Flaw Type Research Articles

Autofocus algorithm for dispersion correction in optical coherence tomography.

Practical clinical optical coherence tomography (OCT) systems require automatic tools for identifying and correcting flaws in OCT images. One type of flaw is the loss of image detail owing to the dispersion of the medium, which in most cases is unknown. We present an autofocus algorithm for estimating the delay line and material dispersion from OCT reflectance data, integrating a previously presented dispersion compensation algorithm to correct the data. The algorithm is based on minimizing the Renyi entropy of the corrected axial-scan image, which is a contrast-enhancement criterion. This autofocus algorithm can be used in conjunction with a high-speed, digital-signal-processor-based OCT acquisition system for rapid image correction.

Parametric study on the impact-echo method using mock-up shafts

In this study, the impact-echo method was employed to evaluate the integrity of shafts, and parametric simulations of the impact-echo method were carried out numerically and experimentally. A one-dimensional finite element study was performed for mock-up shafts that include solid and damaged shafts. The mock-up shaft was made of Monocast, and four types of flaws were considered: (1) axisymmetric voids, (2) non-axisymmetric voids, (3) necks, and (4) bulbs. The reduction in shaft cross-sectional area varied from 30 to 80%. Subsequently, experimental studies were carried out to verify the finite element models using similar mock-up shafts. These experimental studies were carried out in the air and soil, and impact responses were analyzed in both time and frequency domains. It was shown that the results of the experiment were in agreement with those of numerical studies, and the accuracy of the impact-echo method was influenced by the type, size and location of flaws. In addition, it was also revealed that axisymmetric void, non-axisymmetric void, and neck could be detected in the frequency domain when the reduction in shaft cross-sectional area was more than 50%. Alternatively in the time domain, it was possible to identify the flaw reliably when the reduction was 30%. This latter approach provides the better possibility of detecting smaller flaw in shaft.

Elastodynamic inversion for shape reconstruction and type classification of flaws

Two linearized inverse scattering methods are investigated to reconstruct the shape of flaws in the elastic solid. One is based on the Born approximation and the other is based on the Kirchhoff approximation. The Born inversion is sensitive to the volumetric flaw but not to a crack-like flaw. On the other hand, the Kirchhoff inversion reacts to both boundaries of volumetric and crack-like flaws. The combined use of Born and Kirchhoff inversions leads to the classification method of flaw type. The performance of the proposed classification procedure is demonstrated by the numerical simulations and then by the experimental measurements for the two-dimensional models of flaws. An example for the three-dimensional shape reconstruction is also shown by using the numerically calculated backscattering amplitudes.

Structural health monitoring of materials by high critical temperature SQUID

Nowadays, tailoring the material properties is essential for advanced product design in engineering systems. The need to provide key information about micro- and macro-structural behaviour of materials, without destructively sectioning the sample, has spurred the development of nondestructive evaluation methodologies. These techniques are required during material production, quality testing of components during manufacturing, and in-service inspection of structural integrity. To ensure the highest possible operational safety along with an economic efficiency, it is necessary to carry out inspections with a high sensitivity and a proven reliability. Due to its unparalleled magnetic field sensitivity over a wide frequency range and large dynamic range, SQUID-based nondestructive evaluation has unique advantages for materials and structures characterization. We will present an overview of eddy-current nondestructive analysis utilizing high T c SQUIDs with an emphasis on examples relevant to the aeronautical industry. These include the detection of deep-lying defects in multi-layer structures of Al–Ti alloys and damage of extremely lightweight graphite/epoxy composites. Both of these can be successfully treated by this approach where conventional electromagnetic probes often fail. In addition some results based on the volume integral formulation, successfully developed to simulate the response of the system to different type of flaws in Al-alloy planar structures and to solve the inverse electromagnetic problem, will be shown.

