Stress Wave Attenuation Research Articles

Multi-fractured stimulation technique of hydraulic fracturing assisted by the DTH-hammer-induced impact fractures

Hydraulic fracturing is one of the key technologies in geothermal development of hot-dry-rock (HDR) geothermal energy. How to reduce the fracturing pressure and enhance the fractures connectivity of HDR has become a hot issue. In this paper, we propose to use a down-the-hole (DTH) hammer to impact granite, which assist to create new artificial fractures similar to natural fractures or widen the existing fractures near the wellbore, consequently enhancing the connectivity of fractures during hydraulic fracturing. A series of experiments were conducted to verify the feasibility of this stimulation technique, including the observation and quantification of impact fractures, and the determination of fracturing pressure. In addition, numerical simulation of impact stress wave was carried out to explain the distribution of fractures. The results showed that the granite rock mass damage caused by percussive rotary drilling was about 5–7 times larger than that caused by diamond rotary drilling. Meanwhile, the distribution of fractures was characterized by regionalization, which was mainly attributed to the attenuation of impact stress wave, and the numerical results were consistent with the experimental observations. Laboratory experiment results of hydraulic fracturing indicated that the fracturing pressure of granite samples near the wellbore after percussive drilling had been greatly reduced, which was nearly 50% lower than that of conventional drilling. It indicates that the proposed multi-fractured stimulation technique of hydraulic fracturing assisted by the impact fractures induced by a DTH hammer can be a potential solution to solve the problems of hydraulic fracturing.

An Innovative Approach of Using Continuous Impedance-Graded Metallic Composite System for Attenuation of Stress Waves

This paper presents an innovative approach of stress attenuation through a continuous impedance-graded material system for high strain-rate events. High energetic dynamic events such as blasts and impact could cause stress waves—in the form of elastic, plastic, and shock—to propagate in a solid material. An impedance-graded composite is created by arranging different metallic alloys in the reducing order of their impedance through the system. Impedance, which is the product of volumetric mass density and wave velocity, is chosen as the function as it plays a governing role in elastic, plastic, and shock waves. An analytical framework to quantify the stress wave propagation through an impedance-graded multimaterial system is developed based on the principles of shock and elastic wave theories. The numerical simulations carried out using nonlinear finite element code, LS-DYNA, were able to capture and quantify the elastic, plastic, and shock waves and their reflections at different interfaces. It was identified that the final transmitted stress wave, which could comprise elastic, plastic, and shock waves, as well as the reflected tensile elastic wave at each material interface, needs to be controlled in order to develop a robust multimaterial system.

Study on the Decoupled Charge Effect in Deep‐Hole Cumulative Blasting of Coal Seam

Five models of cumulative blasting are established by using ANSYS/LS‐DYNA to study the effect of decoupling coefficient on cumulative blasting to improve coal seam permeability. The formation and migration process of the shaped energy jets with two kinds of decoupling coefficient are compared and analyzed; also, the propagation of explosive stress waves is represented. The result showed that the air in the blast hole is the key to the formation and migration of the condensing jet. The air in the hole also could reduce the attenuation of stress wave in a certain range. However, if the decoupling coefficient is too large, the air in the hole will consume excessive explosive energy, which is also not conducive to energy transfer. Therefore, there is an optimum decoupling coefficient which can minimize the coal crushing area, increase the coal fissure area, and improve the gas extraction rate. Besides, the cumulative blasting tests were carried out in a coal seam. The test results show that decoupling charge could effectively improve coal seam permeability, and the blasting effect was better when the decoupling coefficient is between 1.67 and 2.

