Failure Process Analysis Research Articles

Gas diffusion layer wettability determination by cyclic voltammetry for automotive fuel cells

AbstractFuel cell performance and durability are highly dependent on water management, wherefore wettability properties of the cell's gas diffusion layer (GDL) are important. In this work, we implement a method to determine the GDL wetted surface area, which is based on capacitance measurements by cyclic voltammetry with a pH‐neutral, aqueous electrolyte, to an automotive size fuel cell failure analysis process and demonstrate its benefit. The electrolyte penetrates large pores of the GDL, wherefore, in contrast to the most conventional methods, also inner parts of the GDL are measured. Tenside concentration of the electrolyte and penetration time, polytetrafluoroethylene treatment of the GDL, and properties of the microporous layer highly influence the capacitance values. Thus, the method is sensitive to different GDL morphologies and surface modifications. Different degradation patterns for samples either artificially chemically and mechanically aged or after real‐operation (e.g., prototype vehicle) are detected by the method. For a comprehensive understanding, the obtained results are compared to ex situ degradation analysis data. A comparison to static contact angle measurements, being a state‐of‐the‐art method to determine GDL wettability, reveals a higher sensitivity of the introduced method to detect degradation, in particular, of chemically aged and real‐operation aged GDLs.

Numerical simulation of brittleness effect on propagation behavior of glutenite hydraulic fractures

For hydrocarbon exploitation in tight glutenite, glutenite brittleness plays an important role in fracturing efficiency, but the detailed effect is not entirely clear. In this study, considering rock heterogeneity, RFPA (Rock Failure Process Analysis) was applied to establish the three-dimension computational model. An improved rock brittleness index considering post-peak properties in rock uniaxial compression test was proposed to quantify the glutenite brittleness. Results show that brittle glutenite can generate multiple failure surfaces under the exterior loadings, which can be a mirror to reveal the denser development of natural fractures in the brittle reservoir, while the ductile glutenite is prone to generate a simple shear failure. Hydraulic fractures propagate faster in more brittle glutenite and can generate short branches, which is beneficial to connect the reservoir natural fractures and form complex hydraulic fractures. Brittle glutenite requires a lower treatment pressure than the ductile glutenite, which can provide convenience for ground pumping operation. Hydraulic fractures prefer to propagate in brittle glutenite, rather than the ductile glutenite. The obtained conclusions can provide a reference for the similar practice.

TOUGH-RFPA: Coupled thermal-hydraulic-mechanical Rock Failure Process Analysis with application to deep geothermal wells

This paper presents a coupled thermal-hydraulic-mechanical (THM) model for simulating failure processes in geological media. The coupling between stress/strain and heat transfer describing how thermal expansion affects stress is based on the theory of thermo-elasticity. Additionally, the coupling between stress/strain and fluid flow describing how pore-pressure affects stress is governed by Biot's consolidation theory. Stress/strain-dependent porosity/permeability/capillary-pressure defines the influence of stress on fluid flow and thermal fields. The THM model is implemented in the solver module that links the multiphase, multicomponent fluid flow simulator TOUGH2 with the geomechanical simulator RFPA (Rock Failure Process Analysis), which is based on the concept that heterogeneity leads to non-linearity and progressive failure behavior. Capability of TOUGH-RFPA simulator was verified by two benchmark problems related to hydraulic-mechanical and thermal-mechanical response in a hollow cylinder. Then the applicability of TOUGH-RFPA was demonstrated for modeling cooling-induced damage around a very deep geothermal well with mesoscopic heterogeneous properties, which showed complex damage evolution and fracturing during well cooling and fluid pressurization.

Analysis of cascading failures of power cyber-physical systems considering false data injection attacks

This study considers the performance impacts of false data injection attacks on the cascading failures of a power cyber-physical system, and identifies vulnerable nodes. First, considering the monitoring and control functions of a cyber network and power flow characteristics of a power network, a power cyber-physical system model is established. Then, the influences of a false data attack on the decision-making and control processes of the cyber network communication processes are studied, and a cascading failure analysis process is proposed for the cyber-attack environment. In addition, a vulnerability evaluation index is defined from two perspectives, i.e., the topology integrity and power network operation characteristics. Moreover, the effectiveness of a power flow betweenness assessment for vulnerable nodes in the cyber- physical environment is verified based on comparing the node power flow betweenness and vulnerability assessment index. Finally, an IEEE14-bus power network is selected for constructing a power cyber-physical system. Simulations show that both the uplink communication channel and downlink communication channel suffer from false data attacks, which affect the ability of the cyber network to suppress the propagation of cascading failures, and expand the scale of the cascading failures. The vulnerability evaluation index is calculated for each node, so as to verify the effectiveness of identifying vulnerable nodes based on the power flow betweenness.

