13 research outputs found
Quantitative Electromagnetic Modeling and NDE of Carbon-Carbon Composites
No full textThere is much need for investigating the use of eddy-current inspection with advanced composite materials, including graphite-epoxy and carbon-carbon. One of the problems in evaluating the performance of eddy-current inspection is that it is often difficult to characterize the conductivity of the fiber composite material. For example, when the material is composed of conducting fibers and a nonconducting matrix, as is the case with graphite-epoxy, the overall conductivity is a complicated quantity that depends on fiber conductivity, fiber density, fiber layup order (sample geometry), and the frequency at which the eddy-currents are being excited. Dependency on frequency and layup order, in particular, give the investigator much difficulty in interpreting any eddy-current data from experiments. If these two factors cause a weak effect, there may be a suitable range of frequencies for inspecting the material via application of somewhat standard techniques.</p
Detecting Cracks in Semiconductor Solarcells from Eddy-Current Measurements
No full textAs modern-day solar cells are made increasingly thin and large, it is very important for the manufacturer to be able to detect cracks in the finished product. When the cells are to be installed in spacecraft or large panel arrays, it is difficult or impossible to replace faulty cells. Small cracks present during manufacturing may not cause significant performance problems immediately; the affected cells may fail at a later time due to thermal cycling or physical shock. Visual, thermal, and infrared methods of inspection have many limitations and often miss detecting cracks. In this paper we present experimental results obtained from eddy-current measurements that were used to infer locations of cracks in solarcells. EMF measurements were made using primarily hand-wound sensors excited by various current sources. The eddy-current measurements indicated conducting traces on the cells and cracks in the material that could be visually verified. Our measurements were made under computer control, using phase-sensitive techniques to measure the amplitude and phase of the induced sensor signal. In some cases, digital enhancement of the images improved the detectability of the cracks.</p
A Linear Eddy-Current Array Driven by a Whip Excitation
No full textIn eddy-current inspection, it is often desirable to use an array of sensors. An array can alleviate problems such as excessive scanning time and difficulties in positioning the sensors. Our “whip” excitation source, a single loop of wire carrying current, is used to excite a workpiece for an eight-sensor array of air-core, inductive, pancake coils. The whip is designed such that it produces an excitation similar to that produced by a single uni-directional wire carrying current. This design is accomplished by forming a loop of rectangular cross-section, placed tangentially to the surface of the test material. Figure 1 shows a conceptual drawing of the whip excitation. Due to its proximity to the test material, the closest side of the whip to the material has a much greater excitation effect than the current’s return path.</p
Some Inversion Problems in Nondestructive Evaluation
No full textNondestructive evaluation (NDE) is to materials and structures what CAT scanning is to the human body---an attempt to look inside without opening the body. It is in nature an inversion problem. While such problems present a formidable mathematical challenge, sophisticated new models and software are beginning to yield useful results. In this paper we discuss the solution of several inversion problems using the eddy-current volume-integral code, VIC-3D1. These problems, the reconstruction of flaws in steam-generator tubing, the determination of metallic plate thicknesses, and the reconstruction of conductivity profiles versus depth in metallic materials, are typical of the application of eddy-current NDE to process control and in-service inspection.</p
Detecting Micro-Structure and Flaws in Composites Using Eddy-Current Instrumentation
No full textEddy-current inspection of advanced composite materials appears to overcome some of the difficulties typical of ultrasonic inspection. For example, carbon-carbon material can be inspected using eddy-currents without making the sample wet or using any acoustic coupling substances. Eddy-current inspection is very good for detecting fibers and fiber content because the fibers usually conduct electricity very well. Eddy-current measurements can therefore detect broken fibers in many cases where ultrasonic inspection fails to find the damage (broken fibers are not always good at scattering the ultrasonics). One of the common arguments against using eddy-currents to inspect composite materials is that they are unable to detect delaminations. This paper presents experimental evidence that suggests that delaminations can be detected with eddy-current inspection. Sabbagh [4] offer theoretical reasoning for the delamination behavior. This paper also presents some experimental data that indicates fiber structure, impact damage, and drilled holes in advanced composite material, including carbon-carbon and graphite-epoxy.</p
Deverbal nominals in Kiswahili: Underspecification morphology and the lexicon
