VTechWorks Repository :: Browsing by Author "Dillard, David A."

Browsing by Author "Dillard, David A."

Now showing 1 - 20 of 160

Accelerated Durability Characterization of Laminated Polycarbonate Systems
Riddle, Samuel George (Virginia Tech, 2024-08-27)
Adhesion Mechanics: Durability
Dillard, David A. (2022-12-21)
These slides contain figures prepared over my teaching career, and will appear in Chapter 20 of Advances in Structural Adhesive Bonding (2nd ed., 2023). The figures are provided for reuse under a CC BY-SA 4.0 license.
Adhesion Mechanics: Strength & Energy Metrics
Dillard, David A. (2023-01-21)
These slides contain figures prepared over my teaching career, and will appear in Chapter 14 of Advances in Structural Adhesive Bonding (2nd ed., 2023). The figures are provided for reuse under a CC BY-SA 4.0 license.
Adhesion of novel high performance polymers to carbon fibers: fiber surface treatment, characterization, and microbond single fiber pull-out test
Heisey, Cheryl L. (Virginia Tech, 1993-11-05)
The adhesion of carbon fibers to several high performance polymers, including a phosphorus-containing bismaleimide, a cyanate ester resin, and a pyridine-containing thermoplastic, was evaluated using the microbond single fiber pull-out test. The objective was to determine the chemical and mechanical properties of the fiber and the polymer which affect the fiber/polymer adhesion in a given composite system. Fiber/matrix adhesion is of interest since the degree of adhesion and the nature of the fiber/matrix interphase has a major influence on the mechanical properties of a composite. The surface chemical composition, topography, tensile strength, and surface energy of untreated AU-4 and commercially surface treated AS-4 carbon fibers were evaluated using x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), single fiber tensile tests, and dynamic contact angle analysis. The commercial surface treatment which converted the AU-4 to the AS-4 fiber oxidized the carbon fiber surface. The surface of the AS-4 carbon fiber was further modified using air, oxygen, ammonia, and ethylene plasmas. The AS-4 fiber tow was also characterized following exposure to the aqueous poly(amic acid) solution used to disperse the matrix powder during aqueous suspension prepregging of thermoplastic matrix composites. The air and oxygen plasma treatments significantly oxidized and roughened the surface of the AS-4 carbon fibers. In addition, the air and oxygen plasma increased the the polar component of the AS-4 fiber surface energy. The ammonia plasma increased the concentration of nitrogen on the fiber surface, without significantly altering the fiber topography (at a nlagnification of 50,000X). The atomic oxygen present in the air and oxygen plasma treatments is capable of reacting with both the edge and basal planes in the carbon fiber structure. As a result, the oxygen-containing plasmas progressively ablated the organic material in the carbon fiber surface. Energetic species in the ammonia plasma cleaned the fiber surface and reacted with the carbon fiber surface, increasing the concentration of amine groups in the fiber surface. The ethylene plasma deposited a layer of plasma polymerized polymer on the carbon fiber surface. The AS-4 carbon fibers were coated with poly(amic acid) when the tow was wet with the aqueous suspension prepregging solution. The carbon fiber adhesion of bis(3-maleimido phenoxy) triphenylphosphine oxide was compared to that of Ciba-Geigy's Matrimid 5292 A/B bismaleimide system. With both bismaleimides, the carbon fiber adhesion increased significantly when the fiber received an oxidative commercial surface treatment or was exposed to an air or ammonia plasma prior to bonding. In contrast, the poly(pyridine-bis A) microbond pull-out test results showed that the carbon fiber adhesion of poly(pyridine-bis A) was not affected by the fiber surface chemical composition, fiber surface energy, or topography.
