Detachment Fronts Research Articles

Tuning load redistribution and damage near heterogeneous interfaces

We investigate interface failure of model materials representing architected thin films in contact with heterogeneous substrates. We find that, while systems with statistically isotropic distributions of impurities derive their fracture strength from the ability to develop rough detachment fronts, materials with hierarchical microstructures confine failure near a prescribed surface, where crack growth is arrested and crack surface correlations are suppressed. We develop a theory of network Green’s functions for the systems at hand, and we find that the ability of hierarchical microstructures to control failure mode and locations comes at no performance cost in terms of peak stress and specific work of failure and derives from the quenched local anistotropy of the elastic interaction kernel.

Statistical aspects of interface adhesion and detachment of hierarchically patterned structures

We introduce a three dimensional model for interface failure of hierarchical materials adhering to heterogeneous substrates. We find that the hierarchical structure induces scale invariant detachment patterns, which in the limit of low interface disorder prevent interface failure by crack propagation (‘detachment fronts’). In the opposite limit of high interface disorder, hierarchical patterns ensure enhanced work of failure as compared to reference non-hierarchical structures. While the study of hierarchical adhesion is motivated by examples of fibrous materials of biological interest, our results indicate that hierarchical patterns can be useful in engineering scenarios in view of tuning and optimizing adhesion properties.

The multiple V-shaped double peeling of elastic thin films from elastic soft substrates

In this paper, a periodic configuration of V-shaped double peeling process is investigated. Specifically, an elastic thin film is detached from a soft elastic material by applying multiple concentrated loads periodically distributed with spatial periodicity λ. The original Kendall’s idea is extended to take into account the change in elastic energy occurring in the substrate when the detachment fronts propagate. The symmetric configuration typical of a V-peeling process causes the energy release rate to be sensitive to variations of the elastic energy stored in the soft substrate. This results in an enhancement of the adhesion strength because part of the external work required to trigger the peeling mechanism is converted in substrate elastic energy.A key role is played by both spatial periodicity λ and elasticity ratio E/Eh, between tape and substrate elastic moduli, in determining the conditions of stable adhesion. Indeed, the presence of multiple peeling fronts determines a modification of the mechanism of interaction, because deformations close to each peeling front are also affected by the stresses related to the other fronts. Results show that the energy release rate depends on the detached length of the tape so that conditions can be established which lead to an increase of the supported load compared to the classical peeling on rigid substrates.Finally, we also find that for any given value of the load per unit length, an optimum value of the wavelength λ exists that maximizes the tolerance of the system, before unstable propagation of the peeling front can occur.

Strength of adhesive contacts: Influence of contact geometry and material gradients

The strength of an adhesive contact between two bodies can strongly depend on the macroscopic and microscopic shape of the surfaces. In the past, the influence of roughness has been investigated thoroughly. However, even in the presence of perfectly smooth surfaces, geometry can come into play in form of the macroscopic shape of the contacting region. Here we present numerical and experimental results for contacts of rigid punches with flat but oddly shaped face contacting a soft, adhesive counterpart. When it is carefully pulled off, we find that in contrast to circular shapes, detachment occurs not instantaneously but detachment fronts start at pointed corners and travel inwards, until the final configuration is reached which for macroscopically isotropic shapes is almost circular. For elongated indenters, the final shape resembles the original one with rounded corners. We describe the influence of the shape of the stamp both experimentally and numerically.Numerical simulations are performed using a new formulation of the boundary element method for simulation of adhesive contacts suggested by Pohrt and Popov. It is based on a local, mesh dependent detachment criterion which is derived from the Griffith principle of balance of released elastic energy and the work of adhesion. The validation of the suggested method is made both by comparison with known analytical solutions and with experiments. The method is applied for simulating the detachment of flat-ended indenters with square, triangle or rectangular shape of cross-section as well as shapes with various kinds of faults and to “brushes”. The method is extended for describing power-law gradient media.

A phase-plane analysis of localized frictional waves.

Sliding frictional interfaces at a range of length scales are observed to generate travelling waves; these are considered relevant, for example, to both earthquake ground surface movements and the performance of mechanical brakes and dampers. We propose an explanation of the origins of these waves through the study of an idealized mechanical model: a thin elastic plate subject to uniform shear stress held in frictional contact with a rigid flat surface. We construct a nonlinear wave equation for the deformation of the plate, and couple it to a spinodal rate-and-state friction law which leads to a mathematically well-posed problem that is capable of capturing many effects not accessible in a Coulomb friction model. Our model sustains a rich variety of solutions, including periodic stick–slip wave trains, isolated slip and stick pulses, and detachment and attachment fronts. Analytical and numerical bifurcation analysis is used to show how these states are organized in a two-parameter state diagram. We discuss briefly the possible physical interpretation of each of these states, and remark also that our spinodal friction law, though more complicated than other classical rate-and-state laws, is required in order to capture the full richness of wave types.

