An emerging change across the science, technology, engineering, and mathematics curriculum is the implementation of online, or virtual laboratories as supplements or replacements to both homework assignments and laboratory exercises. To test the effectiveness of such labs, a web-based virtual laboratory on the topic of torsion of engineered and biological materials was developed. The lab contains extensive data sets, videos of experiments, narrated presentations on lab practice and theory and assignments. Flexibility of use is built into the lab by providing the capability for the web-pages to be tailored to the needs of a particular institution. The lab was implemented and evaluated in a standard, sophomore level statics, and strength of materials course. Results of the evaluation show that the virtual lab is clear, helped students with their understanding of torsion concepts, and offered a number of benefits. However students also rated hands-on labs to be more fun and more interesting. ß
We present a numerical study for the steady, coupled, hydrodynamic, heat and mass transfer of an incompressible micropolar fluid flowing over a nonlinear stretching sheet. The governing differential equations are partially decoupled using a similarly transformation and then solved by two numerical techniques – the finite element method and the finite difference method. The dimensionless translational velocity, microrotation (angular velocity), temperature and mass distribution function are computed for the different thermophysical parameters controlling the flow regime, viz the nonlinear (stretching) parameter, b, Grashof number, G and Schmidt number, Sc. All results are shown graphically. Additionally skin friction and Nusselt number, which provide an estimate of the surface shear stress and the rate of cooling of the surface, respectively, are also computed. Excellent agreement is obtained between both numerical methods. The dimensionless translational velocity (f′) for both micropolar and Newtonian fluids is shown to decrease with an increase in nonlinear parameter b. Dimensionless microrotation (angular velocity), g, generally increases with a rise in nonlinear parameter b (in particular in the vicinity of the wall) and decreases with a rise in convective parameter, G. The effects of other parameters on the flow variables are also discussed. The flow regime has significant applications in polymer processing technology and metallurgy.
Moisture diffusion properties of the polyimide HFPE-II-52 were determined using weight gain, weight loss, and swelling experiments over a temperature range of 25-2008C. Below 1008C, diffusivity was measured using standard weight loss and weight gain methods. Above 1008C, diffusivity is found by weight loss experiments performed by placing moisture saturated samples in an oven and recording weight loss dynamically. The diffusivity of the polyimide was found to obey the Arrhenius relation over the entire range of temperature. Weight gain experiments were performed to determine the equilibrium level of moisture absorbed by the polyimide as a function of relative humidity. Swelling experiments were performed to measure swelling strain as a function of moisture absorption.
ABSTRACT:The polyimide HFPE-II-52 was developed at NASA Glenn Research Center for use as a matrix in high temperature composite materials. The unique properties of such composites stem largely from the performance of the matrix at high temperature. Thus, as part of a larger effort to study high temperature composite materials, the linear viscoelastic properties of HFPE are measured and a mathematical model of the properties is developed. In particular, storage, loss, and stress relaxation moduli were obtained from cyclic and transient loading experiments. A Prony series was fit to the relaxation modulus data. As a cross check, the fit to the relaxation modulus was converted to storage and loss moduli and compared with those measured directly. Effects of postcuring and of moisture on the properties are investigated as well. These results provide researchers with a constitutive model for HFPE-II-52 and provide some insight into the performance of HFPE matrix composites at high temperatures.
The shear strength of a 4-ply unidirectional composite laminate consisting of carbon fibers (T650-35) and a polyimide matrix (HFPE-II-52) was measured over a temperature range of 25 to 315 C. The tests were performed using a Iosipescu test sample, modified to provide a more uniform shear stress distribution across the gauge section and loaded with an Arcan type test fixture. The test specimen design is based on the results of an extensive finite element study. Shear strength tests were performed on dry, 50% RH and 100% RH moisture saturated samples. Results of the experiment show that shear strength decreases from approximately 120 MPa at 25 C to 60 MPa at 315 C and that moisture saturated samples have a 5-10 MPa lower shear strength than dry samples.
An electronic imaging system using a curved image sensor can use a faster lens, and cover a greater field of view, than an imaging system using a planar sensor. The simpler lens systems also weight less, a decisive advantage in portable applications.This paper describes a method to fabricate a curved silicon substrate from a flat wafer containing appropriate circuits. To curve the substrate, the processed wafer is diced, by dry-etching from the backside, into 1x1cm tiles. The tiles are separated by 0.5mm gaps, which are bridged, in turn, by a dense array of 45x100μm gold leads formed by electroplating using lithographically defined leads as seeds. Two methods were used to curve the wafer. In the first one, the wafer was bonded with epoxy to a PMMA disk, and then curved by heating the sandwich, under a load of ∼ 230gr, for 1.5 hrs at 130°C in a concave metal mold with a radius of curvature of 7.8cm. In the second method, the wafer was put into a curved metal mold, radius 14cm, loaded with 230gr, and heated to 290°C for 2 hrs. The normal and shear strains accommodated by the flexible interconnects were measured by analyzing their deformation. The experimentally measured strains are compared with a model that calculates the deformation required to deform a flat sheet into a spherical surface.
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