Surface Stress Research Articles

Surface Effect in Nano-Scale Fretting Contact Problems

Abstract The fretting contact behavior of nanostructured materials is significantly influenced by the surface effect. A model of fretting contact between a nano-sized rigid cylindrical indenter and an elastic half-plane is established based on Gurtin–Murdoch (G–M) surface elasticity theory, with which the surface effects on the stress and displacement distributions and the size of stick region (no-slip region) in the contact zone are systematically studied. It is found that the surface effect induces an additional traction besides the external force applied by punch, which could help to smoothen the stress and displacement distributions. The normal surface-induced traction related to the residual surface stress is opposite to the externally applied compression, which results in a material stiffening in the contact zone so that the contact radius, normal displacement, and normal stress decrease compared with their classical counterparts. The tangential surface-induced traction is also opposite to the externally applied frictional stress, consequently leading to reductions of the shear stress and tangential displacement induced by friction in the contact zone. More interestingly, the surface effect leads to three possible states in the contact zone, including complete slip, partial slip, and complete stick, instead of the solely partial slip state in classical fretting contact models without surface effect. Among them, the complete stick due to the action of surface residual stress is more beneficial for inhibiting the wear of contact devices, which can be realized by reducing the indenter size. The present research does not only help one to better understand the physical mechanism in nano-scale fretting contact problems, but should also guide the anti-wear design in nano-electro-mechanical (NEMs) systems.

Effect of Scratching and Bending Degree on the Residual Stress of the 7055 Aluminum Alloy Plate

This study investigated the effects of scratches and bending on the surface stress of aluminum alloy plates with varying thicknesses. The purpose of this study was to provide guidance for surface stress testing of plates with different thicknesses. The “birth and death” element method was used to simulate the generation of surface scratches, while the stress field under the pre-stretched state was obtained through the displacement of one end of the board by 2.5%. The X-ray residual stress analyzer (μ-x360 s) was used to experimentally characterize the stress level on the surface of the plate. The research results showed that the residual stress level near the scratches decreases significantly, while the surface stress level of the plate decreases significantly after bending. The changes in stress values with increasing bending degree for plates of different thicknesses showed that the thicker the plate, the greater the tensile effect on the surface of the plate, resulting in smaller compressive stress. The longitudinal non-uniformity of the plates with different thicknesses was also analyzed, and it was found that the testing error of stress testing results was within ±5 MPa. In conclusion, this study provides useful guidance for the accurate measurement of surface stress on aluminum alloy plates of varying thicknesses, considering the effects of scratches, and bending.

Stability of forming induced residual stresses in stainless steel components under mechanical load

Cold forward extrusion leads to the generation of residual stresses, which can have an advantageous or detrimental effect on the operating behavior of the formed components. In industrial practice, energy-intensive annealing steps are often used to remove unknown stress states and thus prevent unexpected failures. Residual stress-related knowledge can therefore help to save energy and resources. To be able to estimate the effects, not only sign and magnitude need to be known, but also the residual stress stability under typical operating loads. In this paper, stainless steel X6Cr17 (DIN 1.4016, AISI 430) is formed by forward rod extrusion at room temperature. The forming process leads to residual stresses in the formed components, which are subsequently subjected to mechanical testing. To represent different initial states, components with varied die opening angles are produced and tested. A modified 3-point bending setup is used to generate a defined tensile load stress in the part surface. The results show a behavior of the residual stresses, which is depending on the load magnitude, the initial stress state and the work hardening distribution in the part. While the number of cycles has an influence, the largest changes occur in the first stroke.

