Shear Traction Research Articles

Analysis of load-induced top-down cracking initiation in asphalt pavements using a two-dimensional microstructure-based multiscale finite element method

Load-induced top-down cracking is one of the major types of asphalt pavement deterioration; however, its initiation mechanisms have not been fully understood so far, which makes it very difficult to effectively consider this failure pattern in the pavement design procedures. To address this issue, the present study developed a two-dimensional microstructure-based multiscale finite element model, in which material properties on two physical length scales, i.e., the local (mixture level) and global (pavement level) scales, were incorporated in the computation and linked through a homogenization process. A digital image processing (DIP) technology was employed to develop the two-dimensional local-scale representative volume element (RVE) model that considered the realistic heterogeneous microstructure of asphalt concrete (AC), and a bilinear cohesive zone model was applied to simulating the local-scale damage initiation and evolution in the RVEs. Two typical pavement structures, with cement-treated base (CTB) and granular base (GB) respectively, were taken into account to interpret the influence of the global-scale pavement configurations on top-down cracking performance. The results showed that the significant near-surface transverse tensile stress just outside the tire edge could be the primary cause of the top-down cracking. For the pavement with CTB, the top-down cracking was the predominant type of fatigue failure, whereas for the pavement with GB, the bottom-up cracking was the main pattern of fatigue failure. Besides, as the temperature increased, more damage was induced under the same traffic loading due to the reduced tensile strength of AC. It was also found on the local scale that the significant tensile stress within the mortar matrix phase probably acted as the driving force of the microcrack initiation and propagation and the effects of the shear traction on the damage evolution in the AC layer increased with the temperature.

Localized and overall interaction effects of irregular interfacial bonds and elastic edge restraints for sandwich and functionally graded multilayer circular plates with normal/shear tractions

In the present article, interactions of the effects of the interfacial bond’s irregularity, shear and normal tractions, and flexible edge supports on the distributions of the lateral deflections and stress of the laminated/sandwich circular plates are investigated, for the first time. To widen the applicability of the research, the layers are assumed to be functionally graded ones. A layerwise model that unlike the traditional layerwise theories, not only guarantees continuity of the transverse stress components but also takes into account the discontinuity of all of the displacement components at the imperfect or damaged bonds between layers is proposed. The governing equations are derived based on the principle of minimum potential energy and solved based on the Maclaurin transformation analytical method. Results are enhanced through a three-dimensional elasticity correction. Results reveal that while compliance of the edge restraint has a significant role on the magnitude of the global displacements, amount of the discontinuities in the displacement components at the mutual interfaces is mainly affected by both the local and global (mean) bond stiffness. Furthermore, while both the compliance of the elastic edge restraint and irregularity of the bonds increase the stress and displacement components, each combination of these parameters may specifically lead to magnifications of the stresses in a particular layer of the plate.

Dynamic deformation and failure process of quasi-closed-cell aluminum foam manufactured by direct foaming technique

The application of cellular structure for energy dissipation requires the investigation on the deformation and failure process of the foam under dynamic loading. In this study, the split-Hopkinson pressure bar technique with high speed video camera was used to directly observe the dynamic deformation in quasi-closed-cell aluminum foam fabricated by an in-house direct foaming methodology. The characterization of cellular structure was first performed on the developed aluminum foam, indicating that the cell size follows a log-normal distribution. The measurements revealed that the deformation mode varies with the strain rate. After that an experimentally validated X-ray micro-computed tomography based 3D finite element model of the as-fabricated specimen was established to predict the stress distribution and deformation history under impact loading, and correlated to the experimental observations to explore the deformation mechanisms. It was found that shear band is formed after peak stress. The dynamic deformation and failure process of the quasi-closed-cell aluminum foam result from the coexistence of cell wall bending and buckling, cell collapse and shear traction induced tearing breakage.

Surface initiation of rolling contact fatigue at asperities considering slip, shear limit and thermal elastohydrodynamic lubrication

Abstract A numerical investigation was performed, with single axisymmetric asperities passing through lubricated rolling contacts at different slip. Two explanatory and cooperating phenomena were found as to why the damage develops more frequently at negative than positive slip. Metal contact occurred in the inlet, where tractive asperity contacts at negative slip provided a large tensile surface stress outside the contact. As the asperity moved through the contact, sliding supplied it with lubricant and heated the lubricant along the contact. The shear tractions were thus higher near the inlet than the outlet, making them more detrimental for negative than positive slip.

