Instability Phenomena Research Articles

Adiabatic Shear Localization in Metallic Materials: Review

In advanced engineering applications, there has been an increasing demand for the service performance of materials under high-strain-rate conditions where a key phenomenon of adiabatic shear instability is inevitably involved. The presence of adiabatic shear instability is typically associated with large shear strains, high strain rates, and elevated temperatures. Significant plastic deformation that concentrates within a adiabatic shear band (ASB) often results in catastrophic failure, and it is necessary to avoid the occurrence of such a phenomenon in most areas. However, in certain areas, such as high-speed machining and self-sharpening projectile penetration, this phenomenon can be exploited. The thermal softening effect and microstructural softening effect are widely recognized as the foundational theories for the formation of ASB. Thus, elucidating various complex deformation mechanisms under thermomechanical coupling along with changes in temperatures in the shear instability process has become a focal point of research. This review highlights these two important aspects and examines the development of relevant theories and experimental results, identifying key challenges faced in this field of study. Furthermore, advancements in modern experimental characterization and computational technologies, which lead to a deeper understanding of the adiabatic shear instability phenomenon, have also been summarized.

Interaction of microstructure evolution mechanism in Ti-0.3Mo-0.8Ni alloy with initial lamellar microstructure during isothermal compression below the β-transus

The flow behavior and interaction of microstructure evolution mechanisms for a near α Ti-0.3Mo-0.8Ni alloy with as-cast lamellar microstructure are investigated at temperatures of 800∼880 °C in the α+β region with wide strain rate range from 0.01s−1 to 30s−1 during isothermal compression. It is demonstrated that the main reasons of flow softening in the true stress-true strain curves are the kinking of lamellar α, dynamic α→β transformation (DT), dynamic spheroidization (DS), and dynamic recovery (DRV) of β grain. The instability phenomena such as adiabatic shear band (ASB) and flow localization (FL) are the main causes of the stress collapse in the curves at higher strain rate. The dislocation density at the kinking position of initial lamellar α is higher. With the increase of strain rate, the mechanism of the DS changes from flat separation to slat shear. The diffusion of the β-stabilizer results in starting the DT, promoted by the thermodynamic coupling. The interactions between different microstructure evolution mechanisms are extremely complex. In the early stage of hot deformation, the DT promotes the kinking of lamellar α. With the increase of deformation amount, the DT inhibits the DS at higher strain rates, and conversely, the DS promotes the DT. At lower strain rates, the DT and DS accelerate each other in the middle and late stages of hot deformation. The DT and DS are restricted by the process of lamellar α kinking in the middle and late stages of hot deformation.

Jeans analysis in fractional gravity

It has recently been demonstrated (Giusti in Phys Rev D 101:124029, 2020, https://doi.org/10.1103/PhysRevD.101.124029) that characteristic traits of Milgrom’s modified Newtonian dynamics (MOND) can be replicated from an entirely distinct framework: a fractional variant of Newtonian mechanics. To further assess its validity, this proposal needs to be tested in relevant astrophysical scenarios. Here, we investigate its implications on Jeans gravitational instability and related phenomena. We examine scenarios involving classical matter confined by gravity and extend our analysis to the quantum domain, through a Schrödinger–Newton approach. We also derive a generalized Lane–Emden equation associated with fractional gravity. Through comparisons between the derived stability criteria and the observed stability of Bok globules, we establish constraints on the theory’s parameters to align with observational data.

Longitudinal vibrations and their suppression of a tethered satellite system using friction self-excitation mechanism

This paper proposes a friction self-excitation mechanism to regulate the longitudinal vibrations of the tether during the state-keeping phase of a two-body tethered satellite system. A simplified dynamic model is established, consisting of two mass blocks, a constant-length tether, and a conveyor belt. The Stribeck friction model is utilized to describe the effects of friction on the stability of longitudinal vibration. Numerical simulations reveal the instability phenomenon induced by Hopf bifurcation and indicate the potential self-excited vibration under different parameters. Additionally, the paper explores the design approach for the tether by introducing the concept of segmented tethering and its dynamic behavior under different design scenarios. Varying the stiffness of the tether can change the vibration modes of the system. Finally, a ground experiment is carried out to evaluate the dynamic behaviors of the system and to demonstrate the effectiveness of the proposed vibration suppression mechanism.