Blasting Impact Simulation Test and Fragmentation Distribution Characteristics in an Open‐Pit Mine

Based on the split Hopkinson pressure bar (SHPB) test system, dynamic impact tests of coal specimens under different impact pressures were carried out to study the relationship between the impact load and the size of crushed lump coal. Based on the theory of stress wave attenuation, the relationship between the blasting impact load in a single‐hole blasting area of a coal seam and the load applied in an impact failure test of a coal specimen in the laboratory was established. According to the characteristics of the fragmentation distribution of the coal specimens destroyed under a laboratory impact load and the requirement of the minimum cost control of coal blasting in an open‐pit coal mine, the fragmentation size range was divided into three groups: large‐diameter, medium‐diameter, and powder particles. Based on this range, the variation rule of the mass percentage of coal fragments with impact pressure was obtained. Established on the evaluation principle of the blasting effect in an open‐pit coal mine, a good impact fragmentation effect was obtained. The good pressure range is 0.30 MPa ≤ P < 0.90 MPa.

Distribution Characteristics of Fragments Size and Optimization of Blasting Parameters Under Blasting Impact Load in Open-Pit Mine

The size distribution characteristics of fragments formed after loose blasting in open-pit mine will directly affect the economic benefits of the mining enterprise. The impact load test of coal samples with pressure ranging from 0.13MPa to 2.0MPa was carried out by using the split Hope Pressure Bar Test System (SHPB). Based on the theory of stress wave attenuation, the relationship between blasting load and coal samples impact test in the laboratory is established. It is concluded that with the increase of impact pressure, the degree of coal samples breakage increases gradually. According to the requirement of blasting quality in open-pit mine, it is concluded that the impact pressure range with good crushing effect is 0.30MPa ≤P ≤0.90 MPa. Based on image recognition method, the fragmentation distribution characteristics of loose deposits of coal seam blasting in open-pit mine are obtained. Combining the impact load test in the laboratory and the fragmentation distribution of coal seam blasting, the influence area of blasting impact fragmentation is divided. According to the research results, two optimization schemes of blasting parameters are put forward and the blasting effect of the two schemes is verified by numerical simulation using ANSYS/LSDYNA. Through field blasting optimization test in open-pit mine, it is concluded that increasing blasting hole spacing and row spacing can effectively control dust pollution and increase massive coal rate.

ЗАБЕЗПЕЧЕННЯ НАДІЙНОЇ ЕКСПЛУАТАЦІЇ НАФТОВИХ РЕЗЕРВУАРІВ У СКЛАДНИХ ГЕОТЕХНІЧНИХ УМОВАХ ПРИ СЕЙСМІЧНИХ ВПЛИВАХ

ЗАБЕЗПЕЧЕННЯ НАДІЙНОЇ ЕКСПЛУАТАЦІЇ НАФТОВИХ РЕЗЕРВУАРІВ У СКЛАДНИХ ГЕОТЕХНІЧНИХ УМОВАХ ПРИ СЕЙСМІЧНИХ ВПЛИВАХ

A Feasibility Study on Timber Moisture Monitoring Using Piezoceramic Transducer-Enabled Active Sensing.

In recent years, the piezoceramic transducer-enabled active sensing technique has been extensively applied to structural damage detection and health monitoring, in civil engineering. Being abundant and renewable, timber has been widely used as a building material in many countries. However, one of the more challenging applications of timber, in construction, is the potential damage caused by moisture. Increased moisture may cause easier warping of timber components and encourage corrosion of integrated metal members, on top of potentially causing rot and decay. However, despite numerous efforts to inspect and monitor the moisture content of timber, there lacks a method that can provide truly real time, quantitative, and non-invasive measurement of timber moisture. Thus, the research presented in this paper investigated the feasibility of moisture-content monitoring using an active sensing approach, as enabled by a pair of the Lead Zirconate Titanate (PZT) transducers bonded on the surface of a timber specimen. Using a pair of transducers in an active sensing scheme, one patch generated a designed stress wave, while another patch received the signal. While the active sensing was active, the moisture content of the timber specimen was gradually increased from 0% to 60% with 10% increments. The material properties of the timber correspondingly changed under varying timber moisture content, resulting in a measurable differential in stress wave attenuation rates among the different specimens used. The experimental results indicated that the received signal energy and the moisture content of the timber specimens show a parabolic relationship. Finally, the feasibility and reliability of the presented method, for monitoring timber moisture content, are discussed.