Analysis and Modeling of the Mechanical Durability of Proton Exchange Membranes Using Pressure-Loaded Blister Tests
Grohs, Jacob R. (Virginia Tech, 2009-04-10)
Environmental fluctuations in operating fuel cells impose significant biaxial stresses in the constrained proton exchange membranes (PEM). The PEM's ability to withstand cyclic environment-induced stresses plays an important role in membrane integrity and consequently, fuel cell durability. In this thesis, pressure loaded blister tests are used to study the mechanical durability of Gore-Select® series 57 over a range of times, temperatures, and loading histories. Ramped pressure tests are used with a linear viscoelastic analog to Hencky's classical solution for a pressurized circular membrane to estimate biaxial burst strength values. Biaxial strength master curves are constructed using traditional time-temperature superposition principle techniques and the associated temperature shift factors show good agreement when compared with shifts obtained from other modes of testing on the material. Investigating a more rigorous blister stress analysis becomes nontrivial due to the substantial deflections and thinning of the membrane. To further improve the analysis, the digital image correlation (DIC) technique is used to measure full-field displacements under ramped and constant pressure loading. The measured displacements are then used to validate the constitutive model and methods of the finite element analysis (FEA). With confidence in the FEA, stress histories of constant pressure tests are used to develop linear damage accumulation and residual strength based lifetime prediction models. Robust models, validated by successfully predicting fatigue failures, suggest the ability to predict failures under any given stress history whether mechanically or environmentally induced - a critical step in the effort to predict fuel cell failures caused by membrane mechanical failure.
Analysis of a bonded joint using bulk adhesive properties
Osiroff, Talia (Virginia Tech, 1988-12-05)
Adhesives and adhesively bonded structures are being considered as a viable alternative to conventional fastening methods. In order to gain wider acceptance, it is essential to address the issue of the mechanical characterization of adhesive materials and its implementation in the design of bonded joints. While measuring the in-situ properties of the adhesive in a joint is a difficult task, characterizing its bulk properties is a relatively simpler undertaking. The objective of this study was to propose and verify an experimental procedure that would allow the analytical prediction of the viscoelastic behaviour of a bonded joint, using bulk adhesive properties. The Arcan joint geometry was chosen because of the simple state of stress within the adhesive.
A beam test for adhesives
Fior, Valerie F. (Virginia Tech, 1988-07-15)
The strength of materials solution for a new bonded cantilever beam test specimen to determine adhesive shear properties is reviewed and discussed. A parametric analysis for the adhesive shear stress and for the end deflection reveals the specimen dimensions required for reliable bonded adhesive shear properties determination. Recommendations are provided for conducting reproducible tests. A pure and quasi-uniform shear test for stiff adhesives is proposed. Analytical solutions are compared with Finite Element solutions from VISTA and NOVA for the stresses in the adhesive. It appears that the assumption of pure shear is nearly valid even for very stiff and/or very thick adhesives. In order to increase the end point deformations for stiff adhesives, a modified specimen is proposed. Three-dimensional effects through the thickness of the adhesive layer are studied with the program ABAQUS. Experiments were performed using the two methods derived from theory and good correlation between theory and experiment were obtained with some restrictions. For both methods, experimental results underlined the need for defining proper specimen geometry prior to testing. Simple numerical codes are proposed to facilitate this purpose.
Behavior of wood under transverse compression
Kasal, Bohumil (Virginia Tech, 1989-01-05)
The increasing demand on wood and wood products, and the simultaneously decreasing quality of wood as a raw material leads to the increasing significance of wood-based composites such as particleboard or flakeboard. The resulting mechanical and physical properties are to the large extend dictated by the densification of the wood component. To be able to predict the density of the material, the behavior of structural elements must be known. A theory developed for rigid plastic foams was modified and applied to the deformation of wood in transverse compression. A testing procedure for high strain compression over a range of temperatures was developed. In addition, a stochastic model for prediction of high strain behavior was developed. Wood of yellow poplar (Liriodendron tulipuera) was used as the experimental material.
