Elastic leak of a seal

An elastomeric seal may leak by elastic deformation without any material damage. We describe elastic leak using a theoretical model, and watch a seal deform and leak using a transparent experimental setup. The elastomer seals the fluid by forming contact with surrounding hard materials. As the fluid pressure increases, the contact stress also increases but not as much. When the fluid pressure surpasses the contact stress, the elastomer and t he hard materials lose contact in some region, forming a leaking path. The critical fluid pressure for elastic leak depends on the geometry and constraint of the seal, but is insensitive to the rate at which the fluid is injected. Our study points to the significance of elastic deformation in modes of failure that also involve material damage.Engineering and Applied Science

Extrusion, slide, and rupture of an elastomeric seal

Elastomeric seals are essential to two great technological advances in oilfields: horizontal drilling and hydraulic fracturing. This paper describes a method to study elastomeric seals by using the pressure-extrusion curve (i.e., the relation between the drop of pressure across a seal and the volume of extrusion of the elastomer). Emphasis is placed on a common mode of failure found in oilfields: leak caused by a crack across the length of a long seal. We obtain an analytical solution of large elastic deformation, which is analogous to the Poiseuille flow of viscous liquids. We further obtain analytical expressions for the energy release rate of a crack and the critical pressure for the onset of its propagation. The theory predicts the pressure-extrusion curve using material parameters (elastic modulus, sliding stress, and fracture energy) and geometric parameters (thickness, length, and precompression). We fabricate seals of various parameters in transparent chambers on a desktop, and watch the seals extrude, slide, rupture and leak. The experimentally measured pressure-extrusion curves agree with theoretical predictions remarkably well.Engineering and Applied SciencesPhysicsOther Research Uni

Morphology and Grain Texture in As-Deposited and Heat Treated Inconel 718 Structures Produced using Laser-Based Powder Bed Fusion

With increasing interest in the use of powder bed fusion (PBF) processes for additive manufacturing, understanding the relationship between as-deposited and heat treated states and the intrinsic anisotropy of fabricated parts has become critical for its successful application. This phenomenon has been studied and reported extensively for Inconel 718 parts fabricated using PBF for aerospace applications, but few reports exist on the morphology and grain texture of Inconel 718 parts fabricated for oil and gas applications, which have different demands. This work demonstrates that the anisotropy in Inconel 718 parts produced using laser-based PBF is not entirely removed by subsequent heat treatments, and it may be an artifact of the as-deposited grain structure, whose elongated grains may stretch through several melt pools. The as-built material is observed to exhibit some texturing, with (001) being the preferential growth direction. Despite some residual anisotropy, heat treatments are sufficient to provide material qualities that meet specification, even without the use of a HIP (hot isostatic pressing) step. It is hypothesized that similarly elongated grain structures may explain the anisotropy observed in other materials systems employed in PBF additive manufacturing processes.Mechanical Engineerin

A Preliminary Examination of Variability Due to Build Location and Powder Feedstock in Additive Manufacture of Inconel 718 using Laser-Based Powder Bed Fusion

The production of metallic parts by additive manufacturing (AM) is of significant interest to industry, but in the absence of standards, practical design considerations for manufacturing engineers are not widely known. Within the context of powder bed fusion (PBF), many unknowns persist regarding variations in part quality due to part location on the build plate, process consistency, feedstock supplier, and machine manufacturer. In this paper, we investigate the mechanical property variance across the build platform and document the successful use of feedstock powders obtained from several suppliers for the manufacture of Inconel 718 tensile and Charpy specimens, built on an EOS M280 laser-based powder bed fusion system. Particular emphasis is placed on describing the manufacturing process design challenges encountered even for simple geometries. While many advocate that complexity is free when using AM, we find that AM can lead to expensive build failures given the current state of manufacturing process knowledge and that design for additive manufacture is required for successful application of AM techniques.Mechanical Engineerin

Guided assembly of nanostructures

A solid solution can spontaneously separate into phases, e.g. spinodal decomposition, that self assemble into patterns. This process can be guided via external fields to form ordered micro- and nano-structures, e.g., nanodots and nanowires. A Cahn-Hilliard type phase field model is developed that incorporates chemical, interfacial, and elastic energies, including heterogeneous elastic properties, and that couples naturally to externally-imposed mechanical fields. Aggregation in bulk and in thin films under patterned external stress fields are investigated through detailed simulations, which includes a systematic variation of transformation strain, elastic contrast and the magnitude of external load. The patterned external stress fields are shown to directly affect the kinetics and morphology of aggregation through interacting with internal elastic properties. A major contribution of this thesis is the demonstration that the trends observed from simulations can be qualitatively interpreted through Eshelby-type asymptotic estimates for interaction energies

Guided assembly of nanostructures

A solid solution can spontaneously separate into phases, e.g. spinodal decomposition, that self assemble into patterns. This process can be guided via external fields to form ordered micro- and nano-structures, e.g., nanodots and nanowires. A Cahn-Hilliard type phase field model is developed that incorporates chemical, interfacial, and elastic energies, including heterogeneous elastic properties, and that couples naturally to externally-imposed mechanical fields. Aggregation in bulk and in thin films under patterned external stress fields are investigated through detailed simulations, which includes a systematic variation of transformation strain, elastic contrast and the magnitude of external load. The patterned external stress fields are shown to directly affect the kinetics and morphology of aggregation through interacting with internal elastic properties. A major contribution of this thesis is the demonstration that the trends observed from simulations can be qualitatively interpreted through Eshelby-type asymptotic estimates for interaction energies

Swellable elastomeric HNBR-MgO composite : magnesium oxide as a novel swelling and reinforcement filler

In this paper, we introduce a novel reactive rubber composite made by compounding magnesium oxide (MgO) powder with hydrogenated nitrile butadiene rubber (HNBR). This HNBR-MgO composite system initially looks and behaves like rubber, but exposure to water causes it to swell and stiffen. Compared with conventional swellable materials, which lose stiffness significantly upon swelling, the sealing capacity of these novel reactive composites improves significantly with their improved stiffness. Three mixing ratios of HNBR and MgO were examined in this study, and their properties upon reaching equilibrium in water of 82°C were reported. The elastic modulus value tripled, reaching 80 MPa, while doubling in volume for the rubber filled with 40% by volume of MgO. After drying, modulus of this particular composite increased even further to almost 200 MPa while the volume expansion was largely retained (shrinkage of approximately 10%). In this paper, we will show that the increase in elastic modulus and volume increase are related to the reaction of MgO with water to form magnesium hydroxide, absorbing water molecules into the composite and chemically reacting with it in the process.Accepted versio