Linearized Inverse Scattering Methods for Classification of Flaw Type(NDE3)

A classification method of the flaw type is discussed by utilizing the ultrasonic scattering amplitudes from flaws in an elastic body. The method is based on the combination of two linearized inverse scattering methods of Born and Kirchhoff inversions. The Born inversion is sensitive to the volumetric flaw but not to a crack-like flaw. The Kirchhoff inversion reacts to both surfaces of volumetric and crack-like flaws. The combined use of the Born and Kirchhoff inversions leads to the classification of flaw type. To show the versatility of the method, aluminum specimens that contain a circular cavity and artificial cracks are prepared, and the backscattered waveforms are acquired from the experimental measurement. The measured waveforms are processed and the waveforms are fed into the Born and Kirchhoff inversions. The results show that the crack-like flaw can be discriminated from the volumetric flaw. It is also shown that the scattered wave component in the low frequency range is important for the present classification method.

Eddy current flaw characterization in tubes by neural networks and finite element modeling

For quantitative flaw characterization in steam generator tubes, inversion of eddy current testing (ECT) signals in an automated fashion is strongly desired. In this paper, we report our effort to develop a systematic approach for flaw characterization in tubes by the novel combination of neural networks and finite element modeling. Specifically, the finite element model that can predict ECT signals from axisymmetric flaws in tubes was developed, and its accuracy was verified experimentally. Using this model, an abundant synthetic database with 400 ECT signals generated from 200 axisymmetric machined grooves in four types has been constructed with two test frequencies per flaw. For the automated inversion of ECT signals, a total of 22 features have been extracted from each flaw. Then, a set of 10 features has been selected for flaw classification, while the other set of 10 features for flaw sizing. For the determination of the flaw type and the flaw size parameters, we have proposed an intelligent flaw characterization system that adopts two different paradigms of neural networks: probabilistic neural networks for flaw classification and back propagation neural networks for flaw sizing. The performance of this system has been investigated using the synthetic ECT signals in the database. The excellent performance presented here, even though it has been obtained from synthetic flaws representing machined grooves in tubes, demonstrates the high potential of this system to serve as a robust tool for practical flaw characterization in tubes.

Residual flaws due to formation of oxygen bubbles in anodic alumina

Flaws in anodic alumina films are localized sites where the film material is modified morphologically, structurally, or chemically compared with adjacent regions of uniform amorphous alumina. The flaws are preferred sites of pitting and dielectric breakdown, which are of importance to the corrosion resistance and electronic applications of aluminium respectively. Although flaws have been categorized in general terms as either mechanical or residual, there is no detailed understanding of the precise origin and nature of flaws and their mechanisms of influence in pitting and dielectric breakdown. Consideration is given here to the formation of oxygen bubbles within anodic alumina, by oxidation of O 2− ions in the vicinity of the metal/film interface, which are suggested to be one type of residual flaw. The compositions of impurities or second phase particles, which may lead to generation of oxygen bubbles, are examined in the light of systematic investigations of anodizing of binary aluminium alloys.

Numerical simulation of failure prediction for ceramic tools: comparison with forging test results

A fracture prediction criterion for brittle materials has been introduced in the POLLUX finite-element code in order to predict the risk-of-rupture of ceramic tools during a forging operation. POLLUX is a software dedicated to the simulation of forging operations, initially developed by INSA (Lyon). The chosen probabilistic fracture model is based on the weakest-link theory and the statistical theory of Weibull. A surface approach or a volume approach can be retained on the basis of the type of critical flaws in the ceramic. Two different criteria are available in order to characterise the stress state, considering the tensile normal stresses and neglecting the compressive stresses. An identification procedure of the critical flaw type is presented for a particular ceramic material. Statistical parameters of ceramic fracture have been determined experimentally using bending tests performed under environmental conditions close to those of forging. A constitutive equation of the workpiece material has been proposed, issued from torsion tests. In order to validate the model in the case of ceramic tools subjected to multi-axial stress states, a particular configuration has been defined to compare the simulation predictions with the experimental results. A forging test has then been developed, in which a billet of superalloy is formed in a ceramic tool up to its fracture at the temperature of 1423 K. The experimental distribution of tool fracture, according to the strain of the billet, is in good agreement with fracture predictions computed by the simulation.

