Longitudinal Attenuation Research Articles

Full-scale experimental study on fire under vehicle operations in a sloped tunnel

Urban rail systems are developing rapidly, and due to the limited space of the metro tunnel, once a fire occurs will result in disastrous consequences. Full-scale fire experiment is an effective testing method for fire prevention of metro systems. Aiming at the characteristics of a long continuous downhill distance and large lifting height in the sloped metro tunnel, a full-scale fire experiment under vehicle operations was carried out on a metro line. By changing the operated speed and direction of the vehicle, the airflow velocity, ceiling temperature distribution, and smoke layer characteristics in the tunnel were studied. The results show that when a fire occurs in a sloped tunnel, the running vehicle will produce piston effect, and the airflow will be affected by the stack effect and the piston effect simultaneously, which will change the flow field in this area, and then affect the flame shape, ceiling temperature and smoke distribution. Through the dimensionless treatment of the ceiling temperature, a longitudinal attenuation distribution model of the ceiling temperature was established, which reveals the law of fire smoke migration and diffusion under different vehicle operating conditions. The conclusions of this study could technically support the smoke control design in such metro tunnel fire scenarios.

Comparison of Experimental Measurements of Material Grain Size Using Ultrasound

Material grain size is related to metallic material properties and its elastic behaviour. Measuring and monitoring material grain size in material manufacturing and service is an important topic in measurement field. In this paper, three materials, i.e., aluminium 2014 T6, steel BS970 and copper EN1652, were chosen to represent materials with small, medium and large grain size, respectively. Various techniques of measuring material grain size were demonstrated and compared. These techniques include the measurements from material microstructure images, backscattered ultrasonic grain noise using a conventional transducer, longitudinal wave attenuation using ultrasonic arrays and shear wave attenuation using a lead zirconate titanate (PZT) plate. It is shown that the backscattered ultrasonic noise measurement and material attenuation measurement are complementary. The former is pretty good for weak scattering materials, e.g., aluminium, while the latter for materials with large grains, e.g., steel and copper. Consistent measured grain size from longitudinal and shear wave attenuations in steel and copper suggests that shear wave attenuation can be calculated from the measured longitudinal wave attenuation integrated with Stanke–Kino’s model or Weaver’s model, if there is a difficulty to either excite or capture shear waves in practice. The outcome of the paper expects to provide a further step towards the industrial uptake of these techniques.

Effects of axial static stress on stress wave propagation in rock considering porosity compaction and damage evolution

A wave equation of rock under axial static stress is established using the equivalent medium method by modifying the Kelvin-Voigt model. The analytical formulas of longitudinal velocity, space and time attenuation coefficients and response frequency are obtained by solving the equation using the harmonic method. A series of experiments on stress wave propagation through rock under different axial static stresses have been conducted. The proposed models of stress wave propagation are then verified by comparing experimental results with theoretical solutions. Based on the verified theoretical models, the influences of axial static stress on longitudinal velocity, space and time attenuation coefficients and response frequency are investigated by detailed parametric studies. The results show that the proposed theoretical models can be used to effectively investigate the effects of axial static stress on the stress wave propagation in rock. The axial static stress influences stress wave propagation characteristics of porous rock by varying the level of rock porosity and damage. Moreover, the initial porosity, initial elastic modulus of the rock voids and skeleton, viscous coefficient and vibration frequency have significant effects on the P-wave velocity, attenuation characteristics and response frequency of the stress wave in porous rock under axial static stress.

Experimental Research on Mechanical Properties of High‐Temperature Sandstone with Different Cooling Methods

Various tests including the longitudinal wave velocity tests and uniaxial compression tests have been conducted to evaluate the impact of cooling methods (including natural cooling, water cooling, and cooling by liquid carbon dioxide) on mechanical properties of sandstone under the natural status and high temperature. The acoustic emission signals were also monitored during the tests. According to the tests conducted, the sandstone sample density attenuation rate and the longitudinal wave velocity attenuation rate are higher than those of the specimen under natural status while the uniaxial compressive strength and Young’s modulus are lower. Comparing with the sandstone under the natural status, the compression sections of the stress‐strain curves of the high‐temperature sandstone samples treated by three cooling methods are longer with lower strain peak values. The order of the acoustic emission is revealed as follows: the sample cooled by liquid carbon dioxide < the sample cooled by water < sample cooled naturally < the sample under natural status, which suggests that the rapid cooling (cooled by liquid carbon dioxide) produces the severest damage on the sample, followed by the water cooling and natural cooling methods. In addition, the relationship between the sample strength weakening coefficient and the cooling rate is defined based on the statistical data of the cooling time of the high‐temperature specimen under the three cooling methods.