Simultaneous inversion of shear modulus and traction boundary conditions in biomechanical imaging

ABSTRACTWe present a formulation to simultaneously invert for a heterogeneous shear modulus field and traction boundary conditions in an incompressible linear elastic plane stress model. Our approach utilizes scalable deterministic methods, including adjoint-based sensitivities and quasi-Newton optimization, to reduce the computational requirements for large-scale inversion with partial differential equation (PDE) constraints. We address the use of regularization for such formulations and explore the use of different types of regularization for the shear modulus and boundary traction. We apply this PDE-constrained optimization algorithm to a synthetic dataset to verify the accuracy in the reconstructed parameters, and to experimental data from a tissue-mimicking ultrasound phantom. In all of these examples, we compare inversion results from full-field and sparse data measurements.

On the transient dynamic antiplane contact problem in the presence of dry friction and slip

This article models the elastodynamic transient contact between two elastically similar half planes under antiplane loading and in the presence of friction. Contact is maintained along the positive real line under the presence of a certain remote contact pressure. An antiplane shear load is applied, which entails interfacial shear traction that opposes the frictional force entailed by the contact pressure. In order to balance the surface tractions, the surface must be allowed to slip. We derive the closed form solution of the interfacial traction due to a general antiplanar displacement distribution using a variant of the Wiener-Hopf technique. We also find closed-form expressions for the interfacial shear traction due to this remote antiplane load. In combination with the frictional force, this leads to an integral equation the solution to which is the distribution of relative slip. We quantify both this and the magnitude of the interfacial shear tractions under diverse loading, showing that transient loading leads to partial reverse slip of the contact surfaces. We show that the reverse slip tends to vanish over time, and that it is ameliorated if the friction coefficient is reduced.

Micropitting Fatigue Wear Simulation in Conformal-Contact Under Mixed Elastohydrodynamic Lubrication

In this study, a physics-based fatigue wear model is proposed to evaluate the reliability and to predict the life of cumulative micropitting wear for lubricated conformal contacts on rough surfaces. The surface normal load, mean film thickness, and frictional shear traction are simulated by a mixed elastohydrodynamic lubrication (EHL) model for a stress prediction model to calculate the average maximum Hertzian pressure of contact asperities and unit with the statistical contact model and dynamic contact model to obtain the asperity stress cycle number. The wear formula is established through combining a micropitting life prediction model of surface asperities and a mean micropitting damage constant of asperities. The four dominant aspects affecting wear behaviors of the surface contact pairs, working conditions, structure and surface topographies, material properties and lubrication conditions are all taken into account in the model. It is a high-fidelity and comprehensive model that can be used to analyze and optimize the tribological design of rolling–sliding pairs in machinery. The micropitting fatigue wear modeling scheme is validated by comparison of theoretical calculations and available experimental wear data.

General Winkler Model for Kinematic Responses of Shafts in Linear Soil

A simplified model for calculating the seismic responses of the shaft is proposed in this paper. Based on the theory of Winkler elastic foundation beam, the urban shaft is simplified as a vertical beam. The horizontal soil reaction and vertical shear tractions between the shaft circumference and the surrounding soils are considered through horizontal springs and rotating springs on the sidewall of the shaft. The translation and rocking motion of the shaft are considered through horizontal springs and rotating springs at the bottom of the shaft. Then, the dynamic analysis model of the shafts under seismic motion is established, and the control equation of the dynamic response of the shaft in frequency domain is deduced. The analytical solution of the steady state response of the shaft is obtained. Considering the randomness of the earthquake motion, this method can get the shaft kinematic responses under different ground motions efficiently in conceptual design process.

Traction and strain-rate at the base of the lithosphere: an insight into cratonic survival

SUMMARY Cratons are the oldest parts of the lithosphere, some of them surviving since Archean. Their long-term survival has sometimes been attributed to high viscosity and low density. In our study, we use a numerical model to examine how shear tractions exerted by mantle convection work to deform cratons by convective shearing. We find that although tractions at the base of the lithosphere increase with increasing lithosphere thickness, the associated strain-rates decrease. This inverse relationship between stress and strain-rate results from lateral viscosity variations along with the model’s free-slip condition imposed at the Earth’s surface, which enables strain to accumulate along weak zones at plate boundaries. Additionally, we show that resistance to lithosphere deformation by means of convective shearing, which we express as an apparent viscosity, scales with the square of lithosphere thickness. This suggests that the enhanced thickness of the cratons protects them from convective shear and allows them to survive as the least deformed areas of the lithosphere. Indeed, we show that the combination of a smaller asthenospheric viscosity drop and a larger cratonic viscosity, together with the excess thickness of cratons compared to the surrounding lithosphere, can explain their survival since Archean time.

