Venkitanarayanan parameswaran | IIT Kanpur (original) (raw)
Papers by Venkitanarayanan parameswaran
Journal of Applied Mechanics-transactions of The Asme, 2009
Strain, 2008
Abstract: This paper focuses on the application of the digital image correlation (DIC) technique... more Abstract: This paper focuses on the application of the digital image correlation (DIC) technique to determine the stress intensity factor (SIF) for cracks in orthotropic composites. DIC is a full-field technique for measuring the surface displacements of a deforming object and can be applied to any type of material. To determine the SIF from full-field displacement data, the asymptotic expansion of the crack-tip displacement field is required. In this paper the expansion of the crack tip displacement field is derived from an existing solution for strain fields. Unidirectional fibre composite panels with an edge crack aligned along the fibre were tested under remote tensile loading and the displacements were recorded using DIC. The SIF was calculated from the experimental data by fitting the theoretical displacement field using the least squares method. The SIF thus determined was in good agreement with theoretical results and therefore demonstrates the applicability of the derived displacement field and DIC technique for studying fracture in composites.
Journal of Materials Science, 2003
A simple procedure was established to fabricate polyurethane-cenosphere particulate composite mat... more A simple procedure was established to fabricate polyurethane-cenosphere particulate composite materials. Composites having four different volume fractions of cenospheres (hollow ceramic microspheres) ranging from 10 to 40% in increments of 10% were prepared and their mechanical properties were evaluated. A predictive model to estimate the fracture toughness of the composite was developed. The dynamic constitutive behavior of the composite in compression was investigated using the split Hopkinson pressure bar (SHPB) technique in conjunction with high-speed photography. The results of the material characterization indicated that addition of cenospheres decreased the density of the composite. The quasi-static stiffness, both in tension and compression, and the quasi-static fracture toughness of the composite increased with addition of cenospheres. The high strain rate constitutive behavior of 100% polyurethane showed monotonic stiffening whereas the composite at higher cenosphere volume fractions (40%) exhibited a stiffening-softening-stiffening behavior. Scanning Electron Microscopy (SEM) studies were also carried out to determine the failure mechanisms of the composite.
A functionally gradient material (FGM) with discrete property variation is prepared, and the dyna... more A functionally gradient material (FGM) with discrete property variation is prepared, and the dynamic fracture in this material is studied using the technique of photoelasticity combined with high-speed photography. Transparent sheets required for the study are made by ...
Journal of Applied Mechanics-transactions of The Asme, 2002
ABSTRACT Stress field for stationary cracks, aligned along the gradient, in functionally graded m... more ABSTRACT Stress field for stationary cracks, aligned along the gradient, in functionally graded materials is obtained through an asymptotic analysis coupled with Westergaard's stress function approach. The first six terms of the stress field are obtained for both opening mode and shear mode loading. It is observed that the structure of the terms other than r-1/2 and r0 are influenced by the nonhomogeneity. Using this stress field, contours of constant maximum shear stress are generated and the effect of nonhomogeneity on these contours is discussed.
Optics and Lasers in Engineering, 2003
Static and dynamic fracture of interfaces between orthotropic and isotropic materials were studie... more Static and dynamic fracture of interfaces between orthotropic and isotropic materials were studied using photoelasticity. In this study, a bi-material specimen made of PSM-1 ® and Scotchply ® 1002, a unidirectional glass fiber reinforced epoxy composite, was used. Two fiber orientations, fibers parallel to the interface ( α=0°) and fibers perpendicular to the interface ( α=90°) were considered. Center crack bi-material specimens having different crack lengths were loaded quasi-statically and the full-field isochromatics were recorded using a digital camera. The complex stress intensity factor corresponding to each crack length was calculated from the isochromatics and the values were compared to that obtained from boundary collocation method. Dynamic interfacial fracture was studied with an edge crack bi-material geometry for the two different fiber orientations. The isochromatics around the propagating crack were recorded using a digital high-speed camera. The fracture parameters such as crack speed, complex stress intensity factor and energy release rate were extracted from the isochromatics using the asymptotic stress field equations. The complex stress intensity factor obtained from the static experiments was in close agreement with that calculated using the boundary collocation method. The results also indicated that the fiber orientation with respect to the interface influences the fracture parameters for stationary and propagating cracks.
