Vasanth Chakravarthy Shunmugasamy

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.

High strain rate response of rabbit femur bones

Strain rate dependence of the mechanical response of hard tissues has led to a keen interest in their dynamic properties. The current study attempts to understand the high strain rate characteristics of rabbit femur bones. The testing was conducted using a split-Hopkinson pressure bar equipped with a high speed imaging system to capture the fracture patterns. The bones were also characterized under quasi-static compression to enable comparison with the high strain rate results. The quasi-static compressive moduli of the epiphyseal and diaphyseal regions were measured to be in the range of 2-3 and 5-7GPa, respectively. Under high strain rate loading conditions the modulus is observed to increase with strain rate and attains values as high as 15GPa for epiphyseal and 30GPa for diaphyseal regions of the femur. The strength at high strain rate was measured to be about twice the quasi-static strength value. A large number of small cracks initiated on the specimen surface close to the incident bar. Coalescence of crack branches leading to fewer large cracks resulted in specimen fragmentation. In comparison, the quasi-static failure was due to shear cracking.

Influence of clinically relevant factors on the immediate biomechanical surrounding for a series of dental implant designs.

The objective of the present study was to assess the influence of various clinically relevant scenarios on the strain distribution in the biomechanical surrounding of five different dental implant macrogeometries. The biomechanical environment surrounding an implant, i.e., the cortical and trabecular bone, was modeled along with the implant. These models included two different values of the study parameters including loading conditions, trabecular bone elastic modulus, cortical/trabecular bone thickness ratio, and bone loss for five implant designs. Finite element analysis was conducted on the models and strain in the bones surrounding the implant was calculated. Bone volumes having strains in four different windows of 0-200 με, 200-1000 με, 1000-3000 με, and > 3000 με were measured and the effect of each biomechanical variable and their two-way interactions were statistically analyzed using the analysis of variance method. This study showed that all the parameters included in this study had an effect on the volume of bones in all strain windows, except the implant design, which affected only the 0-200 με and >3000 με windows. The two-way interaction results showed that interactions existed between implant design and bone loss, and loading condition, bone loss in the 200-1000 με window, and between implant design and loading condition in the 0-200 με window. Within the limitations of the present methodology, it can be concluded that although some unfavorable clinical scenarios demonstrated a higher volume of bone in deleterious strain levels, a tendency toward the biomechanical equilibrium was evidenced regardless of the implant design.

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.

High Strain Rate Compressive Behavior of Polyurethane Resin and Polyurethane/Al2O3 Hollow Sphere Syntactic Foams

Polyurethane resins and foams are finding extensive applications. Seat cushions and covers in automobiles are examples of these materials. In the present work, hollow alumina particles are used as fillers in polyurethane resin to develop closed-cell syntactic foams. The fabricated syntactic foams are tested for compressive properties at quasistatic and high strain rates. Strain rate sensitivity is an important concern for automotive applications due to the possibility of crash at high speeds. Both the polyurethane resin and the syntactic foam show strain rate sensitivity in compressive strength. It is observed that the compressive strength increases with strain rate. The energy absorbed up to 10% strain in the quasistatic regime is 400% higher for the syntactic foam in comparison to that of neat resin at the same strain rate.

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.

Clay/Polymer nanocomposites: Processing, properties, and applications

Clay/polymer nanocomposites have been extensively studied in recent years. The present state of the art for these materials is summarized in this chapter. The development of fabrication methods for these composites is very challenging because the platelets of nanoclay exist in the form of clusters, which need to be dispersed in the matrix resin in order to obtain any benefit from the high surface area of nanoclay. Incorporation of only a small weight fraction (1–5 %) of nanoclay in polymers provides significant benefits in the properties of composites. Several tensile, flexural, and thermal properties are found to increase by 30–40 % due to the presence of nanoclay in the composite compared to the properties of the neat resin. Entrapment of air porosity at higher nanoclay content can lead to reversal of the trends and can actually reduce the mechanical properties of nanocomposites. Theoretical models have been developed to estimate the properties of nanocomposites by accounting for the microstructure that may include clustered, intercalated, or exfoliated nanoclay. The benefit in mechanical properties obtained from incorporating nanoclay is much greater than that can be achieved with microscale reinforcement at the same loading levels. The current applications of clay/epoxy nanocomposites are in the area of automotive moldings and fire retardant coatings.

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.

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.