Shock buffet onset prediction with flow feature-informed neural network

Transonic shock buffet is a significant self-excited shock oscillations and aerodynamic instability phenomenon induced by shock-boundary layer interaction, which limits the flight envelope and even causes flight accidents. The aviation industry has a significant interest in accurately predicting the shock buffet onset boundary, defined by a specific combination of Mach number and angle of attack. While the current methods of steady and unsteady numerical simulation suffer from a contradiction of efficiency and accuracy. In the current paper, a flow feature-informed neural network (FINN) model is constructed to predict the buffet onset boundary over airfoils. Typical features associated with buffet onset are extracted from the steady base flow and subsequently integrated into the hidden layer of the neural network to impose physical constraints. With the test cases of the NACA0012 airfoil at various Mach numbers, the FINN model can accurately predict the damping representing the unsteady instability margin. Compared to the direct mapping input-output neural network (NN) model, the proposed method with shock wave feature-informed has enhanced accuracy, with an average relative error decreased by 70% at extrapolated Mach numbers. This research demonstrates the effectiveness of the FINN model in predicting the buffet onset, which leverages physics features derived from the more economical steady solution far from the onset boundary at a given predicted Mach number.

GEOTECHNICAL DESIGN FOR THE STABILIZATION WORKS OF THE LETTOMANOPPELLO LANDSLIDE (ABRUZZO, ITALY)

In order to stabilize the landslide of Lettomanoppello, the stabilization works are finalized to reduce/control the causes and to mitigate the economic, social, and environmental risks. A careful geotechnical design, of the stabilization works can allow an economic recovery of the cost by means of a virtuous and profitable process to produce electric energy. The landslide of Lettomanoppello, located in Central Italy (south-west of the city of Pescara) is a large, very complex active landslide. Slope instability phenomena, often triggered by intense rainfall and seismic events are recurrent in this region of Apennines and frequently result in serious damage to the historical-environmental heritage and in dreadful economic losses. The stability of slope is mainly affects by the hydrogeological conditions of the area characterized by a complex drainage system. Various stabilization works were executed along the slope during the years. The paper presents the studies (geological and geotechnical characterizations) and a general plan of the works required to stabilize the entire slope. The general stabilization project includes different types of works: (i) structural works (retaining walls), effective in short term; (ii) drainage works (tunnel for deep water drainage), effective to medium-long term, aimed at reducing/controlling the major causes of the landslide; (iii) protection from erosion at the toe of slope and surface drainage works. The tunnel crosses a horseshoe shape underneath the historical centre of Lettomanoppello and the elevations of its mouths are at 296 m a.s.l. The drainage waters will be conveyed to pipe towards a hydraulic turbine located at 206 m a.s.l. in the lower part of the slope. Furthermore, the geometry of the cut-off retaining walls at the toe slope are conceived as foundations of aeolian turbines.

Unveiling STRs instability in a colorectal cancer FFPE sample: a case report.

In forensic genetics, sometimes formalin-fixed paraffin-embedded (FFPE) biopsy material taken during life is the only biological sample available for individual identification or paternity testing. In most cases, this biological tissue is characterized by the presence of tumor cells characterized by instability and loss of heterozygosity of microsatellites (MSI/LOH) compared to the DNA present in cells of normal tissue.In this case report, two FFPE samples from the same male subject were available for genetic investigation: one sample with colorectal cancer tissue and the other with normal tissue (no cancerous histopathological features). The comparison of the genetic profiles obtained from DNA extracted from the two tissues showed in the tumor tissue the presence of three genomic instability phenomena affecting FGA, CSF1P0, D21S2055 loci, located on three distinct autosomal chromosomes, and one duplication phenomenon affecting the DYS438. Therefore, due to the MSI/LOH phenomena, the genetic profile acquired from the tumor tissue was distorted and thus generated a fictitious genetic profile, not corresponding to the subject's real one (normal tissue free of tumor cells).