Acta Mechanica, 2003
Quasi-static stress fields for a crack inclined to the direction of property gradation in functi... more Quasi-static stress fields for a crack inclined to the direction of property gradation in functionally graded materials (FGMs) are obtained through an asymptotic analysis coupled with Westergaard's stress function approach. The elastic modulus of the FGM is assumed to vary exponentially along the gradation direction. The mode mixity due to the inclination of the property gradient is accommodated in the analysis through superposition of opening and shear modes. The first four terms in the expansion of the stress field are derived to explicitly bring out the influence of nonhomogeneity on the structure of the stress field. Using these stress field contours of constant maximum shear stress, constant maximum principal stress, constant first stress invariant and constant out of plane displacement are generated, and the effect of inclination of the property gradation direction on these contours is discussed.
Optics and Lasers in Engineering, 2003
... 2.2. Determination of stress intensity factor for stationary crack by boundary collocation me... more ... 2.2. Determination of stress intensity factor for stationary crack by boundary collocation method. The bi-material model used for numerical study is shown in Fig. 1(a) with traction T y applied on the upper and lower boundaries. ...
Experimental Mechanics, 1999
An experimental investigation of the ballistic performance of composite armor with geometric modi... more An experimental investigation of the ballistic performance of composite armor with geometric modifications was carried out. The armor was simulated using polymeric materials. Four different geometric modifications were incorporated into the front plate of the armor, and two different adhesives were considered in this study. High-speed photography was employed to observe the real-time evolution of impact damage and to obtain the projectile penetration history. The nature and extent of damage for each modification and adhesive was estimated by postmortem inspection of the impacted armor and was compared to that obtained in unmodified armor of equal weight. The results of the study indicate that the geometric modifications after the nature and extent of damage significantly compared to conventional composite armor. The strong adhesive causes tearing of the back plate, whereas the compliant adhesive results in extensive delamination without any back plate damage. The modifications assist in spreading the damage laterally away from the impact site, thus distributing the load onto a larger area of the back plate. Calculations using a one-dimensional theoretical model also conclude that geometrical modifications improve the ballistic performance of the armor.
A technique for preparing model Functionally Gradient Materials (FGM) using polyester resin and c... more A technique for preparing model Functionally Gradient Materials (FGM) using polyester resin and cenospheres is developed. The cenosphere volume fraction in the polyester matrix is continuously varied through a buoyancy assisted casting process. FGMs having ...
Mechanics Research Communications, 2002
Asymptotic expansion for the out of plane displacement field around a crack propagating along the... more Asymptotic expansion for the out of plane displacement field around a crack propagating along the gradient in a functionally graded material is developed. The irregular behavior of one of the terms in the expansion at low crack speeds is further examined and a remedial solution, which is well behaved at low crack speeds, is proposed. The developed out of plane
Mechanics of Materials, 1999
An asymptotic expansion of the stress ®eld around a crack propagating at constant velocity in a F... more An asymptotic expansion of the stress ®eld around a crack propagating at constant velocity in a Functionally Gradient Material (FGM) is developed. All the three modes of crack propagation are analyzed for FGMs having two dierent types of property variations in the direction of crack propagation. The assumed property variations are (1) exponential variation of shear modulus and mass density and (2) linear variation of the shear modulus with constant mass density. The Poisson's ratio is assumed to be constant throughout the analysis. The analysis reveals that the cracktip stress ®elds retains the inverse square root singularity and only the higher order terms in the expansion are in¯uenced by the material nonhomogeneity. Expression for stresses and strains in the form of a series, in powers of the radial distance from the crack tip, is obtained for the tearing mode of fracture. For the opening and shear modes of fracture, an expression for the ®rst stress invariant under plane stress conditions is obtained in a series form in which the coecient of the ®rst term is proportional to the dynamic stress intensity factor. Contours of constant out of plane displacement, which is of interest in experimental techniques such as the coherent gradient sensing, are also given for dierent levels of nonhomogeneity. The stress ®elds are developed for large scale property variation where transient eects can be neglected. Ó
Journal of Testing and Evaluation, 2000
... (1) Dynamic Photomechanics Laboratory, Department of Mechanical Engineering and Applied Mecha... more ... (1) Dynamic Photomechanics Laboratory, Department of Mechanical Engineering and Applied Mechanics, University of Rhode Island, RI ... Junta recubierta ; Efecto dimensional ; Carga dinámica ;Onda tensión ; Onda elástica ; Propagación onda ; Resistencia cizallamiento ...
