Quasi-static stress fields for a crack inclined to the property gradation in functionally graded materials

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\u27s 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

Dynamic out of Plane Displacement Fields for an Inclined Crack in Graded Materials

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

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

Spontaneous Mixed-Mode Fracture in Bonded Similar and Dissimilar Materials

In this paper, we report on an experimental study of spontaneous, mixed-mode, crack propagation in weakly bonded similar and dissimilar materials. A unique experimental configuration is proposed to induce spontaneous crack growth events along the interfaces. The cracks nucleate from tiny circular holes and are triggered by an exploding wire. They subsequently propagate under the action of a constant, far-field load. Dynamic photoelasticity in conjunction with high speed photography is used to capture the real-time isochromatics associated with crack propagation. In the case of identical materials, crack propagation is anti-symmetric with respect to the crack nucleation point while strong asymmetry is observed for the case of dissimilar materials. In both cases, cracks propagate at constant velocity from the initiation point. The time histories of dynamic stress intensity factors and of energy release rates of the propagating cracks along the bonded similar materials are also reported

Sensitivity and dynamic electrical response of CNT-reinforced nanocomposites

A series of dynamic compressive experiments were performed to experimentally investigate the electrical response of multi-wall carbon nanotube (CNT)-reinforced epoxy nanocomposites subjected to split Hopkinson pressure bar (SHPB) loading. Low-resistance CNT/epoxy specimens were fabricated using a combination of shear mixing and ultrasonication. Utilizing the CNT network within, the electrical resistance of the nanocomposite was monitored using a high-resolution four-point probe method during each compressive loading event. In addition, real-time deformation images were captured using high-speed photography. The percent change in resistance was correlated to both strain and real-time damage. The results were then compared to previous work conducted by the authors (quasi-static and drop weight impact) in order to elucidate the strain rate sensitivity on the electrical behavior of the material. Furthermore, the percent change in conductivity was determined using a Taylor expansion model to investigate the electrical response based on both dimensional change as well as resistivity change during mechanical loading within the elastic regime. Experimental findings indicate that the electrical resistance is a function of both the strain and deformation mechanisms induced by the loading. The bulk electrical resistance of the nanocomposites exhibited an overall decrease of 40-65% and 115-120% during quasi-static/drop weight and SHPB experiments, respectively. © 2012 Springer Science+Business Media, LLC

Electrical behavior of carbon nanotube reinforced epoxy under compression

An experimental investigation was conducted to study the effect of quasi-static and dynamic compressive loading on the electrical response of multi-wall carbon nanotube (MWCNT) reinforced epoxy nanocomposites. An In-situ polymerization process using both a shear mixer and an ultrasonic processor were employed to fabricate the nanocomposite material. The fabrication process parameters and the optimum weight fraction of MWCNTs for generating a well-dispersed percolation network were first determined. Absolute resistance values were measured with a high-resolution four-point probe method for both quasi-static and dynamic loading. In addition to measuring the percentage change in electrical resistance, real-time damage was captured using high-speed photography. The real-time damage was correlated to both load and percentage change in resistance profiles. The experimental findings indicate that the bulk electrical resistance of the nanocomposites under both quasi-static and dynamic loading conditions initially decreased between 40%-60% during compression and then increased as damage initiated and propagated

Subsonic interfacial fracture using strain gages in isotropic-orthotropic bimaterial

An experimental study has been conducted in which strain fields were used to investigate the behavior of subsonic crack propagation along the interface of an isotropic-orthotropic bimaterial system. Strain field equations were developed from available field equations and critically evaluated in a parametric study to identify optimum strain gage location and orientation. Bimaterial specimens were prepared with PSM-1 polycarbonate and Scotchply® 1002 unidirectional, glass-fiber-reinforced, epoxy composite. Dynamic experiments were conducted using these specimens with strain gages mounted on the composite half to obtain values of the dynamic complex stress intensity factor, K=K1 +iK2, in the region of the crack tip while photoelasticity was used on the PSM-1 half. Results show that the trend and magnitude of K obtained using strain gages compare favorably with those obtained using photoelasticity. © 2002 Elsevier Science Ltd. All rights reserved

Experimental fracture mechanics of functionally graded materials: An overview of optical investigations

The experimental efforts towards understanding the fracture behavior of continuously graded Functionally Graded Materials (FGMs) using full-field optical methods are reviewed. Both quasi-static and dynamic fracture investigations involving mode-I and -II conditions are presented. FGM configurations with crack planes perpendicular to, parallel to, and inclined to the direction of compositional gradation are discussed. Different strategies adopted by various investigators to develop polymer-based FGM systems for experimental mechanics studies are also described in this overview. Major theoretical developments that have predated and paralleled the experimental studies have been presented as well. Finally, the paper notes a few potential new directions where further contributions are possible. © Society for Experimental Mechanics 2010

Electrical Response of Carbon Nanotube Reinforced Nanocomposites Under Static and Dynamic Loading

An experimental investigation was conducted to study the effect of quasi-static and dynamic compressive loading on the electrical response of multi-wall carbon nanotube (MWCNT) reinforced epoxy nanocomposites. An in-situ polymerization process using both a shear mixer and an ultrasonic processor were employed to fabricate the nanocomposite material. The fabrication process parameters and the optimum weight fraction of MWCNTs for generating a well-dispersed percolation network were first determined. Absolute resistance values were measured with a high-resolution four-point probe method for both quasi-static and dynamic loading. In addition to measuring the percentage change in electrical resistance, real-time damage was captured using high-speed photography. The real-time damage was correlated to both load and percentage change in resistance profiles. The experimental findings indicate that the bulk electrical resistance of the nanocomposites under both quasi-static and dynamic loading conditions initially decreased between 40%-60% during compression and then increased as damage initiated and propagated. © 2011 Society for Experimental Mechanics