Distinct stick-slip modes in adhesive polymer interfaces

Stick-slip, manifest as intermittent tangential motion between two solids, is a well-known friction instability that occurs in a number of natural and engineering systems. In the context of adhesive polymer interfaces, this phenomenon has often been solely associated with Schallamach waves, which are termed slow waves due to their low propagation speeds. We study the dynamics of a model polymer interface using coupled force measurements and high speed \emph{in situ} imaging, to explore the occurrence of stick-slip linked to other slow wave phenomena. Two new waves---slip pulse and separation pulse---both distinct from Schallamach waves, are described. The slip pulse is a sharp stress front that propagates in the same direction as the Schallamach wave, while the separation pulse involves local interface detachment and travels in the opposite direction. Transitions between these stick-slip modes are easily effected by changing the sliding velocity or normal load. The properties of these three waves, and their relation to stick-slip is elucidated. We also demonstrate the important role of adhesion in effecting wave propagation.Comment: 22 pages, 9 figure

Opposite moving detachment waves mediate stick-slip friction at soft interfaces

Intermittent motion, called stick--slip, is a friction instability that commonly occurs during relative sliding of two elastic solids. In adhesive polymer contacts, where elasticity and interface adhesion are strongly coupled, stick--slip results from the propagation of slow detachment waves at the interface. Using \emph{in situ} imaging experiments at an adhesive contact, we show the occurrence of two distinct detachment waves moving parallel (Schallamach wave) and anti-parallel (separation wave) to the applied remote sliding. Both waves cause slip in the same direction and travel at speeds much lesser than any elastic wave speed. We use an elastodynamic framework to describe the propagation of these slow detachment waves at an elastic-rigid interface and obtain governing integral equations in the low wave speed limit. These integral equations are solved in closed form when the elastic solid is incompressible. Two solution branches emerge, corresponding to opposite moving detachment waves, just as seen in the experiments. A numerical scheme is used to obtain interface stresses and velocities for the incompressible case for arbitrary Poisson ratio. Based on these results, we explicitly demonstrate a correspondence between propagating slow detachment waves and a static bi-material interface crack. Based on this, and coupled with a recently proposed fracture analogy for dynamic friction, we develop a phase diagram showing domains of possible occurrence of stick--slip via detachment waves vis-\'a-vis steady interface sliding.Comment: 40 pages, 13 figure

Design of a low-velocity impact framework for evaluating space-grade materials

Material deformation and failure under impact loading is a subject of active investigation in space science and often requires very specialized equipment for testing. In this work, we present the design, operational analysis and application of a low-velocity ($\sim 100$ m/s) projectile impact framework for evaluating the deformation and failure of space-grade materials. The system is designed to be modular and easily adaptable to various test geometries, while enabling accurate quantitative evaluation of plastic flow. Using coupled numerical methods and experimental techniques, we first establish an operating procedure for the system. Following this, its performance in two complementary impact configurations is demonstrated using numerical and experimental analysis. In the first, a Taylor impact test is performed for predicting the deformed shape of a cylindrical projectile impinging on a rigid substrate. In the second, deformation of a plate struck by a rigid projectile is evaluated. In both cases, physics-based models are used to interpret the resulting fields. We present a discussion of how the system may be used both for material property estimation (e.g., dynamic yield strength) as well as for failure evaluation (e.g., perforation and fracture) in the same projectile impact configuration

Environment-assisted fracture, my friend: The cutting of gummy metals

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Model Checking Multithreaded Programs with Asynchronous Atomic Methods

In order to make multithreaded programming manageable, programmers often follow a design principle where they break the problem into tasks which are then solved asynchronously and concurrently on different threads. This paper investigates the problem of model checking programs that follow this idiom. We present a programming language \spl{} that encapsulates this design pattern. \spl{} extends simplified form of sequential Java to which we add the capability of making asynchronous method invocations in addition to the standard synchronous method calls and the ability to execute asynchronous methods in threads atomically and concurrently. Our main result shows that the control state reachability problem for finite \spl{} programs is decidable. Therefore, such multithreaded programs can be model checked using the counter-example guided abstraction-refinement framework

Slow frictional waves

When modeling physical phenomena, it is common to lump the effects of friction into one or a few parameters. Historically, this phenomenological view of frictional processes was driven by purely practical considerations. As a result of independent developments in miniaturization of components, tribology and improved geophysical observations, the microscopic aspects of friction are now becoming better understood. In particular, the occurrence of stick-slip instabilities has been explained in many systems within the asperity framework. However, in highly adhesive soft polymers, strong van der Waals forces mask the effects of asperity deformation. Hence such surfaces are in more intimate contact and the interface dynamics is governed by pinning and de-pinning of polymer chains. Additionally, the extended nature of the contact surface allows elastic effects to be manifest on the mesoscale (several micrometers). Hence stick-slip in soft polymer surfaces is a complex phenomenon that has hitherto escaped a unified description. I n this work, we isolate individual stick-slip events in a soft polymer surface. Using high-resolution high-speed in situ imaging methods, it is shown that stick-slip in such adhesive surfaces is fundamentally of three kinds, corresponding to the propagation of three different surface waves—detachment pulses, slip pulses and Schallamach waves. Each of theses waves has basic differentiating properties and occurs under certain experimental conditions. The nucleation and propagation of these waves are studied quantitatively, revealing a fundamental relation between stick-slip and slow surface waves. The three observed surface waves have direct analogues in muscular locomotion of many soft-bodied invertebrates. The results raise an important question about the origin and subsequent development of specialized physiology in these organisms. A theoretical description is presented to explain the observations. A unified elastic theory for slow waves involving detachment is developed and is capable of quantitatively reproducing experimental observations. This theory is independent of any assumed friction model. Other simple models are also presented for describing the nucleation and propagation of Schallamach waves. It is finally shown that Coulomb friction cannot account for the existence of waves without detachment—assuming Coulomb friction to hold at the interface results in a completely inconsistent solution. Hence new statistical or microscopic models are necessary for fully explaining wave propagation without interface detachment. Important implications of the results for the phenomenon of slow earthquakes, as well as other recently observed slow fronts is briefly discussed. This work outlines the fundamental link between the propagation of slow surface waves and friction. It is hoped that the findings will also help drive efforts to control stick-slip in soft polymer surfaces

Fifty years of Schallamach waves: From rubber friction to nanoscale fracture

The question of how soft polymers slide against hard surfaces is of significant scientific interest, given its practical implications. Specifically, such sytems commonly show interesting stick-slip dynamics, wherein the interface moves intermittently despite uniform remote loading. \mt{The year 2021 marked the 50$^{th}$ anniversary of the publication of a seminal paper by Adolf Schallamach (\emph{Wear}, 1971)} that first revealed an intimate link between stick-slip and moving detachment waves, now called Schallamach waves. We place Schallamach's results in a broader context and review subsequent investigations of stick-slip, before discussing recent observations of solitary Schallamach waves. This variant is not observable in standard contacts so that a special cylindrical contact must be used to quantify its properties. The latter configuration also reveals the occurrence of a dual wave -- the so-called separation pulse -- that propagates in a direction opposite to Schallamach waves. We show how the dual wave and other, more general, Schallamach-type waves can be described using continuum theory, and provide pointers for future research. In the process, fundamental analogues of Schallamach-type waves emerge in nanoscale mechanics and interface fracture. The result is an on-going application of lessons learnt from Schallamach-type waves to better understand these latter phenomena.Comment: 45 pages, 12 figure