Dinesh Pinisetty

Electrical properties of hollow glass particle filled vinyl ester matrix syntactic foams

Low dielectric constant materials play a key role in modern electronics. In this regard, hollow particle reinforced polymer matrix composites called syntactic foams may be useful due to their low and tailored dielectric constant. In the current study, vinyl ester matrix/glass hollow particle syntactic foams are analyzed to understand the effect of hollow particle wall thickness and volume fraction on the dielectric constant of syntactic foams. The dielectric constant is found to decrease with increase in the hollow particle volume fraction and decrease in the wall thickness. Theoretical estimates are obtained for the dielectric constant of syntactic foams. Parametric studies are conducted using the theoretical model. It is found that a wide range of syntactic foam compositions can be tailored to have the same dielectric constant, which provides possibility of independently tailoring density and other properties based on the requirement of the application.

Unnotched Izod impact characterization of glass hollow particle/vinyl ester syntactic foams

Vinyl ester matrix syntactic foams filled with hollow glass microspheres are characterized for unnotched Izod impact properties. The study is aimed to analyze the effect of wall thickness and volume fraction of the hollow glass microsphere on the impact properties of syntactic foams. The impact strength of syntactic foams was observed to be lower in comparison to the neat vinyl ester resin. The volume fraction of the hollow glass microspheres was found to have a more pronounced effect on the impact strength than the wall thickness. The energy absorbed until failure decreased with increase in the hollow glass microsphere volume fraction. The observed values decreased by 50–72.2% depending on the hollow glass microsphere volume fraction and wall thickness. The failure feature of syntactic foams under the current testing condition is explained using finite element analysis. The failure initiates from the tensile region, propagates through the specimen and is deflected near the compression region. The microstructural failure features are examined using a scanning electron microscope and matrix cracking, hollow glass microsphere-matrix debonding, and crack deflection by hollow glass microspheres are observed to be the failure features. Since the cracks were deflected around the compression zone, all types of syntactic foams showed tensile failure features, which include prominent matrix fracture and lack of hollow glass microsphere crushing. The understanding of the variation of impact properties with respect to the hollow glass microsphere volume fraction and wall thickness can help in tailoring the properties of syntactic foams.

Nanomechanical Characterization of Canine Femur Bone for Strain Rate Sensitivity in the Quasistatic Range under Dry versus Wet Conditions.

As a strain rate-dependent material, bone has a different mechanical response to various loads. Our aim was to evaluate the effect of water and different loading/unloading rates on the nanomechanical properties of canine femur cortical bone. Six cross-sections were cut from the diaphysis of six dog femurs and were nanoindented in their cortical area. Both dry and wet conditions were taken into account for three quasistatic trapezoid profiles with a maximum force of 2000 μN (holding time = 30 s) at loading/unloading rates of 10, 100, and 1000 μN/s, respectively. For each specimen, 254 ± 9 (mean ± SD) indentations were performed under different loading conditions. Significant differences were found for the elastic modulus and hardness between wet and dry conditions (P < 0.001). No influence of the loading/unloading rates was observed between groups except for the elastic modulus measured at 1000 μN/s rate under dry conditions (P < 0.001) and for the hardness measured at a rate of 10 μN/s under wet conditions (P < 0.001). Therefore, for a quasistatic test with peak load of 2000 μN held for 30 s, it is recommended to nanoindent under wet conditions at a loading/unloading rate of 100–1000 μN/s, so the reduced creep effect allows for a more accurate computation of mechanical properties.