Thermal Weakening of Asperity Tips on Fault Planes at High Sliding Velocities

AbstractReal contacts on the scale of ~10 μm support gigapascal shear tractions and normal tractions on rapidly sliding faults. At sliding velocities above ~0.1 m/s, the asperity tips of the contacts become hot and weak. The macroscopic friction depends on the average strength of the asperity tips during their lifetimes of contact. The strength of the asperity tips does not go essentially to zero once they become weak because frictional heating would then cease and the tips would cool and strengthen. Rather, the contact retains finite strength so that frictional heating balances the heat lost from the asperity tip by conduction. Crudely, the macroscopic coefficient of friction at high sliding velocities decreases with the inverse of the square root of velocity rather than the inverse of the velocity in the widely used model of Rice (2006, https://doi.org/10.1029/2005/JB004006). Numerical thermal‐mechanical models support this inference. The finite strength of the asperity tips retards lateral extrusion of weakened material from the tips. Otherwise, extrusion would allow the sliding surfaces to converge establishing new contacts, which would renew the strength of the surface. The macroscopic coefficient of friction with somewhat weakened asperity tips remains high enough that the fault surface becomes hot on a millimeter scale in large crustal earthquakes. Fluid pressurization and eventually millimeter‐scale melting then reduce the macroscopic shear traction to lower values than does asperity tip weakening acting alone.

A new framework for the formulation and validation of cohesive mixed-mode delamination models

A new framework for the formulation and validation of interface cohesive models for mixed mode I-mode II delamination with variable mode-ratio is presented. The approach is based on a free energy decomposition driven by the identification of three main damage modes in the tensile plane of normal and shear tractions and leads to models thermodynamically consistent for any loading path. A model with a bilinear traction-separation law is developed in detail within the proposed framework. The considered model requires a limited number of easily identifiable parameters: the traction-separation laws in the pure modes with their fracture energies, plus two phenomenological parameters, responsible for the mixed-mode interaction between the mode I and mode II failure modes, that directly affect the shape of the damage activation locus, namely an exponent and a constitutive parameter, the latter geometrically defined as an internal angle. The overall fracture energy at any mode-ratio is an outcome of the model, without the need to introduce any empirical laws, and depends on the actually followed loading path. A rigorous validation protocol, including consistency, accuracy and evolutionary tests is proposed and used for the model validation. Several tests proposed in the literature, along different proportional and non-proportional loading paths, with application to the simulation of pure and mixed-mode tests for a variety of composite materials, are considered obtaining in all cases, consistent and accurate results.

A double-Hertz model for adhesive contact between cylinders under inclined forces.

A generalized double-Hertz (D-H) model has been proposed to consider the adhesive contact between an elastic cylinder and an elastic half space under inclined forces. The normal traction is exactly the same as that in the conventional D-H model. The shear traction of finite value is distributed into a slipping zone and a non-slipping zone. In the slipping zone, the shear traction is proportional to the compressive pressure. With the model, adhesive contact behaviour between cylinders has been numerically illustrated. The shear-induced peeling has been demonstrated. The value of the ratio for shear traction to normal traction larger than friction coefficient has been found in part of the non-slipping zone. Those altogether are consistent with experiments.

Investigation of the asperity point load mechanism for thermal elastohydrodynamic conditions

The rolling contact fatigue damage called pitting or spalling develops more frequently in surfaces with negative than positive slip. Since normal line loads do not cause any tensile surface stresses this investigation considers the effects of small point shaped asperities. Shear traction causes tensile stresses at the trailing edge of asperities entering the contact at negative slip. At positive slip the tensile stresses appear at the leading edge when the asperities exit the contact. It was found that the trailing edge of the asperity breaks through the lubrication film at contact entry. This causes negative slip to be more detrimental than positive slip. At negative slip the location of large frictional shear stresses and tension stresses from normal asperity contact coincide.

