Jacky C. Prucz

Thermo-elastic Stresses in Composite Beams with Functionally Graded Layer

Analytical expressions have been derived for the thermo-elastic stresses in a three-layered composite beam system whose middle layer is a functionally graded material (FGM). Continuous gradation of the volume fraction in the FGM layer is modeled in the form of an mth power polynomial of the coordinate axis in thickness direction of the beam. Solutions for beams with linear, quadratic, or cubic distributions of continuous gradation in the FGM layer can be obtained from a single algorithm, just by changing the value of the exponent ‘m’. A numerical scheme of discretizing the continuous FGM layer (in sublayers) and treating the beam as a discretely graded structure has also been developed. Appropriate expressions for the solution have been derived for the case of continuous power law gradation (mth power) of the FGM layer. The discretized FGM layer scheme has been shown to yield results that practically match those predicted analytically by the closed-form model.

Thermoelastic Stresses in Composite Beams with Functionally Graded Layer

Analytical expressions have been derived for the thermoelastic stresses in a threelayered composite beam system having a middle layer of functionally graded material (FGM). Continuous gradation of volume fraction in the FGM layer is taken in the form of the mth power of the coordinate axes in the thickness direction of the beam. Using a single algorithm, solutions for the beams with continuous gradation in the FGM layer of linear, quadratic, or cubic nature, can be obtained just by changing the value of ‘ m’. A numerical scheme of discretizing the continuous FGM layer (in sublayers) and treating the beam as a discretely graded structure has also been discussed. Appropriate expressions for the solution have been derived for the power law gradation (mth power) of the FGM layer. A discretized FGM layer scheme has been shown to give results practically matching with the results obtained analytically.

Microstructure Modeling of Particulate Reinforced Metal Matrix Composites.

The study of microscopic and macroscopic response of a particulate reinforced MMC using finite element analysis is the aim of the current study. In this regard, two types of microstructure models are subjected to FE analysis. In the first part of the work, a technique is presented for the generation of artificial microstructure containing spherical and ellipsoid shaped inclusions. The problem of detection of ellipsoidal intersection is tackled using newly available algorithms. The FE analysis of the artificial microstructure and a summary of the results form the second part of the study. It is seen that the results from the newly developed models agree very well with the published results and that the microstructure generation technique can be reused in many computational micromechanics problems with minimum modifications.

Multi-Fiber Unit Cell for Prediction of Residual Stresses in Continuous Fiber Composites

The mechanical properties of Continuous Fiber Metal Matrix Composite (CFMMC) materials are often affected by the residual stresses that arise during their fabrication process as a result of the mismatch between the Coefficients of Thermal Expansion (CTE) of the fibers and matrix. Three-dimensional finite element Unit Cell Models (UCM) are commonly used to predict such residual stress fields. However, the boundary conditions chosen in the past for these models neglect the effects of interactions between neighboring fibers, which results in poor correlations with experimental results. Previous research shows that the UCM approach usually overestimates the residual stress levels by about 30%. In this paper, two new three-dimensional finite element Multi Fiber Models (MFM) are developed in which the boundary conditions are shifted away from the fiber-matrix interface, in order to account for the effects of neighboring fibers on the stress distribution over such interfaces. One model assumes a hexagonal packing pattern of the neighboring fibers around the fiber-matrix interface where the residual stresses are calculated, whereas the other assumes that the neighboring fibers are packed in a square pattern. The proposed models are examined for two different scenarios regarding the contact surface between the fiber and the matrix, one where there is no bond over the interface and the other where the interface is perfectly bonded. The residual stress predictions of the new MFM models are compared to those of conventional UCM models by using an Alumina/Titanium (Al 2 O 3 /Ti-6Al-4Va) material system to represent, as a test case, a typical CFMMC material. The results indicate that the residual stresses predicted by the MFM models correlate much better with published experimental results than those provided by the UCM models. The effects of the fiber volume fraction and the fiber-matrix bond integrity on the magnitude and distribution of residual stresses are examined for both hexagonal and square packing patterns of neighboring fibers. The analysis demonstrates that all these factors can influence significantly the field of residual stresses that develops in the matrix when the material is cooled-down from its processing temperature. If the fiber-volume fraction is assumed to increase, from 10% to 30% for example, the MFM models predict, as expected, a reduction in the magnitude of such residual stresses, which for the scenario of perfectly bonded interfaces can be as high as 49% for the square fiber packing pattern or 41% for the hexagonal pattern. The numerical results show that the hexagonal fiber-packing pattern predicts, in general, higher residual stresses than the square packing system, so that it should be recommended for design as the more conservative approach.

Dynamic Response of Composite Pressure Vessels to Inertia Loads

A new analytical model has been developed in order to investigate the potential benefits of using fiber-reinforced composites in pressure vessels that undergo rigid-body motions. The model consists of a quasi-static lamination analysis of a cylindrical, filament-wound, pressure vessel, combined with an elastodynamic analysis that accounts for the coupling effects between its rigid-body motion and its elastic deformations. The particular type of motion investigated in this paper is that of an oil-pressurized, tubular connecting rod in a slider-crank mechanism of an internal combustion engine. A comprehensive parametric study has been focused on the maximum wall stresses induced in such a rod by the combined effect of internal pressure and inertia loads associated with its motion. The numerical results illustrate potential ways to reduce these stresses by appropriate selection of material systems, lay-up configurations and geometric parameters.