The effects of loading conditions and specimen environment on the nanomechanical response of canine cortical bone

Bone is a viscoelastic connective tissue composed primarily of mineral and type I collagen, which interacts with water, affecting its mechanical properties. Therefore, both the level of hydration and the loading rate are expected to influence the measured nanomechanical response of bone. In this study, we investigated the influence of three distinct hydration conditions, peak loads and loading/unloading rates on the elastic modulus and hardness of canine femoral cortical bone via nanoindentation. Sections from three canine femurs from multiple regions of the diaphysis were tested for a total of 670 indentations. All three hydration conditions (dry, moist and fully hydrated tissue) were tested at three different loading profiles (a triangular loading profile with peak loads of 600, 800 and 1000 μN at loading/unloading rate of 60, 80 and 100 μN/s, respectively; each test was 20s in duration). Significant differences were found for both the elastic modulus and hardness between the dry, moist and fully hydrated conditions (p≤0.02). For dry bone, elastic modulus and hardness values were not found to be significantly different between the different loading profiles (p>0.05). However, in both the moist and fully hydrated conditions, the elastic modulus and hardness were significantly different under all loading profiles (with the exception of the moist condition at the 600- and 800-μN peak load). Given these findings, it is critical to perform nanoindentation of bone under fully hydrated conditions to ensure physiologically relevant results. Furthermore, this work found that a 20-s triangular loading/unloading profile was sufficient to capture the viscoelastic behavior of bone in the 600- to 1000-μN peak load range. Lastly, specific peak load values and loading rates need to be selected based on the structural region for which the mechanical properties are to be measured.

Fillers and Reinforcements

Syntactic foams are two component materials consisting of matrix resin and hollow particles. Reinforced syntactic foams contain an additional reinforcing material. The density of syntactic foams can be tailored based on the appropriate selection of hollow particle density and volume fraction. Glass hollow particles have been a widely used filler material because their low thermal expansion coefficient provides syntactic foams with high dimensional stability. Hollow fly ash particles, called cenospheres, have also been used as filler material. Fly ash cenospheres are inexpensive and help in developing low cost syntactic foams. However, usually defects are present in the walls of cenospheres and the mechanical properties of cenosphere-filled syntactic foams are not as high as those filled with glass hollow particles at the same density level. Enhancement of mechanical properties of syntactic foams beyond those obtained by tailoring the matrix and hollow particles can be obtained by micro- and nano-scale reinforcement of the matrix material. This chapter discusses hollow particle parameters such as wall thickness and density. Structure and properties of various reinforcements including glass fibers, nanoclay, carbon nanofibers (CNFs), carbon nanotubes (CNTs), and rubber particles are also discussed.

Modeling and Simulation

Development of theoretical models is very important for syntactic foams. Numerous parameters are involved in syntactic foam design, which include matrix and particle material, particle volume fraction and wall thickness, and reinforcement material and volume fraction. To identify the parameters that would result in syntactic foam with desired set of mechanical and thermal properties, theoretical models can be very useful and cut down the need for experimentation. Several existing models applicable to particulate composites can be modified to include the particle wall thickness effect. Multiscale models that can include the nanofibers or particles along with hollow particles are not available yet. It is challenging to model syntactic foams that contain high volume fractions of hollow particle (close to packing limit) because of particle-to-particle interaction effects. Two models are used in this chapter to estimate the properties of multiscale syntactic foams. Both models are applicable to plain syntactic foams containing only hollow particles in matrix. Therefore, semi-empirical approach is adopted and the experimentally measured properties of nanofibers reinforced polymer are assigned to the matrix. The model predictions are validated with experimental results. Finite element analysis is especially illustrative in understanding the behavior of syntactic foams under the applied load. A validated finite element study conducted on a unit cell geometry comprising a hollow particle and a fiber showed that the particle wall thickness plays an important role in determining the stress distribution in microscale reinforced syntactic foam system. For syntactic foams containing thin-walled particles, the location of the maximum von-Mises stress exists within the particle, whereas above a critical wall thickness the location of the maximum stress shifts to the fiber. This pattern illustrates that the location of failure initiation can be tailored in syntactic foams.