Journal of Elasticity, 2002
A generalized elastic solution for an arbitrarily propagating crack in Functionally Gradient Mate... more A generalized elastic solution for an arbitrarily propagating crack in Functionally Gradient Materials (FGMs) is obtained through an asymptotic analysis. The shear modulus and mass density of the FGM are assumed to vary exponentially along the gradation direction. The mode mixity due to the inclination of property gradient is accommodated in the analysis through superposition of the opening and shear modes. Using this asymptotic solution, contours of constant out of plane displacement are generated. The effect of inclination of property gradation direction and the crack speed on these contours is discussed.
Journal of Materials in Civil Engineering, 2004
Asphalt concrete is a heterogeneous material composed of aggregates, binder cement, and air voids... more Asphalt concrete is a heterogeneous material composed of aggregates, binder cement, and air voids, and may be described as a cemented particulate system. The load carrying behavior of such a material is strongly related to the local load transfer between aggregate particles, and this is taken as the microstructural response. Simulation of this material behavior was accomplished using a finite element technique, which was constructed to simulate the micromechanical response of the aggregate/binder system. The model incorporated a network of special frame elements with a stiffness matrix developed to predict the load transfer between cemented particles. The stiffness matrix was created from an approximate elasticity solution of the stress and displacement field in a cementation layer between particle pairs. A damage mechanics approach was then incorporated with this solution, and this lead to the construction of a softening model capable of predicting typical global inelastic behaviors found in asphalt materials. This theory was then implemented within the ABAQUS finite element analysis code to conduct simulations of particular laboratory specimens. Experimental verification of the elastic response has included tests on specially prepared cemented particulate systems, which allowed detailed measurement of aggregate displacements and rotations using video imaging and computer analysis. Model simulations compared favorably with these experimental results. Additional simulations including inelastic behavior of laboratory indirect tension tests have been conducted, and while preliminary in nature these results also compared well with experimental data.
Journal of Mechanical Science and Technology, 2002
In the present study, interfacial cracks between an isotropic and orthotropic material, subjected... more In the present study, interfacial cracks between an isotropic and orthotropic material, subjected to static far field tensile loading are analyzed using the technique of photoelasticity. The fracture parameters are extracted from the full-field isochromatic data and the same are compared with that obtained using boundary collocation method. Dynamic photoelasticity combined with high-speed digital photography is employed for capturing the isochromatics in the case of propagating interfacial cracks. The normalized stress intensity factors for static cracks are greater when α=90° (fibers perpendicular to the interface) than when α=0° (fibers parallel to the interface), and those when α=90° are similar to ones of isotropic material. The dynamic stress intensity factors for interfacial propagating cracks are greater when α=0° than α=90°. For the velocity ranges (0.1c/c s1 K D |, I increases with crack speedc, however, the rate of increase of |K D | with crack speed is not as drastic as that reported for homogeneous materials.
Composites Science and Technology, 2008
Epoxy composites with 200nm thick alumina platelets as reinforcements were prepared and the effec... more Epoxy composites with 200nm thick alumina platelets as reinforcements were prepared and the effect of platelet volume fraction and functionalization of the platelets on the mechanical and fracture properties of the composites was investigated. The use of as-received ...