The shock and spallation behavior of a carbon fiber reinforced polymer composite

Abstract In this work, the shock and spallation behavior of a carbon fiber reinforced polymer (CFRP) composite has been studied using the plate-impact technique. The free surface velocity of the sample was monitored by a dual-laser Doppler pin system (DPS). The results showed that a steady shock front can be achieved and propagated in the composites. However, due to the stress wave dispersion and attenuation, the peak particle velocity was attenuated with propagation distance and became even more apparent as impact velocity increased. In addition, the shock Hugoniot curves were computed and compared with experimental results, and the CFRP was found to be stiffer than the materials cited in the references in the range of 1.5–5.3 GPa. Meanwhile, the spall strengths of the composites were observed to increase with increasing strain rates within the test range. Finally, the soft recovered samples were examined by macroscopic and microscopic analysis and it was found that the fracture surface revealed an envelope of matrix and fiber/matrix interface delamination, indicating that both sites were equally prone to fracture.

Analysis of curved tunnels in soil subjected to internal blast loading

The present study deals with three-dimensional nonlinear finite element (FE) analyses of tunnels with curved alignment in the longitudinal direction, subjected to internal blast loading. Blast load is simulated using the coupled Eulerian–Lagrangian (CEL) analysis tool available in FE software Abaqus/Explicit. The explosive and the surrounding air have been modeled using the Eulerian elements. Soil and concrete lining are modeled using the Lagrangian elements. The trinitrotoluene (TNT) explosive is modeled using Jones–Wilkins–Lee (JWL) equation-of-state (EOS). The Drucker–Prager plasticity model has been used to simulate the strain rate dependent stress–strain response of soil. The concrete damage plasticity model has been used to simulate the strain rate dependent behavior of concrete. The deformation, stress and damage response of the tunnel lining and the surrounding soil are investigated herein. Attenuation of blast-induced stress wave in soil is also studied. It is observed that deformation and damage of tunnel lining and soil are dependent on the location of explosive position inside the tunnel and the radius of curvature of the tunnel. Higher deformation and damage is observed in a tunnel with a lesser radius of curvature. A significant amount of ground heave is observed in all analyses.

PZT sensor array for local and distributed measurements of localized cracking in concrete

An application is developed with surface mounted lead zirconate titanate (PZT) patches for sensing damage in the form of a stress-induced crack in a concrete substrate. A localized crack is introduced in a controlled manner using a fracture test. Full-field displacements obtained using digital image correlation are used for crack penetration and crack width measurements. Electrical impedance (EI) measurements are obtained from the individual PZT patches, which are attached at different locations relative to the crack. Stress wave transmission measurements are performed using the PZT patches as actuator-receiver (AR) pairs. The EI measurements indicate that small, quantifiable changes in the mechanical impedance of the substrate are experienced by the PZT patch in the vicinity of the localized crack, which sensitively detect crack initiation. The stress wave-based measurements are very sensitive to the presence of physical discontinuity created by a localized crack in the stress wave path. A measure of stress wave attenuation, the attenuation factor is developed, to quantify the measured changes in the stress wave produced by the physical discontinuity in concrete upon unloading a stress-induced crack. The physical discontinuity due to a stress-induced crack opening on the order of 10 μm can be detected from the measured changes in the attenuation factor. The physical discontinuity in the concrete associated with a stress-induced crack opening on the order of 100 μm produces a complete attenuation of the stress wave of 120 kHz. The combined use of PZT patches in the EI and the AR modes can be used to detect local changes close to a PZT patch and it allows distributed sensing over the entire volume of a structural element.