Behaviour of the joint between slabs and walls composed of light steel and foam concrete

Aiming at the prefabricated composite structure, a new type of interior joint in the prefabricated slab wall system suitable for light steel-concrete composite structures is proposed. In the paper, the pseudo-static test of the joint was designed and carried out, and its failure pattern was mainly described. In addition, the test data was analysed from the perspectives of load-displacement curve, strength, stiffness, ductility and energy dissipation. The finite element analysis software ABAQUS was used to carry out finite element modelling and calculation of the new joint, and the results were compared and analysed in terms of failure process and load-displacement curve analysis. The results show that it is feasible to apply the joints in this new type of slab wall system to light steel concrete composite structures.

Influence of steel fiber on dynamic failure property of twin fibers pull-out specimens

By using the realistic failure process analysis RFPA^2D-dynamics software, we establish a dynamic numerical model for twin fibers pull-out test. The microscopic non-uniformity of mechanical properties of concrete materials in different phases is accepted to study the influence of steel fiber volume content under dynamic load on mechanical properties. According to the elastic damage constitutive relationship of the meso-element, the dynamic tensile failure process of specimens with various steel fiber volume content, from micro-cracks initiation, propagation to crack penetration, is studied in details. Furthermore, curves of load-displacement and acoustic emission cumulative energy could also be obtained. The results show that variation of steel fiber volume content has significant influence on the failure pattern of the specimen under dynamic load. With the increase of steel fiber volume fraction, the energy consumption level of specimens first increases and then decreases. When the steel fiber volume fraction is 1.6%, the energy consumption reaches the highest, which is about 1.36 times of that of the plain concrete specimens.

Influence of weak inclusions on the fracturing and fractal behavior of a jointed rock mass containing an opening: Experimental and numerical studies

To gain a better understanding of the interaction between weak inclusions and jointed rock masses, a conceptual model containing a joint set and an opening is prepared, and cases involving unfilled and filled openings are considered. The influence of weak inclusions on the fracturing and fractal behavior of these models is investigated by using laboratory experiments, the rock failure process analysis (RFPA) code and the fractal geometry. An overhanging beam model is proposed to explain the initiation mechanism of tensile cracks around the unfilled and filled openings. The RFPA simulations provide deep and quantitative insight into the fracturing behavior. The coalescence patterns of the surrounding rock mass obtained from physical and numerical tests are in reasonable agreement, and can be classified into two categories: shearing along the joint set and cutting through the rock bridges. The inclusion with a low strength exerts little influence on the failure pattern but has an appreciable reduction in the stress values and an appreciable increment in the mechanical properties. The crack coalescence exhibits fractal properties, and there are good correlations between the fractal dimension and mechanical properties of these jointed rock specimens containing unfilled and filled openings.

Numerical Simulation of Rock Uniaxial Compressive Strength and Deformation Failure Law under Different Size Conditions

Rock engineering occupies an important position in the 21st century. In the face of rock engineering disasters, we are only looking for the essential problems through experiments on rocks, but rock experiments cannot be realized in large numbers, so the article uses numerical simulation software RFPA (Realistic Failure Process Analysis) 2D Basic to simulate rock under different size conditions numerically. In this paper, a rock model with a diameter of 50 mm is used for simulation research. Meanwhile, five calculation models of height-to-diameter ratios of 1.0, 1.5, 2.0, 2.5, and 3 are used. Through simulation calculation, we find that the rock model failure is more than complicated when the value of the height-to-diameter ratio is exceedingly low (1), but as the height-to-diameter ratio increases, the failure mode will become simpler. The stress-concentrated failure will be in the form of axial failure. When the height-to-diameter ratio increases (1.5–2), other damage cracks appear on the basis of axial cleavage failure. As the height-to-diameter ratio continues to increase (about 2.5), only shear failure occurs. When the height-to-diameter ratio reaches a relatively high level (3), there will be both axial rip and other damage. When the height-to-diameter ratio is oversize, there will be both axial rip failure and end damage.