Stability analysis of Rayleigh–Bénard–Marangoni convection in fluids with cross-zero expansion coefficient

Cross-zero expansion coefficient Rayleigh–Bénard–Marangoni (CRBM) convection refers to the convective phenomenon where thermal convection with stratified positive and negative expansion coefficients in a liquid layer is coupled with the Marangoni convection. In the Bénard convection, fluids with a cross-zero expansion coefficient contain a neutral expansion layer where the expansion coefficient (α) is zero, and the local buoyancy-driven convection is coupled with the Marangoni convection, leading to unique flow instability phenomena. This paper uses linear stability theory to analyze the CRBM convection in a horizontal liquid layer under a vertical temperature gradient and performs numerical calculations for fluids under different Bond numbers (Bd) in both bottom-heated and bottom-cooled models, obtaining the critical destabilization conditions and modes. In the bottom-heated model, different combinations of buoyancy instability mechanism (BIM), tension instability mechanism, and coupled instability mechanism (CIM) appear depending on the dimensionless temperature for the neutral expansion layer (Tα0) and the Bd. In the bottom-cooled model, two mechanisms occur according to the variation of Tα0: BIM and CIM.

A high-frequency reduced-order model for parameters optimization of AC/DC distribution systems considering random disturbances

Accompanying the rapid development of AC/DC distribution systems, along with the random disturbances introduced by a significant number of distributed generations and loads, the difficulty of system stability analysis has increased. The phenomenon of DC bus voltage instability has become severe, affecting the safe and stable operation of power systems. This paper focuses on the research of multi-terminal AC/DC distribution systems. A reduced-order method for multi-terminal distribution systems is proposed. Through parameter equivalent transformation, the system model is simplified while maintaining its dynamic characteristics. Parameter sensitivity analysis is utilized to identify key parameters influencing the system’s dynamic behavior, achieving an analytical expression of the system’s dynamic characteristics. And stability domain analysis is applied to delineate the variations in system dynamic behavior and responses to random disturbances under different parameter settings. Then a multi-parameter optimization method is designed to enhance system stability while achieving fast system response. Finally, theoretical analysis is validated through software simulation and hardware-in-the-loop experiments.

Diversity and transition of periodic motion of a periodically excited soft-impacting machinery

Dynamics of a periodically excited vibro-impact system with soft impacts is investigated. Essential features of period-one multi-impact motion group and correlated transition characteristics in low-frequency range are discussed in detail by the way of two-parameter bifurcation space providing qualitative domains for different periodic motions. The main focus is given to the effect of sensitive parameters including constraint stiffness k 0, clearance threshold b, and damping parameter ζ on the system response. The low-frequency characteristics in the finite-dimensional parameter space are particularly explored. It is found that the increase of k 0 induces multi-type bifurcation of period-one double-impact symmetrical motion, which induces a rich variety of periodic motions, and period-one multi-impact motion group orbit primarily exist in the small-clearance b and low-frequency ω zone. Based on the evolution irreversibility of adjacent period-one multi-impact orbit, the mechanism of singularies appearing in pairs and two different transition zones (hysteresis and liguliform zones) is studied, the result of which provides a theoretical reference value for the common low-frequency vibration instability phenomenon in the field of mechanical engineering. For small-damping coefficient ζ, period-one multi-impact motion has a large quantity, and the main bridge for the transition of adjacent period-one multi-impact motion is liguliform zone, which embraces period-one multi-impact asymmetrical motion and period-n multi-impact subharmonic motion and a certain chaotic zone. For large-damping coefficient ζ, the amount of period-one multi-impact motion group is reduced, and the main bridge for the transition of adjacent period-one multi-impact motion is hysteresis zone, where adjacent period-one multi-impact orbits can coexist according to initial conditions. As designing and renovating impact mechanical equipment, the reasonable matching law of dynamic parameters can be determined through two-parameter bifurcation space, which is conducive to making the system work in stable periodic motion and obtaining larger instantaneous impact velocity.

Optical pattern formation of laser fields in the Rydberg atomic gases

We study the phenomena of long-wave and short-wave modulation instabilities (LMI and SMI) in Rydberg atomic gases within the framework of the nonlocal nonlinear Schrödinger equations in the local and nonlocal regions (characterized by the ratio of beam radius to radius of Rydberg blockade sphere), respectively. We further reveal the rich soliton dynamics arising from interactions of nonlinear waves triggered by LMI. We also show the variety of spatial self-organization pattern formations and their active manipulation due to SMI. Through a synthesis of theoretical analysis and numerical simulations, the self-organized optical structures reported here provide a way for realizing what we believe to be novel optical patterns and solitons based on Rydberg atomic gases.

