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The long‐term state of stress in the subduction forearc depends on the balance between margin‐normal compression due to the plate‐coupling force and the margin‐normal tension due to the gravitational force on the margin topography. In most subduction margins, the outer forearc is largely in margin‐normal compression due to the dominance of the plate‐coupling force. The inner forearc's state of stress varies within and among subduction zones, but what gives rise to this variation is unclear. We examine the state of stress in the forearc region of nine subduction zones by inverting focal mechanism solutions for shallow forearc crustal earthquakes for five zones and inferring the previous inversion results for the other four. The results indicate that the inner forearc stress state is characterized by margin‐normal horizontal deviatoric tension in parts of Nankai, Hikurangi, and southern Mexico. The vertical and margin‐normal horizontal stresses are similar in magnitudes in northern Cascadia as previously reported and are in a neutral stress state. The inner forearc stress state in the rest of the study regions is characterized by margin‐normal horizontal deviatoric compression. Tension in the inner forearc tends to occur where plate coupling is shallow. A larger width of the forearc also promotes inner‐forearc tension. However, regional tectonics may overshadow or accentuate the background stress state in the inner forearc, such as in Hikurangi. Plain Language Summary The state of stress in the overriding plate between the trench and the volcanic arc of subduction zones depends on frictional coupling between the overriding and subducting plates and gravitational force, which causes lateral compression and tension, respectively. The trench‐ward portion of this so‐called “forearc” region is generally in compression due to the dominant effect of plate coupling, but for the arc‐ward portion, the relative importance of the two forces varies spatially. We constrain the state of stress in the forearc using earthquake data for five subduction zones and inferring results from previous studies for four other subduction zones. The results indicate that the forearc stress state seems to correlate with the downdip depth of plate coupling and the width of the forearc. A relatively shallow downdip extent of coupling in a wide forearc tends to have the arc‐ward portion in tension or neutral stress state as observed in parts of Nankai, Hikurangi, Mexico, and Cascadia although this tendency is impacted by the local tectonic settings. Key Points Focal mechanism inversion results indicate the correlation of inner forearc stress state with the downdip depth of plate coupling Margin‐normal horizontal deviatoric tension in the inner forearc tends to occur where plate coupling is shallow and the forearc is wide The variation in the inner forearc stress state does not require a variation in the subduction fault strength

The stability of cratons has long been attributed to their neutral buoyancy and strong roots. However, recent seismic studies have revealed widespread mid‐lithospheric discontinuities (MLDs) within cratonic roots. Previous studies, based on geophysical and xenolith evidence, mainly suggest that MLDs are formed through metamorphic processes and have low strength. This raises the question of how most cratons remain stable with weak MLDs. Using numerical models and theoretical analysis, we show that the spatial distribution of MLDs can influence cratonic behavior during plate motion. Mantle flow can shear the cratonic root along a weak, laterally continuous MLD, leading to cratonic destruction. However, when the MLDs are laterally intermittent and contain several 10's–1,000's km‐wide gaps, especially near the side boundaries, they can generate a viscous coupling that can counterbalance the basal traction force from mantle flow and contribute to stabilizing the cratons. Plain Language Summary Over billions of years, cratons have remained stable as the old cores of continents on Earth. The long‐term sustainability of cratons is attributed to their neutral buoyancy and strong roots. However, recent seismic studies have observed extensive mid‐lithospheric discontinuities (MLDs) within these cratonic roots. These MLDs are characterized by a significant reduction in seismic velocity of 2%–7%. Previous research primarily suggests that MLDs form through metamorphic processes and have low strength. This raises the question of how cratons are able to maintain their stability in the presence of weak MLDs. Our study, using numerical models and simplified theoretical analysis, demonstrates that the spatial distribution of MLDs can greatly influence the behavior of cratons during plate motion. According to numerical modeling results, lateral gaps in the MLD layer, especially near the side boundaries, can generate viscous coupling that counteracts the basal traction from mantle flow. Our findings indicate that the existence and spatial distribution of MLD gaps have a significant impact on the dynamics and stability of cratons. Key Points Mid‐lithospheric discontinuities (MLDs) are observed to be widespread within the cratonic root and are likely to have low strength The presence of certain wide gap(s) in the weak MLD layer can counterbalance the basal traction from mantle flow Multiple MLD gaps located near the side edges of cratons can further enhance the stability of cratons during plate motion