Failure process analysis of frustum windows for deep-sea manned cabin

ABSTRACT The commonly used acrylic frustum windows are the weakest components in the deep-sea manned cabin of the submersible. The short-term collapse test is typically required by full or scale model test, which can applied to measure the critical pressure but cannot tell when the structure begins to damage and how the damage continues. To make up for the deficiency, it is necessary to develop the design-by-analysis methodology. In this paper, numerical analysis for the pressurisation process of the frustum windows considering two failure criteria is carried out to capture the initial failure pressure and location. An effective way to study the detailed failure status and failure process of window under pressurisation is provided. The results are regressed as functions and compared with the values of conversion factor applied in window’s design based on ASME specification, which will provide reference for safe design based on determining safety factor reasonably.

Height of the Fractured Zone in Coal Mining under Extra‐Thick Coal Seam Geological Conditions

The height of the fractured zone caused by coal mining is extremely significant for safely mining under water, water conservation, and gas treatment. At present, the common prediction methods of overburden fractured zone height are only applicable to thin and medium‐thick coal seams, not suitable for thick and extra‐thick coal seams. In order to determine the overburden fractured zone distribution characteristics of extra‐thick seam mining, failure process analysis method of overlying strata was proposed based on key strata theory. This method was applied to 15 m coal seam of Tongxin coal mine, and fractured zone height was determined to be 174 m for 8100 panel. EH4 electromagnetic image system and borehole televiewer survey were also conducted to verify the theory results. The distribution of the electrical conductivity showed that the failure height was 150–170 m. Observation through the borehole televiewer showed that the fractured zone height was 171 m. The results of the two field test methods showed that the fractured zone height was 150–171 m, and it was consistent with the theory calculation results. Therefore, this failure process analysis method of overlying strata can be safely used for other coal mines.

Numerical experimental study on influence factors of anchoring force of constant resistance bolt

It is one of the most important problems to judge anchoring force of constant resistant bolt in anchoring support. This involves the properties of rock, binder and bolt material as well as the interaction among them in the process of stress. In this paper, a series of numerical experiments have been carried out by using the software "Real Failure Process Analysis (RFPA)", which is suitable for the study of complex mechanism of nonlinear and discontinuous media. Four factors that affect the anchoring force of constant resistance bolt are analyzed, the whole process of mechanical failure of anchored rock with constant resistance bolt is monitored, and the failure mode of anchored rock under different conditions is revealed. Draw a few conclusions with reference significance.

A Numerical Investigation on the Hydraulic Fracturing Effect of Water Inrush during Tunnel Excavation

When a high-pressure water source is located near a tunnel under excavation, water inrush is commonly associated with a hydraulic fracturing effect. To study the hydraulic fracturing effect of water inrush (HFEWI), flow-rock failure process analysis (F-RFPA2D) was adopted to simulate the water inrush process. The simulated results indicated that a stress disturbance area formed in front of the excavation face and that a hydraulic fracture zone formed in front of the karst cavity. Similarly, stress concentrations formed in front of the excavation face and the karst cavity. The hydraulic fracturing effect was characterized by stress concentration, and the local hydraulic crack propagation was the result of stress concentration. In addition, a pore pressure gradient formed in the crack-free area of the surrounding rock, and the occurrence of hydraulic cracking was the root cause of the significant change in water flow. When the hydraulic cracks initially formed and expanded, the zone of crack activity was large. As the cracks continued to expand, the range of activity decreased and finally concentrated directly in front of the excavation face. Additionally, the shapes of the water inrush channel obtained by the experimentation and numerical simulation were basically the same: semielliptical. During the evolution of hydraulic crack initiation, expansion, and penetration, the bottom of the excavated borehole was initially dry and then experienced seepage and water inrush. Finally, the minimum safe thickness of the rock wall was calculated to provide a safety guideline for this type of water inrush.

Floor water inrush analysis based on mechanical failure characters and microseismic monitoring