5814731 Ultrasonic scanning apparatus for nondestructive site characterization of structures using a planar based acoustic transmitter and receiver in a rolling pond

The invention is a carriage type sonic or ultrasonic testing apparatus for flaw and deterioration detection testing in a structure, especially concrete. The apparatus detects both location and type of flaw in a structure. The carriage unit incorporates a rolling pond feature which includes: i) foam-covered tracks and rollers, this soft foam material in addition to its primary function prevents vibration of the transducers while traversing rough surfaces such as weathered concrete. The tracks and rollers form a water-tight seal with each other and the specimen surface; ii) an air-removal brush assembly which maintains contact with a specimen surface to facilitate transmission and reception of ultrasonic energy into and out of a test specimen; iii) an ultrasonic isolator element and optional wave absorbers; and iv) the ultrasonic transducer suspension system. A transducer water bed is continuously maintained while the system is in motion across the concrete. A sealed space surrounds the transducer system, and that space is continuously flooded with water so as to keep the bottom sections of the transducers submerged. This sealed space continuously transports a sufficient amount of fluid (water) along the concrete surface for proper acoustic coupling. The ultrasonic transducer includes a granite wedge as an impedance-matching transducer faceplate material for concrete structural examinations.

CHARACTERIZATION OF INTEGRATED DATA ON INHERENT FLAWS IN ENGINEERING CERAMICS

Integrated data on inherent flaws in engineering ceramics were characterized for quantitative analyses of them. For this purpose, a ceramic strength database utilized on a personal computer was temporarily constructed so that collected data on flaws could be compiled as records in the database. It was clarified that distributions of the equivalent crack length l for several ceramics were nearly fitted to normal distribution functions. The distribution of l was investigated for every type of flaw, and it was found to shift toward a longer length region in the order of embedded, surface, and corner flaws. The flaw data were also correlated with strength and microstructural parameters. In the relation to the strength, the strength was reduced as a whole with the decrease in the crack length, while the lower bound of strength scatter was predicted by a modified relation of the fracture mechanics criterion for long cracks. It was revealed that the flaw size increased with the increase in the grain size or the porosity. The relation of a Petch-Hall type was observed between the grain size and the mean strength estimated by using the mean value of l. Some useful suggestions were offered for the improvement of ceramic strength.

Classification of Ultrasonic Flaw Signals by Means of Time-Frequency Analysis.

An ultrasonic method for flaw type determination is described. Clearly, the lower the frequency of an ultrasonic longitudinal wave becomes, the wider the beam spreads. In the case of a large flaw, there is a difference in the beam path between low and high frequency components. Therefore, the propagation time of a low frequency component is longer than that of a high frequency component. On the other hand, in the case of a small flaw, there is little difference in the propagation time between the components. Hence it is possible to determine the flaw type, by analyzing the propagation time of a particular frequency range. Time-frequency, t-f, analysis is applied to the ultrasonic signals reflected by three types of artificial flaws; side drilled holes, fiat-bottomed holes and slits. The Short Time Fourier Transform, STFT, method of t-f analysis is found to be effective. The results support the feasibility of classification of flaws by means of t-f analysis of the reflection of ultrasonic waves emitted by probes of different diameter.

Practical Experiences with some principles pertinent to CIM

Ceramic injection moulding (CIM) is a promising manufacturing process which attracts growing attention today. The basic principles pertinent to CIM are well known, but as there is a variety of viable manufacturing approaches the final results may be quite different. This paper highlights some practical experience with specific aspects of CIM. One requirement of the binder system is that its components must be chemically compatible with respect to ceramic powders. Stearic acid, a very popular binder constituent, may react with some ceramic powders and seriously disturb the process. A solution to this problem is presented. The imprints of knock out pins on injection moulded ceramic products may show flaws. The origin of and how to remedy this flaw type is discussed. Defect sensitivity is a major issue with ceramics. As well as other ceramic forming processes the injection moulding process may cause specific defects. Fracture strength of injection moulded silicon carbide is compared with data from literature. Some measures to improve the quality are discussed.