Pressure influence on elastic wave attenuation in polycrystalline materials.

Traditionally, the acoustoelastic effect refers to the influence of stress in a solid on an elastic wave's phase velocity. Since the phase velocity can be represented by the real part of the complex wave number, a natural question arises regarding the effect of stress on the imaginary part or dissipation of the wave. In this article, the influence of pressure on the elastic wave's attenuation in polycrystalline materials is modeled. The constitutive behavior of an initially stressed solid is coupled into Weaver's scattering-based attenuation model [J. Mech. Phys. Solids 38, 55-86 (1990)]. As a result, the pressure-dependent longitudinal and shear wave attenuation coefficients are unveiled. As the traditional stress-free attenuation coefficients depend on the degree of single-crystal elastic anisotropy, it is shown that the pressure influence on attenuation depends on the anisotropy of the single-crystal's third-order or nonlinear elastic constants. Analysis of the model indicates linkages between pressure derivatives of velocity and attenuation to the material's linear and nonlinear elastic anisotropy, crystal structure, and type of atomic bonding.

Study on Combustion Characteristics and Evacuation during Intake Airway Fire in Coal Face under Different Ventilation Conditions

ABSTRACT The exogenous fire is one of the major disasters of mines. It is difficult to put them out but easy to result in serious casualties and property losses. Until now, the fire combustion in mines and smoke flow characteristics under multi-factor coupling effect of wind speed, wind direction, fire source and so on are still not clear. Moreover, the interaction mechanism between fire and people escape has still not been revealed. In order to solve the above problems, this paper used the FDS numerical simulation method to study the parameter values of working face temperature and visibility of laneway as the fire occurs as well as their changes, so as to obtain the influences of fire source position and fire power on the smoke spread rate of fire in the laneway. Then, the function expressions of smoke spread rate related to the fire source position and fire power were derived, respectively, and the rationality of theoretical prediction model of longitudinal attenuation of smoke temperature was verified. Furthermore, the fire characteristic parameters and their change rules under the inverted ventilation were studied, and the optimal scheme of wind control was obtained. The results showed that the increase of inverted ventilation speed could accelerate the discharge of smoke flow from the laneway. Moreover, when the inverted ventilation speed was between 2.2 m/s and 2.7 m/s, there was an optimal wind rate for smoke discharge, which could provide relatively safe conditions for evacuation. In addition, when the power of the fire source was 10 MW, the critical wind speed to prevent the smoke flow reversal was 1.6 m/s under the condition of local inverted ventilation. Finally, this study simulated the personnel escape behavior in combination with Pathfinder to build the prediction models of evacuation time and evacuation speed in the laneway and working face. Simultaneously, the necessary time for personnel escape was analyzed, which provided a basis for the safe evacuation of personnel in the case of fire.

Gain-scale ultrasonic properties measurement of cast austenitic stainless steel

Ultrasonic properties of coarse-grained austenite are critical to the quantitative nondestructive testing and evaluation of cast austenitic stainless steel (CASS). However, the orientation-dependent information is quite difficult to acquire from polycrystalline materials. Here a grain-scale measurement technique was developed based on electron backscatter diffraction (EBSD) and water-immersion focusing ultrasonic testing. 2-D angular distributions of the longitudinal wave velocity and attenuation coefficient in (0 1 0) plane were obtained respectively, which were correlated with the top-view and side-view orientation of columnar austenite grains. 〈1 0 1〉 orientation presented the largest velocity 6210 m/s and the smallest attenuation coefficient 1.9 dB/mm, while 〈0 0 1〉 reversed. The data deviation and universality were discussed.