Failure characteristics of the coal seam floor above-confined aquifer are the key to understanding water inrush mechanism during mining. Due to the heterogeneity of coal strata and complex structures of the coal seam floor, it is still difficult to comprehensively reveal failure characteristics of the coal seam floor and accurately determine the floor failure depth during mining above the confined aquifer. In this research, Dongjiahe coal mine, where the coal seam floor is exposed to abundant Ordovician limestone water, is taken as a case study. An improved theoretical model is firstly proposed, in which the whole confined aquifer in the floor is taken into account for stress and failure analysis. Then, RFPA (Rock Failure Process Analysis), a numerical test method considering the heterogeneity of coal and rock, is adopted to simulate the progressive failure process of coal seam floor during mining. Moreover, The microseismic monitoring system is constructed and the 3D visualization is realized through virtual reality, which can sense the initiation and expansion of microdamage in real-time. Based on microseismic monitoring results, we investigate and calculate the floor disturbance depth. By comparing the failure characteristics obtained by different methods, we discuss the pros and cons of the above methods. Finally, a comprehensive analysis method and roadmap are proposed to analyze the failure characteristics of the coal seam floor more accurately based on theoretical calculation, numerical simulation, and microseismic monitoring. The results can provide some reference for the evaluation and prevention of floor water inrush hazards.

Seismic slope stability and failure process analysis using explicit finite element method

Earthquake is one of the primary factors that triggers the failure of slopes and the landslides in the mountainous area. In this study, an efficient method for the seismic slope stability analysis and seismic failure process simulation is proposed. The seismic slope stability and the failure process of a two-dimensional slope model are analyzed via the explicit finite element method with the influence of the dynamic mechanical behavior of soil and the frictional resistance characteristic of the slope rupture surface. A dynamic visco-elasto-plastic constituted model for soil is established and written in FORTRAN as a user subroutine of the finite element method code of ABAQUS. The validation of the visco-elasto-plastic constituted model and the explicit finite element method are compared with the experimental results and the implicit method. The seismic factor of safety and the sliding distance of the soil slopes under the natural earthquake conditions are calculated via the dynamic visco-elasto-plastic constituted model and the explicit finite element method. The influence of the ground motion characteristics on the seismic slope stability is analyzed in this study via the specified elastic response acceleration spectrum. The analysis results of this paper suggest that the seismic slope stability analysis and the progressive failure process of the slope under the seismic condition can be analyzed by the explicit finite element method efficiently. The results show that the dynamic mechanical behavior of soil and the ground motion characteristics are both critical influence factors for the seismic slope stability. The frictional resistance characteristic of the slope rupture surface is the principal influence factor for the sliding distance of the slope, and it also has more influence on the sliding distance of a high slope than that of the small slope.

Numerical simulation on effect of heterogeneity on mode I fracture characteristics of rock

Rock is more sensitive to tensile loading than compressive loading, since the tensile strength of rock is much lower than compressive strength. The fracture characteristics of rock in the tensile state are of great significance to the understanding of rock failure mechanisms. To this end, we have conducted numerical simulation researches on mode I cracking process of rock with varying homogeneity, using the Realistic Failure Process Analysis program. With the increase of homogeneity, cracks are concentrating to the ligament area with a decreasing number of crack bifurcations, and the main crack path is becoming smooth. Crack behaviors and mechanical properties are influenced significantly when the homogeneity index is in the range of 1.5 to 5. When the homogeneity index is greater than 30, they are not affected by rock homogeneity and the rock can be considered as essentially homogeneous. It is considered that the global and local strengths are affected by the distribution of rock mechanical properties at mesoscale, which influence the crack behaviors and mechanical characteristics.

Distance effects of the fault on the surrounding rock mass stability of the main powerhouse at the Huanggou pumped-storage power station

The distance between the fault and the underground cavern affects the surrounding rock mass stability, which is of great importance to the design and construction of the large-scale underground cavern especially the underground powerhouse. This study focuses on the distance effects of the fault on the rock mass stability of the main powerhouse at the Huanggou pumped-storage power station. Firstly, a microseismic monitoring system was constructed to investigate the distribution and mechanism of distance-dependent damage of the surrounding rock masses of the main powerhouse. Then, the rock failure process analysis and strength reduction method (RFPA-SRM) was adopted to reproduce the failure processes of the surrounding rock masses of the main powerhouse at different distance to the fault f34. Finally, by comparing the numerical simulation results with microseismic monitoring results, the distance effects of the fault f34 on the damage mechanism and failure modes were discussed. It can be concluded that when the distance is low, the damage of the surrounding rock masses subjected to excavation evolved from the spandrel toward the fault, eventually causing a V-shaped collapse if the strength reduction continues. As the distance increases, the effects of the fault on the damage mechanism and failure modes are reduced and the safety factor of the underground cavern is improved. The findings are expected to provide references for evaluating the surrounding rock mass stability of an underground powerhouse under complex geological conditions.

