Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P43: Extreme Deformation of Polymers and Soft Matter II: High Speeds, Rupture, and Large DeformationFocus
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Sponsoring Units: DPOLY GSOFT Chair: Alfred Crosby, Univ of Mass - Amherst Room: LACC 503 |
Wednesday, March 7, 2018 2:30PM - 3:06PM |
P43.00001: Extreme Plastic Deformation of Glassy Polymer Thin Films at Ballistic Strain Rates Invited Speaker: Edwin Thomas Polystyrene (PS), is a brittle, glassy solid at room temperature and absorbs little energy during deformation. When confined in a very thin film, the behavior of high molecular weight polymers is modified. Here we show unexpected ductile deformation mechanisms resulting in unprecedented energy absorption by high molecular weight thin freestanding PS films at extreme deformation rates created by impact of micron sized projectiles. We use a laser-induced projectile impact testing apparatus to launch 3.7 micron diameter silica projectiles to impact the substrate-free thin films mounted across a TEM grid to induce high strain rate deformation (~ 107 s-1). For the range of film thicknesses investigated, the ratio of the projectile diameter to the film thickness, D/h, varies from approximately 50:1 to 25:1 to 13:1. The impact produces axisymmetric tensile loading of a thin membrane with the principal stresses along the radial and tangential directions of a circular impact region. The supersonic projectiles initiate deformation zones, crazing and adiabatic heating leading to extensive plastic flow of a viscoelastic melt prior to perforation and film rupture. We investigate films of 75, 150, 290 and 550nm and find a strong thickness dependence of the specific energy absorption. This suggests that the less entangled near-surface region of free standing films likely plays an increased role in nucleating deformation zones and crazes as the film thickness decreases. While other materials adiabatically heat during rapid deformation, only polymers have a load-bearing viscoelastic melt state due to chain entanglements and strong frictional forces from the sliding of the long and aligning chains. These ductile deformation processes result in record high specific energy absorption (3 MJ/kg for the 75 nm PS film at 800 m/s) at extreme strain rates in what is normally considered a brittle material. |
Wednesday, March 7, 2018 3:06PM - 3:18PM |
P43.00002: The Role of Fast Polymer Dynamics as Quantified by Inelastic Neutron Scattering on the Impact Strength in Solid Polymers Kanae Ito, Adam Burns, Kevin Masser, Christopher Soles It is generally understood that there is a link between molecular relaxations in a glassy a polymer and its mechanical toughness. The notion is that these relaxations are important to dissipate the energy of impact and enhance toughness. Decades of research have focused on correlating the mechanical toughness of a polymer with the relaxation processes quantified by techniques such as dynamic mechanical analysis, dielectric spectroscopy, and solid state nuclear magnetic resonance. By correlating the strength and temperature dependence of the relaxations with mechanical performance, the community has tried to developed a primitive understanding of the molecular origins of toughness. However, there is a disconnect because the time and length scale of the molecular relaxations or processes that are invoked to rationalize the toughness are typically several orders of magnitude faster and more localized than the experimental techniques used to characterize the motions, especially in the case of ballistic impact events. We revisit this topic by using inelastic and quasielastic neutron scattering to quantify the polymer relaxations that occur on the ns to ps time scales and show how these fast motions correlate well with impact resistance in a series of different solid polymers. |
Wednesday, March 7, 2018 3:18PM - 3:30PM |
P43.00003: Impulsive Elastic Energy Release from a Resilin-like Elastomer Mark Ilton, Suzanne Cox, Thijs Egelmeers, Gregory P. Sutton, S. N. Patek, Alfred Crosby Small organisms and micro-robotic devices can use elastically-driven motion to achieve astonishing accelerations. To understand the size-scaling limits of elastic performance, we measured the elastic recoil of elastomer bands with mechanical properties similar to the biological protein resilin. By tracking the center-of-mass motion of elastomer bands using high-speed videography, three metrics of kinematic performance were quantified: the maximum velocity, peak acceleration, and duration of energy release. The velocity of the elastomer bands was found to be size-scale independent, while smaller bands demonstrated larger accelerations and shorter durations of elastic energy release. The scaling equations derived from these measurements are consistent with the performance of small organisms which utilize elastically-driven motion. Engineered micro-robotic devices found in the literature do not follow the same size-scaling relationships, which suggests an opportunity for improved design of engineered devices. The scaling relationships we extract for each metric of kinematic performance (velocity, acceleration, duration) determine principles of materials selection for use in elastically-driven motion. |
Wednesday, March 7, 2018 3:30PM - 3:42PM |
