Simulation Of Microstructural Evolution Of Cross-Linked Templated Silica-Aerogels Under Compression

The evolution of a cross-linked templated aerogel mesostructure under compression was simulated using the Material Point Method (MPM) on a model generated from X-ray nano-computed tomographic image. The aerogel mesostructure is that identified in acid-catalyzed surfactant template materials, which after cross-linking with polyurea, demonstrate very high compressive yield strengths and energy absorption capabilities. In MPM each voxel occupied by aerogles in an X-ray tomograph was converted to a material point to generate a MPM model. A parallel version of MPM code, developed in our group, has been used to simulate the response of a cross-linked templated aerogels under compression at high strain rates. For a comparison with experimental data, a long split Hopkinson pressure bar was used to measure the stress-strain relationship of this material at high strain rates. The results from the simulations show that the MPM can effectively model cross-linked templated silica-aerogel considering its microstructure, and capture the elastic, compaction and densification behaviors of the aerogel, and the simulation results agree reasonably well with the experimental data. Simulations also indicate a nearly uniform compression at all three stages of deformation in the aerogel, consistent with experimental observations. Simulations were also conducted to indentify functions of ultra-thin polymer coating and the effect of density or porosity. Models with different porosities indicate that the skeletal wall thickness of both the silica particles and the polymer coating affect the local stress distribution, which in turn could induce different mechanical response under compression. MPM simulations show that the stress-strain behavior of cross-linked templated aerogels under compression follows a power law relation with bulk density.Mechanical & Aerospace Engineerin

Web wrinkling resulting from moment transfer

Considerable research has been focused on the impact of roller misalignment on web instability. Early work focused on the prediction of trough instabilities in the entering span, just upstream of the misaligned roller [1]. Later works involving misaligned [2] and tapered [3] rollers proved that the trough instability was a required precursor for the occurrence of wrinkles on the misaligned or tapered roller. The compressive stress required to induce web wrinkles on a roller can be 2 orders of magnitude larger, in the absolute sense, than the compressive stress required to precipitate trough instabilities in a web span. These works [2][3] found the out-of-plane web deformation due to troughs was responsible for creating the larger compressive stresses that would finally result in wrinkles whenever the misalignment or taper became sufficient. The three works referenced thus far all rely upon an assumption that the friction forces between the web and the upstream roller, which separates the entering and pre-entering spans, are sufficient to prevent moment transfer. The lateral deformation of the web in the pre-entering and entering spans as a result of moment transfer has been another focus of web handling research [4][5]. This publication will focus on the impact of moment transfer on web wrinkling. It will be shown that troughs may now occur in both the pre-entering and entering spans. It will also be shown that wrinkles can precipitate on either the misaligned roller or the roller upstream of the misaligned roller. It will be shown that these behaviors can be predicted and the predictions will be validated by test results. It will be shown that as a result of moment transfer the roller misalignment to induce wrinkles can be less than the misalignment required to induce wrinkles when moment transfer does not occur.Mechanical and Aerospace Engineerin

Explicit simulations of web transport through process machines using periodic media analysis technique

In this paper, the periodic media analysis (PMA) technique in Abaqus/Explicit [1] has been applied to simulate multiple cases of web transport through process machines. The Abaqus models for web transport developed in the Web Handling Research Center (WHRC) at Oklahoma State University (OSU) all have long free upstream spans, which are required to achieve steady state for post-simulation analysis. This increases model size and complexity, induces longer simulation run time and limits applications of simulations. The primary goal of introducing PMA to web handling modeling is to create simulation models which take a shorter time to run. The PMA technique is a Lagrangian technique that offers a Eulerian-like view into a moving structure. Models created using this technique have smaller sizes, and the simulation run time is significantly reduced compared to OSU models [2]. In this work, the moment transfer due to roller misalignment case has been modeled using the PMA technique. The results, e.g. lateral displacements and moment distribution, are compared to OSU model results and experimental results. The comparisons show good agreements. More importantly, the PMA model takes a much shorter time (about 40% less) to conduct. Two more cases, actual wrinkle formation and web with non-uniformity running through rollers, have been modeled using PMA as well. The simulation results from the actual wrinkle formation case agree with the wrinkle failure criterion [3] based upon Timoshenko's shell buckling theory [4]. The web with non-uniformity case indicates the capability of PMA to study complex web structures