Industrial Application of the Diamond-Electrochemical Honing Method

The method of diamond-electrochemical honing (DEH) is a finishing method that provides the required characteristics of the surface roughness, the distribution of the residual surface stress diagrams, which ultimately ensures the formation of a high-strength electroplated coating. This article presents studies of changes in the surface roughness, the residual stresses, and other parameters depending on the selected DEH modes for the parts made of steels 30KhGSA, 30KhN2MFA, 35KhN2MFASH, 23KhGS2MFALU, and St. 50A. The article presents the design of the instrument-khon; with the solution of the issue of the geometric characteristics of the cathode part, the insulating coating and the material of the hone tool guides. The regime support of the DEH was formed on the base of the studies whose results of are given in the article. The selected kinematic-geometric indicators of the hone-tool – a workpiece system make it possible to form the drives of the DEH equipment. The optimal composition of the DEH electrolyte was chosen with the creation of an electrolyte storage and a supply system. The article contains the results that demonstrate the effect of DEH on the reliability and durability of the parts subjected to the use of this method. The data on the technical and economic efficiency of the created industrial site of DEH are presented.

Surface stress monitoring of laser shock peening using AE time-scale texture image and multi-scale blueprint separable convolutional networks with attention mechanism

Surface stress monitoring of laser shock peening using AE time-scale texture image and multi-scale blueprint separable convolutional networks with attention mechanism

Triangular Position Multi-Bolt Layout Structure Optimization

Stress concentration often occurs around bolt holes in load-bearing joint structures of large complex equipment, ships, aerospace and other complex machinery fields, which is an important mechanical factor leading to the failure of joint structures. It is of great engineering significance to study the phenomenon of stress concentration on connected structures for the safety of large and complex equipment; meanwhile, the layout of bolts seriously affects the stress around holes. Many scholars have studied the layout optimization of multi-bolted structures through experiments and simulations, but few algorithms have been applied to the layout optimization of bolted structures. And most of the studied types of multi-bolt structures are symmetrical. Therefore, in this paper, the gray wolf algorithm is used to optimize the layout of nickel steel plate connectors with a bolt layout in triangular position, and the optimal objective function is found based on the hole circumferential stress of the nickel steel plate, maximum shear stress of the bolt and bending stress of the nickel steel plate. Comparing the optimal values obtained by the fruit fly optimization algorithm, particle swarm optimization algorithm, gray wolf optimization algorithm, multiverse optimization algorithm and wind driven optimization algorithm, the accuracy of selecting the gray wolf algorithm for optimization is verified. A multi-bolt connection structure model was established in ABAQUS, and the surface stress before and after optimization was compared to verify the correctness of the gray wolf algorithm applied to the structure layout optimization of the nickel steel flat bolt connection. The results show that under the force of 15 KN, compared with the original bolt structure layout, the optimized upper side nickel steel plate bore peripheral stress is reduced by 73.1 MPa, and the optimization rate is 24%; bolt stress is reduced by 47.7 MPa, and the optimization rate is 12.5%; when the load is less than 18 KN, the optimization effect of both the upper nickel steel plate and bolt group is more than 10%. When the load is greater than 18 KN, the optimization effect is reduced, and when the load is greater than 21 KN, the nickel steel plate has exceeded the yield limit. Due to the existence of fixed constraints, the optimization of the lower nickel steel plate is not obvious. The results of this study can provide data and theoretical support for the layout optimization of the nickel steel flat bolt connection structure, and help to improve reliability analysis and health monitoring in complex assembly fields such as large complex equipment and aerospace.

Investigation on Residual Stress in SA508/Inconel Metal/CF8A Dissimilar Welded Joint for Nuclear Steam Generator Safe End Using Different Processes

The residual stress of a safe-end/nozzle dissimilar metal welded joint in nuclear power plants was investigated by finite element simulation coupled with a measurement method. Cladding, butting, dissimilar metal multipass welding, machinery processes, and in-service condition were all considered to investigate the evolution of residual stress. The numerical simulation matched well with the measured values, validating the effectiveness of the numerical simulation. Tensile residual stress occurred at both inner and outer surfaces due to a double-side-welding sequence. Moreover, the highest stresses in the outer surface were located at the interface between the SA508 and clad layer due to the difference of expansion materials across the welded joint. The machinery process would greatly reduce the residual stress level and produce compressive residual stress in the base metal. When the safe end was subjected to working conditions, the tensile residual stress obviously was reduced especially on the inner surface.