Biaxial Response of Individual Bonds in Thermomechanically Bonded Nonwoven Fabrics
Wijeratne, Roshelle Sumudu (Virginia Tech, 2017-06-29)
Thermomechanically bonded spunbond nonwoven fabrics contain discrete bonds that are formed by melted and fused fibers. Through equi-biaxial tensile testing and simultaneous image capture, the mechanical response of individual bonds was studied through loading in the preferential fiber direction, the machine direction, and in the direction that is perpendicular, the cross direction, of the fabric web. Independent biaxial force and displacement data were collected and analyzed, and the maximum force and stiffness of the bonds in the machine and cross directions were found to be statistically different. After scaling the maximum force and stiffness by a relative basis weight parameter, a fiber orientation parameter, and the width of the bond itself, the peak force and stiffness in the machine and cross directions were found to no longer be statistically different. This indicates that basis weight, fiber orientation, and bond size dictate the biaxial mechanical behavior of the bonds. Furthermore, significant fiber debonding was observed in all the bonds tested, effectively suggesting bond disintegration into the individual component fibers during testing. Digital image correlation, using the captured images, was utilized to calculate local and average Eulerian strains of the bond during the initial stages of the test. The strain experienced by the bonds in the machine direction was always positive and increasing as the biaxial load increased. The strain in the cross direction, however, experienced increasing and decreasing strain. Local strain maps revealed the highly inhomogeneous strain response of the bonds under biaxial loading.
Bifurcations, Multi-stability, and Localization in Thin Structures
Yu, Tian (Virginia Tech, 2020-01-22)
Thin structures exist as one dimensional slender objects (hairs, tendrils, telephone cords, etc.) and two dimensional thin sheets (tree leaves, Mobius bands, eggshells, etc.). Geometric and material nonlinearities can conspire together to create complex phenomena in thin structures. This dissertation studies snap-through, multi-stability, and localization in thin rods and sheets through a combination of experiments and numerics. The first work experimentally explores the multi-stability and bifurcations of buckled elastic strips subject to clamping and lateral end translations, and compares these results with numerical continuation of a perfectly anisotropic Kirchhoff rod model. It is shown that this naive Kirchhoff rod model works surprisingly well as an organizing framework for thin bands with various widths. Thin sheets prefer to bend rather than to stretch because of the high cost of stretching energy. Knowing the bending response of thin sheets can aid in simulating deformations such as creasing. The second work introduces an exact pure bending linkage mechanism for potential use in a bend tester that measures the moment-curvature relationship of soft sheets and filaments. Mechanical rotary pleating is a bending-deformation-dominant process that deforms nonwoven materials into zigzag filter structures. The third work studies what combinations of processing and material parameters lead to successful rotary pleating. The rotary pleating process is formulated as a multi-point variable-arc-length boundary value problem for an inextensible rod, with a moment-curvature constitutive law, such as might be measured by a bend tester, as input. Through parametric studies, this work generates pleatability surfaces that may help avoid pleating failure in the real pleating process. Creased thin sheets are generally bistable. The final work of this dissertation studies bistability of creased thin disks under the removal of singularities. A hole is cut in the disk and, through numerical continuation of an inextensible strip model, this work studies how the crease stiffness, crease angle, and hole geometry affect the bistability.
Characterization and Development of General Material Models for Use in Modeling Structures Bonded with Ductile Adhesives
Cassino, Christopher (Virginia Tech, 2005-04-22)
Structural adhesives are materials that are capable of bearing significant loads in shear, and sometimes tension, over a range of strains and strain rates. Adhesively bonded structures can dissipate large amounts of mechanical energy and can be lighter and more efficient than many bolted or vibration welded parts. The largest barrier to using structural adhesives in more applications is the many challenges engineers are presented with when designing and analyzing adhesively bonded structures. This study develops, characterizes and compares several material models for use in finite element analysis of adhesively bonded structures, in general, and a bonded tongue and groove (TNG) joint in particular. The results indicate that it is possible to develop a general material model for ductile adhesives used in structural applications under quasi-static conditions. Furthermore, the results also show that it is also possible to take bulk material data and apply it to an adhesively bonded specimen provided that the mode of failure of the bulk test specimen closely approximates the mode of failure of the bonded joint.
