Venkitanarayanan Parameswaran

Crack-tip stress fields for dynamic fracture in functionally gradient materials

Abstract An asymptotic expansion of the stress field 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 different 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 Poissons ratio is assumed to be constant throughout the analysis. The analysis reveals that the crack-tip stress fields retains the inverse square root singularity and only the higher order terms in the expansion are influenced 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 first stress invariant under plane stress conditions is obtained in a series form in which the coefficient of the first 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 different levels of nonhomogeneity. The stress fields are developed for large scale property variation where transient effects can be neglected.

Processing and characterization of a model functionally gradient material

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 cenosphere volume fraction varying from 0 to 0.45 over a length of 250 mm are prepared. The overall properties of the FGM are varied by adding plasticizer to the polyester matrix. The physical, elastic and fracture properties of the prepared FGMs are evaluated as a function of location to generate the property profiles. The results of the material characterization indicate that, the quasi-static and dynamic modulus of the material increases and the material density decreases in the direction of increasing cenosphere volume fraction. The quasi-static fracture toughness increases up to a certain volume fraction of cenospheres and then decreases. Fractographic analyses of the fractured specimens indicate a change in the fracture mechanism as the cenosphere volume fraction increases. Estimate of the composite modulus using the Halpin-Tsai model with porosity correction matches closely with the test results.

Dynamic fracture of a functionally gradient material having discrete property variation

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 casting a polyester resin mixed with varying amounts of plasticizer. The mechanical (quasi-static and dynamic) and optical properties of the material are evaluated as a function of the plasticizer content. Results of material characterization show that the fracture toughness increases with increasing plasticizer content, whereas the Youngs modulus decreases. The material fringe constant and the dynamic modulus are observed to be relatively insensitive to plasticizer content. The FGM is then prepared by casting together thin strips having different plasticizer content. The dynamic crack propagation phenomenon is studied for four different property variations along the crack propagation direction, and the effects of these property variations on crack speed, crack jump distance and dynamic stress intensity factor are investigated. Results of this investigation show that increasing the toughness in the direction of crack growth reduces the crack jump distance as compared to on increasing-decreasing toughness variation for the same initial energy.

Degradation of polymer dielectrics with nanometric metal-oxide fillers due to surface discharges

Recent research has indicated that dielectric properties of polymer insulating materials might be improved by the inclusion of nanosized particles dispersed in the polymer matrix. Insulating materials in power apparatus are often exposed to surface discharges in the course of normal operation. Surface degradation due to continued exposure to such discharges may cause deterioration of the surface, and could ultimately lead to catastrophic failure. The current work investigates the effect of inclusion of nanometric particles on the ability of a polymeric dielectric to resist degradation when exposed to surface discharges. The dielectric material used was epoxy resin, while nanosized alumina (Al2O3) and titania (TiO2) were used as fillers. Surface discharge tests were carried out on the specimens. The degraded surfaces were studied using a scanning electron microscope (SEM). Surface roughness measurements were made using a laser surface profilometer (LSP). It was observed that the degradation was greater for unfilled epoxy specimens than that for filled ones. Atomic force microscopy (AFM) and energy dispersive X-ray analysis (EDX) were used to identify surface changes in the dielectric material due to degradation. It has been conclusively shown that addition of even very small volume fractions of nanoparticles increases the resistance of the material to degradation due to surface discharges. A possible mechanism for surface degradation in nanocomposites has been proposed.

Asymptotic Stress Fields for Stationary Cracks Along the Gradient in Functionally Graded Materials

Stress field for stationary cracks, aligned along the gradient, in functionally graded materials is obtained through an asymptotic analysis coupled with Westergaards 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. ©2002 ASME

Improvement in surface degradation properties of polymer composites due to pre-processed nanometric alumina fillers