Seismic stability and failure process analysis of earth-filled dam

Earthquake is one of the primary influence factors for the stability and safety of the earth-filled dam. An efficient way for the seismic stability and failure process analysis of the earth-filled dam is proposed in this paper. In this study, taking the dynamic mechanical behavior of soil and the ground motion characteristics into account, the seismic stability, and failure process of a two-dimensional earth-filled dam is analyzed via the explicit finite element method. A dynamic visco-elasto-plastic constituted model for soil is established and written in FORTRAN as a user subroutine of the finite element method code ABAQUS. The seismic response, residual deformation, and failure process of the earth-filled dam under natural earthquake conditions are analyzed using the dynamic visco-elasto-plastic constituted model and the explicit finite element method. Moreover, the influence of the ground motion characteristics on the seismic stability of the earth-filled dam is analyzed via the specified elastic response acceleration spectrum. The analysis results of this paper suggest that the seismic stability and the progressive failure process analysis of the earth-filled dam under the seismic condition can be analyzed efficiently by explicit finite element method. The dynamic mechanical behavior of soil and the ground motion characteristics are both critical influence factors for the seismic stability analysis of the earth-filled dam. The research work of this study is helpful for the analysis of the failure process of the earth-filled dam collapse.

Numerical Investigation of Fracture Network Formation under Multiple Wells

A two-step fracturing method is proposed to investigate the hydraulic fracture evolution behavior and the process of complex fracture network formation under multiple wells. Simulations are conducted with Rock Failure Process Analysis code. Heterogeneity and permeability of the rocks are considered in this study. In Step 1, the influence of an asymmetric pressure gradient on the fracture evolution is simulated, and an artificial structural plane is formed. The simulation results reflect the macroscopic fracture evolution induced by mesoscopic failure; these results agree well with the characteristics of the experiments. Step 2, which is based on the first step, investigates the influence of preexisting fractures (i.e., artificial structural planes) on the subsequent fracturing behavior. The simulation results are supported by mechanics analysis. Results indicated that the fracture evolution is influenced by pressure magnitude on a local scale around the fracture tip and by the orientation and distribution of pore pressure on a global scale. The constant pressure in wellbore H2 can affect fracture propagation by changing the water flow direction, and the hydraulic fractures will propagate to the direction of higher local pore pressure. Furthermore, the artificial structural planes influence the stress distribution surrounding the wellbores and the hydraulic fracture evolution by altering the induced stresses around the preexisting fractures. Finally, fracture network is formed among the artificial structural planes and hydraulic fractures when multiple wells are fractured successively. This study provides valuable guidance to unconventional reservoir reconstruction designs.

Progress of sodium battery failure research

<p indent="0mm">Sodium batteries have attracted much attention owing to their abundant reserves and low cost of Na raw materials and diverse sources of electrode materials. However, even if no external factors are applied, sodium batteries might fail in a certain probability during the normal operation, which exhibits the failure modes of electrochemical performance decay and the component destruction of the batteries, resulting in these failure consequences such as battery thermal runaway, short circuit, fire etc. In this paper, various failure causes are analyzed for the components of sodium batteries like cathodes, anodes, electrolytes, separators, current collectors and the case of the sodium battery, according to the theoretical failure chain model of failure mode and effect analysis (FMEA) technology. Progress of sodium battery failure research is summarized based on above sodium battery components. Moreover, the failure analysis process of sodium batteries is also proposed. The sodium battery failure analysis system is finally proposed. It will provide the informations for the failure research, safe use, design and manufacture of sodium batteries.

A Crack Propagation Control Study of Directional Hydraulic Fracturing Based on Hydraulic Slotting and a Nonuniform Pore Pressure Field

Hydraulic fracturing techniques for developing deeply buried coal reservoirs face routine problems related to high initial pressures and limited control over the fracture propagation direction. A novel method of directional hydraulic fracturing (DHF) based on hydraulic slotting in a nonuniform pore pressure field is proposed. A mechanical model is used to address crack initiation and propagation in a nonuniform pore pressure field, where cracks tend to rupture and propagate towards zones of high pore pressure for reducing the effective rock stress more. The crack initiation pressure and propagation morphology are analyzed by rock failure process analysis software. The numerical results show that the directional propagation of hydraulic fracturing cracks is possible when the horizontal stress difference coefficient is less than or equal to 0.5 or the slotting deviation angle is less than or equal to 30°. These findings are in good agreement with experimental results, which support the accuracy and reliability of the proposed method and theory.