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The task of pattern recognition is one of the most recurrent tasks that we encounter in our lives. Therefore, there has been a significant interest of automating this task for many decades. Many techniques have been developed to this end, such as neural networks. Neural networks are excellent pattern classifiers with very robust means of learning and a relatively high classification power. Naturally, there has been an increasing interest in further improving neural networks’ classification for complex problems. Many methods have been proposed. In this thesis, we propose two novel ensemble approaches to further improving neural networks’ classification power, namely paralleling neural networks and chaining neural networks. The first seeks to improve a neural network’s classification by combining the outputs of a set of neural networks together via another neural network. The second improves a neural network’s accuracy by feeding the outputs of a neural network into another and continually doing so in a chaining fashion until the error is reduced sufficiently. The effectiveness of both approaches has been demonstrated through a series of experiments. iv

The experimental evaluation of advanced mechanical properties that characterize the behavior of emerging materials for aerospace structures under a broad range of conditions representative of those encountered in service was emphasized in this investigation. The information guided the development of life prediction methodologies which are based on the concept of damage tolerance as a design philosophy. The research effort was subdivided into four interrelated tasks: (a) material characterization tests, (b) experimental procedure and test developments, (c) analytical and modeling developments, and (d) test support activities and data archival. Material characterization tests for fatigue, creep, and crack growth responses were conducted on a titanium matrix composite reinforced with continuous SiC fibers and fabricated from a foil-fiber-foil layup. The molybdenum wire used in the weaving of the fiber mats was a major factor in the development of damage under fatigue and creep test conditions. A model ceramic matrix composite composed of an aluminosilicate glass matrix and SiC fiber tows provided some interesting fatigue and fracture toughness responses. Behavior of monolithic materials were evaluated under creep, creep crack growth, fatigue crack growth, and oxidation conditions. Significant developments in test procedures and systems have been accomplished with emphasis on conducting thermomechanical fatigue (TMF) tests.

Crack growth of Titanium intermetallics at elevated temperatures has been studied. Based on limited amount of test data available in the literature, it is concluded that, for Titanium aluminides at temperatures over 700 C, Ct correlates better with da/dt than the LEFM parameter K. An alternative definition of Ct has also been derived in this study. The new Ct definition is based on a line integral along the outer boundary of the whole structure. Such an alternative definition hsa made it easier to calculate Ct for structural components. Keywords: Titanium aluminides, Creep, Fracture mechanics, Crack growth, High temperature, Alloys. (js)

Analytical and experimental investigations have been performed to determine crack growth behavior of turbine engine disk and blade alloys under various loading conditions typical of service environments. Tests were performed at elevated temperature in vacuum also to evaluate the baseline crack growth behavior. Substructural analysis of creep rupture specimens were conducted to understand the basic mechanism of time-dependent elevated temperature deformation. The work performed can be divided into three categories-- development of experimental techniques, material characterization techniques, and data maintenance and test support activities. The capability of the IDG laser system to monitor extended gage length was analyzed and found feasible. New grips and specimens have been designed and fabricated for high temperature composite testing. Enhancements to existing systems for crack length and displacement measurements were made. A number of computer automation applications are also discussed. Keywords: Laser interferometer system, Compliance, fatigue crack closure; Surface crack; Fatigue crack growth.

Analytical and experimental investigations have been performed to determine crack growth behavior of turbine engine disk and blade alloys under various loading conditions typical of service environments. Tests were performed at elevated temperature in vacuum also to evaluate the baseline crack growth behavior. Substructural analysis of creep rupture specimens were conducted to understand the basic mechanism of time-dependent elevated temperature deformation. The work performed can be divided into three categories -- development of experimental techniques, material characterization techniques, and data maintenance and test support activities. The capability of the IDG laser system to monitor extended gage length was analyzed and found feasible. New grips and specimens have been designed and fabricated for high temperature composite testing. Enhancements to existing systems for crack length and displacement measurements were made. Keywords: Laser interferometer system; Compliance; Fatigue crack closure; Surface crack; Fatigue crack growth.

