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Über dieses Buch

Mechanics of Time-Dependent Materials and Processes in Conventional and Multifunctional Materials represents one of eight volumes of technical papers presented at the Society for Experimental Mechanics Annual Conference on Experimental and Applied Mechanics, held at Uncasville, Connecticut, June 13-16, 2011. The full set of proceedings also includes volumes on Dynamic Behavior of Materials, Mechanics of Biological Systems and Materials; MEMS and Nanotechnology; Optical Measurements, Modeling and, Metrology; Experimental and Applied Mechanics, Thermomechanics and Infra-Red Imaging, and Engineering Applications of Residual Stress.

Inhaltsverzeichnis

Frontmatter

Using Remendable Polymers for Aerospace Composite Structures

The work described in this research focuses on the development of a single-component remendable polymer suitable for integration into aerospace composite structures. The structural polymer can be used as a replacement matrix material in fiberreinforced composites and allows for in-situ site-specific healing of delamination and matrix cracking when combined with a small volume fraction of heat-assisting materials such as magnetic particles. Whereas previous studies focused on the optimal volume fraction and composition of magnetic particles, and the healing of cracks in carbon-fiber composite coupons, current studies focus on the barriers to adopt this material in a aircraft manufacturing environment. These barriers include the (1) the extensive labor involved in producing a limited quantity of Mendomer (1-3 grams), (2) the evolution and entrapment of voids for all but exquisitely controlled environments, and (3) the low melting temperature (ca. 125°C) of the material when compared against high-temperature matrix systems. Proprietary steps have been formulated to increase raw material yield, reduce viscosity, and increase the glass transition temperature. Interests have also been motivated by reducing costs and adhering to conventional composite fabrication techniques. Research has also involved the investigation of automated damage detection to locate the site of damage for further healing, followed by automated healing since the ultimate goal of this research is to develop an autonomously healing composite system. However, a remendable composite is in itself valuable where its successful incorporation will reduce the need for part replacement and maintenance as well as increase the longevity and reliability of the structure.

Terrisa Duenas, Jennifer Vander Vennet, Akhilesh Jha, Karen Chai, Christian Nelsen, A. John Ayorinde, Ajit Mal

Study of the Effect of Interface Enthalpy on Nanocomposite Viscoelasticity

Polymer Nanocomposites (PNC) are a fascinating category of material systems, in which nanoscopic fillers are found to induce remarkable improvements in the properties of the polymeric matrix.[1] This improvement is often determined by the interfacial energetic interactions which dictate the matrix chain mobility, filler dispersion, and distribution. The PNC community has widely studied the glass transition temperature (Tg) with emphasis laid primarily on the effects of confinement on the matrix polymer.[2, 3, 4] Dispersion/flocculation has also received attention in the elastomeric nanocomposite community in relation to the wetting behavior of fillers. [5] A consolidated view of both these phenomena is yet to be presented in terms of the interfacial energetics. In this study we investigate bare or short chain silane modified filler systems, in which enthalpic effects dominate. This is a first step towards understanding the effect of surface energetics on the thermomechanical properties of PNCs, such as the Tg and viscoelasticity. We subsequently seek to develop a predictive model for the same invoking an informatics based approach [6], which employs Materials Quantitative Structure-Property Relationships (MQSPR) coupled with a Finite Element Method, to link disparate length scales, reducing the need for detailed calculations.

B. Natarajan, H. Deng, D. Gai, M. Krein, C. M. Breneman, L. C. Brinson, L. S. Schadler

Sterilization effect on structure, thermal and time-dependent properties of polyamides

This article studies the effect of different sterilization techniques on structure, thermal and time-dependent mechanical properties of polyamide 6 materials. We used two different types of polyamide 6 material: conventional polyamide 6 with monomodal molecular mass distribution and modified with bimodal one. Samples were prepared and properly shaped in order to observe the mentioned features before and after sterilization with three different techniques, namely sterilization with autoclave, ethylene oxide, and hydrogen peroxide plasma. Optical microscopy, differential scanning calorimetry and torsional creep testing were used for the properties investigation. As a main criterion to evaluate the effect of sterilization durability of materials was used. There was observed significant difference between monomodal and bimodal PA6 materials regarding their morphology and consequently thermal and mechanical properties: bimodal material exhibits more complex structure than chemically identical monomodal one. The results of analysis did not show considerable influence on both materials structure and thermal properties. However, there was observed strong effect on creep behavior. The most significant effect is detected for monomodal material sterilized by ethylene oxide, while creep compliance of the bimodal material did not change much due to any of applied sterilization techniques. Thus, modification the complexity of the material inherent structure improves sensitivity to sterilization.

G. Kubyshkina, B. Zupančič, M. Štukelj, D. Grošelj, L. Marion, I. Emri

Microstructural Evolution of Nafion During Uniaxial Deformation Monitored by X-ray Scattering

Fuel cells enable direct chemical to electrical conversion of fuel to electricity, providing an efficient and clean process. Proton Exchange Membrane Fuel Cells (PEMFC), in which protons from hydrogen or methane cross a membrane to react with oxygen producing electricity, are the preferred transportable fuel cell. Nafion is the membrane of choice for Proton Exchange Membrane Fuel Cells (PEMFC) because its unique microstructure allows rapid transport of protons in a hydrated environment while maintaining mechanical integrity. The teflon-like backbone is hydrophobic while the sulfonated side chains are hydrophilic. In the presence of water molecules this causes the side chains to aggregate into clusters which contain most of the water while the backbone remains relatively dry. Extensive studies have been conducted in order to deduce the size and shape of the microstructural features in order to gain insight into its superior electrochemical and mechanical characteristics, and how they can be further improved (e.g. [1-7]). The shape of these regions (sphere, cylinders, ribbons etc) and their evolution with deformation is still a matter of debate. Here the microstructural evolution is monitored during uniaxial tensile testing via small and wide angle x-ray scattering. Two dimensional scattering profiles are recorded along with stress and strain as a function of time for monotonic, cyclic, and stress relaxation loading histories. These profiles are then reduced to amplitude, location, and orientation for each of the major structural peaks. The scattering data are interpreted in conjunction with existing literature in order to understand the rate and deformation dependent microstructural evolution and its relation to the rate dependent elastic-plastic stress-strain behavior exhibited by Nafion.