P43.00004: Dynamics of High-Speed Adhesive Detachment Katharine Jensen The science of adhesion is often focused on how things stick, but many open questions also remain about the physics of how sticky stuff unsticks. We use optical microscopy and high-speed imaging to measure the surface profiles of soft, sticky, solid silicone gels as they undergo rapid, high-strain deformation from the final moments of adhesion through detachment and subsequent relaxation. We observe that the soft adhesive substrates always detach from a single point on an adhered, rigid sphere, independent of the sphere size and the initial contact area. After detachment, the height of the relaxing surface decays as a power law in time and the surface profile is self-similar, both suggestive of a detachment singularity that is distinctly different from liquid break-off. We further explore the competition between surface and bulk effects by varying the material properties of the adhesive substrates, as well as the size and surface energy of the spheres. |
Wednesday, March 7, 2018 3:42PM - 3:54PM |
P43.00005: Damage and Rupture in Random Fiber Networks Sai Deogekar, Catalin Picu Fibrous networks occur ubiquitously in biological and synthetic materials. Failure in such materials can occur due to the rupture of fibers or, more often, due to the rupture of bonds. Material parameters such as network architecture, fiber and bond properties and degree of heterogeneity in the network determine the dominant failure mechanism. In this work, we identify the key parameters controlling the strength of random fiber networks and propose scaling laws relating strength to structural parameters and properties of the bonds. A ductile to brittle transition is evidenced and the system parameters controlling the transition are discussed. The results provide guidelines for network design. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P43.00006: Rupture of Polymers by Chain Scission Yunwei Mao, Brandon Talamini, Lallit Anand One of the distinguishing features of elastomeric materials, which consist of a network of flexible polymeric chains, is that the deformation response is dominated by changes in entropy. Accordingly, most classical theories of rubber-like elasticity consider only the entropy and neglect any changes in internal energy. On the other hand, the fracture of strongly cross-linked elastomers is essentially energy dominated, as argued in the well-known Lake-Thomas model for the toughness of elastomers. However, a single model unifying these two phenomena is still lacking. We provide a rational yet simple model for deformation and fracture of cross-linked polymers, based on two ingredients: (i) a non-Gaussian statistical mechanics model of polymer chains that accounts for the increase in energy due to the deformation of molecular bonds; (ii) a chain scission criterion based on the bond deformation energy attaining a critical value. Using this model, we can estimate the rupture stretch of elastomeric materials from fundamental quantities describing the polymer network. We use this model to relate the flaw sensitivity of elastomers to an intrinsic material length scale. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P43.00007: Direct visualization of the molecular damage precursors to crack nucleation Jasper Van der Gucht, Hanne van der Kooij, Simone Dussi, Joris Sprakel Loading a solid with a subcritical stress can lead to sudden fracture after a long period of seemingly quiescent stability. Contrasting hypotheses exist to explain delayed brittle fracture, yet to date, unambiguous experimental proof of the mechanisms underlying this unpredictable mode of failure remains absent. To elucidate the damage processes that precipitate crack nucleation, we use quantitative micromechanical mapping at the site of crack nucleation within an elastomer by means of Laser Speckle Imaging. This allows us to visualize the damage processes in the moments prior to crack nucleation. We find that the local rigidity of the rubber gradually degrades and becomes zero at the exact moment when a macroscopic crack becomes visible. This pre-fracture damage zone grows in space and time along a self-catalytic pathway, exhibiting signs of a critical transition. Our results paint a microscopic picture of the elusive origins of delayed fracture through damage accumulation. |
Wednesday, March 7, 2018 4:18PM - 4:30PM |
P43.00008: Embedding topography in Flexible Substrates enables control of crack propagation. Christopher Maiorana, Mitchell Erbe, Travis Blank, Guy German There is a current need to ensure that conformal electronics remain robust. The ability of these circuits to function even while damaged would therefore be beneficial. Our research explores the ability to control crack propagation in compliant membranes upon which flexible electronics can be printed. This control could prevent cracking in regions critical to device functionality. Our results first demonstrate that single layer membranes containing topographical channels can redirect crack pathways by up to 30 degrees. Here, cracks propagate along the thinnest part of the film when channels are sufficiently deep. Coating a second layer results in membranes that appear homogeneous and uniform in thickness. The embedded topography however also governs crack propagation; reorienting crack pathways by up to 45 degrees. Here cracks follow delamination pathways between the two layers. However, crack reorientation results in mode 2 fracture; limiting the extent of this control. The use of step wise pathways with alternating angled and parallel segments, along with parallel embedded channels, help regain control of crack propagation. |