Estimating the longitudinal maximum gas temperature attenuation of ceiling jet flows generated by strong fire plumes in an urban utility tunnel

This study is centred on longitudinal maximum gas temperature attenuation of ceiling jet flows generated by strong fire plumes in a utility tunnel. A theoretical correlation is derived by incorporating the approximate boundary layer thickness. To obtain related coefficients, a series of full-scale fire tests are also conducted in a utility tunnel under construction. Four Existing models for the estimation of longitudinal temperature decay are compared and analysed in detail. It is seen that none of them are applicable to make satisfactory predictions in current situations, while good agreement is observed between our theory and test data. Additionally, impinging conditions of strong plumes under different ceiling clearances above fuel are divided into three states. We find Stanton numbers in temperature attenuation were overestimated by Delichatsios, and they are reassessed under these states. In these one-dead-end cases, it is also found the end wall has a great impact not only on the upstream flow pattern, but also on the characteristics of downstream flow. As the fire moves away from the dead end, the downstream gas temperature decays more slowly and Stanton number becomes smaller. In an enclosed case, a new expression has also been provided following the same derivation procedure. The present work positively provides a tool for the assessment of thermal environment, heat exposure and estimation of fire detection time in engineering practice.

High throughput rapid detection for SLM manufactured elements using ultrasonic measurement

Both pore parameters and elastic properties are important for the elements manufactured by the Selective Laser Melting (SLM) technology; however, their measurements are usually destructive and time-consuming. Thus, the ultrasonic longitudinal and transverse wave velocities (dual-mode velocities) method is extended here; then it is combined with the longitudinal wave attenuation to develop an ultrasonic high throughput rapid detection for SLM manufactured elements. As the focal transducer is used, a multi-Gaussian beam model is introduced to correct the diffraction attenuation. Twenty-eight parameters can be obtained by a single ultrasonic experiment, including the pore parameters and their distributions, the two-point spatial correlation function, and the Young’s modulus. Ti-6Al-4V samples with different porosities manufactured by SLM technology are used to conduct the ultrasonic experiment. The experimental results agree well with the results of metallographic, tensile experiment and micro-CT results. This method provides a useful tool for nondestructively, rapidly and accurately evaluating the pore parameters and elastic properties of SLM manufactured elements.

Flame characteristic and ceiling temperature distribution under the effect of curved sidewall

This experimental study focused on fire source combustion characteristics and the distribution law of fire temperature field in the utility tunnel caused by side wall constraint effect. The oil pans are placed at 0 m, 0.15 m, 0.3 m, 0.45 m from the side wall and the center of the model to study the influence of the side wall of the utility tunnel on the flame height, flame shape and temperature distribution of the ceiling. The flame height increases with the fire source gradually approaching the side wall from the center line. According to the flow path of the fire plume, the effective height of the ceiling is corrected. It is found that the longitudinal attenuation of temperature in the center of the ceiling shows a power exponential attenuation law. The temperature distribution under the tunnel ceiling near the fire source area is asymmetrical. When the distance from the fire source center to the side wall increases to 0.4 m and the longitudinal distance increases to 1.2 m, the temperature distribution tends to be symmetrical.

Ultrasonic Attenuation in Polycrystalline Materials in 2D

Grains in a polycrystalline material, typically a metal, act as scatterers of ultrasonic waves and thus give rise to attenuation of the waves. Grains have anisotropic stiffness properties, typically orthotropic or cubic. A new approach is proposed to calculate attenuation in a 2D setting starting from the scattering by an anisotropic circle in an isotropic surrounding. This problem has recently been solved, giving explicit, simple expressions for the elements of the transition (T) matrix (which gives the relation between the the incoming and scattered fields) when the circle is small compared to the ultrasonic wavelengths. The T matrix can be used to calculate the total scattering cross section, which in turn can be used to estimate the attenuation in the material. Explicit expressions for the attenuation coefficient for longitudinal and transverse waves are obtained for a cubic material, and contrary to results in the literature these expressions are valid also for strong anisotropy. For the longitudinal attenuation coefficient a comparison with recent FEM results for Inconel 600 gives excellent agreement.