Meredith N. Silberstein, J. David Londono, Mary C. Boyce

Coupled Non-Fickian Diffusion and Large Deformation of Hydrogels

Solvent migration in swelling polymer shows complex behaviour, as the interface of wet (rubbery) region moves along with solvent diffusion into the dry (glassy) region, which is accompanied by local deformation. This extrinsic mechanism has led to novel three-dimensional (3D) polymer micro-actuators using direct solvent delivery via microfluidic channels. Here we present experimental techniques to quantify the non-Fickian diffusion in the swelling polymer in an attempt to predict the dynamics of local deformation in such solvent driven micro-actuators. We recorded the evolving diffusion front of solvent in poly(ethylene glycol) diacrylate (PEG-DA) hydrogel upon wetting. In order to measure diffusivity of solvent in the polymer, magnetic resonance imaging (MRI) was used. Simulation result from the theory shows good agreement with Case II non-Fickian swelling experiment. We expect that our experimental methods for such coupled diffusion and deformation will help better capture the underlying physics of hydrogel behaviour and provide fundamental basis in exploration of various hydrogel applications.

Howon Lee, Jiaping Zhang, Jiaxi Lu, John Georgiadis, Hanqing Jiang, Nicholas Fang

Accelerated Testing Methods for Oxidative Aging of Polymeric Composites

Controlling damage progression in oxidative environments is critical for enhancing the long-term durability of polymeric resins and composites. Traditional methods for characterization of these materials for their practical service life require several thousands of hours of isothermal aging. Therefore, there is a need for accelerated testing methods in order to reduce the prohibitive cost in this testing process. Both elevated temperatures and pressure environmental conditions can accelerate oxidative aging of the materials. This paper presents methods to characterize oxidation progress and damage growth in a polymeric matrix composite based on stress-assisted diffusion and sample miniaturization. In this approach, microscale specimens are fabricated using micro-fabrication techniques. The specimens are isothermally aged at controlled stress levels that accelerate both the oxidation and damage growth in the specimen. Coupling effects of temperature and stress on the oxidative aging are investigated based on the presented method. Due to the small scale of the specimen, the number of specimens that can be tested in parallel grows significantly. Micro-fabrication techniques also allow integration of instrumentation for measurement of the specimen response during aging, thereby reducing the additional effort, time and expense of acquiring and processing the data from the specimens during the traditional long-term aging tests.

Nan An, Kishore V. Pochiraju, Gyaneshwar P. Tandon

Degradation of Shape Memory Polymer Due to Water and Diesel Fuels

Shape-Memory-Polymers (SMP) is smart materials have the ability to memorize original shape; and can be used as sensors, transplants, and structural materials in engineering applications. Despite its practical importance, limited work is available on its degradation behavior. This study has been carried out to evaluate degradation behavior of Veriflex-SMP on exposures to water and diesel fuels separately. It is found that “glass transition temperature, T

g

” decreases due to immersion in liquids; and immersion facilitates acceleration of breakage of the existing bonds and the formation of new bonds, thereby increasing the mobility of polymeric chains; and the immersion medium effectively plasticize SMPs by reducing storage modulus and decreases structural integrity of SMP. Upon exposure to diesel fuel for several weeks, “T

g

” goes below room temperature; and SMP goes back to its original shape without the need of the application of external energy. Stress relaxation tests shows that stress decay is found to be much faster and to a much lower value in degraded samples and increases with increase of immersion time. From Fourier Transform Infrared tests formation of hydrogen bonding between SMP and the solvents in which it is immersed, is the main reason for SMP degradation; although hydrogen has a minor effect on SMP structure but has an obvious influence on glass transition temperature, T

g

.

M. N. H. Nahid, M. A. Wahab, Kun Lian

Structural Enhancement of Framing Members Using Polyurea

This paper demonstrates the potential for using field applied structural coatings to reinforce traditional framing members and standard building ties, thereby providing an improved and continuous foundation to roof load pathway. Tension tests are performed on rafter top plate model joint connections, some of which were reinforced with a hurricane tie, to establish how much of a difference a polyurea coating made as the joints between the stud and top plate, and top plate and rafter, were loaded to failure. Polyurea provides universal strengthening compared to hurricane ties with the added advantage that members and joints can be protected from a multitude of threats including corrosion due to moisture, damage due to flood; and, with selfextinguishing properties, fire. Results show that the failure mode of the structures tested can be controlled by using different types of field or factory applied polymer coatings. The addition of the coating allowed both unreinforced and reinforced configurations to withstand higher loads (200-400% more). In general, the polyurea delayed the onset of failure and significantly strengthened every configuration by increasing the amount of work/energy required to pull it apart; in some cases, by almost 800%.

