Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A30: Fracture in Soft MaterialsFocus
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Sponsoring Units: GSOFT GSNP Chair: John Kolinski, Ecole polytechnique federale de Lausanne Room: BCEC 162B |
Monday, March 4, 2019 8:00AM - 8:36AM |
A30.00001: ‘Sideways’ and stable crack propagation in a silicone elastomer Invited Speaker: Matt Pharr This talk will describe a peculiar form of fracture that we have found in a highly stretchable silicone elastomer. Namely, under certain conditions a crack will deviate from its ‘standard’ trajectory and instead propagate perpendicular to that trajectory. The crack arrests stably, allowing the material ahead of the crack front to continue to sustain load, and thereby enabling enormous stretchabilities. We call this phenomenon ‘sideways’ cracking. To explain this behavior, we first perform finite element simulations that demonstrate a propensity for sideways cracking, even in isotropic elastomers. Next, we provide a hypothesis on the origin of sideways cracking that invokes microstructural anisotropy. To substantiate this hypothesis, we transversely pre-stretch samples to various extents prior to fracture testing, as to determine the influence of microstructural arrangement (chain alignment) on fracture energy. We conclude by describing how a number of loading conditions, such as sample geometry and strain rate affect this phenomenon. Overall, this talk aims to provide fundamental mechanical insight into basic phenomena associated with fracture of elastomers. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A30.00002: How Supertough Gels Break Itamar Kolvin, John Kolinski, Jian Ping Gong, Jay Fineberg Fracture of soft materials typically takes place at large stretches. This condition challenges our view of how things break, which is based on small-strain linear elasticity. In this talk, I will show how we directly visualized rupture of tough double-network gels at >50% strain. During fracture, crack tip shapes obey a x∼y1.6 power law, in contrast to the parabolic profile observed in low-strain cracks. A new length scale emerges from the power law that scales directly with the stored elastic energy and diverges when the crack velocity approaches the shear wave speed. Our results show that double-network gels undergo brittle fracture and provide a testing ground for large-strain fracture mechanics. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A30.00003: Quantitative Analysis of Spiropyran Mechanophore Activation in Multiple Network Elastomers Chyi-Huey Yeh, Yinjun Chen, Costantino Creton Multiple network elastomers (MNE) have been used as model systems to characterize the crack tip behavior in soft tough materials. These materials contain reinforcing swelled network chains that are largely responsible for the high fracture toughness of MNEs. By incorporating a spiropyran mechanophore as a chromatic force sensor in the swelled network, stress and energy density localization around a crack tip can be optically detected and quantified. Also, regions with a history of loading and unloading around a crack tip can be identified based on the reversible changes in absorbance of merocyanine isomers (spiropyran in its activated form) in the loading and unloading states. This allows for a more accurate identification of the onset of crack propagation, which occurs before reaching the peak stress. Based on the experimental results, a perspective can be constructed on how statistical molecular information, provided by the mechano-activation of spiropyran, can inform macroscopic modeling of soft tough materials. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A30.00004: Depinning dynamics of crack fronts Julien Chopin, Aditya Bhaskar, Atharv Jog, Laurent Ponson Nacre, bones or rationally designed artificial materials are all heterogeneous solids with mechanical properties far exceeding those of their constitutive components. Understanding the role of microscale heterogeneities on the macroscale fracture behavior of solids remains a query, especially when toughness gradients are large. In this talk, I will present a combined experimental and theoretical study of the micro-instabilities of a crack front taking place in heterogeneous materials between two successive equilibrium positions. The instabilities are triggered by pinning the front by isolated tough obstacles of controlled strength and size. We show that the depinning dynamics is controlled by a nonlocal line elasticity and the rate dependency of the dissipative mechanisms taking place within the process zone. We model the behavior by an overdamped equation of motion involving a characteristic material speed and provide an analytical solution which captures quantitatively all our experimental observations. The implications of our results on the energy dissipated during fast fracture events and the fracture behavior of materials with randomly distributed obstacles will be discussed. |
Monday, March 4, 2019 9:12AM - 9:48AM |