Inclusion of nano-sized alumina and titania fillers in epoxy resin has been found to increase the... more Inclusion of nano-sized alumina and titania fillers in epoxy resin has been found to increase the resistance of the material to surface degradation, when exposed to surface discharges. In this paper, the work was extended to include Alumina platelets as opposed to spherical γ -alumina nanoparticles. Nanoparticles with diameter of range 40-47 nm and platelets 200 nm in thickness and 5-10 µm in length were used as fillers to prepare polymer nanocomposites. After preparation, the polymer composites were investigated and characterized by Fourier Transform Infra-Red (FTIR) spectroscopy and Nuclear Magnetic Resonance (NMR) spectroscopy. The specimens were then subjected to degradation by continuous exposure to surface discharges under high electric stresses. The degraded surfaces were studied using a Scanning Electron Microscope (SEM). Surface roughness measurements were made using a Laser Surface Profilometer (LSP). Additionally, surfactants like Sodium Dodecyl Sulfate (SDS), functionalization of particles with 3-glycidoxypropyltrimethoxysilane (GPS), and other particle processing techniques were utilized in preparation of nanocomposites being investigated. Nanocomposite specimens prepared with preprocessed nanoparticles were exposed to surface degradation tests in order to determine their effect on the bulk material properties. It was concluded that polymer composites with spherical nanoparticles as fillers have better resistance to surface degradation over unfilled epoxy, as well as polymer composites prepared with platelets. While use of surfactants like SDS in preparation of the nanocomposite failed to enhance its surface degradation properties as compared to unprocessed nanoparticles, pre-processing of the nanoparticles before use were seen to yield better results.
IEEE Transactions on Dielectrics and Electrical Insulation, 2009
In this work, the complex permittivity of epoxy resin is measured. Earlier, we have shown that th... more In this work, the complex permittivity of epoxy resin is measured. Earlier, we have shown that the inclusion of nanometric alumina particles (Al2O3), both as-received and pre-processed, improves the ability of a polymeric dielectric (epoxy) to resist degradation when exposed to surface discharges. In this work, we use dielectric spectroscopy to characterize neat epoxy (unfilled) and epoxy nanocomposites prepared with as-received and pre-processed Al2O3 nanoparticles. The dielectric spectroscopy measurements and analyses are carried out in the frequency range of 10-3 Hz to 103 Hz and temperature range of 25degC to 90degC. Analyses of the data for neat epoxy indicate the presence of low frequency dispersion below 100 Hz. It is observed that the inclusion of nanoparticles lowers the effective real and imaginary permittivity of the composite material, at low temperatures. At higher temperatures, low permittivities are exhibited only by composites prepared with particles functionalized with silane before use. It is therefore seen that not only the presence of filler particles, but also the nature of the interface affects the dielectric properties.
Journal of Applied Mechanics-transactions of The Asme, 2009
Strain, 2008
Abstract: This paper focuses on the application of the digital image correlation (DIC) technique... more Abstract: This paper focuses on the application of the digital image correlation (DIC) technique to determine the stress intensity factor (SIF) for cracks in orthotropic composites. DIC is a full-field technique for measuring the surface displacements of a deforming object and can be applied to any type of material. To determine the SIF from full-field displacement data, the asymptotic expansion of the crack-tip displacement field is required. In this paper the expansion of the crack tip displacement field is derived from an existing solution for strain fields. Unidirectional fibre composite panels with an edge crack aligned along the fibre were tested under remote tensile loading and the displacements were recorded using DIC. The SIF was calculated from the experimental data by fitting the theoretical displacement field using the least squares method. The SIF thus determined was in good agreement with theoretical results and therefore demonstrates the applicability of the derived displacement field and DIC technique for studying fracture in composites.
Journal of Materials Science, 2003
A simple procedure was established to fabricate polyurethane-cenosphere particulate composite mat... more A simple procedure was established to fabricate polyurethane-cenosphere particulate composite materials. Composites having four different volume fractions of cenospheres (hollow ceramic microspheres) ranging from 10 to 40% in increments of 10% were prepared and their mechanical properties were evaluated. A predictive model to estimate the fracture toughness of the composite was developed. The dynamic constitutive behavior of the composite in compression was investigated using the split Hopkinson pressure bar (SHPB) technique in conjunction with high-speed photography. The results of the material characterization indicated that addition of cenospheres decreased the density of the composite. The quasi-static stiffness, both in tension and compression, and the quasi-static fracture toughness of the composite increased with addition of cenospheres. The high strain rate constitutive behavior of 100% polyurethane showed monotonic stiffening whereas the composite at higher cenosphere volume fractions (40%) exhibited a stiffening-softening-stiffening behavior. Scanning Electron Microscopy (SEM) studies were also carried out to determine the failure mechanisms of the composite.