Hollow Glass Microspheres in Thermosets—Epoxy Syntactic Foams

Rapid growth in applications of syntactic foams in transportation and oil industry has motivated research and development activities in polymer matrix syntactic foams. While a wide range of matrix and particle materials have been used in fabricating syntactic foams, this chapter specifically focuses on the properties of epoxy matrix glass hollow particle-filled syntactic foams. Among mechanical properties, tensile, compressive, and flexural properties are discussed. In addition, dielectric and thermal properties are also discussed. Data on these properties have been extracted from all available studies and compared with respect to the syntactic foam density to identify the common trends. All these properties of syntactic foams can be tailored either by the hollow particle volume fraction or by the wall thickness. The possibility of using these two parameters independently enables developing syntactic foams with two or more properties tailored at the same time. Such possibility allows developing multifunctional syntactic foams and tailoring their properties for a wide range of applications.

Image Based Model Development and Analysis of the Human Knee Joint

Developments in medical imaging and finite element analysis techniques have made it possible to conduct personalized studies on patients. The field of medical implants is especially benefitting from these advancements, where patient specific geometries can be created and analyzed. The present work is focused on using image based techniques for construction of solid models of human knee joints for finite element analysis. Accurate 3D solid models of the human cadaveric knee joint are developed based on a sequence of high resolution MRI images obtained from a Siemens 7T machine. The approach involves identification of various components of the knee joint such as the femur, tibia, femoral and tibial cartilage, and menisci of the tibio-femoral knee joint; construction of a 3D model; smoothing the geometries; meshing of geometry; and then performing finite element analysis. The focus of the present work is on understanding the effect of menisci on the stress and strain distribution in the knee joint. Availability of such image based modeling and analysis methods would help in designing effective meniscal implants.

Processing and Microstructure of Syntactic Foams

This chapter discusses processing methods for reinforced syntactic foams and the effect of processing parameters on the structure and properties of syntactic foams. Enhancement of the mechanical properties of syntactic foams can be achieved by incorporation of micro- or nano-scale reinforcements into the matrix material. Dispersion of nanoparticles and nanotubes and nanofibers in polymer resins is challenging. Mechanical, shear, and ultrasonic mixing techniques have been used for obtaining wetting and dispersion of nanoscale reinforcement in the matrix. Nanoparticle reinforcement may also provide unintentional effect of increased matrix porosity by stabilizing gas bubbles in polymer matrix, if the processing method is not carefully designed. The processing methods are also required to be efficient in promoting wetting of reinforcement by the matrix resin, breaking clusters without fracturing the reinforcement material, and obtaining uniform distribution of reinforcement in the matrix resin. In addition, the hollow particles should not be excessively fractured during the manufacturing process. This chapter provides an overview of various processing methods and the issues encountered during fabrication of reinforced syntactic foams.

Dynamic Mechanical Properties

Studying the influence of temperature and loading frequency on the behavior of syntactic foams is important because of its diverse set of applications. Dynamic mechanical analysis (DMA) is a widely used technique for measuring viscoelastic properties of materials over a range of temperatures and loading frequencies. The storage modulus and loss modulus determined in a DMA experiment measure the capacity of a material to store and dissipate energy, respectively. In general, the storage modulus of syntactic foams decreases with increasing temperature. This response was consistent between plain and reinforced syntactic foams. Study of storage modulus of vinyl ester/glass hollow particle syntactic foams at three different temperatures concluded that the neat resin has higher storage modulus than the syntactic foams below glass transition temperature (T g) but this trend is reversed above T g. Also, the room temperature (30 °C) storage modulus of syntactic foams increases with the increase in the wall thickness of hollow particles. The addition of nanoclay increased the storage modulus of epoxy matrix syntactic foams. The effect was attributed to the toughening of matrix resin by nanoclay particles. However, increased stiffness of nanoclay reinforced syntactic foams resulted in decreased loss modulus. Cyanate ester matrix syntactic foam with 4 vol. % of nanoclay showed higher storage modulus than the plain syntactic foams, owing to the restricted movement of the polymer chains which can be attributed to the good interaction between the nanoclay and the matrix resin.