A30.00005: The Rules of Roughness: Understanding the Dynamic Generation of 3D Complexity in Fractures Invited Speaker: William Steinhardt There is a disconnect between our theoretical understanding of brittle fracture, where 2D (effectively 1D) mathematical descriptions generate idealized fractures that are flat, smooth, and stable, and most brittle fractures we encounter in natural or manmade materials (rocks, bones, ceramics), which have significant 3D complexity. However, much of the basic physics that governs this complexity is not well understood. We have developed an experimental system to study 3D fracture mechanics by observing hydraulic fractures in brittle hydrogels. Heavily cross-linked hydrogels have been shown to be a good model system for brittle materials, with the benefits of highly tunable rheology, transparency, and low breakdown pressures. Studying hydraulic fractures allows us to match the refractive index of the interior of the fracture to the bulk, which combined with high speed photography and scanning laser sheet illumination, enables us to resolve the fracture dynamics in three dimensions at up to 1000 volumes per second. We observe that macro-scale roughness comes in the form of step-like perturbations of the fracture front, resulting from material heterogeneity, which leave in their wake a curved linear scar known as a step line. Our dynamic three-dimensional observations of these steps and their interactions allow us to understand the surprisingly elegant topological rules that govern their growth and interaction. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A30.00006: Tearing path: instabilities for crack paths in thin sheets cut by blunt objects Benoit Roman, Eugenio Hamm, Iryna Sivak A thin sheet is a very soft object, which therefore undergoes very large deformation when torn. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A30.00007: The Virtual Frame Technique (VFT): direct imaging of fast cracks in soft elastomers Samuel Dillavou, Shmuel Rubinstein, John Kolinski Many phenomena of interest in nature and industry are rapid, making direct imaging both challenging and cost-prohibitive. Dynamic cracks can propagate at the sound speed of a material and are ubiquitous in the earth sciences and engineering applications; thus fracture epitomizes such phenomena. In soft materials, the sound speed is of order several meters per second, and a dynamic crack will entirely rupture a cm-scale sample in about 10 msec requiring rapid imaging for direct visualization. Here we present the Virtual Frame Technique (VFT), a simple, useful, and accessible form of compressed sensing that leverages the dynamic range of the camera’s sensor to increase the frame acquisition rate by up to 6 orders of magnitude. We discuss the requirements for use of the VFT and its performance when employed with several commercially available conventional and high-speed cameras. Even Modern cell phones can achieve imaging rates of over a million fps using the VFT. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A30.00008: Controlling fracture cascades through twisting and quenching Vishal Patil, Ronald Heisser, Norbert Stoop, Emmanuel Villermaux, Jorn Dunkel Fracture fundamentally limits the structural stability of macroscopic and microscopic matter, from beams and bones to microtubules and nanotubes. Despite substantial recent experimental and theoretical progress, fracture control continues to present profound practical and theoretical challenges. While bending-induced fracture of elongated rod-like objects has been intensely studied, the effects of twist and quench dynamics have yet to be explored systematically. Here, we show how twist and quench protocols may be used to control such fracture processes, by revisiting Feynman's observation that dry spaghetti typically break into three or more pieces when exposed to large pure bending stresses. Combining theory and experiment, we demonstrate controlled binary fracture of brittle elastic rods for two distinct protocols based on twisting and nonadiabatic quenching. Our experimental data for twist-controlled fracture agree quantitatively with a theoretically predicted phase diagram, and we establish novel asymptotic scaling relations for quenched fracture. Due to their general character, these results are expected to apply to torsional and kinetic fracture processes in a wide range of systems. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A30.00009: Rifts in Rafts Kha-I To, Daniel Hexner, Vincenzo Vitelli, Sidney Robert Nagel Two-dimensional particle rafts are single-layers of aggregated sub-millimeter polydisperse particles floating at an air-fluid interface. The material failure of such rafts under an applied extensional load has a morphology that appears to be distinct from other known fracture modes. At higher extensional shear rates, numerous small-scale cracks are distributed diffusively throughout the entire system; at low strain rates, the distance between adjacent cracks increases. The characteristics of this distributed failure also depend on the surface tension and viscosity of the underlying fluid. To decrease the influence of secondary flows, we perform experiments by changing the liquid level in the tanks with inclined walls so that we are able to increase the area accessible to the rafts as the liquid height changes. This results in an expansion in quasi-1D (with and without boundaries) and isotropic 2-D expansion in the linear and cylindrical geometries respectively. We simulate this behavior with a model based on weak interparticle forces coupled to an expanding underlying metric. |
Monday, March 4, 2019 10:36AM - 10:48AM |
A30.00010: Cracking and self-healing in soft, shrinkable hydrogel packings Han-Jae Jeremy Cho, Michael P Howard, Nancy B Lu, Rebekah A Adams, Sujit Datta We aim to better understand cracking in granular, shrinkable packings. Such packings are relevant to agriculture and CO2 sequestration; they also have potential uses in non-fouling films, biosensors, cosmetics, and drug delivery platforms. We use hydrogel particle packings as a model system and experimentally observed how these packings dry, shrink, and crack at an individual-particle level. Most remarkably, we observed a behavior where cracks can form but eventually self-heal. Furthermore, using discrete-element simulations that we developed, we found the precise range of individual-particle shrinkabilities, capillary forces, and contact forces needed to produce this self-healing behavior. Our results inform ways to control crack evolution, which could ultimately pave the way to engineering crack behavior for a wide variety of applications. |
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