David J. Alldredge, John A. Gilbert, Houssam Toutanji, Thomas Lavin, Madhan Balasubramanyam

Experiments and Models for the Time Dependent Mechanics of Nanoscale Polymeric Structures and Nanocrystalline Metal Films

To date, micro and nanoscale experiments have been mostly focused on the length scale dependent mechanical behavior of nanostructures and nanostructured thin films but have not been able to address their time and rate dependence. This inefficiency stems from the use of high resolution electron microscopes which are slow imaging tools, and quite often are of detrimental effect to the integrity of polymeric materials. Optical methods have been revisited in the recent years and were modified to accommodate micro and nanoscale specimens in order to obtain high resolution deformations and their time evolution at time scales varying from microseconds to days [1-4]. This research, conducted at the University of Illinois, has developed new approaches to investigate the time-dependent mechanical behavior of metallic thin films for MEMS and polymeric nanostructures in an effort to understand the important deformation processes at small scales. The extended (internal) surfaces in nanocrystalline metal films and the large surface-to-volume ratios in polymeric nanostructures favor material transport mechanisms that are not important in bulk or large grain materials because they do not result in appreciable strains. On the contrary, in nanoscale polymeric fibers for instance such processes result in large material deformations and sustained ductilities in a large range of loading rates. This presentation will summarize experimental work conducted with polymeric nanofibers and nanocrystalline metals and some early modeling efforts to rationalize the measured time- and ratedependent mechanical behavior.

Ioannis Chasiotis

Study of Damage Evolution in High Strength Al Alloy using X-Ray Tomography

Many micromechanics-based damage models were developed to mimic the macroscopic response of materials through matching measures such as toughness and failure strain [1]. However, there is a lack of microstructural experimental data to identify the roles of the initiation, growth and coalescence of voids to damage and failure. This paper is aimed to experimentally investigate the microstructure of the material and understand the damage processes leading to failure. Experiments using X-Ray Computed Tomography (XRCT) 3D imaging technique with in-situ loading were conducted [2 - 5].

Helena Jin, Wei-Yang Lu, Alejandro Mota, James W Foulk, George Johnson, Nancy Yang, John Korellis

Corrosion Behavior of SS 304 with Ball Milling and Electrolytic Plasma Treatment in NaCl Solution

Electrolytic plasma process (EPP) was used as a fast annealing treatment on ball milled surfaces. With localized melting of EPP, the surface morphology and the chemical composition can be drastically changed. Annealed, ball milled and EPP treated stainless steel 304 samples were examined and EPP treatment has been shown to improve the overall corrosion and localized corrosion. Materials were characterized by X-Ray Diffraction, SEM. The 24 hours of exposure in NaCl solution, the sample open-circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy were measured to estimate the anti corrosion properties.

J. Liang, M. A. Wahab, S. M. Guo

Experimental Studies of Phase Transformation in Shape Memory Alloys

This paper presents experimental studies to examine stress-induced martensitic phase transformation during the superelastic deformation of the shape memory alloy Nickel-Titanium. The phase transformation, which is solid-to-solid and diffusionless, occurs between austenite, a B2 cubic crystal structure, and martensite, a monoclinic crystal structure during loading and unloading at constant ambient temperature. To examine the complex local thermo-mechanical interactions that affect transformation behavior, we utilize a temporally- and spatially- simultaneous combination of strain and thermal mapping using three-dimensional digital image correlation and infrared thermography, respectively. This combined experimental approach enables full-field, quantitative maps of strain and temperature fields over the specimen surface, allowing the investigation of factors including cycling, strain rate, texture, and local temperature variations. The effects of these factors on fundamental transformation properties, such as the stresses required for phase nucleation and propagation, accumulated residual plastic strain, total strain accommodated by phase transformation, the evolution of martensitic volume fraction, and the amount of hysteresis, are discussed.

Kyubum Kim, Samantha Daly

Measurement of Energy Loss in Thin Films Using Microbeam Deflection Method

A technique developed for studying the energy loss behavior of submicron to nanometer scale thin metal films on substrate is presented. The test microstructure was designed the triangular cantilever beam and fabricated by the standard CMOS processes, which can improve stress distribution non-uniform problem and the thickness regime of deposited metal thin film on its surface could reduce to several nanometers. In order to reduce the measure error and calculation complex due to the contact force, the driving system was used electrostatic force to making the paddle cantilever beam bend and the deflection of paddle cantilever beam due to the electrostatic force was measured by a capacitance change. The deflection of the paddle beam can be measured from the capacitance value. A force equilibrium calculate method (include sample compliance force, force due to the film, force due to the gravity and electrostatic force) could determine the stress and strain of the deposited films easily. The anelastic behavior and internal friction of 200~500 nm Al thin film were studied using the dynamic frequency response of the paddle structure generated by electrostatic force under vacuum pressure. The result show the measurement system used here can accurately measures the loss mechanism of thin film using dynamic response which give potential to study the grain boundary motion and dislocation motion in the nano-scale thin films.

F.-C. Hsu, C.-J. Tong, M.-T. Lin, Y.-C. Cheng

Multiscale Characterization of Water-, Oil- and UV-Conditioned Shape-Memory Polymer under Compression

Shape memory polymers (SMPs) are an emerging class of active polymers that may be used for reconfigurable structures on air vehicles. In this study, epoxy-based SMPs were conditioned separately in simulated service environments relevant to Air Force missions, namely, (1) exposure to UV radiation, (2) immersion in jet-oil, and (3) immersion in water. The mechanical properties and shape recovery abilities of the unconditioned and conditioned SMPs were examined using large-scale compressive tests and small-scale nano-indentation tests. Results show that all the conditioned SMPs exhibit a decrease in Tg as compared to the unconditioned one. Under compressive loading, the SMPs undergo significant plastic deformation prior to fracture, except the UV conditioned sample. Environmentally conditionings generally result in higher moduli and yield strength of the SMPs. Environmental conditionings, in particular the UV exposure and water immersion, also affect the shape recovery abilities of the SMPs if the recovery temperatures are set low.