Large‐signal stability analysis for hybridized vehicular power supply systems of rolling stocks

AbstractHybridized vehicular power supply (HVPS) are acquiring widespread adoption in various applications such as providing power for passenger or heavy‐duty electric vehicles, more‐electric ships, electrified rolling stocks, and more. Stability analysis is a critical issue for HVPS, but it presents significant challenges due to their prominent high‐dimensional and nonlinear characteristics. This paper presents a large‐signal stability analysis scheme for HVPS of rolling stocks, enabling the assessment of system stability under sudden load changes. Based on the state‐space averaging method, a mathematical model for the large‐signal behavior of the system is established. Subsequently, the small‐signal stability parameter range of the system is determined by analyzing the eigenvalues of the Jacobian matrix. To further analyze the stability of the system during large‐signal disturbances, a nonlinear decoupling transformation approach is employed to decompose the original high‐order state equations into multiple lower‐order equations. For the decoupled lower‐order subsystems, their stability is analyzed using phase portrait, and the region of attraction (ROA) is determined using the inverse trajectory method. Based on the stability analysis results, the controller is enhanced to bring the points outside the ROA back within it, thereby mitigating the transient instability phenomenon of the system. The effectiveness of the proposed method was demonstrated using two case studies.

Nonlinear friction dynamics of water-lubricated bearings under transient shocks considering cavitation and turbulence effects

This study proposes a transient elastohydrodynamic mixed lubrication (EHML) model for water-lubricated bearings (WLBs) that incorporates cavitation and turbulence effects to evaluate the nonlinear friction dynamic performance under transient shock loads. The generalized average Reynolds equation is discretized using the control volume technique (CVM), and the constrained system is solved using the Fischer-Burmeister-Newton-Schur (FBNS) method. The validity of the model is verified by a comparison of experimental data from published literature. On this basis, the effects of cavitation, turbulence, shock load amplitude, direction, time, and rotational speed on WLB nonlinear friction dynamics characteristics are investigated. The results show that cavitation induces hydrodynamic loss under transient shock conditions, significantly increases contact time and load, and exacerbates the hydrodynamic instability phenomenon. In contrast, the turbulence effect effectively reduces frictional contact. The stability of WLBs under transient shock conditions is closely related to the load offset angle. Reducing the load offset angle improves the anti-shock stability of the bearings. Nevertheless, an increase in the amplitude and duration of the shock load may result in a deterioration of the frictional contact behavior. Increasing the rotational speed appropriately favors accelerating the lubrication regime transition and improving WLB’s anti-shock stability. This study provides a reference for enhancing WLB anti-shock performance and optimizing structural design.

Investigations into Hydraulic Instability during the Start-Up Process of a Pump-Turbine under Low-Head Conditions

To investigate the hydraulic characteristics during the start-up process of a full-flow pumped storage unit under low-head conditions, numerical simulations were conducted to study the dynamic characteristics during the process, providing a detailed analysis of the dynamic behavior of the internal flow field during the transition period as well as the associated variation in external performance parameters. Study results revealed a vortex-shedding phenomenon during the initial phase of the start-up process. These vortices restrict the flow, initiating a water hammer effect that abruptly elevates the upstream pressure within the runner. As the high-pressure water hammer dissipated, the flow rate rapidly increased, leading to a secondary but relatively weaker water hammer effect, which caused a momentary drop in pressure. This series of events ultimately resulted in significant oscillations in the unit’s head. After the guide vanes stop opening, the vortex structures at the runner inlet and outlet gradually weaken. As the runner torque continues to decline, the unit gradually approaches a no-load condition and enters the S-shaped region. Concurrently, pressure pulsations intensify, and unstable vortex formations reemerge along the leading and trailing edges of the runner blades. The escalated flow velocity at the runner’s exit contributes to the elongation of the vortex band within the draft tube, ultimately configuring a double-layer vortex structure around the central region and the pipe walls. This configuration of vortices precipitates the no-load instability phenomenon experienced by the unit.