A functionally gradient material (FGM) with discrete property variation is prepared, and the dyna... more A functionally gradient material (FGM) with discrete property variation is prepared, and the dynamic fracture in this material is studied using the technique of photoelasticity combined with high-speed photography. Transparent sheets required for the study are made by ...
Journal of Applied Mechanics-transactions of The Asme, 2002
ABSTRACT Stress field for stationary cracks, aligned along the gradient, in functionally graded m... more ABSTRACT Stress field for stationary cracks, aligned along the gradient, in functionally graded materials is obtained through an asymptotic analysis coupled with Westergaard's stress function approach. The first six terms of the stress field are obtained for both opening mode and shear mode loading. It is observed that the structure of the terms other than r-1/2 and r0 are influenced by the nonhomogeneity. Using this stress field, contours of constant maximum shear stress are generated and the effect of nonhomogeneity on these contours is discussed.
Optics and Lasers in Engineering, 2003
Static and dynamic fracture of interfaces between orthotropic and isotropic materials were studie... more Static and dynamic fracture of interfaces between orthotropic and isotropic materials were studied using photoelasticity. In this study, a bi-material specimen made of PSM-1 ® and Scotchply ® 1002, a unidirectional glass fiber reinforced epoxy composite, was used. Two fiber orientations, fibers parallel to the interface ( α=0°) and fibers perpendicular to the interface ( α=90°) were considered. Center crack bi-material specimens having different crack lengths were loaded quasi-statically and the full-field isochromatics were recorded using a digital camera. The complex stress intensity factor corresponding to each crack length was calculated from the isochromatics and the values were compared to that obtained from boundary collocation method. Dynamic interfacial fracture was studied with an edge crack bi-material geometry for the two different fiber orientations. The isochromatics around the propagating crack were recorded using a digital high-speed camera. The fracture parameters such as crack speed, complex stress intensity factor and energy release rate were extracted from the isochromatics using the asymptotic stress field equations. The complex stress intensity factor obtained from the static experiments was in close agreement with that calculated using the boundary collocation method. The results also indicated that the fiber orientation with respect to the interface influences the fracture parameters for stationary and propagating cracks.
Acta Mechanica, 2003
Quasi-static stress fields for a crack inclined to the direction of property gradation in functi... more Quasi-static stress fields for a crack inclined to the direction of property gradation in functionally graded materials (FGMs) are obtained through an asymptotic analysis coupled with Westergaard's stress function approach. The elastic modulus of the FGM is assumed to vary exponentially along the gradation direction. The mode mixity due to the inclination of the property gradient is accommodated in the analysis through superposition of opening and shear modes. The first four terms in the expansion of the stress field are derived to explicitly bring out the influence of nonhomogeneity on the structure of the stress field. Using these stress field contours of constant maximum shear stress, constant maximum principal stress, constant first stress invariant and constant out of plane displacement are generated, and the effect of inclination of the property gradation direction on these contours is discussed.
Optics and Lasers in Engineering, 2003
... 2.2. Determination of stress intensity factor for stationary crack by boundary collocation me... more ... 2.2. Determination of stress intensity factor for stationary crack by boundary collocation method. The bi-material model used for numerical study is shown in Fig. 1(a) with traction T y applied on the upper and lower boundaries. ...
Experimental Mechanics, 1999
An experimental investigation of the ballistic performance of composite armor with geometric modi... more An experimental investigation of the ballistic performance of composite armor with geometric modifications was carried out. The armor was simulated using polymeric materials. Four different geometric modifications were incorporated into the front plate of the armor, and two different adhesives were considered in this study. High-speed photography was employed to observe the real-time evolution of impact damage and to obtain the projectile penetration history. The nature and extent of damage for each modification and adhesive was estimated by postmortem inspection of the impacted armor and was compared to that obtained in unmodified armor of equal weight. The results of the study indicate that the geometric modifications after the nature and extent of damage significantly compared to conventional composite armor. The strong adhesive causes tearing of the back plate, whereas the compliant adhesive results in extensive delamination without any back plate damage. The modifications assist in spreading the damage laterally away from the impact site, thus distributing the load onto a larger area of the back plate. Calculations using a one-dimensional theoretical model also conclude that geometrical modifications improve the ballistic performance of the armor.