J. T. Fulcher, H. E. Karaca, G. P. Tandon, D. C. Foster, Y. C. Lu

Shape Memory Polymer based Cellular Materials

We propose the concept of periodic cellular materials with programmable effective properties and present initial results from a computational study of a prototypical material that exhibits this behavior. Nonlinear FEA shows that programmed geometric imperfections at the cell level can be used to modify the effective compressive storage modulus of shape memory polymer (SMP) based periodic cellular materials after they have been manufactured. The ability of SMPs to freeze a temporary deformation for an extended period of time and the low modulus of these materials in the rubbery regime allow us to freeze controlled and reversible imperfections at the cell level following the typical temporary shape programming process for SMPs. Small geometric imperfections (2% global strain) are observed to produce variations of up to 40% in the effective initial compressive storage modulus in the prototypical material.

D. Restrepo, N. D. Mankame, P. D. Zavattieri

Influence of Mechanical Properties and Loading Conditions on the Recovery of Shape Memory Polymers

This work presented a parameter study to investigate the influence of material properties and loading conditions on the recovery performance of amorphous shape memory polymers using a recently developed thermoviscoelastic model. The model incorporated the time-dependent effects of both structural relaxation—using a nonlinear Adam-Gibbs model—and viscoelasticity. The model can predict well the unconstrained strain recovery response and stress evolution during constrained recovery process. The materials properties and the loading parameters, including the cooling rate, the annealing time, and the heating rate, were varied one by one to compare the effects on the start and end temperatures and recovery time of the unconstrained recovery response and on the stress hysteresis of the constrained recovery response. The results confirmed experimental observations that unconstrained strain recovery response was mostly influenced by viscoelasticity, while the constrained recovery response resulted from the interaction of many different mechanisms, including structural and stress relaxation, thermal expansion, the modulus of rubbery and glass state. The results also showed that the cooling and heating rates had the largest influence on both recovery responses.

Rui Xiao, Xiang Chen, Thao. D. Nguyen

Fatigue Cycling of Shape Memory Polymer Resin

Shape memory polymers have attracted great interest in recent years for application in reconfigurable structures (for instance morphing aircraft, micro air vehicles, and deployable space structures). However, before such applications can be attempted, the mechanical behavior of the shape memory polymers must be thoroughly understood. The present study represents an assessment of viscoelastic and viscoplastic effects during multiple shape memory cycles of Veriflex-E, an epoxy-based, thermally-triggered shape memory polymer resin. The experimental program is designed to explore the influence of multiple thermomechanical cycles on the shape memory performance of Veriflex-E. The effects of the deformation rate and hold time at elevated temperature on the shape memory behavior are also investigated.

A. J. W. McClung, G. P. Tandon, J. W. Baur

Higher Rate Testing of Long Fiber Filled Polypropylene

The properties of long fiber-reinforced thermoplastics at high rates are needed to model the impact behavior of materials and structures. Testing at high rates introduces several issues, namely, bulk material property measurement, equipment capacity due to the higher failure loads, proper load introduction into the material before failure, achieving dynamic equilibrium within the timeframe of the test event, and natural oscillatory vibrations within the specimen. This paper describes a program to develop a specimen suitable for higher rate testing of long fiber reinforced thermoplastics. Two general configurations, with variations, were analyzed using finite-element analysis methods: (1) a radial shoulder-loaded configuration and (2) a mechanical wedge grip-loaded configuration. The selected configuration minimized stress concentrations and had a high probability of failure in or near the gage section. Long glass fiber filled polypropylene was tested at rates from 0.4 to 400/s (0.004 to 6.4 m/s) using variations of the selected specimen configuration. The gage widths were 5, 10, and 15mm. The tensile strength increased with increasing strain rate for all three sizes. The strengths at a given rate were similar for all three sizes. The failure strain of the 15mm wide specimen was lower than the 5mm and 10mm at 0.4/s and 40/s. The stiffness increased with specimen width and rate. The maximum practical strain rate decreased with increasing specimen width. Quality data were obtained from 5mm wide specimens at rates up through 400/s. However, the stiffness and failure strain varied from those of the wider specimens. The 15mm specimen configuration produced data at rates above 100/s, although some issues still remain regarding data interpretation.

Susan I. Hill, Peter Phillips

Characterization of Elastomeric Composite Materials for Blast Mitigation

In this work, we seek to develop elastomeric composite materials capable of shock mitigation through material design by small-scale heterogeneity. The host elastomeric material is a polyurea system that is a lightly cross-linked two-phase polymer, which consists of the diamine component Versalink P-1000 as the soft segment and the diisocyanate component Isonate 143L as the hard segment. This study evaluates the impact of additives in the form of untreated and surface treated milled glass fibers. The properties of the resultant elastomeric composite materials are mechanically and thermally characterized using durometer testing, dynamic mechanical analysis (DMA) testing, and differential scanning calorimetry (DSC) testing in order to determine the hardness, storage and loss moduli, and glass transition temperature of the composites, respectively. Preliminary results indicate that the dynamic mechanical properties of the material can be significantly altered through such modifications. The work described here is part of an ongoing effort to understand the impact of additives on the ultimate properties and performance of the host elastomeric material.

