Vikas Srivastava

Metallic glasses: viable tool materials for the production of surface microstructures in amorphous polymers by micro-hot-embossing

Metallic glasses possess unique mechanical properties which make them attractive materials for fabricating components for a variety of applications. For example, the commercial Zr-based metallic glasses possess high tensile strengths (?2.0 GPa), good fracture toughnesses (?10?50 MPa) and good wear and corrosion resistances. A particularly important characteristic of metallic glasses is their intrinsic homogeneity to the nanoscale because of the absence of grain boundaries. This characteristic, coupled with their unique mechanical properties, makes them ideal materials for fabricating micron-scale components, or high-aspect-ratio micro-patterned surfaces, which may in turn be used as dies for the hot-embossing of polymeric microfluidic devices. In this paper we consider a commercially available Zr-based metallic glass which has a glass transition temperature of Tg ? 350??C and describe the thermoplastic forming of a tool made from this material, which has the (negative) microchannel pattern for a simple microfluidic device. This tool was successfully used to produce the microchannel pattern by micro-hot-embossing of the amorphous polymers poly(methyl methacrylate) (Tg ? 115??C) and Zeonex-690R (Tg ? 136??C) above their glass transition temperatures. The metallic glass tool was found to be very robust, and it was used to produce hundreds of high-fidelity micron-scale embossed patterns without degradation or failure.

Experimental Evaluation of the Dynamic Shear Strength of Adhesive-Bonded Lap Joints

An experimental method utilizing a novel specimen geometry was developed to determine dynamic shear strength of adhesive-bonded lap joints using the classical Split Hopkinson Pressure Bar (SHPB) technique in compression. The specimens were loaded dynamically at four different loading rates, and the transmitted load through the joint was obtained from the time-resolved strain history assuming one-dimensional stress wave propagation. The shear strength of the joint was determined from the maximum transmitted load, assuming the load was transferred predominantly as shear load through the adhesive-bonded joint. The shear strength of a lap joint bonded using a general purpose epoxy adhesive was obtained at loading rates varying from quasi-static to 2300 N/µs. The results indicate that as the loading rates are increased to 1000 N/µs the shear strength of the particular adhesivebonded lap joint increases to three times its static value, after which it stabilizes.

Numerical Analysis of Experimental Data of Subsea Jumper Vortex Induced Vibrations

Vortex Induced Vibrations (VIV) may cause fatigue damage to subsea jumpers that are exposed to bottom currents. ExxonMobil Upstream Research Company (URC) has been conducting research on VIV of subsea jumpers since 2011. Model tests conducted on an “M” shaped jumper in 2012 showed that VIV can occur in subsea jumpers over a wide range of bottom current speeds and directions [1]. Presently, there is no well-established industry practice for assessing subsea jumper VIV and determining the need for its suppression. In this paper, we present two different methods for characterizing jumper VIV response based on the model test data described in [1]. Specifically, these methods consist of spectral analysis of local response and modal scalar analysis of global response. These methods are used to analyze measured response over a wide range of towing speeds and towing directions. A brief summary of the findings are provided and some general conclusions are drawn.Copyright

A Thermo-Mechanically Coupled Large-Deformation Theory for Amorphous Polymers Across the Glass Transition Temperature

Amorphous thermoplastic polymers are important engineering materials; however, their nonlinear, strongly temperature- and rate-dependent elastic-viscoplastic behavior is still not very well understood, and is modeled by existing constitutive theories with varying degrees of success. There is no generally agreed upon theory to model the large-deformation, thermo-mechanically-coupled, elastic-viscoplastic response of these materials in a temperature range which spans their glass transition temperature. Such a theory is crucial for the development of a numerical capability for the simulation and design of important polymer processing operations, and also for predicting the relationship between processing methods and the subsequent mechanical properties of polymeric products. In this manuscript we briefly summarize a few results from our own recent research [1–4] which is intended to fill this need.Copyright

Effect of Loading Rate and Geometry Variation on the Dynamic Shear Strength of Adhesive Lap Joints

Dynamic and quasi-static experiments were performed using a novel lap joint specimen to evaluate the shear strength of adhesive bonded lap joints at different loading rates, length to width ratios and lap areas. Dynamic shear strength was determined by subjecting the lap joints to stress wave loading in a Split Hopkinson Pressure Bar (SHPB) apparatus. All joints were bonded by a general-purpose epoxy adhesive (Armstrong A-12®). The shear strength of the joint was determined using maximum transmitted load through the joint, assuming that the load was predominantly transferred through shear.

Stress Analysis of a Cryogenic Corrugated Pipe

One method to develop offshore gas reserves is to use a floating LNG plant (FLNG) on site and export the LNG via tankers. This alternative requires the use of a reliable LNG transfer system between the FLNG and the tanker under offshore conditions. One such system involves a flexible cryogenic hose whose main body is a pipe-in-pipe hose made of two concentric corrugated 316L stainless steel pipes (C-pipe) with flanged terminations. Thermal insulation is achieved by maintaining vacuum between the inner and outer corrugated stainless steel pipes. In addition, the hose assembly contains two outer layers of helical armor wires to sustain the axial load. Given the complexity and novelty of the transfer system, a finite element study was performed on the inner C-pipe — the critical fluid containment layer. The effects of strain hardening of corrugations due to cold forming and temperature were modeled. Finite element (FE) analyses of the C-pipe under axial, bending, and internal pressure loading were carried out to evaluate global load-deformation and local stress responses. Comparisons of full-scale tests at room and cryogenic temperatures to simulation predictions including the novel material model showed good agreement. However, fatigue life predictions for the C-pipe that were based on local stresses and sheet metal fatigue S-N curves did not agree with the full-scale fatigue test results. The results indicated that the spatial variation in strain hardening due to corrugation forming and biaxial local stresses during pipe deformation could play important roles in the fatigue response of the C-pipe.Copyright

A Large-Deformation Theory for Thermally-Actuated Shape-Memory Polymers and its Application

The most common shape-memory polymers are those in which the shape-recovery is thermally-induced. A body made from such a material may be subjected to large deformations at an elevated temperature above its glass transition temperature ϑg . Cooling the deformed body to a temperature below ϑg under active kinematical constraints fixes the deformed shape of the body. The original shape of the body may be recovered if the material is heated back to a temperature above ϑg without the kinematical constraints. This phenomenon is known as the shape-memory effect. If the shape recovery is partially constrained, the material exerts a recovery force and the phenomenon is known as constrained-recovery.Copyright

On Modeling the Mechanical Behavior of Amorphous Polymers for the Micro-Hot-Embossing of Microfluidic Devices

In this paper, a brief summary of some of our recent work [1, 2] is presented, with the goal of developing an engineering science-based process-simulation capability for micro-hot-embossing of amorphous polymers. To achieve this goal: (i) a three-dimensional thermo-mechanically-coupled large deformation constitutive theory has been developed to model the temperature and rate-dependent elastic-viscoplastic response of amorphous polymers; (ii) the material parameters in the theory were calibrated by using new experimental data from a suite of simple compression tests on Zeonex-690R (cyclo-olefin polymer), that covers a wide range of temperatures and strain rates; (iii) the constitutive model was implemented in the finite element program ABAQUS/Explicit; and (iv) the predictive capability of the numerical simulation procedures were validated by comparing results from the simulation of a representative micro-hot-embossing process against corresponding results from a physical experiment.Copyright