Mahfuz, Hassan

With the goal of improving chemical detection methods for buried improvised explosive
devices (IED’s), the intention of this study is to show that functionalized nano-particles
improve the sensing properties of a polymer applied to gas sensors. The approach was
reinforcing the polymer, Nafion, with acid-functionalized carbon nanotubes (CNT’s).
Ammonia was chosen as the analyte for its similarity to IED byproducts without the
dangers of toxicity or explosion. Two sensor platforms were investigated: Quartz crystal
microbalances (QCM’s) and microcantilevers (MC’s). Preliminary evaluation of treated
QCM’s, via frequency analyzer, showed improvements in sensitivity and fast reversal of
adsorption; and suggested increased stability. Tests with coated MC’s also supported the
findings of QCM tests. Amplitude response of MC’s was on average 4 times greater
when the Nafion coating contained CNT’s. Quantitative QCM testing with gas-flow
meters showed that with CNT inclusion: the average number of moles adsorbed increased
by 35% (>1.2 times frequency response); sensitivity improved by 0.63 Hz/ppt on average; although the detection threshold decreased marginally; but reusability was
much better after extended exposures to concentrated ammonia.

Three-dimensional photoelastic stress analysis techniques are used to determine the stress distribution in
a metal-to-metal contact bolted flange. The flange belongs
to a thin-walled stage support casing of a jet aircraft
engine. Of special interest is the state of stress
experienced at flange separation due to axial and bending
loads during severe in-flight maneuvering. Details of model
development, data collection and discussion of results for
the stresses in the bolts and in the vicinity of the flange
are presented.

Equations of stress-difference elasticity, derived from the equations of equilibrium and compatibility for a two-dimensional stress field, are solved for arbitrarily digitized, singly and multiply connected domains. Photoelastic data determined experimentally along the boundary provide the boundary values for the solution of the three elliptic partial differential equations by the finite difference method. A computerized method is developed to generate grid mesh, weighting functions and nodal connectivity within the digitized boundary for the solution of these partial differential equations. A method is introduced to digitize the photoelastic fringes, namely isochromatics and isoclinics, and to estimate the values of sigma1 - sigma2, sigma x - sigma y and tau xy at each nodal point by an interpolation technique. Interpolated values of the stress parameters are used to improve the initial estimate and hence the convergence of the iterative solution of the system of equations. Superfluous boundary conditions are added from the digitized photoelastic data for further speeding up the rate of convergence. The boundary of the domain and the photoelastic fringes are digitized by physically traversing the cursor along the boundary, and the digitized information is scanned horizontally and vertically to generate internal and boundary nodal points. A linear search determines the nodal connectivity and isolates the boundary points for the input of the boundary values. A similar scanning method estimates the photoelastic parameters at each nodal point and also finds the points closest to the tint of passage of each photoelastic fringe. Stress values at these close points are determined without interpolation and are subsequently used as superfluous boundary conditions in the iteration scheme. Successive over-relaxation is applied to the classical Gauss-Seidel method for final enhancement of the convergence of the iteration process. The iteration scheme starts with an accelerating factor other than unity and estimates the spectral radius of the iteration matrix from the two vector norms. This information is used to estimate a temporary value of the optimum relaxation parameter, omega[opt], which is used for a fixed number of iterations to approximate a better value of the accelerating factor. The process is continued until two successive estimates differ by a given tolerance or the stopping criteria are reached. Detailed techniques of developing the code for mesh generation, photoelastic data collection and boundary value interpolation to solve the elliptic boundary value problems are presented. Three separate examples with varying stress gradients and fringe patterns are presented to test the validity of the code and the overall method. Results are compared with the analytical and experimental solutions, and the significant improvement in the rate of convergence is demonstrated.