How Braze Flaws Affect the Thermal-Hydraulic Performance of the Tore Supra Phase-III Outboard Pump Limiter: A Case Study of the Effects of Nonuniform Thermal Resistance on the Peak Heat Flux to the Coolant for Tubes with One-Sided Heating

The Phase-III Outboard Pump Limiter is a heat sink made of pyrolytic graphite armor brazed to water-cooled copper tubes. Around the inner wall of the tube wall, some of the water can be in the subcooled boiling regime. The central issue analyzed here is how the heat flow in the tube changes when the thermal resistance along the heated portion of the tube is redistributed. Cracks or braze flaws in the joint between the tile and tube cause this redistribution. Severe cracks or flaws reduce the power-handling capability of this assembly because the local peak heat fluxes increase and, for a given critical heat flux (CHF), the safety margin decreases. There were some surprises. The increase in local peak heat flux for the most common type of flaw encountered in the fabrication of this limiter was negligible up to a flaw size of ∼ 50 %. The examples presented are intended as a case study that illuminates the more general problem of how correlations for heat transfer and for CHF developed for uniform circumferential heating are applied to a case of one-sided heating.

Effect of Cleaning and Abrasion‐Induced Damage on the Weibull Strength Distribution of Sapphire Fiber

Fractographic analysis revealed the presence of concurrent flaw populations in sapphire fibers which were tensile tested in the as‐received condition (sized and unsized) and after various cleaning procedures. The following flaw populations were identified: surface flaws attributed to handling and abrasion damage (type A), volume or internal flaws attributed to shrinkage voids which form during the manufacturing process (type B), localized fiber surface reaction flaws introduced during the flame‐cleaning procedure (type C), and self‐abrasion surface flaws intentionally introduced on unsized fibers (type D). The strength distribution associated with each flaw type was characterized using a censored data Weibull analysis for both the least‐squares and maximum‐likelihood estimation methods. The strength distribution for type C (flame‐cleaning) flaws exhibited an approximately 20% degradation in strength compared to the distribution for type A flaws. The strength distribution for type D (self‐abrasion) flaws exhibited an approximately 35% degradation in strength compared to the strength distribution for type A flaws. This result underscores the need for fiber sizings to prevent damage during shipping and handling. However, higher purity sizings and/or improved procedures for sizing removal are required to mitigate cleaning‐induced fiber strength degradation during composite fabrication.

Numerical study of defects in glass reinforced composite transducer shells

Fatigue in composite shells of flextensional transducers can be investigated experimentally but in service results transposition is uncertain. This study can also be carried out numerically. However, a simple model rarely allows to describe the fracture in fatigue of an existing structure. The approach that is suggested here aims (starting from a modal analysis of a damaged structure in situ and according to results of a numerical computation by finite elements) at identifying the type of flaw in a qualitative and quantitative way. This paper first presents the finite element used to deal with this problem. Second the approach is validated on a shell example. Finally an experiment on a simple G.R.P. plate is used to compare numerical and experimental results to validate the approach

The interaction of chromate species with aluminium supporting air-formed and anodic films—II. Impedance studies

