Pitting corrosion initiation in AISI 316 austenitic stainless steel

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The initiation of pitting corrosion on AISI 316 stainless steel has been examined from a phenomenological viewpoint with emphasis on the role of the metal in this complex, interaction. A modified potentiodynamic technique was used to prepare specimens corresponding with a series of different points on the anodic polarisation curve for the material in 0.05 M sulphuric acid alone and with additions of 0.1 M sodium chloride solutions-. The specimens were subsequently examined using standard metallographic, techniques. It was found that suitable pit nuclei, called 'pit sites', are manifest as a result of the initial interaction of the metal with the solution at the rest potential, (ER), i.e., at potentials far below the potential range in which catastrophic pitting processes normally occur. It was further found that these pit sites were manifest even in the absence of chloride ions for which there is no subsequent catastrophic pitting process, Estimation of pit site density (NA) for the different stages of the E-i curve and the use of a simple stereological model permit a statistical interpretation of the localisation of the phenomenon to particular areas of the metal surface. The statistical argument is extended to show that the breakdown potential for chloride media is associated with the development of a catastrophic condition which does not apply if chloride ions are absent and it is deduced that the breakdown potential is essentially indeterminate.FONINVES, Caracas (Venezuela

Construction of Exact Solutions to Nahm's Equations for the Multimonopole

We construct high rank solutions to Nahm's equations for boundary conditions that correspond to the Dirac multimonopole. Here, the spectral curve is explicitly known and we achieve the integration by constructing a basis of polynomial tuples that forms a frame for the flow of the eigenline bundle over the curve

Active Damping Using Distributed Anisotropic Actuators

A helicopter structure experiences substantial high-frequency mechanical excitation from powertrain components such as gearboxes and drive shafts. The resulting structure-borne vibration excites the windows which then radiate sound into the passenger cabin. In many cases the radiated sound power can be reduced by adding damping. This can be accomplished using passive or active approaches. Passive treatments such as constrained layer damping tend to reduce window transparency. Therefore this paper focuses on an active approach utilizing compact decentralized control units distributed around the perimeter of the window. Each control unit consists of a triangularly shaped piezoelectric actuator, a miniature accelerometer, and analog electronics. Earlier work has shown that this type of system can increase damping up to approximately 1 kHz. However at higher frequencies the mismatch between the distributed actuator and the point sensor caused control spillover. This paper describes new anisotropic actuators that can be used to improve the bandwidth of the control system. The anisotropic actuators are composed of piezoelectric material sandwiched between interdigitated electrodes, which enables the application of the electric field in a preferred in-plane direction. When shaped correctly the anisotropic actuators outperform traditional isotropic actuators by reducing the mismatch between the distributed actuator and point sensor at high frequencies. Testing performed on a Plexiglas panel, representative of a helicopter window, shows that the control units can increase damping at low frequencies. However high frequency performance was still limited due to the flexible boundary conditions present on the test structure

Markov Processes, Hurst Exponents, and Nonlinear Diffusion Equations with application to finance

We show by explicit closed form calculations that a Hurst exponent H that is not 1/2 does not necessarily imply long time correlations like those found in fractional Brownian motion. We construct a large set of scaling solutions of Fokker-Planck partial differential equations where H is not 1/2. Thus Markov processes, which by construction have no long time correlations, can have H not equal to 1/2. If a Markov process scales with Hurst exponent H then it simply means that the process has nonstationary increments. For the scaling solutions, we show how to reduce the calculation of the probability density to a single integration once the diffusion coefficient D(x,t) is specified. As an example, we generate a class of student-t-like densities from the class of quadratic diffusion coefficients. Notably, the Tsallis density is one member of that large class. The Tsallis density is usually thought to result from a nonlinear diffusion equation, but instead we explicitly show that it follows from a Markov process generated by a linear Fokker-Planck equation, and therefore from a corresponding Langevin equation. Having a Tsallis density with H not equal to 1/2 therefore does not imply dynamics with correlated signals, e.g., like those of fractional Brownian motion. A short review of the requirements for fractional Brownian motion is given for clarity, and we explain why the usual simple argument that H unequal to 1/2 implies correlations fails for Markov processes with scaling solutions. Finally, we discuss the question of scaling of the full Green function g(x,t;x',t') of the Fokker-Planck pde.Comment: to appear in Physica

Harvey Cushing: early use of tendon transfers for repair of foot deformity: A historical case report

We describe 4 cases of tendon transfers for correction of foot deformities, which were performed by Harvey Cushing in 1898