Characterization and Lifetime Performance Modeling of Acrylic Foam Tape for Structural Glazing Applications
Townsend, Benjamin William (Virginia Tech, 2008-09-12)
This thesis presents the results of testing and modeling conducted to characterize the performance of 3M™ VHB™ structural glazing tape in both shear and tension. Creep rupture testing results provided the failure time at a given static load and temperature, and ramp-to-fail testing results provided the ultimate load resistance at a given rate of strain and temperature. Parallel testing was conducted on three structural silicone sealants to compare performance. Using the time temperature superposition principle, master curves of VHB tape storage and loss moduli in shear and tension were developed with data from a dynamic mechanical analyzer (DMA). The thermal shift factors obtained from these constitutive tests were successfully applied to the creep rupture and ramp-to-fail data collected at 23°C, 40°C, and 60°C (73°F, 104°F, and 140°F), resulting in master curves of ramp-to-fail strength and creep rupture durability in shear and tension. A simple linear damage accumulation model was then proposed to examine the accumulation of wind damage if VHB tape is used to attach curtain wall glazing panels to building facades. The purpose of the model was to investigate the magnitude of damage resulting from the accumulation of sustained wind speeds that are less than the peak design wind speed. The model used the equation derived from tensile creep rupture testing, extrapolated into the range of stresses that would typically be generated by wind loading. This equation was applied to each individual entry in the data files of several real wind speed histories, and the fractions of life used at each entry were combined into a total percentage of life used. Although the model did not provide evidence that the established design procedure is unsafe, it suggested that the accumulation of damage from wind speeds below the peak wind speed could cause a VHB tape mode of failure that merits examination along with the more traditional peak wind speed design procedure currently recommended by the vendor.
Characterization of Mixed-Mode Fracture Testing of Adhesively Bonded Wood Specimens
Nicoli, Edoardo (Virginia Tech, 2010-07-19)
The primary focus of this thesis was to investigate the critical strain energy release rates (G) for mixed-mode (I/II) fracture of wood adhesive joints. The aims of the study were: (1) quantifying the fracture properties of two material systems, (2) analyzing the aspects that influence the fracture properties of bonded wood, (3) refining test procedures that particularly address layered orthotropic systems in which the layers are not parallel to the laminate faces, of which wood is often a particular case, and (4) developing testing methods that enhance the usefulness of performing mixed-mode tests with a dual-actuator load frame. The material systems evaluated experimentally involved yellow-poplar (Liriodendron tulipifera), a hardwood of the Magnoliaceae family, as adherends and two different adhesives: a moisture-cure polyurethane (PU) and a phenol/resorcinol/-formaldehyde (PRF) resin. The geometry tested in the study was the double cantilever beam that, in a dual-actuator load frame, can be used for testing different levels of mode-mixity. The mixed-mode loading condition is obtained by applying different displacement rates with the two independently controlled actuators of the testing machine. Characteristic aspects such as the large variability of the adhesive layer thickness and the intrinsic nature of many wood species, where latewood layers are alternated with earlywood layers, often combine to confound the measures of the critical values of strain energy release rate, Gc. Adhesive layer thickness variations were observed to be substantial also in specimens prepared with power-planed wood boards and affect the value of Gc of the specimens. The grain orientation of latewood and earlywood, materials that often have different densities and elastic moduli, limits the accuracy of traditional standard methods for the evaluation of Gc. The traditional methods, described in the standards ASTM D3433-99 and BS 7991:2001, were originally developed for uniform and isotropic materials but are widely used by researchers also for bonded wood, where they tend to confound stiffness variations with Gc variations. Experimental analysis and analytical computations were developed for quantifying the spread of Gc data that is expected to be caused by variability of the adhesive layer thickness and by the variability of the bending stiffness along wooden beams.
Characterization of photocurable networks in real-time and post-exposure
Zumbrum, Michael Allen (Virginia Tech, 1990)
Several novel characterization techniques were developed to gain a molecular-level understanding of the effects of exposure intensity and exposure time on photopolymer network formation. These techniques enable detailed characterization of photopolymer behavior in real-time and post-exposure. In situ dynamic mechanical analysis was performed to observe the changes in composite modulus during photopolymerization of thin acrylate films supported on stainless steel grid sheets. Vitrification phase transformations were monitored in real-time via remote sensing dielectric spectroscopy. The relationship between exposure intensity and vitrification time revealed the exposure conditions necessary to shift the crosslinking rate from reaction kinetics controlled to diffusion kinetics controlled. The effect of exposure intensity and exposure time on chemical conversion was ascertained via Fourier transform infrared microspectroscopy mapping experiments. The relationship between exposure intensity and time at a given conversion level revealed an increased occurrence of radical isolation at higher conversions. Furthermore, the exposure necessary to maintain a fixed conversion indicated greater reciprocity failure at lower conversions, indicative of classical radical termination kinetics.
