Detection of water ingress in composite sandwich structures: a magnetic resonance approach

Water ingress inside honeycomb sandwich panels during service has been linked to in-flight failure in some aircraft. There is an ongoing effort to develop nondestructive testing methods to detect the presence of water within the panels. Magnetic resonance (MR) represents an attractive approach in that it is sensitive to moisture. Using a unilateral MR sensor, testing can be applied directly to the surface of the panel. The viability of MR is demonstrated through laboratory imaging of both water within sandwich panels, as well as the adhesive itself. The detection of water using a one-sided handheld MR sensor is presented. It is shown that simple detection, as well as spatial localization of water within sandwich panels is possible.     

Temperature sensitivity of coaxial probe complex permittivitymeasurements: experimental approach

An experimental investigation of the temperature sensitivity of the Teflon dielectric semirigid coaxial probe used in complex permittivity measurements is presented. Measurements are performed over the frequency range extending from 100 MHz to 26.5 GHz using 2.2 mm and 3.6 mm coaxial probes at a number of temperatures. An acute sensitivity of the probe-tip geometry to temperature is revealed, along with its effect on measured complex permittivity. Measurements are further complicated by the nonlinear thermal phase response of the probe, which results in the appearance of hysteresis in the measured complex permittivity during thermal cycling. The potential for removing these errors through temperature correction and the use of the thermally stable probes is discussed     

Teaching transmission lines: a project of measurement and simulation

A junior-year project for a course in electromagnetic waves is described, including the theory, hardware, and basic measurements. The essence of the project is to simulate transmission-line properties based upon theory developed in the classroom and to measure those properties in the laboratory for comparison. The equipment chosen is readily available and inexpensive, but is used here to illustrate concepts usually requiring an expensive vector network analyzer. The transmission-line properties of insertion loss, input impedance, and crosstalk are measured as a function of frequency on Category 5 cable. The transmission-line phase shift, propagation velocity, attenuation, characteristic impedance, impedance under various transmission-line configurations, and crosstalk are modeled and measured. Measured and theoretical results are in good agreement, reinforcing the strength of the underlying theory for the student. Evaluation of the project over a three-year period with more than 120 students is very positive in terms of developing confidence in and understanding of this abstract material.     

Thru characteristics of a coaxial gap (FDTD model and measurements)

Thru characteristics of a coaxial cable interrupted by a small gap are modeled and measured. Finite-difference time-domain (FDTD) modeling is applied in cylindrical coordinates to semirigid coaxial cable and to the intervening gap material. Both dispersive and nondispersive gap materials are investigated. Gap loss and phase shift are accurately predicted by this two-dimensional model which accounts for TEM and TM modes in the gap and coaxial apertures. An application of the model is to establish reference data for thin sample permittivity or moisture measurements     

Distributed Fiber-Optic Sensor for Dynamic Strain Measurement

A novel Brillouin-based distributed sensing technique is presented that can obtain the strain profile of the entire sensing fiber with a single optical pulse. Experimentally obtained spectra show that this comb excited pump signal is capable of strain and temperature measurement in a dynamic system. The results show the expected sinusoidal strain profile for a 12 s periodic strain cycle applied to the test fiber. The strain distribution for the entire fiber was obtained in 256 mus , thereby demonstrating for the first time, the potential to measure dynamic strains up to 3.9 kHz with 12 m resolution. Increased acquisition speed comes at the expense of achievable spatial resolution.     

Properties and use of cycled grown OMVPE GaAs:Zn,GaAs:Se,and GaAs:Si layers for high-conductance GaAs tunnel junctions

Heavily doped GaAs layers for high conductance GaAs tunnel junctions have been grown by atmospheric pressure organometallic vapor phase epitaxy (OMVPE) using Zn as the dopant for thep ⁺ regions and either Se or Si as the dopant for then ⁺ regions. At a growth temperature of 700° C using a “cycled” growth technique for the Zn-dopedp ⁺⁺-GaAs layer, both the conductance and the peak current density of the tunnel diode has been increased by a factor of ∼65 compared to a tunnel junction with a continuously grown Zn-doped p⁺-GaAs. The conductance of the tunnel junction, which is maximized at a growth temperature of 650° C using cycled growth, is comparable to the best reported values for tunnel junctions grown by molecular beam epitaxy. Cycled growths forn ⁺ Se-doped regions are found to reduce the conductance of a tunnel junction by more than two orders of magnitude. However, cycled growth for the n⁺-GaAs regions with Si doping show no conductance degradation. A model based on incorporation sites of these dopants during OMVPE growth of GaAs is presented to account for the experimental observations.     

Designing Static Fields for Unilateral Magnetic Resonance by a Scalar Potential Approach

We present a method for designing single-sided magnets suitable for unilateral magnetic resonance (UMR) measurements. The method uses metal pole pieces to shape the field from permanent magnets in a target region. The pole pieces are shaped according to solutions to Laplace's equation, and can be designed using a combination of analytical methods and numerical optimization. The design leads to analytical expressions for the pole piece shapes and magnetic field. Here, we develop the method in Cartesian, polar, and spherical coordinates, and discuss the merits of each system. Finite magnet size has a substantial effect on the field quality in many cases, according to our simulations. We found that in order to achieve a compact magnet in which the static field closely matches that specified, a full 3-D design approach is necessary. A magnet designed by our method produces a static field with a constant gradient over a region 2 cm in diameter and 2 mm thick. This leads to a compact cylindrical magnet just over 11 cm in diameter, topped with a single metal pole piece. The design is validated through simulation. The simulated field is found to agree closely with that specified analytically through the design procedure     

The integral equation model and surface roughness signatures insoil moisture and tillage type determination

A proposed application of RADARSAT, C-band, HH-polarized, satellite-based, synthetic aperture radar (SAR) identifies agricultural tillages and soil moisture through multi-incidence angle observations. A procedure to accomplish this has been developed using the integral equation (IE) model and a roughness signature for each anticipated tillage type. Through a comparison of the IE model, using each roughness signature over a range of moisture levels, to the measured backscatter one can select the case with the least error. This procedure yields little confusion between rough and smooth tillages using scatterometer measurements of test plots where extensive moisture and roughness ground truth were obtained. There is, however, significant confusion in classification of tillages with similar roughness characteristics. The bare-field moisture level was determined with a mean error of 1.8% and a standard deviation of 5.8%, when the initial moisture level was constrained to ±10% of the actual level