Atmospheric Dynamic Response to Coupling Currents to Wind Stress over the Gulf Stream

Atmospheric near-surface stress and boundary layer wind responses to surface currents are examined with high resolution coupled atmosphere–ocean models over the Gulf Stream during winter. Because the ocean and atmosphere are linked through surface stress, the two fluids can cause dramatic changes through feedback processes. When the current feedback is included, we find that the current gradient in the cross-wind direction drives the stress curl pattern and wind curl pattern to have minima and maxima at locations matching those of the ocean surface vorticity pattern. Furthermore, we find the large- (>30 km) and small-scale, or submesoscale (<30 km), stress curl and wind curl responses to ocean surface vorticity are complimentary; however, the large- and small-scale wind divergence responses are counteractive. These responses (commonly called coupling coefficients) are found to depend on the relative position to the Gulf Stream maximum current. Throughout the atmospheric boundary layer, we find including the current feedback also leads to changes in the atmospheric secondary circulation on either side of the Gulf Stream extension. The winter seasonal means suggest the current feedback will impact climate, and investigating individual events, such as an atmospheric front passing over the Gulf Stream, suggests the current feedback will also impact the intensity of weather.

A continuous surface cap model and computational framework for shale under high confining pressure

Shale oil and gas development is still prevailing over the global energy industry and an accurate prediction on the mechanical behaviors of shale is essential. Conventional constitutive models for rocks often employ yield surfaces such as Mohr-Coulomb or Drucker-Prager. Such yield surfaces only contain a shear surface but lack a hardening cap, which could lead to inaccurate predictions on shale stresses and strains under high confining pressure.To overcome the limitations, this paper built a constitutive model for shale based on the theory of continuous surface cap. The model considers shale bedding plane inclinations and treats shales as transversely isotropic materials. A continuous surface cap is employed as the yield surface and corresponding stress return algorithm is established for the plastic update on stresses and strains. Strain rate effects and damage are also modeled. A computing framework calculating stresses based on given strain increment is also established. The model is verified by standard compressive tests on shales samples with different bedding plane inclinations under different confining pressure (up to 65 MPa). A good agreement between model predictions and test measurements is found. The comparison to conventional Drucker-Prager model also proves the capability of the proposed model.

Interaction between depth variation and turbulent diffusion in depth-averaged vorticity equations

Steady, depth-averaged, shallow water vorticity transport equations including advection, surface and bed shear stresses, and turbulent diffusion effects are written out in vorticity-velocity and stream function formalisms. The Boussinesq approximation is used to represent turbulent stresses in the effective stress tensor. We consider two different forms of the curl of the effective stress tensor: its complete form and the commonly used form neglecting the terms expressing interaction with variable water depth. After deriving the two equations in vorticity-velocity formalism, we recast the equations into stream function formalism, revealing all the internal effects associated with variable water depth. We examine the differences between the models through analytical solutions of the stream function equations for simple but realistic flows. The solutions are validated with CFD simulations.Graphical abstract

Characterizing the biomechanical transmission effects of elastic compression stockings on lower limb tissues by using 3D finite element modelling