On factors controlling precursor slip fronts in the laboratory and their relation to slow slip events in nature

AbstractSpatial variations in frictional properties on natural faults are believed to be a factor influencing the presence of slow slip events (SSEs). This effect was tested on a laboratory frictional interface between two polymethyl methacrylate (PMMA) bodies. We studied the evolution of slip and slip rates that varied systematically based on the application of both high and low normal stress (σ0=0.8 or 0.4 MPa) and the far‐field loading rate (VLP). A spontaneous, frictional rupture expanded from the central, weaker, and more compliant section of the fault that had fewer asperities. Slow rupture propagated at speeds Vslow∼0.8 to 26 mm s−1 with slip rates from 0.01 to 0.2 μm s−1, resulting in stress drops around 100 kPa. During certain nucleation sequences, the fault experienced a partial stress drop, referred to as precursor detachment fronts in tribology. Only at the higher level of normal stress did these fronts exist, and the slip and slip rates mimicked the moment and moment release rates during the 2013–2014 Boso SSE in Japan. The laboratory detachment fronts showed rupture propagation speeds Vslow/VR∈ (5 to 172) × 10−7 and stress drops ∼ 100 kPa, which both scaled to the aforementioned SSE. Distributions of asperities, measured using a pressure sensitive film, increased in complexity with additional normal stress—an increase in normal stress caused added complexity by increasing both the mean size and standard deviation of asperity distributions, and this appeared to control the presence of the detachment front.

Effects of loading angles on stick–slip dynamics of soft sliders

When soft gels move across a hard surface, stick–slip frictional sliding is mediated by propagation of adhesion and detachment fronts. Here we experimentally investigate the sliding dynamics of an extended frictional interface between soft Silicone gel and hard PMMA and identify three distinct sliding regimes. We directly visualize the interface and show that a minute manipulation of the initial loading angle results in a sharp bifurcation between the different sliding states. The phase diagram as well as universal scaling relations governing the dynamics is presented.

Sensitivity of detachment extent to magnetic configuration and external parameters

Divertor detachment may be essential to reduce heat loads to magnetic fusion tokamak reactor divertor surfaces. Yet in experiments it is difficult to control the extent of the detached, low pressure, plasma region. At maximum extent the front edge of the detached region reaches the X-point and can lead to degradation of core plasma properties. We define the ‘detachment window’ in a given position control variable C (for example, the upstream plasma density) as the range in C within which the front location can be stably held at any position from the target to the X-point; increased detachment window corresponds to better control. We extend a 1D analytic model [] to determine the detachment window for the following control variables: the upstream plasma density, the impurity concentration and the power entering the scrape-off layer (SOL). We find that variations in magnetic configuration can have strong effects; increasing the ratio of the total magnetic field at the X-point to that at the target, , (total flux expansion, as in the super-x divertor configuration) strongly increases the detachment window for all control variables studied, thus strongly improving detachment front control and the capability of the divertor plasma to passively accommodate transients while still staying detached. Increasing flux tube length and thus volume in the divertor, through poloidal flux expansion (as in the snowflake or x-divertor configurations) or length of the divertor, also increases the detachment window, but less than the total flux expansion does. The sensitivity of the detachment front location, zh, to each control variable, C, defined as , depends on the magnetic configuration. The size of the radiating volume and the total divertor radiation increase and , respectively, but not by increasing divertor poloidal flux expansion or field line length. We believe this model is applicable more generally to any thermal fronts in flux tubes with varying magnetic field, and similar sources and sinks, such as detachment fronts in stellarator divertors and solar prominences in coronal loops.

Scalar model for frictional precursors dynamics.