Bianisotropy is common in electromagnetism whenever a cross-coupling between electric and magnetic responses exists. However, the analogous concept for elastic waves in solids, termed as Willis coupling, is more challenging to observe. It requires coupling between stress and velocity or momentum and strain fields, which is difficult to induce in non-negligible levels, even when using metamaterial structures. Here, we report the experimental realization of a Willis metamaterial for flexural waves. Based on a cantilever bending resonance, we demonstrate asymmetric reflection amplitudes and phases due to Willis coupling. We also show that, by introducing loss in the metamaterial, the asymmetric amplitudes can be controlled and can be used to approach an exceptional point of the non-Hermitian system, at which unidirectional zero reflection occurs. The present work extends conventional propagation theory in plates and beams to include Willis coupling and provides new avenues to tailor flexural waves using artificial structures.

Scientific progress in the context of seismic precursors reveals a systematic mechanism, namely lithosphere–atmosphere–ionosphere coupling (LAIC), to elaborate the underlying physical processes related to earthquake preparation phases. In this study, a comprehensive analysis was conducted for two earthquakes that occurred on the sea coast through tidal force fluctuation to investigate ocean–lithosphere–atmosphere–ionosphere coupling (OLAIC), based on oceanic parameters (i.e., sea potential temperature and seawater salinity), air temperature and electron density profiles. The interrupted enhancement and diffusion process of thermal anomalies indicate that the intensity of seismic anomalies in the atmosphere is affected by the extent of land near the epicenter. By observing the evolution of the ocean interior, we found that the deep water was lifted and formed upwelling, which then diffused along the direction of plate boundaries with an “intensification-peak-weakening” trend under the action of the accelerated subduction of tectonic plates. Furthermore, the analysis shows that the seismic anomalies have two propagation paths: (i) along active faults, with the surface temperature rising as the initial performance, then the air pressure gradient being generated, and finally the ionosphere being disturbed; (ii) along plate boundaries, upwelling, which is the initial manifestation, leading to changes in the parameters of the upper ocean. The results presented in this study can contribute to understanding the intrinsic characteristics of OLAIC.

To address the buckling of flat steel plate webs in traditional concrete-filled steel plate composite coupling beams, this study proposes a novel composite coupling beam with axial rib corrugated webs. The corrugated plates provide greater out-of-plane stiffness compared to flat steel plates. Cyclic loading tests are conducted on specimens with a span-to-depth ratio of 1.5:1. The working mechanism of the coupling beam is investigated through stress–strain analysis of the steel plates and internal force analysis of the coupling beam. A calculation method based on an equivalent section is proposed for the ultimate shear capacity of the beam. The design of stiffener plates at the connection between the coupling beam and the wall piers is also discussed. The results show that the proposed coupling beam exhibits excellent energy dissipation performance. Additionally, the proposed shear capacity calculation method is highly applicable, with an error margin within 10%.

To accurately characterize the rolling force and exit thickness ratio of bimetallic composite plates under the combined influence of gradient temperature and deformation during the rolling process, we propose a thermomechanical coupling calculation method based on the finite difference method (FDM). This approach accounts for the effects of non-uniform distributions of strain rate, strain, and temperature on the deformation resistance of elements during rolling. The deformation zone of the plate is discretized into multiple elements along the thickness and rolling direction. Initially, a two-dimensional finite difference method (2D FDM) is employed to determine the strain rate, strain, and temperature field distributions of the composite plate, which are then used to construct the deformation resistance matrix of the elements. Subsequently, using Orowan’s non-uniform deformation theory, we analyze the rolling process of the composite plate, introducing a Coulomb friction assumption at the interface between the two plates to form the corresponding differential equations. Finally, the deformation resistance matrix is applied to the deformation elements, facilitating the coupling of the two physical fields and deriving a computational model for the rolling force and exit thickness ratio of bimetallic composite plates under gradient temperature rolling. Using the hot rolling of stainless steel/carbon steel composite plates as an example, we validated the mathematical model through experiments and Abaqus finite element simulations. With errors under 10% under the same parameters, the mathematical model’s accuracy and effectiveness were confirmed, providing a theoretical basis for mill design and process parameter settings.