Spatial-temporal distribution of debris flow impact pressure on rigid barrier

Grain composition plays a vital role in impact pressure of debris flow. Current approaches treat debris flow as uniform fluid and almost ignore its granular effects. A series of flume experiments have been carried out to explore the granular influence on the impact process of debris flow by using a contact surface pressure gauge sensor (Tactilus (R), produced by Sensor Products LLC). It is found that the maximum impact pressure for debris flow of low density fluctuates drastically with a long duration time while the fluctuation for flow of high density is short in time, respectively presenting logarithmic and linear form in longitudinal attenuation. This can be ascribed to the turbulence effect in the former and grain collisions and grain-fluid interaction in the latter. The horizontal distribution of the impact pressure can be considered as the equivalent distribution. For engineering purposes, the longitudinal distribution of the pressure can be generalized to a triangular distribution, from which a new impact method considering granular effects is proposed.

Ultrasonic wave propagation predictions for polycrystalline materials using three-dimensional synthetic microstructures: Attenuation.

Ultrasonic attenuation plays a crucial role in inspection for heterogeneous materials such that theoretical models are critical for improved measurements. In this article, several assumptions often used in these models are examined with respect to their influence on attenuation. Here, dream.3d software is used to generate 10 ensembles with different volumes, each containing 50 realizations of equiaxed grains with cubic single-crystal symmetry, from which attenuations are calculated. Comparisons are then made with attenuation values derived from classical theories. These theories often decouple the spatial and tensorial components of the microstructure, assume statistical isotropy, and use a spatial correlation function that has a specific exponential form. The validity of these assumptions is examined by calculation of the spatial statistics to obtain the attenuations in their most general form. The results of Voigt-averaged results for nickel at 15 MHz show that the longitudinal and transverse attenuations are about one-third and one-fourth of those obtained from the theory, respectively. Such a difference is attributed to the relevant spatial correlation functions. The results also show a slight anisotropy in the attenuation. Finally, for microstructures with narrow grain size distributions and weak texture, the decoupling assumption is shown to be valid.

Longitudinal Ultrasound Velocity and Attenuation in Isotopically Enriched Cadmium

Longitudinal Ultrasound Velocity and Attenuation in Isotopically Enriched Cadmium

A simultaneous non-destructive characterisation method for grain size and single-crystal elastic constants of cubic polycrystals from ultrasonic measurements

A systematic method was proposed to simultaneously determine both the grain size and elastic constants of a crystallite for cubic polycrystalline materials from ultrasonic velocities, backscattering and attenuation by model-based inversion. The direct model for backscattering was based on single scattering assumption while attenuation was under the frame of the far-field scattering model. At the beginning of the inversion, metal density and ultrasonic velocities, including longitudinal and shear velocities, were measured. Longitudinal backscattering and attenuation coefficients were extracted from the immersion scanning to implement model-based inversion. There were two independent steps to reconstruct grain size and single-crystal elastic constants. The grain sizes were obtained first from the attenuation-to-backscattering ratio, which was exclusively dependent on grain geometry. Furthermore, the anisotropy factor of a cubic crystal can be reconstructed from backscattering amplitude and attenuation coefficients. Since there are only three independent elastic constants for a cubic symmetry crystallite, all three can be determined from experiments. Finally, this method was validated using a pure copper sample. The determined grain size is reasonably close to that identified by optical microscopy and the reconstructed elastic constants are in close agreement with those offered in the literature. Thus, this study provides a practical non-destructive approach for microstructure characterisation.

Characterization of photoreceptor degeneration in the rhodopsin P23H transgenic rat line 2 using optical coherence tomography.