A technique for preparing model Functionally Gradient Materials (FGM) using polyester resin and c... more A technique for preparing model Functionally Gradient Materials (FGM) using polyester resin and cenospheres is developed. The cenosphere volume fraction in the polyester matrix is continuously varied through a buoyancy assisted casting process. FGMs having ...
Mechanics Research Communications, 2002
Asymptotic expansion for the out of plane displacement field around a crack propagating along the... more Asymptotic expansion for the out of plane displacement field around a crack propagating along the gradient in a functionally graded material is developed. The irregular behavior of one of the terms in the expansion at low crack speeds is further examined and a remedial solution, which is well behaved at low crack speeds, is proposed. The developed out of plane
Mechanics of Materials, 1999
An asymptotic expansion of the stress ®eld around a crack propagating at constant velocity in a F... more An asymptotic expansion of the stress ®eld around a crack propagating at constant velocity in a Functionally Gradient Material (FGM) is developed. All the three modes of crack propagation are analyzed for FGMs having two dierent types of property variations in the direction of crack propagation. The assumed property variations are (1) exponential variation of shear modulus and mass density and (2) linear variation of the shear modulus with constant mass density. The Poisson's ratio is assumed to be constant throughout the analysis. The analysis reveals that the cracktip stress ®elds retains the inverse square root singularity and only the higher order terms in the expansion are in¯uenced by the material nonhomogeneity. Expression for stresses and strains in the form of a series, in powers of the radial distance from the crack tip, is obtained for the tearing mode of fracture. For the opening and shear modes of fracture, an expression for the ®rst stress invariant under plane stress conditions is obtained in a series form in which the coecient of the ®rst term is proportional to the dynamic stress intensity factor. Contours of constant out of plane displacement, which is of interest in experimental techniques such as the coherent gradient sensing, are also given for dierent levels of nonhomogeneity. The stress ®elds are developed for large scale property variation where transient eects can be neglected. Ó
Journal of Testing and Evaluation, 2000
... (1) Dynamic Photomechanics Laboratory, Department of Mechanical Engineering and Applied Mecha... more ... (1) Dynamic Photomechanics Laboratory, Department of Mechanical Engineering and Applied Mechanics, University of Rhode Island, RI ... Junta recubierta ; Efecto dimensional ; Carga dinámica ;Onda tensión ; Onda elástica ; Propagación onda ; Resistencia cizallamiento ...
Journal of Elasticity, 2002
A generalized elastic solution for an arbitrarily propagating crack in Functionally Gradient Mate... more A generalized elastic solution for an arbitrarily propagating crack in Functionally Gradient Materials (FGMs) is obtained through an asymptotic analysis. The shear modulus and mass density of the FGM are assumed to vary exponentially along the gradation direction. The mode mixity due to the inclination of property gradient is accommodated in the analysis through superposition of the opening and shear modes. Using this asymptotic solution, contours of constant out of plane displacement are generated. The effect of inclination of property gradation direction and the crack speed on these contours is discussed.
Journal of Materials in Civil Engineering, 2004
Asphalt concrete is a heterogeneous material composed of aggregates, binder cement, and air voids... more Asphalt concrete is a heterogeneous material composed of aggregates, binder cement, and air voids, and may be described as a cemented particulate system. The load carrying behavior of such a material is strongly related to the local load transfer between aggregate particles, and this is taken as the microstructural response. Simulation of this material behavior was accomplished using a finite element technique, which was constructed to simulate the micromechanical response of the aggregate/binder system. The model incorporated a network of special frame elements with a stiffness matrix developed to predict the load transfer between cemented particles. The stiffness matrix was created from an approximate elasticity solution of the stress and displacement field in a cementation layer between particle pairs. A damage mechanics approach was then incorporated with this solution, and this lead to the construction of a softening model capable of predicting typical global inelastic behaviors found in asphalt materials. This theory was then implemented within the ABAQUS finite element analysis code to conduct simulations of particular laboratory specimens. Experimental verification of the elastic response has included tests on specially prepared cemented particulate systems, which allowed detailed measurement of aggregate displacements and rotations using video imaging and computer analysis. Model simulations compared favorably with these experimental results. Additional simulations including inelastic behavior of laboratory indirect tension tests have been conducted, and while preliminary in nature these results also compared well with experimental data.