No full textThis study provides an explicit account of the lexical entries and rules required to generate five common types of Kiswahili nouns based on verbal roots and stems, as illustrated by the data below, all derived from -kata 'cut'. (UNFORMATTED TABLE OR EQUATION FOLLOWS)\vbox{\halign{#\hfil&&\enspace#\hfil\cr&a.&Action Nominals in -{\it o}:&{\it mkato/mikato\/}&`cut(s)'\cr&b.&Transitive Verbals in -{\it a}:&{\it mkata/wakata}&`cutter(s)'\cr&c.&Relic Agentives in -{\it i}:&{\it mkati/wakati}&`cutter(s)'\cr&d.&Productive Agentives in -{\it aji}:&{\it mkataji/wakataji}&`cutter(s)'\cr&e.&Patient Nominals in -{\it e}:&{\it mkate/mikate}&`lump(s), loaf/loaves'\cr}}(TABLE/EQUATION ENDS)Underspecification Theory as developed by Archangeli & Pulleyblank (1986) is adopted to aid in this descriptive task. A rigorous analysis of the Kiswahili phoneme set is provided in this framework, with underspecified and fully specified representations of all phonemes as well as explicit default and complement rules. A detailed account of various types of prefix allomorphy is included, both for inherent nouns and for derived nominals.The apparent simplicity of the data above is deceptive, even disregarding the account provided of the stem and suffix allomorphy associated with the Relic Agentives. Issues confronted in the description include semantic drift, blocking, percolation, questions of derivational source, and archaic roots. A particular focus of the study is to streamline the lexical entries by avoiding the explicit specification of redundant features of all kinds.An innovation in the research is the application of the principles of Underspecification Theory beyond the phonology to capture redundancies in and among the morphological, semantic, and syntactic features and representations. To develop and illustrate this approach, a set of four binary morphological features is proposed to capture the Kiswahili noun-class system. A further innovation is the use of Lexical Cross-References (LXRefs) which capture generalizations relating to morphological complexes while allowing redundant information to be filled in automatically.Made available in DSpace on 2011-05-07T13:59:06Z (GMT). No. of bitstreams: 2
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Inversion of Eddy Current Data and the Reconstruction of Flaws, Part 1: Acquisition of Data
No full textMeasuring the eddy currents in a material induced by an exciting field can provide useful information about the shape of the material. Several methods of nondestructive evaluation using eddy currents do not utilize a uniform exciting field over the area of interest [1]. When a non-uniform exciting field is used, the presence or absence of a flaw in the material is detected. However, some applications require more specific information about the size and shape of the flaw. If reconstruction of the flaw is required, current mathematical algorithms [2,4] require that the magnetic field due to eddy currents induced by a uniform exciting field be accurately measured. The magnetic fields can be measured by placing small inductive pickup coils in the vicinity of the material. Several different frequencies can be used to take advantage of the skin depth effect in conductors. Low frequencies can be used to look for flaws relatively deep beneath the surface; high frequencies can be used to look for “shallow” flaws.</p
Solutions of Benchmark Problems in Eddy-Current NDE
No full textWe present solutions of some benchmark problems, (‘Benchmark Problems in Eddy-Current NDE,’ by Harold A. Sabbagh and Stephen K. Burke, presented at QNDE in 1991), for the computation of ΔZ in eddy-current NDE: - Rectangular slot in a thick plate (900 Hz) - Rectangular slot in a thick plate (7 kHz) - Cracks in a thin plate - Cracks in a double plate system</p
Application of Volume-Integral Models to Steam Generator Tubing
No full textThe nuclear power industry faces the serious challenge of convincing a skeptical public and regulatory agencies that it can operate safely and efficiently. Nondestructive evaluation (NDE) plays a significant role in this task, and computer modeling is playing a significant role in NDE. The industry now realizes the value of using such modeling to replace expensive experimental tests, as well as to design equipment, and interpret results. Eddy-currents have a traditional place in the inspection of steam generator tubing, and the industry seeks improved tools for such inspections. In this paper, we describe progress in developing a general axisymmetric model that will be part of the volume-integral code, VIC-3D1. This model will be capable of analyzing tubes with tube supports and rolled-expansion transition zones. Features such as magnetite, sludge, etc., will be included, and materials may be either ferromagnetic or non-magnetic. The model described in this paper will include only differential (or absolute) bobbin coils. Flaws can be of three types: (1) axisymmetric (such as circumferential rings), (2) the usual thin, axially-oriented, crack that is part of VIC-3D’s present library, and (3) user-defined flaws, such as inter-granular attack (IGA).</p
The Booster/Delta nexus : Henry Miller and his friends in the literary world of Paris and London on the eve of the Second World War.
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