PurposeTo characterize the optical coherence tomography (OCT) appearances of photoreceptor degeneration in the rhodopsin P23H transgenic rat (line 2) in relation to the histological, ultrastructural, and electroretinography (ERG) findings.Materials and methodsHomozygous rhodopsin P23H transgenic albino rats (line 2, very-slow degeneration model) were employed. Using OCT (Micron IV®; Phoenix Research Labs, Pleasanton, CA, USA), the natural course of photoreceptor degeneration was recorded from postnatal day (P) 15 to P 287. The OCT images were qualitatively observed by comparing them to histological and ultrastructural findings at P 62 and P 169. In addition, each retinal layer was quantitatively analyzed longitudinally during degeneration, compared it to that observed in wild type Sprague-Dawley (SD) rats. The relationships between the ERG (full-field combined rod-cone response, 3.0 cds/m2 stimulation) findings and OCT images were also analyzed.ResultsIn the qualitative study, the two layers presumably corresponding to the photoreceptor inner segment ellipsoid zone (EZ) and interdigitation zone (IZ) were identified in the P23H rat until PN day 32. However, the photoreceptor inner and outer segment (IS/OS) layer became diffusely hyperreflective on OCT after P 46, and the EZ and IZ zones could no longer be identified on OCT. In contrast, in the SD rats, the EZ and IZ were clearly distinguished until at least P 247. The ultrastructural study showed partial disarrangements of the photoreceptor outer segment discs in the P23H rats at P 62, although a light-microscopic histological study detected almost no abnormality in the outer segment. In the quantitative study, the outer retinal layer including the outer plexiform layer (OPL) and the outer nuclear layer (ONL) became significantly thinner in the P23H rats than in the SD rats after P 71. The thickness of the IS/OS layer was maintained in the P23H rats until P 130, and it became statistically thinner than in the SD rats at P 237. The longitudinal attenuation in the amplitude of the a- and b-waves of ERG was significantly correlated with the thickness of the combined OPL and ONL but not with that of the IS/OS layer.ConclusionOCT showed the degenerated photoreceptor IS/OS layer in rhodopsin P23H transgenic rats (line 2) as a diffuse hyperreflective zone, even in the early stage, with the partially disarranged and destabilized OS discs recognizable by ultrastructural assessment but not by a histological study. The amplitude of the a- and b-waves mainly depends on the thickness of the OPL and ONL layer rather than the thickness of the photoreceptor IS/OS layer in P23H rats.

Orientation relationship between ferrite and austenite and its influence on ultrasonic attenuation in cast austenitic stainless steel

Cast austenitic stainless steel (CASS) is widely used in important engineering components, which has a two-phase microstructure, i.e.austenite and ferrite. With slow cooling rate during solidification procedure, the austenite grain is coarse and the morphology of ferrite is complex. Due to the remarkable elasticity anisotropy of austenite, the resulting structural noise makes the recognition of macroscopic defects quite difficult in ultrasonic testing. To improve the signal-to-noise ratio, the ultrasonic testing frequency is generally small, about 0.5-2.0 MHz, and the ultrasonic scattering effect of ferrite is ignored. However, for submillimeter or even smaller defect and damage near the surface, the ultrasonic testing frequency should be increased to achieve a higher resolution. In these cases, how the ferrite influences the ultrasonic wave propagation behavior and the testing result is still not conclusive. Therefore, CASS Z3CN20-09M is studied as an example in this paper. Based on ultrasonic propagation modeling and “in situ” experimental design, the crystal orientation relationship between ferrite and austenite in CASS is studied and the factors influencing the ultrasonic scattering attenuation are clarified. The results would be helpful for clarifying the ultrasonic response mechanism of CASS and critical for the quantitative evaluation of small defects and early-stage damage.<br/>The orientation relationship between ferrite and austenite and its influence on ultrasonic scattering attenuation in CASS are studied. The crystal orientations and their relationships between two phases are characterized by the EBSD technique. A two-dimension anisotropic model is built based on the morphology of ferrite, and the ultrasonic propagation is calculated by the time domain finite difference method. The influences of orientation relationship and morphology on the longitudinal wave attenuation are analyzed and verified by “in-situ” experiments. Results show that ferrite grains with bar or island shape are distributed on the austenite grains. The orientation relationship between ferrite and austenite is mainly Kurdjumov-Sachs relationship, and only a minority of ferrite and austenite satisfy the Nishiyama-Wassermann relationship. Numerical simulation of the ultrasonic propagation under a testing frequency of 15 MHz indicates that the orientation relationships between two phases and ferrite morphologies present collaborative effects on the ultrasonic scattering attenuation, which could not be ignored. The factors influencing the ultrasonic attenuation in <101> austenite grain are quantitatively analyzed. It is found that in single austenite grains of CASS, the inhomogeneity of crystal orientation, the orientation relationship between austenite and ferrite and the ferrite morphology play an important role in determining the total ultrasonic attenuation.<br/>The results would provide supports for clarifying the ultrasonic response mechanism of CASS and developing the quantitative evaluation methods.

