the theoretical results. The heterogeneity of rock is the real reason that causes irrecoverable damage on the Kaiser effect of acoustic emission in cyclic loading. The Kaiser effect reflects the damaged state in rocks rather than the previous stress imposed on it. Applications for using the Kaiser effect to estimate geostress were discussed here. It is shown that the commonly used uniaxial loading method for estimating geostress is not in accordance with the theoretical and experimental results. The analysis is of importance to use the Kaiser effect correctly for estimating geostress or in other fields.

個別要素法の岩石破壊に対する新たな適用の試み

Results from distinct element method (DEM) simulations are presented for the uniaxial compression test, cyclic loading test considering the Kaiser effect, and hydraulic fracturing, and the applicability of DEM to model rock fracture is discussed. Each simulation was in good agreement with the actual experimental results. It is shown that DEM is an effective technique to mimic fracture within a heterogeneous material. However, it was found that there are unresolved issues to be considered in the application of DEM to rock fracturing. (1) For proper simulation using DEM, appropriate micro-parameters are needed. In spite of efforts by many researchers, there are no effective procedures to find an optimum set of parameters. (2) Conventional DEM simulations assume generation of new cracks to be acoustic emission (AE) events, whereas slip occurring at the preexisting crack surfaces also generates AE. To better understand the mechanism of rock fracture, it is necessary to discuss the generation mechanisms of such AE events. (3) For a field scale simulation, the important problem of grain size, as well as input parameters, has not yet been resolved.

Violation of the Kaiser effect by hydraulic-fracturing-related microseismicity

Application of a load to different materials and rocks induces acoustic and seismic emission. If the load has a cyclic character, then as a rule such an emission will vanish for loads less than the maximum previously reached load level. This is the so-called Kaiser effect, which is an expression of memorized stress history of materials and rocks. Usually, this effect is assumed to be valid also in the case of fluid-induced seismicity. Here, we show that microseismicity occurring during hydraulic fracturing violates the Kaiser effect. In material sciences, such a breakdown of the Kaiser effect is termed the felicity effect. We observe that the felicity effect can be directly related to reopening of the hydraulic fracture forced by fluid injection in loading cycles. In contrast, closing of the hydraulic fracture following injection cessations seems to be governed by the Kaiser effect. We explain such a contrasting behaviour of microseismicity by an aseismic character of a slow fracture closing and by rubbing and pore pressure diffusion during a quick fracture reopening. This violation of the Kaiser effect for tensile fracturing due to hydraulic forcing on rocks is similar to the absence of this effect in the case of shear fracturing by seismogenic processes. Possibly, the felicity effect is a common feature of ongoing active faulting.

Experimental Verification and Theoretical Analysis on Felicity Effect of Acoustic Emission in Concrete

The experiment of acoustic emission (AE) on concrete specimens under uniaxial cyclic loading was conducted. The Kaiser effect of acoustic emission in concrete and the Felicity effect, which manifest the memorizing ability to the maximum previous stress level of Kaiser effect, were validated by the experiment. The mechanism of Felicity effect was analyzed based on the theory of statistical damage mechanics and a tentative AE factor constitutive model of brittle material under uniaxial cyclic loading was suggested. The curve of constitutive model is in good coincidence with the curve from the experiment. The experimental results showed that the Felicity effect became clearer along with the increasing of stress level. Each loading cycle would cause new damage inside the material, and the response of material to the new loading cycle is different from the previous cycle.

Relation between Kaiser effect levels and pre-stresses applied in the laboratory

This study aims to investigate the relation between the Kaiser effect (KE) levels and pre-stresses applied to rock samples in laboratory. For the purpose, a number of test specimens were obtained from different types of rock and a mortar block. Deformation and crack propagation stages were identified for each sample group. Based on the thresholds of these stages, the specimens were subjected to selected pre-stresses under uniaxial and axisymetric triaxial loading under laboratory conditions. Following the laboratory preloadings, acoustic emission (AE) experiments were carried out on the specimens under uniaxial loading conditions to determine the KE levels. The comparisons made between the laboratory pre-stresses and the KE levels suggest that the KE level is not equal to one of the pre-stress values or their differences, and should be related to combined effect of axial and confined pre-stresses, and some rock dependent characteristics.

On the Kaiser effect in bulk metallic glasses

The Kaiser effect appears during the testing of samples in the repetitive loading and unloading regimes and is found in the fact that the acoustic emission (AE) originates in each subsequent loading only when the stress exceeds its maximum value achieved in the preceding loading. We observed the Kaiser effect in measurements of AE during uniaxial compression of samples of the bulk metallic glasses Zr41Ti14Cu12.5Ni10Be22.5 and Zr52.5Ti5Cu17.9Ni14.6Al10 at room temperature. It is shown that the Kaiser effect recovers after annealing below the glass transition temperature. Mechanisms of structure reconstructions providing the Kaiser effect and its recovery are considered within the framework of polycluster model.

Experimental and numerical study of the directional dependency of the Kaiser effect in granite

Experimental and numerical study of the directional dependency of the Kaiser effect in granite