Studies have primarily simulated the interface pressure and skin surface stress produced by compression textiles or garments with a single pressure level. However, only a few studies have reported the biomechanical transmission behaviors of compression fabrics from the skin to deeper soft tissues of the human body, especially within the context of compression fabrics providing multiple pressure levels. Therefore, to address this limitation, this study developed novel 3D finite element (FE) models to characterize and visualize the interface pressure exerted at the skin surface by elastic compression stockings (ECSs), a type of typical compression textiles, with either low- or high-pressure levels, and to analyze the resulting stress and stress transmission effects within lower limb tissues. The results of the validated FE models indicated that the simulated interface pressure favorably agreed with the measured pressure data. A close relationship existed between the pressure magnitudes and the lower limb surface curvatures, with the highest interface pressure occurring at the anterior and posterior bony regions of the ankle with greater surface curvatures than those of the posterior calf. The internal tissue stress distributions largely varied, which may be one of the potential causes for the inconsistency observed between the claimed pressure doses and those practically delivered inside. Overall, this study serves to improve the understanding of the working mechanisms of the ECS-lower limb system, allowing for improved functional material design and evaluation of the pressure effectiveness of compression textiles and garments used in physiotherapy.

Effect of internal curing on shrinkage of calcium sulfoaluminate cement concrete and calculation of the shrinkage stress at early age considering creep

Effect of internal curing on shrinkage of calcium sulfoaluminate cement (CSA) concrete in both sealed condition and dry/wet cycles was studied using four CSA concretes with water-to-binder ratios (w/b) of 0.62, 0.42, 0.30, 0.24 respectively corresponding to strength grade of CSA30, CSA50, CSA80, CSA100, and ceramsite as internal curing agent. Results indicate that lower w/b results in higher autogenous shrinkage and the total shrinkage of CSA100 at age of 14d under sealed condition reaches as high as 550 με, while this figure is only 100 με for CSA30. The concrete with lower w/b exhibits higher deformation amplitude during subsequent dry-wet cycles and the dimensional variations during dry-wet cycles for CAS100 and CSA30 are respectively 80 με and 30 με. It is found internal curing can effectively reduce both the autogenous shrinkage and the deformation amplitude during dry-wet cycles, especially for the concrete with lower w/b. The internally cured CSA100 exhibits 14d total shrinkage under sealed condition of 100 με and the deformation amplitude during the subsequent three dry-wet cycles of 25 με, with reduction rate of respectively 82% and 69% compared to its normally cured counterpart. Moreover, shrinkage stress of concrete cured for 3d under sealed condition and subsequently for 25d under drying condition was calculated based on ring test results considering creep behavior. It is shown that for CSA100 at Day 28, the shrinkage stress of the inner surface in the ring test is 2.32 MPa while this figure greatly drops to 0.43 MPa for its internally cured counterpart.

Effect of rolling on fretting fatigue assessment of cylindrical contact in partial slip regime

The present research work deals with the fatigue assessment of an Al 7075-T651 subjected to fretting fatigue testing characterised by cylindrical contact. A small rolling of the pads is experimentally observed and, consequently, the contact surface stresses are different to the hertzian one. Therefore, the methodology, recently proposed by some of the present authors for the fatigue assessment of fretting-affected metallic components, is here employed for the first time in conjunction with a FE numerical model able to take into account the effect of the rolling of the pads, in order to find out both the crack nucleation orientation and the lifetime for each specimen of the experimental campaign analysed.

A Novel BioNEMS Sensor Based on Tunable Plasmonic Modes of a Grating MIM Structure

In this paper, a novel optical BioNEMS (Bio Nano-Electro-Mechanical-Systems) sensor using a nanomechanical plasmonic structure is proposed to develop a high performance label-free biosensor. The present device is composed of a functionalized BioNEMS cantilever coated by a gold layer as a part of the proposed plasmonic grating waveguide. This plasmonic system is based on the light interaction with a metallic nanostructure as a metal-insulator-metal (MIM) Bragg grating where an air gap acts as the insulator. Passing patient samples through the functionalized surface of the nanocantilever, the biological interactions release a stress on the surface because of binding events. The cantilever can be thus deflected by inducing a differential surface stress. Therefore, the air gap distance (the insulator width) between two parts of the plasmonic grating structure is changed to excite surface plasmon polaritons in a different wavelength. Furthermore, the present biosensor is analyzed by numerical and analytical approaches and its functional characteristics are obtained as follows: optical sensitivity of 7872 nm/RIU, figure of merit (FOM) of 394 RIU^-1, mechanical sensitivity of 1.2 µm/Nm^-1. According to the results, the proposed label-free identification method can be used as a sensitive platform for detection of biomarkers in point of care testing (POCT) applications.