The impedance behaviour of aluminium, supporting either an air-formed or an anodic film, has been monitored during immersion in chromate/dichromate solution. The impedance was obtained over a wide range of frequency (10 kHz–10 mHz) with particular attention being paid to the low frequency region (1 Hz–10 mHz). At high frequencies ( > 10 Hz) the impedance response is from the film dielectric; any changes with time are associated generally with the penetration and chemically induced disaggregation of the alumina film by Cr 6+ species. The low frequency behaviour largely reflects local processes proceeding at flaws in the films. Specifically, the impedance behaviour at 0.1 Hz has been examined to assess reaction rates during the immersion period and differences in the rates are revealed for aluminium supporting air-formed films and anodic alumina films. The variation in the reaction rates evident during the immersion period is attributed to the type and geometry of persistent flaws in the respective specimens and the related effect of thinning of the anodic alumina film during immersion in the chromate solution. It is evident that Cr 6+ species are reactive to alumina films, with chromate species encouraging healing of anodic sites on aluminium supporting an air-formed film and blocking of cathodic sites on aluminium supporting an anodic alumina film.

Acoustic emission monitoring of uniaxial pressing of ceramic powders

Powder compaction is a critical step in a classical method for the production of zero-defect ceramic products. We have studied the development of this step to determine whether acoustic emission can be used to detect the presence of flaws in a compact. In uniaxial pressing at room temperature, the creation and extension of flaws occur principally during the ejection of the compacts. The type and extent of flaws depend on the mechanical properties of the powder used and on the pressing conditions. Acoustic emission was used to monitor the consolidation by uniaxial pressing of Al 2O 3 powders and a UO 2PuO 2 powder. It was possible to detect flaws in the compacts, to determine their type for a given powder, and to identify the type of powder used by a direct examination of cumulative counts of acoustic emission events and by a statistical analysis of amplitude distributions. Hence, the introduction of this technique in a fabrication process offers the possibility of detecting the presence of defects in compacts as soon as they are created. This would decrease the difficulties involved in reuse in the process of the powder from defective products, and would limit further flaw development during sintering, which are two major problems in the nuclear industry.

Time domain BIE applied to flaw type recognition : Review of Progress in Quantitative Nondestructive Evaluation, La Jolla, California (United States), 15–20 Jul. 1990. Vol. 10 A, pp. 75–82. Edited by D.O. Thompson and D.E. Chimenti, Plenum Press (1991). ISBN 0-306-43903-4

Time domain BIE applied to flaw type recognition : Review of Progress in Quantitative Nondestructive Evaluation, La Jolla, California (United States), 15–20 Jul. 1990. Vol. 10 A, pp. 75–82. Edited by D.O. Thompson and D.E. Chimenti, Plenum Press (1991). ISBN 0-306-43903-4

The nature of strength-controlling structural flaws in formed coke

Curing and carbonizing a raw briquette of char gives a formed coke whose tensile strength varies markedly with the (UK Snibston, rank 902 coal-derived) blend of char and binder used to make the briquette. The strength increases considerably with decrease of (input) char particle size despite extensive crushing during compaction, and appreciably but discontinuously with increase of binder percentage. A previous theoretical picture is extended to show that the char particle size indirectly determines the ‘basic’ size of flaws in the final coke, that this basic size can be reduced by (sufficient) binder under pressure acting proportionately as a ‘flaw-size modifier’ and that hairline fractures arising late during briquetting are the principal source of the flaws in question. Such (Griffith) flaws control the tensile strength and three types are found to operate in sequence as binder percentage is increased. Strength control passes from one flaw type to another, giving an interval of constant strength within an otherwise increasing trend of strength and, being contrary to ‘common sense’, this highlights the accepted view that subtle factors such as flaws control the strength of a brittle solid.

Swedish approach and experience with inspections of austenitic components in the primary circuit of BWR's

It is essential to know why an inspection is required and what type and size of flaw is expected to be detected. A very basic knowledge it seems. However, many national codes and standards are not fully adequate in this respect. The selection of inspection items as well as the acceptance criteria of detected defects are often defined in very general terms. Degradation processes are very little reflected in the standard ISI selection scope which creates demands on additional inspection programs. The swedish authority, the Swedish Nuclear Power Inspectorate (SKI) have issued a new code for in-service inspections which deals with this issues. This paper describes the basics of the new code, the implementation and the experience gained from in plant inspections at Barsebäck Nuclear Power plant.