Discrete differential geometry-based model for nonlinear analysis of axisymmetric shells

In this paper, we propose a novel one-dimensional (1D) discrete differential geometry (DDG)-based numerical method for geometrically nonlinear mechanics analysis (e.g., buckling and snapping) of axisymmetric shell structures. Our numerical model leverages differential geometry principles to accurately capture the complex nonlinear deformation patterns exhibited by axisymmetric shells. By discretizing the axisymmetric shell into interconnected 1D elements along the meridional direction, the in-plane stretching and out-of-bending potentials are formulated based on the geometric principles of 1D nodes and edges under the Kirchhoff–Love hypothesis, and elastic force vector and associated Hessian matrix required by equations of motion are later derived based on symbolic calculation. Through extensive validation with available theoretical solutions and finite element method (FEM) simulations in literature, our model demonstrates high accuracy in predicting the nonlinear behavior of axisymmetric shells. Importantly, compared to the classical theoretical model and three-dimensional (3D) FEM simulation, our model is highly computationally efficient, making it suitable for large-scale real-time simulations of nonlinear problems of shell structures such as instability and snap-through phenomena. Moreover, our framework can easily incorporate complex loading conditions, e.g., boundary nonlinear contact and multi-physics actuation, which play an essential role in the use of engineering applications, such as soft robots and flexible devices. This study demonstrates that the simplicity and effectiveness of the 1D discrete differential geometry-based approach render it a powerful tool for engineers and researchers interested in nonlinear mechanics analysis of axisymmetric shells, with potential applications in various engineering fields.

Instabilities in a two-dimensional granular fault gouge: Particle dynamics and stress fluctuations

Predicting stress fluctuations in granular media under steady-state shear loading is crucial for applications ranging from geophysical processes to construction engineering. Stress fluctuations emerge from particle rearrangement, usually enabled by frictional slip and force-chain buckling. Existing models used to predict stress fluctuations are largely phenomenological, often accounting for the force chain phenomena implicitly through the introduction of internal variables, or explicitly through assumptions of force chain mechanics. Improper consideration of particle mechanics or mesoscale effects can lead to inaccurate predictions of shear strength and instability, making it difficult to predict the onset of yielding, shear band formation, and other instabilities. Furthermore, while recent advancements in machine learning methods have established links between microscale behavior and macroscopic stress drops in granular fault gouges, their predictive capabilities are limited due to their black-box nature. To gain a deeper understanding of stress fluctuations, and ultimately predict them in a physics-informed manner, it is necessary to examine how system energetics change with stress fluctuations. In this paper, we analyze stress fluctuations in a 2D granular fault gouge loaded under quasistatic, steady-state shear. We track the flow of potential energy between force networks and understand how energy and force networks vary with stress rises and drops. We derive an analytical, dynamic force chain model from first principles to illustrate how interactions between force networks lead to the emergence of localized instability phenomena. Finally, we offer insights into how these localized instabilities ultimately enable shear stress fluctuations at the continuum scale.

Research on the flame oscillation characteristics of scramjet combustor equipped with strut fuel injector

The combustor is the core component of the scramjet engine, and stable combustion is the prerequisite for the efficient operation of the scramjet engine. This paper investigates combustion stability in a combustor with a thin strut for oxygen supplementation at the trailing edge. LES (Large eddy simulation) was carried out under the combustor inlet conditions of Ma= 2.8, Tt= 1680 K, and Pt= 1.87 MPa. The results show that as the amount of oxygen supplementation inside the combustor increases, the oscillation forward propagation characteristics of the flame front inside the combustor are formed. First, the combustion instability phenomenon of the flame inside the combustor is analyzed, and then the oscillation frequency of the flame is analyzed by FFT (Fast Fourier Transform) of the flame front and pressure and DMD (Dynamic Mode Decomposition) of the velocity field within one cycle. Finally, the flame forward propagation mechanism inside the combustor is analyzed by pressure-velocity-heat release distribution. The results show that the injection of excess oxygen inside the combustor forms a strong coupling phenomenon of flow and combustion, and the premixed gas downstream of the trailing edge of the strut forms a state of slow combustion to explosion, forming a flame propagation forward and periodic oscillation phenomenon.