Modulation of the Oceanic Mesoscale Activity by the Mesoscale Thermal Feedback to the Atmosphere

Abstract Ocean mesoscale thermal feedback (TFB) is the influence of mesoscale sea surface temperature (SST) anomalies on the overlying atmosphere and its feedback to the ocean. Over the past few decades, TFB has been shown to affect the atmosphere by inducing low-level wind and surface stress anomalies and modulating ocean–atmosphere heat fluxes ubiquitously over the global oceans. These anomalies can alter the climate variability. However, it is not clear yet to what extent heat and momentum flux anomalies modulate the mesoscale ocean activity. Here, using coupled ocean–atmosphere mesoscale simulations over a realistic subtropical channel centered on the equator in which the TFB can be turned off by spatially smoothing the SST as seen by the atmosphere, we show that TFB can damp the mesoscale activity, with a more pronounced effect near the surface. This damping appears to be sensitive to the cutoff filter used: on average, the surface mesoscale activity is attenuated by 9% when smoothing the SST using an ∼1000-km cutoff but by only 2% when using an ∼350-km cutoff. We demonstrate that the mesoscale activity damping is primarily caused by a sink of available eddy potential energy that is controlled by the induced-anomalous heat fluxes, the surface stress anomalies having a negligible role. When TFB is neglected, the absence of sink of potential energy is partly compensated by a more negative eddy wind work. We illustrate that TFB filtering in a coupled model must be done carefully because it can also impact the large-scale meridional SST gradients and subsequently the mean large-scale wind stress curl and ocean dynamics.

Electro-Chemical Actuation of Nanoporous Metal Materials Induced by Surface Stress

Similar to biological muscles, the actuator materials can function as artificial muscles by directly converting an external stimulus in the form of electrical or chemical energy into a mechanical response through the reversible changes in material dimensions. As a new type of high surface-area actuator materials, nanoporous metals represent a novel class of smart electrodes that undergo reversible dimensional changes when applying an electronic voltage on the surface. The dimensional changes in nanoporous metal/polymer composite still originate from the surface stress of nanoporous metal. Additionally, this surface stress can be modulated by the co-adsorbed sulfate counter-ions that are present in the doped polymer chains coating matrix upon the application of an external potential. Nanoporous metals fabricated by dealloying have received extensive attention in many areas, such as catalysis/electrocatalysis, energy conversion/storage, and sensing/biosensing. In this review, we focus on the recent developments of dealloyed nanoporous metals in the application of actuation. In particular, we summarize the experimental strategies in the studies and highlight the recent advances in the actuator materials. Finally, we conclude with outlook and perspectives with respect to future research on dealloyed nanoporous metals in applications of actuation in electrochemical or chemical environment.

Rolling structure from bilayer nanofilm by mismatch

A continuum theoretical scheme for self-rolling nanotubes from bilayers by mismatch is obtained by considering surface elasticity, surface stress, and symmetry lowering effects. For an ultrathin nanofilm with only several nanometers in thickness, isotropic mismatch, and isotropic surface stress usually induce anisotropic rolling behavior. The isotropic Timoshenko formula should be modified anisotropically to explain the mechanical behavior of anisotropic rolling structure of nanotubes accurately. The nanofilm rolls up in tangential direction while remaining straight in cylindrical direction theoretically. Therefore, in this paper the anisotropic shape of nanotubes is taken into consideration. Along the cylindrical direction, although it maintains straight and its residual strain is uniform, the stress varies in the radial direction due to the Poisson’s effect of tangential strain. The results of the current theory applied to Si–Si nanotube, InAs–GaAs nanotube, and InGaAs–Cr nanotube systems show good agreement with the experimental data. Beside the surface elasticity effect and surface stress effect, the symmetry breaking and the anisotropic rolling structure are of great importance in theoretically describing the mechanical behavior of rolling-up of nanotubes.