Wednesday, March 7, 2018 4:30PM - 4:42PM |
P43.00009: Strain hardening of glassy polymers : theory and simulation Didier Long, Luca Conca, Paul Sotta, Alain Dequidt Glassy polymers submitted to an applied stress undergo yield at deformations of a few percent and stresses of some 10 MPa, followed by a slow drop in stress under plastic deformation corresponding to the strain-softening regime. Some polymers of high molecular weight display an increase of stress in the large amplitude regime of deformation. The typical slope of stress versus strain in this regime, GR, is of order 107 – 108 Pa well below Tg. We propose a theory which unifies these effects and what has been observed regarding dynamical heteroegenties. The model is solved in 3D with a spatial resolution on the nanoscale. Simulation results are in agreement with experimental data, such as the elastic modulus (G’∼1 GPa Pa and its temperature dependence), the yield stress and the yield behavior (strain softening), and the strain hardening regime (GR∼10 MPa) without introducing new adjustable paramters as compared to the scale of dynamical heterogeneties. In particular we show that the strain hardening mechanism suppresses the development of shear bands on the sacle of a few tens of nanometers, making the materials tougher. Our model allows in principle to describe various thermo-mechanical histories regrading polymer samples. |
Wednesday, March 7, 2018 4:42PM - 4:54PM |
P43.00010: Topological Structure and Mechanics of Glassy Polymer Networks Robert Elder, Timothy Sirk The influence of chain-level network architecture (i.e., topology) on the mechanics of polymer networks is explored with coarse-grained molecular simulations and graph-theoretic concepts. A modified Watts-Strogatz model is used to control the graph properties of the networks, and the corresponding physical properties are studied with simulations. The topology of dynamically-cured networks is compared with the modified Watts-Strogatz model, and found to agree surprisingly well: the dynamically-assembled topologies were intermediate between lattice and random topologies, due to the restriction of finite chain length. Further, the linear and nonlinear stress response, bond breaking, and non-affine displacements of glassy networks are analyzed as a function topological disorder. The architecture strongly affects the flow stress, onset of chain scission, and ultimate stress, while the modulus and yield point are unchanged. Internal restrictions imposed by topological disorder alter the chain-level dynamics in the flow regime, and the degree of coordinated chain failure at the ultimate stress. The properties are sensitive to even incremental changes to topology, so the overall network architecture, beyond simple defects, is predicted to be a meaningful physical parameter for mechanics. |
Wednesday, March 7, 2018 4:54PM - 5:06PM |
P43.00011: Buckling Mechanics Metrology for Brittle Polymeric Thin Films Mitchell Rencheck, Ricardo Rodriguez, Chelsea Davis Thin films are widely commercially used in Industry and require a thorough understanding of their mechanical properties for application. For these commercial films, rapid and facile mechanical characterization is essential. There is currently a lack of mechanical characterization techniques for films with thicknesses in the meso-scale (~5µm to ~100µm). Therefore, a rapid technique that quantifies the modulus of brittle, meso-scale films is desirable. Here, the buckling mechanics of a thin plate are utilized to relate the critical buckling load and the elastic modulus (Pcr ~ EI/a2) where Pcr is the critical buckling load, E is the elastic modulus, I is the moment of inertia, and a is the sample length. Film geometries are varied from, 6mm < width < 18mm and 15mm < length < 45mm, and the critical buckling load is measured for each film. A custom mechanical frame is used to apply a uniaxial compressive force to a 60μm thick cellulose acetate film until buckling occurs. The load to induce buckling changes with geometry, and a constant elastic modulus is observed for all films independent of film geometry. It is shown that this technique has the potential to rapidly characterize the elastic modulus of brittle micron thick films. |
Wednesday, March 7, 2018 5:06PM - 5:18PM |
P43.00012: Mechano-Responsive Lateral Buckling of Miniaturized Beams Standing on Flexible Polymer Substrate Jung gun Bae, Won Bo Lee An elastomeric beam standing on a flexible substrate was fabricated using 3D printing and soft lithography, then lateral buckling generated in the part of the wall when this beam is under pure bending is investigated. It is also observed changes in the morphology of wrinkling along the applied strain and geometry of the wall, and analyzed in scaling concepts. Furthermore, the degree of lateral buckling is controlled through the tip design in the ratchet structure and it is verified with finite element simulation. Here, geometric and material nonlinearity due to large deformation of rubber-like polymer is considered. Furthermore, instability mode at the edge of the wall is also discussed. |
Wednesday, March 7, 2018 5:18PM - 5:30PM |
P43.00013: Buckling of an elastic fiber confined to a thin elastic membrane Adam Fortais, Kari Dalnoki-Veress
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