NASA Tech Briefs, February 1989

This issue contains a special feature on shaping the future with Ceramics. Other topics include: Electronic Components & and Circuits. Electronic Systems, Physical Sciences, Materials, Computer Programs, Mechanics, Machinery, Fabrication Technology, Mathematics and Information Sciences, and Life Sciences

An investigation into the prognosis of electromagnetic relays.

Electrical contacts provide a well-proven solution to switching various loads in a wide variety of applications, such as power distribution, control applications, automotive and telecommunications. However, electrical contacts are known for limited reliability due to degradation effects upon the switching contacts due to arcing and fretting. Essentially, the life of the device may be determined by the limited life of the contacts. Failure to trip, spurious tripping and contact welding can, in critical applications such as control systems for avionics and nuclear power application, cause significant costs due to downtime, as well as safety implications. Prognostics provides a way to assess the remaining useful life (RUL) of a component based on its current state of health and its anticipated future usage and operating conditions. In this thesis, the effects of contact wear on a set of electromagnetic relays used in an avionic power controller is examined, and how contact resistance combined with a prognostic approach, can be used to ascertain the RUL of the device. Two methodologies are presented, firstly a Physics based Model (PbM) of the degradation using the predicted material loss due to arc damage. Secondly a computationally efficient technique using posterior degradation data to form a state space model in real time via a Sliding Window Recursive Least Squares (SWRLS) algorithm. Health monitoring using the presented techniques can provide knowledge of impending failure in high reliability applications where the risks associated with loss-of-functionality are too high to endure. The future states of the systems has been estimated based on a Particle and Kalman-filter projection of the models via a Bayesian framework. Performance of the prognostication health management algorithm during the contacts life has been quantified using performance evaluation metrics. Model predictions have been correlated with experimental data. Prognostic metrics including Prognostic Horizon (PH), alpha-Lamda (α-λ), and Relative Accuracy have been used to assess the performance of the damage proxies and a comparison of the two models made

Pulsatile Electropolishing of Nitinol Stents

Alloys that oxidize easily such as those containing titanium or chromium present a challenge to electropolishing because the polarization that dissolves the metal species produces positive ions, these oxidize and form stable surface layers of metallic oxides that prevent further dissolution. This is usually overcome with the use of acid solutions that dissolve the metallic oxide. This thesis aims to shift the primary control of the electropolishing e_ect from electrolyte variables to a combination of potential variation and hydrodynamic interference. Traditionally this is achieved with one continuous mass removal process that operates after a steady state of dissolution is established, generally requiring hydro_uoric or phosphoric acid to achieve titanium dioxide breakdown. The resulting concentration gradient is heavily a_ected by electrolyte variables such as viscosity and electrical resistance, while the electrical polarization is constrained by the metallic oxide reaction rate which creates a complex net of interdependent variables that can be di_cult to tune. A rapidly changing electric _eld was applied to modulate the alloying element dissolution rates. In tandem with the electropolishing development, stages prior to the electropolishing step were selectively removed to simplify the process. Utilizing a three electrode system and an external potentiostat controller to permit greater _exibility, a variety of alternating current pulsatile waveforms were investigated and the resulting e_ect on surface topology was observed using SEM and AFM microscopes. Di_erential pulse voltammogram yielded a feedback parameter on surface composition, and various pulse parameters were adjusted to optimize for surface smoothness, and identify the primary control variable. An electropolishing method is presented which achieves a :50% reduction in the Sa surface roughness value to an area average of 45 nm on a laser cut tubular stent geometry. It is shown that this method can be adapted to eliminate the need for chemical etching or mechanical polishing prior to electropolishing. The resulting polished surface displays corrosion resistance equivalent or better than other electropolished Nitinol surfaces from literature with a breakdown potential >1V vs SCE, and a similarly high repassivation potential. Balancing the charge in the anodic and cathodic pulses was the key to minimizing the resulting surface roughness, and eliminating micropits. Nitinol is a nearly binary alloy of NiTi and a charge transfer ratio of 1 yielded the smoothest surfaces at current densities around :1 A/cm2. The initial surface condition was found to be irrelevant to electropolishing control with respect to oxide composition, provided enough mass was removed to fully dissolve the initial layers of mixed composition