Analysis of the stress wave effect during coal breakage by a high-pressure abrasive air jet

In view of the defects of borehole collapse, inhibition of gas desorption and migration of gas existing in hydraulic fracturing and other hydraulic permeability–increasing measures for soft coal seams with low-permeability technology is proposed for coal breakage by a high-pressure abrasive gas jet for relieving pressure and increasing permeability. The comparative analysis of gas jet flow field structure between convergent nozzle and Laval nozzle has been given by numerical simulation. For Laval nozzle, the expansion wave and compression wave alternate and move forward steadily in gas jet and vanish when potential core length reaches maximum. So, the Laval nozzle can form more stable flow filed structure of gas jet and avoid shock wave in gas jet. Furthermore, a high-speed camera is adopted to analyze the jet structure and verify the conclusion of numerical simulation. Based on thermodynamic theory, this article calculates and analyzes the critical local sound velocity and pressure generated from the stress wave during the process of coal breakage by the gas jet. Furthermore, experimental coal breakage by a high-pressure abrasive gas jet is carried out. The high-pressure abrasive gas jet impacts the coal body as a quasi-static load and a dynamic load and forms corrosion pits on the surface of the coal body. Penetrating cracks are formed within the coal in the pattern of the loaded stress wave which leads to coal breakage. The effects of porosity and permeability on the propagation of the stress wave in coal are analyzed by establishing the dispersion equation for the spread of the stress wave in coal. The results show that porosity has a significant effect on wave velocity and that the attenuation of the stress wave is intensified with an increase in porosity. Moreover, the stress wave attenuation is more obvious at high frequency. The effect of permeability on the wave velocity is not significant at low frequencies. In contrast, at high frequency and relatively low permeability, the wave velocity increases with the permeability, and the attenuation of the wave velocity initially increases and then decreases. When the permeability is greater than 10−11 m2, the wave velocity is not affected by the permeability. However, the stress wave is not attenuated.

Dynamic load mitigation using dissipative elastic metamaterials with multiple Maxwell-type oscillators

Dissipative elastic metamaterials have attracted increased attention in recent times. This paper presents the development of a dissipative elastic metamaterial with multiple Maxwell-type resonators for stress wave attenuation. The mechanism of the dissipation effect on the vibration characteristics is systematically investigated by mass-spring-damper models with single and dual resonators. Based on the parameter optimization, it is revealed that a broadband wave attenuation region (stopping band) can be obtained by properly utilizing interactions from resonant motions and viscoelastic effects of the Maxwell-type oscillators. The relevant numerical verifications are conducted for various cases, and excellent agreement between the numerical and theoretical frequency response functions is shown. The design of this dissipative metamaterial system is further applied for dynamic load mitigation and blast wave attenuation. Moreover, the transient response in the continuum model is designed and analyzed for more robust design. By virtue of the bandgap merging effect induced by the Maxwell-type damper, the transient blast wave can be almost completely suppressed in the low frequency range. A significantly improved performance of the proposed dissipative metamaterials for stress wave mitigation is verified in both time and frequency domains.

Research on Attenuation of Shock Induced Stress Wave in Multi-Layered Thin Cylindrical Rods

The attenuation of stress wave induced by impact load in multi-layered thin cylindrical rods has been investigated and analyzed. Firstly, based on stress wave propagation theory, the one dimension solution of the response of stress wave in three-layered media has been given. Secondly, a three-layered thin cylindrical rod has been established through FEM, and the propagation and attenuation of stress wave in it has been analyzed. The analytical and numerical results showed that the stress wave attenuation could be achieved by using multi-layered media.

Determining the Viscosity Coefficient for Viscoelastic Wave Propagation in Rock Bars

Rocks with microdefects exhibit viscoelastic behavior during stress wave propagation. The viscosity coefficient of the wave can be used to characterize the attenuation as the wave propagates in rock. In this study, a long artificial bar with a readily adjustable viscosity coefficient was fabricated to investigate stress wave attenuation. The viscoelastic behavior of the artificial bar under dynamic loading was investigated, and the initial viscoelastic coefficient was obtained based on the amplitude attenuation of the incident harmonic wave. A one-dimensional wave propagation program was compiled to reproduce the time history of the stress wave measured during the experiments, and the program was well fitted to the Kelvin–Voigt model. The attenuation and dispersion of the stress wave in long artificial viscoelastic bars were quantified to accurately determine the viscoelastic coefficient. Finally, the method used to determine the viscoelastic coefficient of a long artificial bar based on the experiments and numerical simulations was extended to determine the viscoelastic coefficient of a short rock bar. This study provides a new method of determining the viscosity coefficient of rock.

