Space-time evolution rules of acoustic emission location of unloaded coal sample at different loading rates

By using MTS815 rock mechanics test system,a series of acoustic emission(AE) location experiments were performed under unloading confining pressure,increasing the axial stress.The AE space-time evolution regularities and energy releasing characteristics during deformation and failure process of coal of different loading rates are compared,the influence mechanism of loading rates on the microscopic crack evolution were studied,combining the AE characteristics and the macroscopic failure modes of the specimens,and the precursory characteristics of coal failure were also analyzed quantitatively.The results indicate that as the loading rate is higher,the AE activity and the main fracture will begin earlier.The destruction of coal body is mainly the function of shear strain at lower loading rate and tension strain at higher rate,and will transform from brittleness to ductility at critical velocities.When the deformation of the coal is mainly plasticity,the amplitude of the AE ringing counting rate increases largely and the AE energy curves appear an obvious ''step'',which can be defined as the first failure precursor point.Statics of AE information shows that the strongest AE activity begins when the axial stress level was 92-98%,which can be defined as the other failure precursor point.As the loading rate is smaller,the coal more easily reaches the latter precursor point after the first one,so attention should be aroused to prevent dynamic disaster in coal mining when the AE activity reaches the first precursor point.

Uniaxial compression CT and acoustic emission test on the coal crack propagation destruction process

Acoustic emission test and CT scanning are important techniques in the study of coal crack propagation. A uniaxial compression test was performed on coal samples by integrating CT and acoustic emission. The test comparison analyzes the acoustic emission load and CT images for an effective observation on the entire process, from crack propagation to the samples＇ destruction. The box dimension of the coal samples＇ acoustic emission series and the CT images were obtained through calculations by using the authors＇ own program. The results show that the fractal dimension of both the acoustic emission energy and CT image increase rapidly, indicating coal and rock mass has entered a dangerous condition. Hence, measures should be taken to unload the pressure of the coal and rock mass. The test results provide intuitive observation data for the coal meso-damage model. The test contributes to in-depth studies of coal or rock crack propagation mechanisms and provides a theoretical basis for rock burst mechanism.

Test of the Cosmic Transparency with the Baryon Acoustic Oscillation and Type Ia Supernova Data

We use the latest baryon acoustic oscillation and Union 2.1 type Ia supernova data to test the cosmic opacity between different redshift regions without assuming any cosmological models. It is found that the universe may be opaque between the redshift regions 0.35 0.44, 0.44 0.57 and 0.6-0.73 since the best fit values of cosmic opacity in these regions are positive, while a transparent universe is favored in the redshift region 0.57-0.63. However, in general, a transparent universe is still consistent with observations at the lo confidence level.

Rejection of Direct Blast Interference Based on Signal Phase-Matching Array Processing

In bistaic acoustic testing, there will be strong direct blast interference. An algorithm based on signal phase-matching array processing that rejects direct blast interference in bistatic acoustic testing has been studied, through which the object scattering signal is accurately extracted. Characteristics of bistatic acoustic testing and signal phase matching processing principle are fully integrated in this algorithm. Firstly, the direct blast interference is calculated from the receiving signal based on three subarrays signal phase matching processing. Secondly, the direct blast is rejected by subtraction from the receiving signal. In this way the limitations of the high signal to noise ratio that signal phase matching processing required for direct calculating the object scattering signal can be avoided. Simulation and sea trial results show that, when the ratio of signal to interference is greater than -20 dB, this algorithm of direct blast interference rejection based phase matching signal processing can accurately extract the object scattering signal.

Time analysis for aero-engine acoustic modes exploiting block sparsity

Acoustic Mode Analysis(AMA)for aero-engines can offer valuable insights for the design of silent engines as well as for fault diagnosis.Commonly,this is done in the(spatial)Fourier domain,necessitating the use of multiple uniformly spaced microphones to ensure adequate resolution.Recent works show that sub-Nyquist estimation is feasible using sparse reconstruction frameworks,although such modelling generally introduces an estimation bias that has to be compensated for.Moreover,there is a growing interest in monitoring mode amplitude over continuous time,as it can offer crucial insights for diagnosing operational conditions.In this work,we introduce a Block Orthogonal Matching Pursuit(BOMP)method for continuous time mode analysis,exploiting the underlying structural sparsity of the signal model.Specifically,the(pseudo)‘0ànorm penalty is employed to induce sparsity in the wavenumber domain,whereas a block structure is imposed as a constraint to monitor the amplitude variation in the time domain.The effectiveness of the BOMP is evaluated using both numerical simulations and experimental measurements,indicating the proposed method's preferable performance as compared to the classic Least Absolute Shrinkage and Selection Operator(LASSO)and Orthogonal Matching Pursuit(OMP)methods.