Journal of Mechanical Science and Technology, 2002
In the present study, interfacial cracks between an isotropic and orthotropic material, subjected... more In the present study, interfacial cracks between an isotropic and orthotropic material, subjected to static far field tensile loading are analyzed using the technique of photoelasticity. The fracture parameters are extracted from the full-field isochromatic data and the same are compared with that obtained using boundary collocation method. Dynamic photoelasticity combined with high-speed digital photography is employed for capturing the isochromatics in the case of propagating interfacial cracks. The normalized stress intensity factors for static cracks are greater when α=90° (fibers perpendicular to the interface) than when α=0° (fibers parallel to the interface), and those when α=90° are similar to ones of isotropic material. The dynamic stress intensity factors for interfacial propagating cracks are greater when α=0° than α=90°. For the velocity ranges (0.1c/c s1 K D |, I increases with crack speedc, however, the rate of increase of |K D | with crack speed is not as drastic as that reported for homogeneous materials.
Composites Science and Technology, 2008
Epoxy composites with 200nm thick alumina platelets as reinforcements were prepared and the effec... more Epoxy composites with 200nm thick alumina platelets as reinforcements were prepared and the effect of platelet volume fraction and functionalization of the platelets on the mechanical and fracture properties of the composites was investigated. The use of as-received ...
Inclusion of nano-sized alumina and titania fillers in epoxy resin has been found to increase the... more Inclusion of nano-sized alumina and titania fillers in epoxy resin has been found to increase the resistance of the material to surface degradation, when exposed to surface discharges. In this paper, the work was extended to include Alumina platelets as opposed to spherical γ -alumina nanoparticles. Nanoparticles with diameter of range 40-47 nm and platelets 200 nm in thickness and 5-10 µm in length were used as fillers to prepare polymer nanocomposites. After preparation, the polymer composites were investigated and characterized by Fourier Transform Infra-Red (FTIR) spectroscopy and Nuclear Magnetic Resonance (NMR) spectroscopy. The specimens were then subjected to degradation by continuous exposure to surface discharges under high electric stresses. The degraded surfaces were studied using a Scanning Electron Microscope (SEM). Surface roughness measurements were made using a Laser Surface Profilometer (LSP). Additionally, surfactants like Sodium Dodecyl Sulfate (SDS), functionalization of particles with 3-glycidoxypropyltrimethoxysilane (GPS), and other particle processing techniques were utilized in preparation of nanocomposites being investigated. Nanocomposite specimens prepared with preprocessed nanoparticles were exposed to surface degradation tests in order to determine their effect on the bulk material properties. It was concluded that polymer composites with spherical nanoparticles as fillers have better resistance to surface degradation over unfilled epoxy, as well as polymer composites prepared with platelets. While use of surfactants like SDS in preparation of the nanocomposite failed to enhance its surface degradation properties as compared to unprocessed nanoparticles, pre-processing of the nanoparticles before use were seen to yield better results.
IEEE Transactions on Dielectrics and Electrical Insulation, 2009
In this work, the complex permittivity of epoxy resin is measured. Earlier, we have shown that th... more In this work, the complex permittivity of epoxy resin is measured. Earlier, we have shown that the inclusion of nanometric alumina particles (Al2O3), both as-received and pre-processed, improves the ability of a polymeric dielectric (epoxy) to resist degradation when exposed to surface discharges. In this work, we use dielectric spectroscopy to characterize neat epoxy (unfilled) and epoxy nanocomposites prepared with as-received and pre-processed Al2O3 nanoparticles. The dielectric spectroscopy measurements and analyses are carried out in the frequency range of 10-3 Hz to 103 Hz and temperature range of 25degC to 90degC. Analyses of the data for neat epoxy indicate the presence of low frequency dispersion below 100 Hz. It is observed that the inclusion of nanoparticles lowers the effective real and imaginary permittivity of the composite material, at low temperatures. At higher temperatures, low permittivities are exhibited only by composites prepared with particles functionalized with silane before use. It is therefore seen that not only the presence of filler particles, but also the nature of the interface affects the dielectric properties.