Influence of Stress Relaxation on Watertight Integrity of Hybrid Bolted Joints

Stress relaxation and creep are major concerns when loading composite materials. Due to the viscoelactic nature of the matrix material, composite materials tend to lose initial loads at a decreasing rate. This is especially true through the thickness of the material, where the behavior of the material is dominated by the matrix. Of particular interest to the current study presented in this thesis is the investigation of stress relaxation in bolted composite/metal hybrid connections. It is ultimately desired to be able to use bolted composite/metal hybrid connections for naval applications, where it is important to maintain as much of the initial preload in the connection as possible in order to maintain watertight integrity. In order to quantify the stress relaxation in bolted hybrid connections, it was decided to study the connections at a sub-component level. Several different effects were studied in the connections, including reloading effects, possible advantages of using tapered head bolts, and environmental effects. All tests were run for a time period of at least 3-month in order to get an estimate of long-term stress relaxation effects in the bolted connections. Test results showed that the load curves could be fit to a power law equation using the method of least squares. Reloading tests showed that some of the preload in the connections could be maintained with periodic retightening of the bolts. The tests also showed a large temperature dependence in the connections that were reloaded multiple times. Connections that are reloaded can maintain more of their initial preload, but are extremely sensitive to temperature shift, even small shifts of only 5 degrees Fahrenheit. Temperature shifts cause the connections to move at a much greater stress relaxation rate. In general, reloading the connections will help the connections to maintain their initial preloads, but great care has to be taken to avoid any temperature changes in the connections. Tapered head bolts were tested in some connections and compared to results obtained from non-tapered head bolts. Little to no advantage was seen when using tapered head bolts over non-tapered head bolts. The connections using tapered head bolts had roughly the same stress relaxation rate as the connections using non-tapered head bolts. Environmental testing has recently been started on the hybrid connections, and results are not yet available. Pilot tests were inconclusive, as they were run at 150 degrees Fahrenheit, and thus thermal expansion effects are indistinguishable from stress relaxation effects. Moisture also may have gotten into the gaged bolts and effected the pilot test results. The environmental tests should provide an accurate description of what happens to the bolted hybrid connections in naval applications. Once completed, environmental test results will be presented to the project sponsor

Active Galactic Nuclei as High Energy Engines

Active Galactic Nuclei are considered as possible sites of cosmic ray acceleration and some of them have been observed as high energy gamma ray emitters (Blazars). There naturally comes an appealing idea that the acceleration of the highest energy cosmic rays in the AGNs has a signature in the form of gamma ray emission and high energy neutrino emission through the collisions of very high energy protons with soft photons. Moreover it is often said that electrons cannot reach enough energy through Fermi acceleration to account for the highest energy photons observed with ground Cerenkov telescopes. In this paper, we discussed these points and show that the fast variability of the flares recently observed rules out the assumption of a Fermi acceleration of protons. We show that Fermi acceleration of electrons is enough to account for the gamma spectra, their shape, cut-off and their variability. Moreover the spectral break is nicely explained by invoking an gamma-ray photosphere. Nevertheless we give estimates of the high energy cosmic ray generation in AGNs and of the resultant neutrino flux, that turns out to be very sensitive to the spectral index of the proton distribution.Comment: 17 pages, 1 figure, accepted for publication in Astrophysics Particle

Largest eigenvalue distribution in the double scaling limit of matrix models: A Coulomb fluid approach

Using thermodynamic arguments we find that the probability that there are no eigenvalues in the interval (-s,\infty) in the double scaling limit of Hermitean matrix models is O(exp(-s^{2m+1})) as s\to+\infty.Here m=1,2,3.. determine the m^{th} multi-critical point of the level density:\sigma(x)\sim b[1-(x/b)^2]^{m-1/2} and b^2\sim N.Furthermore,the size of the transition zone where the eigenvalue density becomes vanishingly small at the tail of the spectrum is \sim N^{(m-3/2)/(2m+1)} in agreement with earlier work based on the string equation.Comment: 10 pages, no figures, to appear in J.Phys. A Lett. 199

Can distributed delays perfectly stabilize dynamical networks?

Signal transmission delays tend to destabilize dynamical networks leading to oscillation, but their dispersion contributes oppositely toward stabilization. We analyze an integro-differential equation that describes the collective dynamics of a neural network with distributed signal delays. With the gamma distributed delays less dispersed than exponential distribution, the system exhibits reentrant phenomena, in which the stability is once lost but then recovered as the mean delay is increased. With delays dispersed more highly than exponential, the system never destabilizes.Comment: 4pages 5figure