Stress Wave Attenuation in Aluminum Alloy and Mild Steel Specimens Under SHPB Tensile Testing

Investigations on the effect of intensity of incident pressure wave applied through the striker bar on the specimen force histories and stress wave attenuation during split Hopkinson pressure bar (SHPB) tensile testing are presented. Details of the tensile SHPB along with Lagrangian x–t diagram of the setup are included. Studies were carried out on aluminum alloy 7075 T651 and IS 2062 mild steel. While testing specimens using the tensile SHPB setup, it was observed that the force calculated from the transmitter bar strain gauge was smaller than the force obtained from the incident bar strain gauge. This mismatch between the forces in the incident bar and the transmitter bar is explained on the basis of stress wave attenuation in the specimens. A methodology to obtain force histories using the strain gauges on the specimen during SHPB tensile testing is also presented. Further, scanning electron microscope images and photomicrographs are given. Correlation between the microstructure and mechanical properties is explained. Further, uncertainty analysis was conducted to ascertain the accuracy of the results.

A Designer’s Approach for Estimation of Nuclear‐Air‐Blast‐Induced Ground Motion

A reliable estimate of free‐field ground displacement induced by nuclear‐air‐blast is required for design of underground strategic structures. A generalized pseudostatic formulation is proposed to estimate nuclear‐air‐blast‐induced ground displacement that takes into account nonlinear stress‐strain behaviour of geomaterials, stress‐dependent wave propagation velocity, and stress wave attenuation. This proposed formulation is utilized to develop a closed‐form solution for linearly decaying blast load applied on a layered ground medium with bilinear hysteretic behaviour. Parametric studies of closed‐form solution indicated that selection of appropriate constrained modulus consistent with the overpressure is necessary for an accurate estimation of peak ground displacement. Stress wave attenuation affects the computed displacement under low overpressure, and stress‐dependent wave velocity affects mainly the occurrence time of peak displacement and not its magnitude. Further, peak displacements are estimated using the proposed model as well as the UFC manual and compared against the field data of atmospheric nuclear test carried out at Nevada test site. It is found that the proposed model is in good agreement with field data, whereas the UFC manual significantly underestimates the peak ground displacements under higher overpressures.

Experimental investigation of the dynamic behavior of metaconcrete

A study investigating the dynamic material behavior of a new type of heterogeneous composite called metaconcrete is presented. Instead of sand, stone and gravel in a conventional concrete, the metaconcrete mortar is filled with heavy, spherical aggregates that are encapsulated in a soft coating. This locally resonant metamaterial exhibits band gaps that lead to the attenuation of applied stress waves at the inclusion’s eigenfrequencies. Plate impact experiments are conducted to investigate the viability of the underlying physical concepts for the attenuation of stress waves. To reduce the effects of porosity and inhomogeneity, the mortar material is substituted by an epoxy matrix and a lead core encapsulated in polyurethane serves as the aggregate. Strain gauges applied between the aggregates on the specimen surface are used to evaluate the performance of the metaconcrete microstructure. Initial conditions were varied with respect to impact velocities (18 m/s–100 m/s) and flyer plate thickness (0.8 mm–25.4 mm) to study the dependence of the attenuation properties for a range of impact configurations. Moreover, the resonant frequency of the aggregates was measured using high-speed photography and digital image correlation (DIC) and shown to be in good agreement with analytical predictions. The metaconcrete microstructure lead to a reduction in maximum strain of up to 72% compared to a homogeneous epoxy specimen, showing the potential for metaconcrete to be used in damage mitigation of structures subjected to dynamic stress wave loading.