K. Schaaf, S. Nemat-Nasser

Experimental arrangement for measuring the high-strain-rate response of polymers under pressures

This study aims to investigate the high-strain-rate shear response of viscoelastic elastomeric coatings at large strains and under elevated levels of hydrostatic pressure. Results of this study shed light on the combined effects of deformation rate and pressure which might promote a transition from viscoelastic to glassy behavior. This work utilizes a Split Hopkinson Pressure Bar (SHPB) apparatus in conjunction with a customized version of the recently proposed Shear Compression Specimen (SCS) which consists of a polymer gage section with two metal ends that remain essentially rigid during deformation. Detailed finite element simulations were used to customize the adopted specimen, to determine its proper dimensions and promote its functionality. The customized specimen permits subjecting the tested specimen to a state of uniform pressure and shear stress, while allowing for measuring pressure, shear stress and shear strain directly. Results obtained using the customized specimen, which are included in this paper, illustrate its usefulness in measuring the effect of high-strain-rate, large strain and hydrostatic pressure on the shear stress-strain response of viscoelastic elastomers.

Maen Alkhader, Wolfgang Knauss, Guruswami Ravichandran

Simulation of impact tests on polycarbonate at different strain rates and temperatures

The use of lighter and impact resistant materials, such as polymers, in vehicular systems is an important motivation for the automotive industry as these materials would make vehicles more fuel-efficient without compromising safety standards. In general, polymers exhibit a rich variety of material behavior originating from their particular microstructural (long molecular chains) behavior that is strongly temperature, pressure, and time dependent. To capture such intricate behavior, a number of polymer constitutive models have been proposed and implemented into finite element codes in an effort to solve complex engineering problems (see [1] for a review of these models). However, developing improved constitutive models for polymers that are physically-based is always a challenging area that has important implications for the design of polymeric structural components.

J. L. Bouvard, C. Bouvard, B. Denton, M. A. Tschopp, M. F. Horstemeyer

Experimental Investigation of Dynamic Mechanical Properties of Polyurea-Fly Ash Composites

Polyurea has been the material of choice in many applications due to its thermomechanical properties. These applications span a wide spectrum from abrasion-resistant coating to reinforcement against blast damage in structures, ships, and vehicles. The improved observed performance is linked to its microstructure, a lightly crosslinked (elastomeric) block copolymer. The constitutive modeling of such materials is generally based on a wide variety of experimental measurements, including dynamic mechanical analysis. Furthermore, the response under stress-wave propagation may be measured through ultrasonic tests. In this work we present our research towards improvement and modification of the dynamic mechanical properties of polyurea through inclusion of various size and distributions of fly ash hollow spherical particles. The extensive experimental results are reported and a micromechanical homogenization model is presented. The extent of application of such model will be established and alternative modeling techniques which include the possible inertia effects at high rates of deformation will be surveyed.

Alireza V. Amirkhizi, Jing Qiao, Wiroj Nantasetphong, Kristin Schaaf, Sia Nemat-Nasser

Damage & Fracture of High-Explosive Mock Subject to Cyclic Loading

We use four-point bend specimen with a single shallow edge notch to study the fracture process in Mock 900-21, a PBX 9501 high explosive simulant mock. Subject to monotonic loading we determine quantitatively the threshold load for macroscopic crack initiation from the notch tip. The four-point bend specimen is then subject to cyclic loading in such a way that during the first cycle, the applied force approaches but does not exceed the threshold load determined from the monotonic loading test and in the subsequent cycles, the overall maximum deformation is maintained to be equal to that of the first cycle. It is expected and is also confirmed that no macroscopic damage and cracking occur during the first cycle. However, we observe that sizable macroscopic crack is generated and enlarged during the subsequent cycles, even though the applied force never exceeds the threshold load. Details of the process of damage formation, accumulation, and crack extension are presented and the mechanical mechanism responsible for such failure process is postulated and discussed.

C. LIU, P. J. Rae, C. M. Cady, M. L. Lovato

An Evaluation Of A Modified Iosipescu Specimen For Measurement Of Elastic-Plastic-Creep Properties Of Solder Materials

There are various specimen configurations available in the literature for characterizing the mechanical behavior of solder interconnect materials. An ideal test specimen should use a simple geometry which produces a reasonably uniform material response throughout the gage zone and minimizes the complexity of data reduction to extract material model constants. In the thermomechanical micro scale (TMM) test used in this study, we use a simple, notched shear specimen, based on a concept originally proposed by Iosipescu [1967] [1], which produces a reasonably uniform shear stress field throughout the solder joint [Reinikainen et al.,1998] [2]. Our modified Iosipescu specimen comprises of two oxygen free, high conductivity (OFHC) copper platens soldered together and loaded in simple shear, as in a lap-shear specimen. The solder joint in this specimen is only 180 microns wide to capture the length scale effects of functional solder interconnects. This study examines the effects of dimensional variabilities of this modified Iosipescu test setup on the shear stress and strain distributions in the solder specimen. Variabilities encountered in these specimens include: (i) fillets at the V-notches, caused by excess solder; (ii) offset between the two copper platens along the loading direction; (iii) taper of the solder joint due to lack of parallelism of the edges of the copper platens; and (iv) misalignment between the specimen centerline and loading axis of the TMM test frame due to mounting variability. Detailed parametric studies of these four dimensional variations in the TMM specimen are conducted using a simple two-dimensional finite element model for measurement of elastic-plastic-creep properties of solder materials.

Subhasis Mukherjee, Abhijit Dasgupta

Temperature Effect on Poisson’s Ratio of Woven Composites

Monotonic tensile tests were conducted following ASTM Standards D3039 (Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials) and D3518 (Standard Test Method for In-Plane Shear Response of Polymer Matrix Composite Materials by Tensile Test of a ±45° Laminate), on non-hybrid plain weave composite materials. Strips (6.35mm×25mm×250mm) of non-hybrid IM-7 Graphite/SC-79 epoxy called GR for short, non-hybrid S-2 Glass/SC-79 epoxy called GL for short specimens were tensile tested. The tests were conducted at -60°C, -20°C, 75°C and 125°C. The Poisson’s ratios were measured using strain gages. It was observed that temperature had a small effect on the Poisson’s ratio.