Research on the Support Technology for Deep Large-Section Refuge Chambers in Broken Surrounding Rock in a Roadway

The phenomenon of peripheral rock instability is more common in crushed bedrock roadways, and the fundamental reason for this lies in the significantly different characteristics of its peripheral rock stress field. Taking the newly dug belt inclined shaft of PingDingShan TianAn Coal Co., Ltd. No. 6 Mine as the engineering background, a mechanical model of a broken perimeter rock roadway was established by using classical rock mechanics theory. Stress distribution around the roadway of the broken perimeter rock medium was systematically analyzed, and radial and tangential stress formulas of the broken perimeter rock were deduced. Through the formula calculation, it was deduced that there was a stress drop in the intact surrounding rock outside the disturbed zone, and the radial stress of the intact surrounding rock in its deep part was relatively increased, while the tangential stress was relatively decreased. The existence of crushed surrounding rock increased the minimum principal stress and decreased the maximum principal stress of the unfractured surrounding rock, which proves that a well-maintained disturbed zone can play a lining role. Thus, a “U-shaped steel + inverted arch + bottom arch linkage beam + floor bolt compensation” support program was proposed. This joint support program easily forms a closed support structure, which is more effective in controlling the deformation of tunnel perimeter rock. The support structure can effectively resist the deformation of the surrounding rock and enhance bottom drum resistance. Through numerical simulation, it was concluded that the horizontal displacement of the two gangs was reduced by 70%, and the displacement of the top and bottom plates was reduced by 77% after optimization of the support, which effectively controlled the stability of the broken surrounding rock.

Temperature-Driven Instabilities in High-Pressure Vessel Flat Plates: A Thermal Buckling Study

In the realm of high-pressure vessel simulation, conventional finite element method (FEM) approaches, as per ASME standards, may inadequately predict the behavior of flat surfaces under elevated temperatures. This study challenges the efficacy of shell-type mesh modeling for high-temperature flat plates, demonstrating that the thermal conditions within such high-pressure vessels can induce thermal instability and buckling, not accounted for by traditional FEM methods recommended by ASME. Through comprehensive analytical investigations, we reveal that traditional shell-type meshing techniques, while suitable for certain applications, fail to capture the intricate thermal stresses and deformation patterns inherent in high-temperature flat plate configurations. Our analysis delineates distinct stability regimes governed by key design parameters, including plate thickness, operating temperature, and geometric dimensions, profoundly impacting the structural integrity of heating plates under thermal loading. Specifically, we found that increasing the plate thickness enhances resistance to thermal buckling, clamping the plate edges raises the critical buckling temperature, and selecting materials with lower thermal expansion coefficients improves stability. These findings provide engineers with critical insights necessary for optimizing the design and performance of high-temperature equipment. This includes the design of high-pressure vessels with flat surfaces for heating materials, flanges in high-temperature environments, and fins in heat exchangers across various industries such as oil and gas, pyrolysis, and power plants. The findings presented herein serve as a valuable reference for engineers seeking to comprehend and mitigate instability phenomena in solid mechanics, offering practical guidance for developing robust and reliable high-temperature structures in demanding industrial environments.

Does a polymer film due to Rayleigh-instability affect interfacial properties measured by microbond test?

ABSTRACT The microbond (MB) test, which is primarily used to characterise the interface of fibrous composites, requires a large number of droplets to be tested and analysed in order to make a reliable conclusion about the fibre–droplet interface. The conventional method of depositing single droplets on fibre and performing the MB test can be improved by depositing multiple droplets using the Rayleigh plateau instability phenomenon (an additional film is formed between the droplets). Although the latter method has significant advantages and higher statistical reliability, the role of the additional film affecting MB test results has not been investigated. In this work, both methods are experimentally evaluated for glass and flax fibres with two different resin systems and the interfacial constants, namely critical stress for damage initiation and critical energy release rate, are validated by finite element (FE) models. The study reveals that the thickness of the additional film shows incorrect interfacial shear strength (IFSS) when determined from simple force-displacement data ( ≈ 18% increase for the fibre-droplet system in this study). The FE models confirm that the damage onset at the interface occurs at a higher force with this method, but the interfacial strength constants remain the same as with the conventional method.