Role of oversized dopant potassium on the nanostructure and thermoelectric performance of calcium cobaltite ceramics

The impact of the non-stoichiometric addition of potassium (K) on the nanostructure and thermoelectric performance of misfit layered calcium cobaltite (Ca3Co4O9) ceramics is reported. The samples were prepared with the designed nominal composition of Ca3Co4O9Kx (x = 0, 0.05, 0.1, 0.15, and 0.2). The K addition promoted the crystal growth and improved the crystal texture. The nanostructure and chemical analysis revealed the segregation of K at the Ca3Co4O9 grain boundaries, while the Ca3Co4O9 grain interior was free of K. At the optimal doping level, the dopant K grain boundary segregation reduced the electrical resistivity and simultaneously increased the Seebeck coefficient, resulting in a large increase in the power factor. At 320 K, the sample Ca3Co4O9K0.1 achieved the power factor of 930 μW K−2 m−1, which is 2.25 times higher than 412 μW K−2 m−1 from pristine Ca3Co4O9 and by far, the highest power factor at room temperature regime for the Ca3Co4O9 ceramics. The impact of the dopant segregation on the ionic diffusion along the grain boundaries and its resultant thermoelectric performance enhancement of Ca3Co4O9 ceramics are discussed.

Effect of FWD Testing Position on Modulus of Subgrade Reaction

This paper discusses the variation of the Modulus of subgrade reaction (k) backcalculated from slab deflection basins, interactive with the location of the Falling Weight Deflectometer (FWD) load pulse, and curling of slabs due to daily temperature variations. The k-value was calculated following the AASHTO design guides procedures, while deflection basins were recorded at an interval of 3 to 4 hours along the day on an instrumented concrete pavement test section in West Virginia. The state of deformation of the slabs are continuously monitored, through dowel bar bending measurements and records of the temperature gradient profiles through the slab thickness, as well as joint openings every 20 minutes. The results indicated that the backcalculated k-values are greatly affected by the positive temperature gradient, and the least variation in (k) was found in the slab center. In order to minimize errors in back-calculations of k-values, it is recommended to perform the FWD test for recording deflection basins in the interior of the slab during late evening or in the early morning.

High Lift Circulation Controlled Helicopter Blade

Circulation control techniques have a long history of applications to fixed wing aircraft. General aviation has used circulation control to delay flow separation and increase the maximum lift coefficient achievable with a given airfoil. These techniques have been gradually expanded to other applications, such as ground vehicles, to reduce drag. Circulation control technology can, potentially, be applied also to each blade of the main rotor in a helicopter, in order to increase the lift capacity of the rotor. Applications of circulation control technologies to fixed wing aircraft have demonstrated the potential of a three-fold increase in the lift coefficient, as compared to a conventional airfoil. This finding would suggest that a rotorcraft equipped with circulation control of the main rotor blades could, conceivably, lift up a payload that is approximately three times heavier than the maximum lift capacity of the same helicopter without circulation control. Alternatively, circulation control could reduce the required rotor diameter by up to 48%, if the maximum lift capacity remains unaltered. A High Lift, Circulation Controlled Helicopter Blade will be undergoing initial testing in the subsonic wind tunnel facility at West Virginia University. Two-dimensional elliptic airfoil models with air blowing slots for circulation control will be used as specimens in these tests in order to determine the aerodynamic changes, especially in lift and drag forces, achievable with various blowing slot configurations. Based on the results of the wind tunnel testing, an improved, detailed design will be developed for the entire main rotor of a helicopter with circulation control.Copyright

A Parametric Study on Particulate Al-SiC Composite Bolted Joints

The main objective of this study is to predict theoretically the stress distributions around the holes in a bolted joint made of particulate metal matrix composite and to investigate the associated load transfer efficiencies both for a single and double lap bolted joints. A three-dimensional finite element parametric model has been developed to examine the effects of various design parameters on the structural performance of such joints. The main feature of this model is explicit modeling of the sliding interfaces between the connected plates and the washers, and those between the hole and the bolt. The model response showed an excellent agreement with a closed form solution as well as experimental data. The results indicated that unsymmetric configuration of single lap joints causes bending as the load is applied, which is opposite of the double lap joints. This research quantifies the relationship between the stress developed around the hole and washer diameter, tightening pressure, and clearance between the bolt and hole. It was also observed that variations in Youngs modulus have no significant effect on the stress concentration around the hole.Copyright

Fatigue of quasi-isotropic composite cylinders under tension-tension loading

Life prediction techniques for structural composites are based on test data obtained from coupon specimens. A tension fatigue life prediction methodology based on a through-thickness damage accumulation model is used to predict the fatigue failure of composite coupon specimens. Its applicability to thin-walled cylindrical specimens is in vestigated by comparing the fatigue behavior of composite coupon specimens to that of composite thin-walled cylindrical specimens. AS4/3501-6 graphite-epoxy coupon speci mens and thin-walled cylinder specimens with the same layup sequence of [0/ ± 45/90] s were tested under static and fatigue loading conditions. Reasonably good agreement is found between the measured and predicted lives of the coupon specimens. Although the ultimate stresses of coupon and cylindrical specimens are different, it is observed that their fatigue lives at the same percentage of ultimate stress are nearly identical, independent of the coupon edge effect. It is concluded that the fatigue life prediction methodology for coupon specimens could be extended to estimate the fatigue life of the thin-walled graphite/epoxy composite cylinders used in this study.