Recent experiments indicate that frictional sliding occurs by nucleation of detachment fronts at the contact interface that may appear well before the onset of global sliding. This intriguing precursory activity is not accounted for by traditional friction theories but is extremely important for friction dominated geophysical phenomena as earthquakes, landslides or avalanches. Here we simulate the onset of slip of a three dimensional elastic body resting on a surface and show that experimentally observed frictional precursors depend in a complex non-universal way on the sample geometry and loading conditions. Our model satisfies Archard's law and Amontons' first and second laws, reproducing with remarkable precision the real contact area dynamics, the precursors' envelope dynamics prior to sliding, and the normal and shear internal stress distributions close to the interfacial surface. Moreover, it allows to assess which features can be attributed to the elastic equilibrium, and which are attributed to the out-of-equilibrium dynamics, suggesting that precursory activity is an intrinsically quasi-static physical process. A direct calculation of the evolution of the Coulomb stress before and during precursors nucleation shows large variations across the sample, explaining why earthquake forecasting methods based only on accumulated slip and Coulomb stress monitoring are often ineffective.

Nucleation and propagation of solitary Schallamach waves.

We isolate single Schallamach waves--detachment fronts that mediate inhomogeneous sliding between an elastomer and a hard surface--to study their creation and dynamics. Based on measurements of surface displacement using high-speed in situ imaging, we establish a Burgers vector for the waves. The crystal dislocation analogs of nucleation stress, defect pinning, and configurational force are demonstrated. It is shown that many experimentally observed features can be quantitatively described using a conventional model of a dislocation line in an elastic medium. We also highlight the evolution of nucleation features, such as surface wrinkles, with consequences for interface delamination.

Effects of Neutral Particles on the Stability of the Detachment Fronts in Divertor Plasmas

given by the recycling of the ion flux plus auxiliary gas puffing, decayed exponentially with the local mean free path of the ionization reaction or the geometric mean of mean free paths of the ionization and charge exchange reactions. In this paper, in order to focus on the effects of neutrals on detachment fronts, we adopted a time dependent self-consistent neutral model. The neutral flux is assumed to be composed of convection with a constant flow velocity and diffusion involving the charge exchange reaction [3]. Source term due to volume recombination reaction and loss/source term due to the cross-field neutral transport are involved in it. The total number of ion and neutral particles becomes conserved. The geometry of the 1D divertor model and the plasma fluid equations are shown in Sec. 2.1. Comparison between old neutral model and new one is shown in Sec. 2.2. As simulation results, we first show the difference in the neutral density profiles resulted from each transport mechanism introduced in the neutral model in Sec. 3.1. In Sec. 3.2, we show a simulation result of the detached regime and the flows of particles and heat from the core plasma. Also is shown there the conditions of the recycling rate and neutral loss time constant where the detached regime occurs. Finally, in Sec. 4, we present a conclusion.

Transition from static to dynamic macroscopic friction in the framework of the Frenkel–Kontorova model

A new generation of experiments on dry macroscopic friction has revealed that the transition from static to dynamic friction is essentially a spatially and temporally non-uniform process, initiated by a rupture-like detachment front. We show the suitability of the Frenkel–Kontorova model for describing this transition. The model predicts the existence of two types of detachment fronts, explaining both the variability and abrupt change of velocity observed in experiments. The quantitative relation obtained between the velocity of the detachment front and the ratio of shear to normal stress is consistent with experiments. The model provides relations between kinematic and dynamic parameters of the transition process.

Family-Vicsek scaling of detachment fronts in granular Rayleigh-Taylor instabilities during sedimentating granular/fluid flows

When submillimetric particles are confined in a fluid such that a compact cluster of particles lie above the clear fluid, particles will detach from the lower boundary of the cluster and form an unstable separation front giving rise to growing fingers of falling particles. We study this problem using both experiments and hybrid granular/fluid mechanics models. In the case of particles from 50 to 500 microns in diameter falling in air, we study the horizontal density fluctuations at early times: the amplitude of the density difference between two points at a certain horizontal distance grows as a power law of time. This happens up to a saturation corresponding to a power law of the distance. The way in which the correlation length builds up to this saturation also follows a power law of time. We show that these decompaction fronts in sedimentation problems follow a Family-Vicsek scaling, characterize the dynamic and Hurst exponent of the lateral density fluctuations, respectively z ∼ 1 and ζ ∼ 0.75, and show how the prefactors depend on the grain diameter. We also show from similar simulations with a more viscous and incompressible fluid, that this feature is independent of the fluid compressibility or viscosity, ranging from air to water/glycerol mixtures.