Characterization of Sulfonated Perfluorocyclobutane /Poly(Vinylidene Difluoride)-co-Hexafluoropropylene (PFCB/PVDF-HFP) Blends for Use as Proton Exchange Membranes
Finlay, Katherine A. (Virginia Tech, 2013-04-22)
The research herein focuses on the characterization of a PFCB/PVDF-HFP (70:30 wt:wt) blend fuel cell membrane including the constitutive and morphological properties, how these properties predict the stresses incurred under fuel cell operating conditions, and how these properties change over time under fuel cell operating conditions. Characterization was performed to mimic temperature and moisture conditions found in operating fuel cells to understand how these materials will behave in service. This included thermal and hygral expansion, mass uptake, and the stress relaxation modulus. These constitutive properties were chosen for characterization such that a model could be created to predict the stresses incurred during fuel cell operation, and examine how these stresses may change under different operating conditions and over time. Based on the results of this model, lifetime predictions were made resulting in recommendations to further extend the operating time of this membrane beyond the DOE 5000 hr requirement. Stress predictions are useful, however if the material properties are changing over time under the fuel cell operating conditions, they may no longer be valid. Therefore, PFCB/PVDF-HFP membranes were conditioned for different amounts of time under conditions similar to those commonly found in operating fuel cells. These conditioned membranes were then characterized and compared with solvent exchanged membranes, the same materials used for previous material characterization. The properties examined included stress relaxation modulus, bi-axial strength, mass uptake, water diffusion, and proton conductivity. To further understand any changes noted in these properties after different environmental exposures, morphological analysis was performed. This included small angle x-ray scattering, infrared spectroscopy, transmission electron microscopy, and differential scanning calorimetry. It was initially found that the proton conductivity decreased severely when the material was immersed at high temperatures over short time periods. This was consistent with changes noted in other properties, and morphological analysis showed a decrease in the ionic network as well as an increase in the phase separation of the PFCB block copolymer as well as the PVDF-HFP crystallinity. These large morphological changes could be very detrimental while in service, resulting in early termination of the fuel cell. However, it was also noted that if these materials are annealed at high temperature (140"C), the negative property changes are abated. This abatement is again tied to the morphology of the material, as annealing the material at high temperature creates stronger physical crosslinks, and induces a small amount of chemical crosslinking via condensation of the sulfonic acid groups, thus allowing the stress predictions performed earlier to have greater validity. Therefore, it is important to not only understand the properties of a material during characterization, but also the underlying polymer structure, and how this structure can change over time, as all of these items control the long term material performance while in service.
Characterization of the Interfacial Fracture of Solvated Semi-Interpenetrating Polymer Network (S-IPN) Silicone Hydrogels with a Cyclo-Olefin Polymer (COP)
Murray, Katie Virginia (Virginia Tech, 2011-02-02)
As hydrogel products are manufactured and used for applications ranging from biomedical to agricultural, it is useful to characterize their behavior and interaction with other materials. This thesis investigates the adhesion between two different solvated semi-interpenetrating polymer network (S-IPN) silicone hydrogels and a cyclo-olefin (COP) polymer through experimental, analytical, and numerical methods. Interfacial fracture data was collected through the application of the wedge test, a relatively simple test allowing for the measurement of fracture properties over time in environments of interest. In this case, the test was performed at discrete temperatures within range of 4Ë C to 80Ë C. Two COP adherends were bonded together by a layer of one of the S-IPN silicone hydrogels. Upon the insertion of a wedge between the two adherends, debonding at one of the two interfaces would initiate and propagate at a decreasing rate. Measurements were taken of the debond length over time and applied to develop crack propagation rate versus strain energy release rate (SERR) curves. The SERR values were determined through the application of an analytical model derived for the wedge test geometry and to take into account the effects of the hydrogel interlayer. The time-temperature superposition principle (TTSP) was applied to the crack propagation rate versus SERR curves by shifting the crack propagation rates with the Williams-Landel-Ferry (WLF) equation-based shift factors developed for the bulk behavior of each hydrogel. The application of TTSP broadened the SERR and crack propagation rate ranges and presented a large dependency of the adhesion of the system on the viscoelastic nature of the hydrogels. Power-law fits were applied to the master curves in order to determine parameters that could describe the adhesion of the system and be applied in the development of a finite element model representing the interfacial fracture that occurs for each system. The finite element models were used to validate the analytical model and represent the adhesion of the system such that it could be applied to future geometries of interest in which the S-IPN silicone hydrogels are adhered to the COP substrate. [Files modified per J. Austin, July 9, 2013 Gmc]