The design of high performance aircraft engines requires the development of reliable life-predication methodology to aid in the evaluation and safety of the critical gas-turbine engine components. A comprehensive multi- task research and development program was conducted to establish a sound life- prediction methodology. The program consisted mainly of : (1) Experimental investigations and analytical modeling of crack growth behavior; (2) Experimental investigation of mechanical properties and performance characteristics of structural materials; (3) Environmental effect on elevated temperature mechanical behavior; and (4) Development of test techniques, upgrading, and maintenance of the AFWAL/MLLN Mechanical Test Facility. Keywords: Automated test control; Material characterization; Nickel base superalloys; Laser interferometry; Electric potential.

Analytical and experimental investigations have been performed to determine crack growth behavior of metals under conditions typical of the service environments of aircraft gas turbine engines. To evaluate baseline crack growth behavior, investigations have also been conducted at elevated temperature in vacuum. The work performed can be divided into three general categories -- development of experimental and automation techniques, material characterization testing, and analytic studies. An automated laser interferometric displacement-measurement system using linear array cards has been developed. Enhancements of existing system for the measurement of crack lengths and displacements are described. A number of computer automation applications are also described. Fatigue and creep crack growth tests and creep rupture and hot corrosion tests were performed. Description of the experimental techniques and the results for these tests are included. Results of analytical investigations in both material modeling and methodology development are presented, including discussions of applications within the laboratory. Keywords: Nickel base superalloys; Single crystals; and Interferometry.

Available methods for accelerated testing of corrosion behavior yield results which are not sufficiently accurate or reliable for predicting the service life of aircraft components and materials which degrade or fail due to environmental attack. Research has been conducted in controlled atmospheres on the localized environmental enhancement of crack-growth rates of aerospace alloys in order to provide the basis for development of realistic accelerated corrosion tests. Slow-strain-rate, corrosion-fatigue, and rising-load experiments have been conducted on high-strength 4340 steel and 7075-T6 aluminum alloy using accelerating pollutants such as sulfur dioxide, nitrogen dioxide, surface salt, and ambient to 100%-relative-humidity (RH) air in a specially designed atmospheric chamber. (Author)

A borax-nitrite based inhibitor has been developed for incorporation into the Air Force Rinse Facility at MacDill Air Force Base. Chromate-based products, in combination with polyphosphates, have been reasonably effective against corrosion of ferrous and nonferrous metals and alloys and are presently the most widely used inhibitors. However, the use of chromates has been the subject of ecological concern, and the investigation discussed herein involves alternatives to chromates. The performance of nonchromate commercial inhibitors has been tested and a borax-nitrite based formulation developed which inhibits the corrosion of several ferrous and nonferrous metals and alloys. This mixture contains no chromates and offers important advantages over the chromate-polyphosphate combination. Corrosion-fatigue and crack-growth studies on aluminum alloys have shown a reduction of more than one order of magnitude in crack-growth rate in the presence of this inhibitor. Infact, the crack growth decelerates to the air-sample level with the use of the inhibitor, even in an aqueous environment of 0.1M NaCl solution. (Author)

A multifunctional borax-nitrite-base inhibitor has been developed for preventing corrosion damage in urinal, galley, and bilge areas of aircraft. Most of the corrosion tests were conducted in a synthetic urine consisting of more than twenty aggressive ingredients which was formulated in the laboratory. The corrosion behavior of this synthetic urine has been studied and found to be comparable to that of natural urine. A number of commercial inhibitors have been screened, and corrosion tests have been conducted with over two hundred formulations consisting of soluble, nontoxic, organic, and inorganic compounds. Several formulations have been developed which inhibit the corrosion of the most aggressive medium (urine) encountered in the bilge and related areas of aircraft. The results of corrosion-fatigue and crack-growth studies on aluminum alloys and high-strength steels confirm a reduction of nearly one-half an order of magnitude in the crack-growth rates in the presence of one of these formulations. (Author)