One-Dimensional Time Dependent Analysis of the Detachment Front in a Divertor Plasma: Roles of the Cross-Field Transport

Results of one-dimensional time dependent analysis of a detachment front in a PDD plasma are reported. Effects of the cross-field particle and energy trasport in a divertor region on a partially detached plasma are particularly discussed. It was found that such transport effects in the divertor region can prevent the detachment fronts from moving upstream, and that the position of the detachment front in a steady state is thermally unstable against the neutral density in the divertor region.

Short-Time Dynamics of Frictional Strength in Dry Friction

We present an experimental study of the onset of local frictional motion along a long, spatially extended interface that separates two PMMA blocks in dry frictional contact. At applied shear forces significantly below the static friction threshold, rapid precursory detachment fronts are excited, which propagate at near sound speeds along the interface. These fronts initiate from the interface edge and arrest prior to traversing the entire sample length. Along the fronts’ path, we perform real-time measurements of the real contact area at every spatial point within the interface. In addition, the motion (slip) of the material adjacent to the interface is simultaneously measured at chosen locations. Upon their arrival at each spatial point along their path, these fronts instantaneously (within 4 μs) reduce the net contact area. Net slip is only initiated after this contact area reduction occurs. Slip is initially rapid and progresses at its initial velocity for a constant (60 μs) duration. Slip dynamics then undergo a sharp transition to velocities an order of magnitude slower, which remain nearly constant until slip arrest. We demonstrate that this scenario can be quantitatively explained by a model of interface weakening caused by instantaneous fracture-induced heating. Sustained rapid slip occurs in this weakened phase. Once the interface cools beneath its glass temperature the sharp transition to slow slip takes place. A similar fracture-induced weakening scenario might be expected in additional classes of materials.

Dynamics of Transition from Static to Kinetic Friction

We propose a model for a description of dynamics of cracklike processes that occur at the interface between two blocks prior to the onset of frictional motion. We find that the onset of sliding is preceded by well-defined detachment fronts initiated at the slider trailing edge and extended across the slider over limited lengths smaller than the overall length of the slider. Three different types of detachment fronts may play a role in the onset of sliding: (i) Rayleigh (surface sound) fronts, (ii) slow detachment fronts, and (iii) fast fronts. The important consequence of the precursor dynamics is that before the transition to overall sliding occurs, the initially uniform, unstressed slider is already transformed into a highly nonuniform, stressed state. Our model allows us to explain experimental observations and predicts the effect of material properties on the dynamics of the transition to sliding.

Crack-like processes governing the onset of frictional slip

We perform real-time measurements of the net contact area between two blocks of like material at the onset of frictional slip. We show that the process of interface detachment, which immediately precedes the inception of frictional sliding, is governed by three different types of detachment fronts. These crack-like detachment fronts differ by both their propagation velocities and by the amount of net contact surface reduction caused by their passage. The most rapid fronts propagate at intersonic velocities but generate a negligible reduction in contact area across the interface. Sub-Rayleigh fronts are crack-like modes which propagate at velocities up to the Rayleigh wave speed, V R, and give rise to an approximate 10% reduction in net contact area. The most efficient contact area reduction (~20%) is precipitated by the passage of ‘slow detachment fronts’. These fronts propagate at ‘anomalously’ slow velocities, which are over an order of magnitude lower than V R yet orders of magnitude higher than other characteristic velocity scales such as either slip or loading velocities. Slow fronts are generated, in conjunction with intersonic fronts, by the sudden arrest of sub-Rayleigh fronts. No overall sliding of the interface occurs until either of the slower two fronts traverses the entire interface, and motion at the leading edge of the interface is initiated. Slip at the trailing edge of the interface accompanies the motion of both the slow and sub-Rayleigh fronts. We might expect these modes to be important in both fault nucleation and earthquake dynamics.

Detachment fronts and the onset of dynamic friction.

The dynamics of friction have been studied for hundreds of years, yet many aspects of these everyday processes are not understood. One such aspect is the onset of frictional motion (slip). First described more than 200 years ago as the transition from static to dynamic friction, the onset of slip is central to fields as diverse as physics, tribology, mechanics of earthquakes and fracture. Here we show that the onset of frictional slip is governed by three different types of coherent crack-like fronts: these are observed by real-time visualization of the net contact area that forms the interface separating two blocks of like material. Two of these fronts, which propagate at subsonic and intersonic velocities, have been the subject of intensive recent interest. We show that a third type of front, which propagates an order of magnitude more slowly, is the dominant mechanism for the rupture of the interface. No overall motion (sliding) of the blocks occurs until either of the slower two fronts traverses the entire interface.