Insulating materials in power apparatus are often exposed to surface discharges in the course of normal operation, resulting in deterioration of the material surface. In an earlier work, the authors have shown that the inclusion of nanometric particles (Al2O3) improves the ability of a polymeric dielectric (epoxy) to resist degradation when exposed to surface discharges. In the current work, the effect of pre-processing the alumina nanoparticles before preparation of the composite, is investigated. Laser Surface Profilometry (LSP) was used to measure the degradation of the composite specimens after exposure to surface discharges. The use of a surfactant, viz. Sodium Dodecyl Sulfate (SDS) was found to be ineffective. However, the simple action of heating the nanoparticles before use, improved the resistance of the bulk composite to surface discharges. Further, the particles were functionalized using 3-glycidoxy-propyltrimethoxysilane (GPS). This process greatly enhanced the ability of the nanocomposite to resist surface degradation. In fact, best results were obtained when the particles were first heated and then coated with GPS. Fourier Transform Infra-Red (FTIR) spectroscopy and other techniques were used to investigate chemical changes at the particle-epoxy interfaces. A direct correlation was observed between the improvement of the resistance of the composite to surface degradation and the ability of the pre-processed nanoparticles to form strong bonds with the neighboring epoxy. Effect of pre-processing particles of larger dimensions (platelets) was negligible compared to nanoparticles, indicating the possible importance of the interfacial surface to volume ratio of the fillers.

Processing and mechanical characterization of lightweight polyurethane composites

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.

On the size and dielectric properties of the interphase in epoxy-alumina nanocomposite

In this work we have proposed a combined experimental-numerical technique to elicit important quantitative information about the dielectric properties of the interphase region surrounding a nanoparticle embedded in a polymer matrix. The method involves performing permittivity measurements through dielectric spectroscopy and combining with concurrent unit cell based Finite Element solutions of the electrostatic fields. The analysis has to be supplemented with some information about the degree of agglomeration in the nanocomposites which are estimated from electron microscopy done at several resolutions. For an epoxy matrix with 40 nm alumina nanoparticles, our analysis indicates the presence of an interphase region about 200 nm thick and having permittivity lower than that of epoxy. Heating the nanoparticles prior to the synthesis of the nanocomposite makes the interphase region thicker and lowers its permittivity slightly. At higher volume fractions of the nanoparticles, agglomeration sets in and creates regions of even higher volume fraction within the matrix. Our analysis also revealed that at 2% volume fraction of alumina nanoparticles, 600 nm sized agglomerates exist in the matrix leading to local particle volume fraction as high as 5%.

An experimental investigation of dynamic crack propagation in a brittle material reinforced with a ductile layer

Abstract The fracture behavior of a dynamically loaded edge crack in a brittle-ductile layered material, as a function of applied loading rate, was experimentally investigated. Layered specimens were prepared by sandwiching a thin layer of ductile aluminum between two thick layers of brittle Homalite-100. The layers were bonded using Loctite Depend 330 adhesive, and a naturally sharp edge crack was introduced in one of the Homalite-100 layers. These single-edge notched specimens were loaded in dynamic three-point bending using a modified Hopkinson bar. The fracture process was imaged in real time using dynamic photoelasticity in conjunction with digital high-speed photography, and the applied load and load-point displacement histories were determined from the strain signals recorded at two locations on the Hopkinson bar. The results of this study indicated two distinct mechanisms of dynamic failure, depending on the applied loading rate. At lower loading rates, the starter crack arrested on reaching the aluminum layer and then caused delamination along the aluminum–Homalite interface. On the contrary, as the loading rate was increased, interfacial delamination was followed by crack re-initiation in the Homalite layer opposite to the initial starter crack. It was determined that the times required for crack initiation, delamination and crack re-initiation decreased as the loading rate was increased. However, it was also observed that the applied load values associated with each event increased with increasing loading rate. These observations indicate that both the dynamic failure process and plausibly the failure mode transition are affected by the rate-dependent properties of Homalite, aluminum and the interfacial bond. Finally, based on the measured peak loads and the observed failure mechanisms it was concluded that the incorporation of a thin ductile reinforcement layer can increase both the overall fracture toughness and strength of a nominally brittle material.

Dielectric spectroscopy of epoxy resin with and without nanometric alumina fillers

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.