Simultaneous extraction of tensile and shear interactions at interfaces

A simple beam analysis is developed that accounts for the tensile and shear interactions between the surfaces of two beams that have been brought into contact. The generally coupled differential equations for the relative normal and tangential displacements between the interacting surfaces and the normal and shear components of the traction between the two surfaces are decoupled when a balance condition involving the bending stiffness and height of each beam is satisfied. This condition also allows the normal and shear components of the J-integral and the crack tip displacement to be obtained from measurements of the applied loads along with load point displacement and rotation of each beam. As a result, the derivative of each component of the J-integral with respect to its associated crack tip displacement simultaneously provides both the normal and shear traction–separation relations at any value of the mode-mix without invoking any assumption on the particular form. As an application of this method, the normal and shear traction–separation relations for a silicon/epoxy interface were determined over nominal mode-mixes ranging from −53° to 87.5° using non-symmetric end-loaded split (ELS) and end-notched flexure (ENF) specimens. It is found that many of the assumptions made previously in developing traction–separation relations based on potential or damage-based approaches do not apply to this particular interface, which should motivate future theoretical developments.

Chemo-thermo-mechanically Coupled Crystal Plasticity Simulation of Stress Evolution in Thermally Strained β-Sn Films

Whisker formation in tin films is a mode of stress relaxation, but the exact conditions causing them are yet to be established. In this work, a three-dimensional full-field chemo-thermo-mechanically coupled crystal plasticity simulation of thermally strained tin films was performed to evaluate the stress evolution and connect it to the redistribution of vacancies. Spatial heterogeneity in the hydrostatic stress along the grain boundary network (that served as the primary conduit for mass transport) was observed, which became more homogeneous towards the film surface. Normal and shear tractions on the columnar grain boundaries were evaluated as they might be crucial to breaking of the oxide layer (formed on the film surface) especially when inclined grain boundaries are present. With such an advanced multi-physics framework, several crystallographic and geometrical factors influencing whisker formation can be analyzed thereby leading to a better understanding of the factors modulating the nucleation and growth of such whiskers.

Tidal stress modeling of Ganymede: Strike-slip tectonism and Coulomb failure

High-resolution Galileo data of Ganymede's complex surface provide strong and ubiquitous evidence of strike-slip motion: en echelon structures, strike-slip duplexes, laterally offset pre-existing features, and strained craters. In a previous study, we performed a detailed mapping of these strike-slip morphologies within nine regions of Ganymede: Dardanus Sulcus, Tiamat Sulcus, Nun Sulci, Byblus Sulcus, Nippur and Philus Sulci, the Transitional Terrain of Northern Marius Regio, Anshar Sulcus, Arbela Sulcus, and Uruk Sulcus. Strike-slip indicators were inferred in various combinations at each site, in addition to extensional processes. However, the prominence of strike-slip indicators suggests shearing has been an important process to the structural development of Ganymede's surface. Moreover, tidal stresses on Ganymede, under particular circumstances, may have been sufficient to induce Coulomb failure and generate strike-slip faulting. Here we investigate the role of both diurnal and non-synchronous rotation (NSR) tidal stresses on Ganymede using the numerical code SatStress. We resolve normal and shear tractions onto discrete fault segments of specified orientation and assess Coulomb failure potential for the nine inferred fault zones. Testing a range of subsurface fault depths (z = 0–5 km) and ice friction (μf = 0.2–0.6), we find that tidal stress models of combined diurnal and NSR stress readily promote Coulomb failure within each studied fault zone. High friction (μf = 0.6) limits failure depths to ∼1 km, while low friction (μf = 0.2) extends failure depths to ∼2 km, consistent with elastic thickness estimates. We also compare each fault zone's predicted shear sense to the inferred shear sense from our structural mapping efforts and find compatible senses of shear among six of the nine regions that exhibit notable fault offset and/or prevalent inferences of en echelon, duplexes, and strained craters. In addition, principal stress orientations and failure feasibility computed on a global scale suggest that combined diurnal and NSR tidal stresses promote Coulomb failure within the shallow (< 2 km) icy lithosphere across much of Ganymede. Coulomb failure is limited near the equator by large compressive NSR stresses, but stresses at mid- to high-latitudes readily promote Coulomb failure along a wide range of fault orientations. Based on these results, we infer that nonsynchronous rotation may have assisted the formation and evolution of strike-slip structures on Ganymede.