Rotating friction circular plates are the main components of a friction clutch. The vibration and temperature field of these friction circular plates in high speed affect the clutch operation. This study investigates the thermoelastic coupling vibration and stability of rotating friction circular plates. Firstly, based on the middle internal forces resulting from the action of normal inertial force, the differential equation of transverse vibration with variable coefficients for an axisymmetric rotating circular plate is established by thin plate theory and thermal conduction equation considering deformation effect. Secondly, the differential equation of vibration and corresponding boundary conditions are discretized by the differential quadrature method. Meanwhile, the thermoelastic coupling transverse vibrations with three different boundary conditions are calculated. In this case, the change curve of the first two-order dimensionless complex frequencies of the rotating circular plate with the dimensionless angular speed and thermoelastic coupling coefficient are analyzed. The effects of the critical dimensionless thermoelastic coupling coefficient and the critical angular speed on the stability of the rotating circular plate with simply supported and clamped edges are discussed. Finally, the relation between the critical divergence speed and the dimensionless thermoelastic coupling coefficient is obtained. The results provide the theoretical basis for optimizing the structure and improving the dynamic stability of friction clutches.

A twin-electrode TIG-MIG (T-TIG-MIG) indirect arc welding method was proposed in this paper. The arc behavior and droplet transfer process were preliminarily investigated; moreover, the process stability was assessed, and bead-on-plate welding was conducted. Results showed T-TIG-MIG indirect arc burnt between a wire and two tungsten electrodes and was essentially formed by the coupling of two single-electrode TIG-MIG indirect arcs. The wire feeding speed (WFS) determined the equilibrium position of the wire end, and the vicinity of the tungsten tips was an ideal position for arc shape and droplet detachment, where the arc was more concentrated with a higher coupling degree. With the increase of the welding current, the arc length and stiffness increased gradually; so did the process stability and the spreadability of the weld bead. When the current exceeded the critical current, the droplet transfer mode changed into streaming spray transfer, since the electromagnetic force and the arc pressure increased considerably. Compared to conventional cold-wire T-TIG welding under the same current, the wire deposition rate of T-TIG-MIG indirect arc welding increased by about 186%, while the range of the heat-affected zone reduced by about 41%.

Aiming at the problem of significant attenuation of electromagnetic acoustic emission signals in the detection of pore-etching defects on the bottom plate of storage tanks, this study reveals the mechanism of electromagnetic acoustic emission and its propagation and attenuation law by constructing a numerical model of electromagnetic-acoustic coupling. Based on the finite element method, a numerical model of Q235 steel plate with pore-etching defects is established, and the generation mechanism of electromagnetic acoustic emission in the defective region under the excitation of strong pulsed current is elucidated. The study shows that: the pulsed current generates Lorentz force perturbation at the edge of the defect, which excites the broadband acoustic emission signal; the acoustic wave propagation shows a significant dispersion effect, with the high-frequency component decaying rapidly in the near-field region (0-25mm), the middle- and low-frequency components forming the dominant modes in the transition region (25-50mm), and finally tending to decay stably in the far-field region (>50mm). The reliability of the model is experimentally verified, and a quantitative relationship between the excitation current and the attenuation coefficient is established. The results provide a theoretical basis for optimising the excitation parameters and sensor layout, which is of great engineering significance for improving the defect detection accuracy of the tank bottom plate.

The nonaxisymmetric magnetoelastic nonlinear coupling free vibration study is performed for a conductive thin annular plate in the nonuniform toroidal magnetic field generated by a long straight current carrying wire in this article. From the electromagnetic theory, expressions for the magnetic field, electromagnetic force and torque acting on the plate are deduced. According to Hamilton principle, nonaxisymmetric magnetoelastic nonlinear vibration equation is derived. The displacement functions for plate under three different boundary conditions are solved, which is combined with Galerkin integral method for derivation of nondimensional coupling nonlinear differential equations. The method of multiple scales is introduced to solve the coupling equations and achieve the second-approximation analytical solution, and then, expressions for the first three mode nondimensional natural frequencies of plate are obtained. In numerical examples, diagrams of electromagnetic characteristics and the first three frequencies under magnetic field and modal coupling effect are presented, which shows the influence of different parameters, e.g., current intensity, plate size and time on natural frequencies and electromagnetic forces. The variation of system singularity stability is discussed, and the obtained analytical results are also validated. The results indicate that current, plate size and time parameters have obvious influence on natural frequencies, which also shows quite different variations under different boundaries. Additionally, initial conditions have significant effects on natural frequencies, which becomes more complicated under modal coupling effect. In nonaxisymmetric vibration case, electromagnetic forces show complicated changing rules along radial and circumferential directions. Furthermore, system equilibrium point will be changed by the induced nonuniform magnetic field.