A feasibility study on real-time evaluation of concrete surface crack repairing using embedded piezoceramic transducers

Surface cracking in large-scale concrete structures are inevitable, and some cracks shall be repaired to prevent further structural damages. Various methods of repairing surface cracks in concrete structures have been studied, however, evaluation of repairing of those cracks, especially through real time means, have not much been reported. In this paper, a feasibility study was performed on evaluation of repairing concrete surface crack in real-time using embedded piezoceramic-based transducers, called Smart Aggregates (SA). Using the active sensing approach, a pair of SAs in a concrete structure can communicate through generating and detecting stress wave. In the experimental study, three concrete beam specimens were investigated. In each specimen, four smart aggregates were embedded in the beam with two designed surface cracks. Subsequently, these two cracks were repaired by using mortar and Carbon Fiber Reinforced Polymer (CFRP) sheet. Due to interfacial properties among the three different interfacial conditions: the healthy concrete structure, the concrete structure with a crack, and the concrete structure with a repaired crack, the stress wave attenuation characteristics differ. The received signals of SAs among those three interfacial conditions were compared in time domain. In addition, a wavelet packet-based energy analysis was performed to quantitatively indicate the energy value of the received signal. Experimental results shows that the signal energy values computed by the wavelet packet-based energy analysis has potential to determine the crack occurrence in concrete structures and to help evaluate the repair effects.

Modeling of the attenuation of stress waves in concrete based on the Rayleigh damping model using time-reversal and PZT transducers

Wave-based concrete structural health monitoring has attracted much attention. A stress wave experiences significant attenuation in concrete, however there is a lack of a unified method for predicting the attenuation coefficient of the stress wave. In this paper, a simple and effective absorption attenuation model of stress waves in concrete is developed based on the Rayleigh damping model, which indicates that the absorption attenuation coefficient of stress waves in concrete is directly proportional to the square of the stress wave frequency when the damping ratio is small. In order to verify the theoretical model, related experiments were carried out. During the experiments, a concrete beam was designed in which the d33-model piezoelectric smart aggregates were embedded to detect the propagation of stress waves. It is difficult to distinguish direct stress waves due to the complex propagation paths and the reflection and scattering of stress waves in concrete. Hence, as another innovation of this paper, a new method for computing the absorption attenuation coefficient based on the time-reversal method is developed. Due to the self-adaptive focusing properties of the time-reversal method, the time-reversed stress wave focuses and generates a peak value. The time-reversal method eliminates the adverse effects of multipaths, reflection, and scattering. The absorption attenuation coefficient is computed by analyzing the peak value changes of the time-reversal focused signal. Finally, the experimental results are found to be in good agreement with the theoretical model.

Enhanced stress wave attenuation of single-walled carbon nanotube lattice via mass mismatch-induced resonance

Nanoscale homogeneous single-walled carbon nanotube lattice exhibits extraordinary attenuation performance and ultrahigh specific energy absorption upon high-velocity impact. However, upon relatively low impact velocity (for example, < 1000 m/s), this system does not possess a good stress wave attenuation performance where severe damage may still occur. To overcome this disadvantage, a new attenuation mechanism is introduced to carbon nanotube lattice via the creation of mass mismatch, i.e. carbon nanotube dimer lattice. Under specific mass ratio, a strong inherent resonance happens, resulting in an evident attenuation of the propagating pulse. The joint effect of mass mismatch-induced resonance and dissipative nature of carbon nanotube lattice make it ideal for both high and low-velocity impacts. Additionally, specific energy absorption is improved due to the mass reduction. This work reveals the possibility to achieve much higher energy dissipation by exploiting the system's inherent dynamics, suggesting a new strategy of lightweight design of protective devices and the potential of technological applications of nanoscale nonlinear granular crystals.