Composite Materials and Structural Glass: Adhesion Phenomena

This study aims to investigate the adhesion phenomena between glass and composite materials, which - if they work in synergy – can increase the performance of glass structures, as regard both structural elements and the manufacture of joints. The experimental programme consisted of shear traction tests on all samples: glass-glass singlelap shear adhesion, glass-GFRP double-lap shear adhesion and glass-SRP shear adhesion. Different types of adhesive and various interface geometries were also tested to evaluate the ultimate force and identify fracture patterns with different bonding lengths. It was possible to identify the effective bonding length for each type of resin used to prepare test samples, by taking fracture load and average peel stress into account. Bonding length variations were also recorded. This enabled the values of bonding length to be verified analytically. Based on experimental results, a formula for the evaluation of delamination resistance and the optimal bonding length is proposed.

Extension and Dynamics of the Andes Inferred From the 2016 Parina (Huarichancara) Earthquake

AbstractTheMw6.1 2016 Parina earthquake led to extension of the south Peruvian Andes along a normal fault with evidence of Holocene slip. We use interferometric synthetic aperture radar, seismology, and field mapping to determine a source model for this event and show that extension at Parina is oriented NE‐SW, which is parallel to the shortening direction in the adjacent sub‐Andean lowlands. In addition, we use earthquake source models and GPS data to demonstrate that shortening within the sub‐Andes is parallel to topographic gradients. Both observations imply that forces resulting from spatial variations in gravitational potential energy are important in controlling the geometry of the deformation in the Andes. We calculate the horizontal forces per unit length acting between the Andes and South America due to these potential energy contrasts to be 4–8 ×1012 N/m along strike of the mountain range. Normal faulting at Parina implies that the Andes in south Peru have reached the maximum elevation that can be supported by the forces transmitted across the adjacent foreland, which requires that the foreland faults have an effective coefficient of friction0.2. Additionally, the onset of extension in parts of the central Andes following orogen‐wide compression in the late Miocene suggests that there has been a change in the force balance within the mountains. We propose that shortening on weak detachment faults within the Andean foreland since ∼5–9 Ma reduced the shear tractions acting along the base of the upper crust in the eastern Andes, leading to extension in the highest parts of the range.

Wrinkling and ratcheting of a thin film on cyclically deforming plastic substrate: Mechanical instability of the solid-electrolyte interphase in Li–ion batteries

Lithium–ion batteries (LIBs) in consumer electronics, electric cars and large-scale energy storage systems are often susceptible to capacity fading due to mechanical degradation of the solid-electrolyte interphase (SEI) layer on the electrodes. Here we present an analytical model to describe SEI wrinkling and ratcheting behaviors during cyclic lithiation and delithiation of LIBs. The SEI-electrode system is modeled as a bilayer structure consisting of a thin film resting on a plastic substrate. Surface instability is found in such a system under cyclic plastic deformation induced by lithiation and delithiation. A linear perturbation analysis is performed to determine the critical wrinkling strain and wavenumber. The interfacial shear traction induced by surface wrinkling can further lead to plastic ratcheting, and the wrinkling amplitude increases with each lithiation/delithiation cycle. A phase diagram is plotted to characterize and predict different system behaviors, e.g., elastic, elastic wrinkling, shakedown without wrinkling, shakedown with wrinkling, and ratcheting. A series of finite element simulations are performed to validate the theoretical predictions. The analysis suggests that the mechanical instabilities of the SEI, including wrinkling and ratcheting, can be prevented by several strategies, such as introducing an artificial SEI with a sufficiently large stiffness and thickness, and/or with a tensile pre-stress in the SEI.

Methods to solve half-plane partial slip contact problems

There exists a family of methods for finding the extent of partial slip in contact problems between elastically similar bodies, capable of idealisation by half-planes. Closed form solutions are given to problems subject to a constant normal load and subsequent application of either an increasing shear force or differential bulk tension parallel with the surface. The starting point may be either a sliding contact (as is customary), or a fully adhered contact. The corrections required to impose a point-wise interpretation of Coulomb’s law of friction may be either in the form of a shear traction distribution or as dislocation arrays. The latter, when applied to the fully adhered contact, has the merit of automatically preserving the locked-in relative surface strains.