Blast-induced rock damage control in Fangchenggang nuclear power station, China

In the process of blasting excavation, stress wave propagation and gas expansion can basically induce damage to surrounding rocks, which is detrimental to rock mass integrity and engineering safety. In this case, evaluation and control of blast-induced effects are essential to the safety of nearby buildings and integrity of bedrock in blasting field. In Fangchenggang nuclear power station of China, the drill-and- blast method was employed for bedrock excavation. In order to reduce the blast-induced damage zone, the wave propagation and associated damage to rock mass should be carefully investigated. In this paper, the wave propagation regressively obtained from field monitoring data was presented based on empirical formula （e.g. Sadovsloj formula）. The relationship between the peak particle velocity （PPV） at a distance of 30 m away from the charge hole and charge per delay in blast design was derived. Meanwhile, the acoustic tests before and after blasting were conducted to determine the damage depth of rock mass. The charge per delay in blast design was then calibrated based on the blast-induced wave propagation regularity. The results showed that a satisfactory effect was achieved on blast-induced damage control of rock mass. This could be helpful to rock damage control in similar blasting projects.

An Investigation on the Relationship between Secretory Otitis Media andAllergy in 2702 Pupils

in order to elucidate the relationship between secretory otitis media (SOM) and allergy,the tympanogram and allergic skin test with house dust, mite, and Japanese cedar pollen allergen were made in 2702 pupils in Shiraoi, Hokkaido, Japan. The correlation between the tympanogram type and the results of allergic skin was analyzed. This study suggests that there is no positive relationship between SOM and allergy.

Effects of installations on the low frequency vibration response of specimens in acoustic fatigue tests

The low frequency vibration response of a specimen in acoustic fatigue tests depends not only on the dynamic characteristics and the boundary conditions of the specimen itself, but also on the test unit which couples the specimen to a given sound field. Further, the latter can even be dominant instead the former in some circumstances. This fact is shown in the paper by using the experimental results and the theoretical analysis of the acoustic-induced vibration of a boundary clamped rectangular thin plate. In analysing the systems of acoustic fatigue test, an approach of electro-mechano-acoustical analogous circuit is used. The application of the approach can give an estimation of the effects on the low frequency vibration modes of various parameters in a system quantitatively. This supplies a theoretical basis and a means for the rational layout of acoustic fatigue tests.

FLOW NOISE MEASUREMENT OF SURFACE SHIP WITH TOWED MODEL

In this article, a new acoustic test technique using towed model was introduced to study flow noise caused by a surface ship. The project of model test was be properly designed for acoustic signal collecting and with the help of appropriate data processing method different kinds of acoustic sources could be successfully identified. A lot of work about fuid noise could be carried on with the towed model, and the noise corresponding to low frequency which is especially interested for its long distance radiating with small attenuation could also be studied in this way.

Effects of High Temperature Oxidation on Mechanical Properties of Ti_3AlC_2

The oxidation tests of Ti_3AlC_2 were conducted at 1100 and 1200？C in air for 48 and 360 h, respectively,and the effects of high temperature oxidation on the flexural strength and hardness of Ti_3AlC_2 were investigated. The microstructure, grain size and phase compositions of Ti_3AlC_2 substrate didn＇t change after oxidation, hence the oxide removed Ti_3AlC_2 substrate maintained its initial flexural strength and hardness. However, the flexural strength of oxide retained Ti_3AlC_2 decreased by about 5%. Acoustic emission monitoring indicated that during the process of three-point bending test, the formed Al_2O_3 scale on Ti_3AlC_2 surface fractured firstly in a cleavage manner, then the substrate/oxide interface cracked,and finally the Ti_3AlC_2 substrate fractured. The mechanical degradation was caused by the preferential formation of cracks in brittle Al_2O_3 scale as well as at defective and lacunose grain boundaries of the substrate where stress concentration generated. The mechanical degradation was insensitive to oxidation temperature and time in the present conditions. In addition, the surface hardness increased significantly after oxidation due to the formed hard Al_2O_3 scale on the surface of Ti_3AlC_2 substrate.

Brittleness and fracture mechanism of sintered Dy-doped NdFeB magnets

The brittleness and fracture behaviors of the sintered and the two-stage aged Dy-doped NdFeB magnets were studied by a unique method of acoustic emission testing and Vickers hardness indentation method in this paper.A detailed analysis on the crack propagation mechanism along the grain boundary of the main grain phase(Nd,Dy)_(2)Fe_(14)B was done.By comparing the acoustic emission energy count value(E_(n))with the Vickers hardness indentation load(P),it is shown that there is a linear relationship between En and P for both the sintered and the two-stage aged Dy-doped NdFeB magnets.According to the slope of En versus P linear lines,it can be found that the two-stage aged Dy-doped NdFeB magnet is more brittle than the sintered one.It is due to that the Ndrich grain boundary phase of the two-stage aged Dy-doped NdFeB magnet is formed as thin film and uniformly distributes around the main grain phase,which plays a significant role in increasing the intrinsic coercive force of the magnet,but decreasing its interface binding strength.Therefore,the resistance of crack propagation along the grain boundary decreases and the brittleness increases.