Ultrasonic sacttering unified theory for polycrystal material with grain sizes distribution

The existing unified theory of ultrasonic scattering can model the attenuation and phase velocity in the frequency domain by using the microstructure and mechanical properties of polycrystalline materials. However, this theory does not consider the influence of grain size distribution, thus degrading the calculation accuracy in the forward modeling. A new unified theory, which is mainly corrected by considering the grain size distribution, is developed. First, the second-order Keller approximation and the full-field Green's function are used to calculate the wave equation of inhomogeneous medium and derive the average wave in the medium, respectively. Second, the method of the truncated lognormal distribution is used to describe the grain size distribution and construct the weighted spatial correlation function. Finally, the new unified theory of ultrasonic scattering is established to reveal the influence of grain distribution on ultrasonic scattering.<br/>Using the new unified model, the effects of the grain distribution widening on the ultrasonic scattering while the average grain size is unchanged, are analyzed for the longitudinal wave and the shear wave. The attenuation increases in the Rayleigh scattering region and the geometric scattering region, while there is less attenuation variation in the stochastic scattering region and two adjacent transition regions. The phase velocity varies strongly in the stochastic-geometric transition region, while the variation is relatively small in other scattering zones. Experiments are conducted by using a 304 stainless steel specimen. The results show that when the grain distribution characteristics are considered, the discrepancy between the longitudinal wave attenuation spectrum and experimental results, and that between the phase velocity spectrum and experimental results are reduced by 49% and 64%, respectively; for the shear wave, these discrepancies are reduced by 12% and 4%, respectively.<br/>From all above aspects, the accuracy of the new model is higher than that of the traditional model. The new unified theory proposed in this paper can effectively correct the discrepancy of the attenuation spectrum and phase velocity spectrum caused by the grain size distribution and provide a theoretical basis for inverse problem of grain distribution. Also, the theory can be extended to materials containing elongated grains, macroscopic texture or multiple phases.

Измерение коэффициента затухания ультразвуковых волн в неоднородных материалах при одностороннем доступе

The article considers an improved method for measuring the ultrasonic wave attenuation coefficient in objects made of materials with a heterogeneous structure under the conditions of unilateral access to objects. The method is primarily aimed at assessing the structural condition of critical bulky objects made of concrete, cast iron, and other intricately structured materials with a view to prevent possible accidents and disasters. The ultrasonic wave attenuation coefficient is determined by processing the realizations of structural noise resulting from the reflection of a probing signal from numerous heterogeneities in the object. The article suggests an improved algorithm for processing the realizations of structural noise, the use of which makes it possible to obtain better accuracy of measurements, to expand the applicability limits of the method, and to simplify the received signals processing procedure. Matters concerned with optimizing the probing signal parameters and the transducer characteristics are studied. A number of experiments were carried out for checking the efficiency of the proposed method and of the new algorithm for processing structural noise realizations. Measurements of the longitudinal ultrasonic wave attenuation coefficient in a specially fabricated concrete block were carried out. The measurements were carried out using two methods: the classic shadow method and the proposed method based on processing the structural noise realizations. The experiments showed good results: the measurement accuracy has been improved by almost 10 times compared with the previous version of the proposed method. The most appropriate application field of the proposed method is monitoring the structural state of critical bulky objects made of materials with a heterogeneous structure like dams and bridge supports under the conditions in which it is impossible to apply other ultrasonic non-destructive testing methods, e.g., those based on ultrasonic wave velocity measurements.

Temperature dependence of hypersound attenuation in silica films via picosecond acoustics

We report picosecond acoustic measurements of longitudinal sound dispersion and attenuation in an amorphous SiO2 layer at temperatures from 20 to 300 K over frequencies ranging from about 40 to 200 GHz. The sample is a radio frequency cathodic sputtered silica layer grown on a sapphire substrate with an aluminum film transducer deposited on top. Acoustic attenuation is evaluated from the simultaneous analysis of three successive echoes using transfer matrix calculation. Results are found to follow rather well a model combining coupling to thermally activated relaxations of structural defects and interactions with thermal vibrations. This leads to a nontrivial variation of the attenuation coefficient with frequency and temperature. The number density of relaxing defects in the SiO2 layer is found to be slightly higher than that in bulk v-SiO2. In contrast, similar anharmonic contribution to acoustic absorption is observed in both systems. The velocity variations are also measured and are compared to the dynamical velocity changes deduced from the sound attenuation.