Characterization of the Viscoelastic Fracture of Solvated Semi-Interpenetrating Polymer Network Silicone Hydrogels
Tizard, Geoffrey Alexander (Virginia Tech, 2010-07-22)
The unique compressive, optical, and biocompatible properties of silicone hydrogels allow them to be used in a wide variety of applications in the biomedical field. However, the relatively weak mechanical behavior, as well as the highly deformable nature of these elastomeric materials, presents a myriad of challenges when attempting to understand their constitutive and fracture properties in order to improve hydrogel manufacturing and performance in applications. In this thesis, a series of experimental techniques were developed or adapted from common engineering approaches in order to investigate the effects of rate and temperature on the viscoelastic constitutive and fracture behavior of two solvated semi-interpenetrating polymer network silicone hydrogel systems. Viscoelastic characterization of these material systems was performed by implementing a series of uniaxial tension and dynamic mechanical analysis shear tests in order to generate relevant master curves and corresponding thermal shift factors of such properties as shear relaxation modulus, dynamic moduli, and the loss factor. Concurrently, the cohesive fracture properties were studied by utilizing a "semi-infinite" strip geometry under constrained tension in which thin pre-cracked sheets of these cured hydrogels were exposed to several different loading conditions. Fracture tests were performed over a relevant range of temperatures and crosshead rates to determine and generate a master curve of the subcritical strain energy release rate. Experimental methods utilizing high-speed camera images and digital image correlation to monitor viscoelastic strain recovery in the wake of a propagating crack were explored. The results from this thesis may prove useful in an investigation of the interfacial fracture of these hydrogel systems on several different polymer substrates associated with an industrial manufacturing problem.
A chemical and mechanical evaluation of interfacial fracture in dicyandiamide cured epoxy/steel adhesive systems
Vrana, Mark A. (Virginia Tech, 1995)
The interfacial fracture performance of dicyandiamide cured epoxy/steel adhesive systems was thoroughly investigated. Fracture mechanics based testing was utilized to study several variables which were believed to influence the epoxy/steel interphase region, specifically the elasomeric toughener concentration, the dicyandiamide concentration, and the cure temperature. Bulk mechanical measurements were conducted to provide background information for comparison with the fracture data, and surface analyses were carried out on the neat adhesives and failed fracture specimens to provide insight into the locus and causes of failure. The addition of toughener drastically impacted the morphological, bulk mechanical, and adhesive properties in these latent cure systems. Modulus values decreased and bulk fracture toughness values increased with increasing toughener content. Static double cantilever beam (DCB), fatigue DCB, and notched coating adhesion (NCA) interfacial fracture performances all increased. X-ray photoelectron spectroscopy (XPS) and tunneling electron microscopy (TEM) analyses of the failed specimens revealed that chemical changes were more prominent at the epoxy/steel interphase than in the bulk of the materials. Morphological variations were also apparent with toughener level variations, but for a single formulation no differences between the bulk and intephase morphologies were seen. Evaluations were conducted on a series of elastomer modified model epoxy formulations cured with varying amounts of dicyandiamide. The modulus and bulk fracture toughness values were shown to be independent of dicyandiamide concentration, whereas the adhesive performance was greatly influenced. For increases in the concentration of dicyandiamide, single lap shear (SLS) failure strength values increased while quasi-static DCB and NCA test performances decreased. Fatigue DCB results showed improved adhesive performance at both high and low levels of dicyandiamide content. The results of the failure surface evaluations suggest that dicyandiamide variations produce significant chemical changes only in the epoxy/steel interphase region, and not in the bulk. Analyses were conducted on all of the above systems using two additional cure temperatures. The purpose of this work was to alter the dicyandiamide solubility, and possibly the dicy/epoxy reaction mechanisms, and to determine what influence these changes had on the interfacial fracture performance. In general it was found that performance increased as the cure temperature was increased.