Identification of Elastic Response of Asphalt Pavements Layers Using Deflectometric Tests With Different Load Levels

It is a unusual practice in national new pavement designs and rehabilitations to consider the nonlinear elastic behavior of the materials that constitute or will constitute the pavement layers. However, the linear elastic response is a simplification of the actual pavement layers behavior, since the materials used in the structure have stiffness dependent on the stress state (granular layers and subgrade) or on the temperature and time of load application (asphalt concrete). In view of this, this paper aims to study the elastic behavior of the pavement structures of two monitored sites in the city of Santa Maria/RS, through tests with FWD equipment, applying four different levels of loading on the full extension of these two sites. In these structures, the pavements showed a tendency to stiffening with increasing load acting at initial measuring distances (near the FWD load application point), which represent the elastic compression of all layers that make up the pavement. The measuring distances farther from the load application point, referring to the elastic compression of the subgrade, indicated, in most cases, a tendency to linear behavior of the load-deflection relationship in both sites. The backcalculated resilience modulus confirmed the impressions drawn from the load-deflection relationships, indicating the nonlinear elastic behavior of the granular layers (base and sub-base) of the analyzed sites, with resilience modulus directly proportional to the increase of the confining stress. The subgrade of the experimental sites exhibited varied behavior, and could be simplified by linear elasticity, without considerable loss. The same fact happened for the asphalt concrete material used in the pavement of site 1. For site 2, the backcalculated modulus indicated asphalt concrete stiffness dependent on the vertical surface stress increment at the center of the load plate.

Modeling and Simulation of the Hysteretic Behavior of Concrete under Cyclic Tension-Compression Using the Smeared Crack Approach.

Concrete structures under wind and earthquake loads will experience tensile and compressive stress reversals. It is very important to accurately reproduce the hysteretic behavior and energy dissipation of concrete materials under cyclic tension-compression for the safety evaluation of concrete structures. A hysteretic model for concrete under cyclic tension-compression is proposed in the framework of smeared crack theory. Based on the crack surface opening-closing mechanism, the relationship between crack surface stress and cracking strain is constructed in a local coordinate system. Linear loading-unloading paths are used and the partial unloading-reloading condition is considered. The hysteretic curves in the model are controlled by two parameters: the initial closing stress and the complete closing stress, which can be determined by the test results. Comparison with several experimental results shows that the model is capable of simulating the cracking process and hysteretic behavior of concrete. In addition, the model is proven to be able to reproduce the damage evolution, energy dissipation, and stiffness recovery caused by crack closure during the cyclic tension-compression. The proposed model can be applied to the nonlinear analysis of real concrete structures under complex cyclic loads.

Traveling wave solutions to the inclined or periodic free boundary incompressible Navier-Stokes equations

This paper concerns the construction of traveling wave solutions to the free boundary incompressible Navier-Stokes system. We study a single layer of viscous fluid in a strip-like domain that is bounded below by a flat rigid surface and above by a moving surface. The fluid is acted upon by a bulk force and a surface stress that are stationary in a coordinate system moving parallel to the fluid bottom. We also assume that the fluid is subject to a uniform gravitational force that can be resolved into a sum of a vertical component and a component lying in the direction of the traveling wave velocity. This configuration arises, for instance, in the modeling of fluid flow down an inclined plane. We also study the effect of periodicity by allowing the fluid cross section to be periodic in various directions. The horizontal component of the gravitational field gives rise to stationary solutions that are pure shear flows, and we construct our solutions as perturbations of these by means of an implicit function argument. An essential component of our analysis is the development of some new functional analytic properties of a scale of anisotropic Sobolev spaces, including that these spaces are an algebra in the supercritical regime, which may be of independent interest.