Yougashwar Budhoo

Detection and Damage Monitoring in Composite Structures Using Piezoelectrics

Piezoelectric-fiber-composite Transducers (PFCT) are an ideal choice for composite structures application, as they are highly flexible, easily embeddable, and have high compatibility with composite structures; and they provide manufacturing flexibility. The major objective of this research is to use PFCTs as an embedded sensor within the composite structures. As embedded sensors PFCTs are able to perform important functions, such as: (i) monitor the stress/strain levels inside the structures continuously and (ii) detect the damages inside the composite structures. Initially, tensile tests are carried out to investigate the effect of the embedded PFCT sensor on the tensile strength of an impregnated glass fiber-epoxy prepreg composite laminate. It is found that by embedding a PFCT sensor inside the composite structure reduces the ultimate strength and modulus of the overall structure about 2.5% and 7.5% respectively. Then tests are performed to investigate the ability of the embedded PFCT sensor to detect the changes in the applied stress/strain. It was found that these sensors are effectively able to detect the changes in the applied stress/strain. A linear relationship has been observed between the applied input mechanical stress and the sensor generated voltage output. Finally, experiments are performed using the embedded PFCT sensors to detect the damages inside the composite structure using the modal analysis and impact methods. From the results of these experiments, it is concluded that embedded PFCT sensors are able to detect the damages using the above methods effectively.

H. P. Konka, M. A. Wahab, K. Lian

Representative Volume Element Analysis for the Evaluation of Effective Material Properties of Fiber and Particle Loaded Composites with Different Shaped Inclusions

In this paper effective material properties are predicted for composites with different shape and size of inclusions such as cylindrical fibers, spherical and elliptical particles and cylindrical fibers with hemispherical ends. The analysis is based on a numerical homogenization technique using finite element method in connection with three-dimensional representative volume element models. Investigations are carried out to study the influence of various parameters like volume fraction, aspect ratio and particle distribution. Results are discussed and compared.

V. K. Srivastava, U. Gabbert, H. Berger

Time Dependent (Creep) Deformation of Thin Elastomers at Cold Temperature and Effective Strain Analysis of Their Laminates

Rolling resistance from tires accounts for 20% of the fuel used in the average car. In applications like automobile tires, heterogeneous “inner liners” are being used as barriers to retain air and to reduce rolling resistance. However, there is great uncertainty in estimating the life of those components under extreme, synergistic environments (e.g., rate and time dependent, cyclic, thermal, and mechanical loading). In the present work, the behavior of such thin films under extreme cold temperature (-40C) has been investigated experimentally. The expected life was estimated both experimentally and by using a parametric creep model. Failure mechanisms were studied extensively and interesting phenomena (such as time-delayed longitudinal fracture) are documented. In order to develop a better understanding of the mechanisms driving crack formation in field tested tires of inner liners, actual tire specimens were also tested. An experimental Arcan fixture was used in conjunction with digital image correlation (DIC) to determine the full-field strain state of the inner liner material under simulated loading conditions. The normalized component sums of the effective strains were computed, and it was found that these strain resultant vectors correspond to directions of observed cracks in field testing.

Jon-Michael Adkins, Prasun Majumdar, Kenneth Reifsnider

Time and Temperature Response of Composite Overwrap Cylinders

Stress relaxation and creep of composite cylinders are investigated based on anisotropic viscoelasticity. The analysis accounts for ply-by-ply variation of material properties, ply orientations, and temperature gradients through the thickness of cylinders subjected to mechanical and thermal loads. Fiber reinforced composite materials generally illustrate extreme anisotropy in viscoelastic behavior. Accordingly, viscoelastic characteristics of composite cylinders are quite different from those of isotropic cylinders. Viscoelastic effects of the composite can result in a drastic change of stress and strain profiles in the cylinders over a period of time, which is critical for structural durability of composite cylinders. The developed analysis can be applied to composite pressure vessels and flywheels.

Jerome T. Tzeng

High Temperature, Non-contact, Electro-magnetic Mechanical Apparatus for Creep Testing

We describe the design and implementation of a second generation Electromagnetic Mechanical Apparatus (EMMA-2) capable of Ultra High Temperature (UHT) creep testing. EMMA-2 uses a variable magnetic field to apply stress to ribbon specimens that are self-heated with DC current. EMMA-2 is capable of continuous non-contact creep testing and operates in a controlled atmosphere. Theoretical models are presented for mechanical stress and temperature, as related to specimen geometry, electrical current, magnetic flux density, and combined with creep models behavior to predict the accessible range of temperature and stress. This unique test apparatus allows for characterization of Ultra High Temperature Ceramics (UHTCs) without physically contacting test samples. Initial testing has been performed on a set of ZrB

2

-SiC (30 Vol.%) ceramics at temperatures ranging from 1700-2100°C.

Sindhura Gangireddy, John W. Halloran, Zachary N. Wing

Experiments and Predictions of Large Deformation and Failure in Thermomechanical Loading Environments

The response of 304L stainless steel to combined mechanical and thermal loadings is studied to enable the development of validated computational simulation methods for predicting deformation and failure in coupled thermomechanical environments. Experimental coupling was accomplished on axisymmetric tubular specimens that were mechanically loaded by internal pressurization and thermally loaded asymmetrically by side radiant heating. Mechanical characterization experiments of the 304L stainless steel tube material was completed for development of a thermal elastic-plastic material constitutive model used in the finite element simulations of the validation experiments. The design and implementation of the experimental methodology and results of preliminary experiments were presented at 2010 SEM Annual Conference [1, 2].