Comparing In Situ and Bulk Constitutive Properties of a Structural Adhesive
Grohs, Joshua Walter (Virginia Tech, 2007-07-20)
In the continuing quest for more efficient designs, structural adhesives are being used in place of, or with, traditional fastening methods; however designing with adhesives is refined as traditional methods. To obtain the adhesive design properties, tests are often performed on bulk tensile and bonded shear specimens. Questions remain about the relationship between properties obtained from in situ adhesive joints and bulk adhesive specimens. As a result, an experimental plan was developed which characterized both the linear and nonlinear region of bulk and in situ adhesive performance of a two-part acrylic adhesive from Dow Chemical Company. A standard uniaxial tensile test was used for the bulk normal, while an Iosipescu shear test was used to characterize the bulk shear performance. In situ testing was performed on a napkin-ring specimen loaded in both tension and torsion. Stress-strain relationships in both shear and normal were developed and bulk and in situ adhesive performance was compared. Observations from testing were: 1. Bulk shear and in situ shear tests showed similar performance in both the linear and nonlinear regions. 2. Modulus of elasticity in bulk adhesive tests was similar to the effective modulus of elasticity in in situ tests. 3. Prediction of normal yield strengths of the in situ adhesive through simple failure theory models proved to be inaccurate. Stress singularities, loading imperfections, and potentially a hydrostatic sensitivity were considered possible explanations. 4. Adhesive showed sensitivity to voiding and surface flaws when loaded in a tensile configuration, refinement of specimen fabrication minimized these effects.
Computational Design of Transparent Polymeric Laminates subjected to Low-velocity Impact
Antoine, Guillaume O. (Virginia Tech, 2014-11-07)
Transparent laminates are widely used for body armor, goggles, windows and windshields. Improved understanding of their deformations under impact loading and of energy dissipation mechanisms is needed for minimizing their weight. This requires verified and robust computational algorithms and validated mathematical models of the problem. Here we have developed a mathematical model for analyzing the impact response of transparent laminates made of polymeric materials and implemented it in the finite element software LS-DYNA. Materials considered are polymethylmethacrylate (PMMA), polycarbonate (PC) and adhesives. The PMMA and the PC are modeled as elasto-thermo-visco-plastic and adhesives as viscoelastic. Their failure criteria are stated and simulated by the element deletion technique. Values of material parameters of the PMMA and the PC are taken from the literature, and those of adhesives determined from their test data. Constitutive equations are implemented as user-defined subroutines in LS-DYNA which are verified by comparing numerical and analytical solutions of several initial-boundary-value problems. Delamination at interfaces is simulated by using a bilinear traction separation law and the cohesive zone model. We present mathematical and computational models in chapter one and validate them by comparing their predictions with test findings for impacts of monolithic and laminated plates. The principal source of energy dissipation of impacted PMMA/adhesive/PC laminates is plastic deformations of the PC. In chapter two we analyze impact resistance of doubly curved monolithic PC panels and delineate the effect of curvature on the energy dissipated. It is found that the improved performance of curved panels is due to the decrease in the magnitude of stresses near the center of impact. In chapter three we propose constitutive relations for finite deformations of adhesives and find values of material parameters by considering test data for five portions of cyclic loading. Even though these values give different amounts of energy dissipated in the adhesive, their effect on the computed impact response of PMMA/adhesive/PC laminates is found to be minimal. In chapter four we conduct sensitivity analysis to identify critical parameters that significantly affect the energy dissipated. The genetic algorithm is used to optimally design a transparent laminate in chapter five.

Browsing by Author "Dillard, David A."

Results Per Page

Sort Options