Explicit simulations of cambered web steering

Cambered webs are common in the web handling industry. The mechanics analyses of stressed cambered webs have been reported by several publications [1]-[3]. The majority of the test data that exist demonstrate that cambered webs steer towards their longer side. A closed form solution [4] and numerical methods [5]-[9] have been focused on the lateral behavior of the cambered web as well, but have provided no explanation of steering toward the longer side. The work that has been done focused on analyzing or modeling a cambered web span. The results from the current work demonstrate that camber in a web causes slippage between webs and rollers that produce lateral steerage. To better understand cambered web response under tension, studying the lateral mechanics of a cambered web passing over aligned rollers is the major focus of this work. Abaqus/Explicit [10] has been used to model cambered web and the transit of the web over a series of rollers. An Abaqus user defined subroutine, VUAMP, has been used to develop the first successful simulation of a web position guide interacting through contact friction with a web. This capability was needed such that a cambered web could be presented with known orientation and initial conditions to a test span where the web steering behavior resulting from camber could be studied. Simulation results are compared to experimental results [3]. The boundary conditions, which govern the steering of a cambered web in a test span, have been concluded based upon this analysis.Mechanical and Aerospace Engineerin

Simulation of the Evolution of the Nanostructure of Crosslinked Silica-Aerogels under Compression

Silica-aerogels are ultra-low-density assemblies of silica nanoparticles, and possess superior acoustic, specific energy absorption and thermal insulation properties. A new class of aerogels encapsulated with polymer is classified as crosslinked silica-aerogels. Manufacturing of such crosslinked silica-aerogel structures, depending on the type and shape of the nanoparticles, the polymer cross-linker and the chemistry in use, yields structures with vastly different morphologies and a wide range of mechanical behavior. With this, it has become necessary to understand the nanostructure / macroscopic properties relationship. Modeling of the aerogel material properties from mesoscale and up approach is needed, which is not considered by the current phenomenological models based on continuum material assumption. Most of the existing simulation methodologies face difficulties mainly due to complex nanostructures, large distortions, and extensive contact. A relatively new numerical method called Material Point Method (MPM) can circumvent these problems. For example, MPM has been used effectively in modeling the microstructural evolution of the bulk metallic glass foam with 70% porosity, where 3D X-Ray microtomography was used first to obtain the representative volume element (RVE) of the closed-cell foam . Due to the particle description of matter, MPM is a very suitable for silica-aerogel simulations. In this regard, an approach based on X-Ray nano-computed tomography (n-CT) will be used to model cross-linked aerogel mesostructure. The voxel information from the 3D tomography will be used to generate material points in MPM. The parallel version (using Structured Adaptive Mesh Refinement Application Infrastructure) of MPM code will be used to simulate the response of the model under compression. In this paper, the MPM is used to model a crosslinked templated silicaaerogel (X-MP4-T045) in compression, and the simulation results are compared with the compressive stress-strain curve obtained experimentally. This work will focus on the deformation mechanisms in crosslinked templated silica-aerogel such as the elastic buckling, compaction and densification, as well as the dependence of mechanical properties on the porosity effect for this crosslinked templated silica-aerogel

Explicit simulations of wrinkle formation due to web non-uniformity

It is very common to laminate different materials together and then form, transport and process a web with non-uniform structure in the product manufacturing industry. The periodic media analysis (PMA) method in Abaqus/Explicit [1] has been applied to simulate web wrinkle formation due to web non-uniformity during web transportation. This is a further application of previous PMA simulation models [5]. In this work, an experiment has been conducted to test a web structure including two materials with significant differences in terms of thickness and material properties running through rollers. Wrinkles have been observed during the experiment. Based upon the experimental setup, a web handling model is generated using the PMA method. This model can capture wrinkle formation due to web non-uniformity which agrees with experimental observation. The model and information provided by this model can be used to study wrinkle formation due to similar root causes and explore solutions to prevent wrinkles from occurring in future applications

Simulation of the microstructural evolution of a polymer crosslinked templated silica aerogel under high-strain-rate compression

Surfactant-templated mesoporous silica aerogels (or nanofoams) with their entire skeletal framework nanoencapsulated conformally by a thin polyurea layer are emerging as materials with high specific strength and high energy absorption. In this paper a modified split Hopkinson pressure bar was used to investigate their mechanical behavior under dynamic compression at high strain rates. The evolution of the mesoporous structure under such dynamic impact conditions was simulated using the Material Point Method (MPM). The material point model was generated from X-ray micro-computed tomography whereas each voxel was converted to a material point corresponding to the local skeletal density of the material. Simulation results agree well with the experimental data, indicating that the MPM can effectively model the compression of complex mesoporous structures. Simulations indicate a nearly uniform deformation at all three stages of compression: the elastic region, compaction and the final densification due to the low ratio of pore size to wall thickness and random distribution of the pores. Simulations have also indentified the function of the conformal polymer coating as a reinforcing factor, showing that different porosities, obtained by varying the skeletal wall thickness, affect the local stress distribution. Eventually, simulations confirm that the stress-strain behavior of aerogels under compression follows a power-law relationship with the initial bulk density, consistent with experimental results. © 2011 Elsevier B.V. All rights reserved