Bonnie R. Antoun, J. Franklin Dempsey, Gerald W. Wellman

Compliance Plot Analysis of Nonlinear Response of PMMA During Nanoindentation

Application of nanoindentation testing to polymeric materials has produced linear viscoelastic properties with the hope of extending results to the nonlinear behavior range. Analysis has indicated that levels of stress well into the nonlinear range are present directly under the indenter. This paper examines how this high stress behavior can be characterized for PMMA material recently tested by nanoindentation. Although stress levels are low compared to many structural materials, a characterization by a compliance plot is made by analogy with the treatment used for metals and other elastic-plastic materials. It illustrates the nature of the stress build-up process during nanonidentation of this polymeric material.

R. J. Arenz

An Incremental formulation for the linear analysis of viscoelastic beams: Relaxation differential approach using generalized variables

This paper is concerned with the development of a new incremental formulation in the time domain for linear, non-aging viscoelastic materials undergoing mechanical deformation. The formulation is derived from linear differential equations based on a discrete spectrum representation for the relaxation function. The incremental constitutive equations are then obtained by finite difference integration. Thus the difficulty of retaining the stress history in computer solutions is avoided. The influence of the whole past history on the behaviour at any time is given by a pseudo second order tensor. A complete general formulation of linear viscoelastic stress analysis is developed in terms of increments of midsurface strains and curvatures and corresponding stress resultants. The generality allowed by this approach has been established by finding incremental formulation through simple choices of the tensor relaxation components. This approach appears to open a wide horizon (to explore) of new incremental formulations according to particular relaxation components. Remarkable incremental constitutive laws, for which the above technique is applied, are given. This formulation is introduced in a finite element discretization in order to resolve complex boundary viscoelastic problems.

Claude Chazal, Rostand Mouto Pitti, Alaa Chateauneuf

Modeling the Nonlinear Viscoelastic Behavior of Polyurea Using a Distortionmodified Free Volume Approach

The pressure-dependent behavior of polyurea was examined under monotonic loading in the confined compression configuration. Additional data from Arcan shear and uniaxial compression was used to respectively complete parameter selection for the linear and nonlinear behavior and then validate it. The distortion-modified free volume approach was examined as a model of nonlinear viscoelasticity for the polyurea. The bulk and shear relaxation behavior were both pressure dependent. Under ramp loadings, the shear and tensile responses were quite nonlinearly viscoelastic but the bulk response was almost linearly elastic.

G. Chevellard, K. Ravi-Chandar, K. M. Liechti

An Incremental Constituve Law for Damaging Viscoelastic Materials

An incremental formulation suitable for modelling of materials with damaging viscoelastic behaviours is proposed in this work. A constitutive law based on linear viscoelasticity coupled with strain dependent damage is developed. The viscoelastic model is represented by a generalized Maxwell’s chain. It is governed by a set of internal stress variables attached to the branches of the Maxwell’s chain. The damage evolution is governed by values gained by a pseudo strain. The coupled law is turned into an incremental form suitable for the numerical analysis of damaging time dependent structures. Taking advantage of the incremental form, the coupled damaging viscoelastic law is implemented as a step-by-step procedure. The calculation procedure consists of a damaging elastic step followed by a number of damaging viscoelastic steps. The damage variable is adjusted at each step, according to the value gained by the pseudo strain. Exemplary calculations are worked out for two cases of uniaxial and biaxial variable or cyclic loadings. The results show the efficiency of the incremental model. It is worth noticing that the time increment used for the calculations is not necessarily small. As a consequence, precise analysis of damaging time dependent structures can be performed for low calculation cost.

Omar Saifouni, Rostand Moutou Pitti, Jean-Francois Destrebecq

Reliability Analysis of Mixed Mode Cracking with Viscoelastic Orthotropic Behaviour

The reliability analysis applied to viscoelastic and orthotropic materials, in the case of mixed mode configuration, is studied in this work. The M integral, separating mixed mode during creep crack initiation in viscoelastic field, is used in the analytical approach. The main development, based on conservative law, and a combination of real and virtual displacement fields, is proposed. In order to provide mixed mode configuration, a Compact Tension Shear (CTS) specimen is used in the numerical process. Simultaneously the fracture and the viscoelastic procedures are coupled with reliability analysis in order to take account for model and parameter uncertainties. In this case, the random parameters related to model factors, elastic constants are defined in the reliability analysis of time dependent fracture materials subjected to complex loading. As results, the reliability levels are computed and discussed according to various mixed-mode loading scenarios

Rostand Moutou Pitti, Alaa Chateauneuf, Claude Chazal

Long-term Life Prediction of CFRP Structures Based on MMF/ATM Method

The accelerated testing methodology (ATM) for the fatigue life prediction of CFRP laminates developed and verified theoretically and experimentally in the previous studies is expanded to the fatigue life prediction of the structures made of CFRP laminates in this paper. The procedure of MMF/ATM method combined with our developed ATM and the micromechanics of failure (MMF) by Professor Sung-Kyu Ha and others is proposed for the fatigue life prediction of the structures made of CFRP laminates. The time and temperature dependent MMF/ATM critical parameters of CFRP are determined by measuring the static and fatigue strengths at elevated temperatures in the longitudinal and transverse, tension and compression directions of unidirectional CFRP. The fatigue strengths of quasi-isotropic CFRP laminates with a central hole under compression load as an example of CFRP structures are measured at elevated temperatures, and these experimental data are compared with the predicted results by using the MMF/ATM critical parameters of CFRP. As results, it was cleared that MMF/ATM method has the possibility to be the strong tool to the fatigue life prediction of the structures made of CFRP laminates.

Yasushi Miyano, Masayuki Nakada, Hongneng Cai

Non-local Solutions to Direct and Inverse Problems in Mechanics: A Fractional Calculus Approach

In this paper we present a new non-local model of continuum mechanics based on fractional calculus. The model encompasses in a single unified framework both classical continuum mechanics and non-local theories for continua with discontinuities and long range forces. Our theoretical framework can be used to solve both direct and inverse problems of importance in practical applications. We present results to two direct mechanical problems: (1) the deformation of an infinite bar subjected to a self-equilibrated load distribution, and (2) the propagation of longitudinal waves in a thin finite bar. We also show results to the inverse problem of magnetic resonance elastography using our proposed model.

C. S. Drapaca, S. Sivaloganathan

Study on Crystallinity Dependency of Creep Deformation on GFRTP of Polyoxythlene (POM)

It has clarified that the viscoelastic property of some kind of thermoplastic resin is dependent on crystallinity in until now. Concretely, it was confirmed that "Time-temperature superposition principle" was established on the creep deformation in them. And also, in various FRTP, in making fiber volume fraction to be a factor for creep deformation, it was clarified that "Time-temperature-fiber volume fraction superposition principle" was established. In this paper on the effect of crystal on creep deformation, the crystallinity was made to be a factor, and it clarified that "Time - temperature -crystallinity superposition principle" was established in POM resin and GFRPOM.

S. Somiya, K. Yamada, T. Sakai

Numerical simulation of hot imprint process of periodical lamellar microstructure into polycarbonate

Thermoplastic polymers are frequently used in the industry. They represent the most important group of polymeric materials. Most of all, injection molding, extrusion, spinning and hot embossing are used for plastic processing. Today polymers are used in precision systems. So it is necessary to create new powerful modeling and analyzing tools. A finite element model for hot imprint process of periodical microstructure into polycarbonate has been developed. In the finite element model polycarbonate is assumed to be a nonlinear elasto-plastic material. The model covers the main three steps of hot imprint process: polycarbonate heating, imprinting and demolding. Periodical lamellar microstructure was chosen as die in the hot imprint process, because it is common structure in the practice. The model is solved using the heat transfer and the solid stress-strain application modes with thermal contact problem between die and polycarbonate. This multiphysics polycarbonate hot imprint model includes the heat transport, structural mechanical stresses and deformations resulting from the temperature distribution. Finite-element simulation of the hot imprint process has been performed using COMSOL Multiphysics. Nonlinear elasto-plastic model was created. It allows evaluation of temperature distributions and stresses in the polycarbonate during hot imprint process. Obtained theoretical results were compared with experimental.

Rimvydas Gaidys, Birutė Narijauskaitė, Arvydas Palevičius, Giedrius Janušas

Rapid Characterization of Visco-elastic Properties of Polymeric Materials

As polymers are widely used in electronic packaging, their visco-elastic behavior must be characterized in order to predict the behavior of package assemblies during manufacturing and operation. Current known testing methods for visco-elastic properties are often too time-consuming or too complex to be implemented as the specimen preparation and the testing conditions are critical to reliability and repeatability of measurements.

Yejin Kim, Bongtae Han

Estimation of Fatigue life of Cortical Bone Considering Viscoelastic Properties and Damage Mechanics

Recently, the Osteoporosis victims increase in the senior citizen. Therefore, the danger of the stress fracture due to the decrease in bone strength is pointed out. Especially, the damage accumulation behavior of the bone in the cyclic load becomes a problem for the fatigue of the bone. Moreover, it is necessary to consider the viscoelastic property for the prediction of the fatigue behavior, because the bone is a viscoelastic material. In this study, our objective is to estimate the fatigue behavior that considers the viscoelasticity behavior and the damage accumulation behavior as damage mechanics. The viscoelastic properties were revealed with the strain rate tests, and we calculated the 3-mode Maxwell parameters with these results. The damage accumulation behaviors were measured with the Acoustic Emission method. Using these results, we could estimate the fatigue behavior of cortical bone with the viscoelastic parameter and cumulative AE energy.

Takenobu Sakai, Keita Yasui, Shuichi Wakayama

Crack initiation and viscoplasticity in polyethylene joint replacement components

Ultrahigh molecular weight polyethylene (UHMWPE) is an abrasion resistant and bioinert polymer widely used as a bearing material in total joint replacements. Recent reports of fracture and crack initiation in these systems make the prediction of crack initiation a primary concern. Past work in assessing the resistance to crack propagation in UHMWPE has typically ignored the creeping (quasi-static) constitutive contribution to the process of failure. We conducted constant load experiments on pre-notched fracture specimens and observed the elapsed time to crack initiation and subsequent crack velocity as a function of the applied load. A hyperelastic-viscoplastic constitutive model was calibrated to three uniaxial tensile experiments; one at a constant crosshead velocity, and two delayed yield creep (viscoplastic) experiments at an engineering stress either slightly or moderately below the short-term yield strength, until the strain saturated or failure. The crack initiation phase of the fracture experiment was modeled in ABAQUS, predicting the time-dependent J-integral and average molecular chain stretch to be single-valued at the experimentally obtained crack initiation times for the three tested boundary conditions. The crack initiation time and propagation velocity were also found to scale with the applied load in agreement with an analytical power-law viscous fracture model.

Jevan Furmanski, P. Abhiram Sirimamilla, Clare M. Rimnac
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