Non Destructive Method Theory - Basic Principles - https://www.tinker.af.mil/Portals/106/Documents/Technical%20Orders/AFD-101516-33B-1-1.pdf AF338-1-1-EC-CP4Sc0-Indice ROCarneval

Capítulo 4 - MÉTODO DE INSPEÇÃO POR CORRENTES PARASITAS

traduzido do livro: AIR FORCE TO 33B-1-1 / ARMY TM 1-1500-335-23 / NAVY (NAVAIR) 01-1A-16-1 - Manual Técnico - Métodos de Inspeção Não Destrutiva, Teoria Básica

  1. INTERPRETAÇÃO DE CP
    1. Detecção de Descontinuidades
      1. Avaliação de Indicações de Trincas
        1. Critério de Aceitação-Rejeição
        2. Condições Afetando a Avaliação de Descontinuidades
        3. Descontinuidades
        4. Deformação Metálica Superficial
        5. Espaçamento entre Metais
        6. Arranões, Abaulamentos e Corrosão Pitiforme
        7. Taxa de Deflexão
        8. Estimativa da Dimenssão da Trinca
      2. Efeito da Velocidade e do Caminho da Varredura
        1. Análise da Resposta do Sinal no Plano de Impedâncias
        2. Indicações na Tela Digital ou no Registro Gráfico
        3. Indicações Empregando Escâner de Varredura de Furos de Rebite
        4. Indicações de Escâner Automáticamente Indexados
      3. Aberturas, Grandes Furos e Recortes em Peças
        1. Localização e Orientação da Trinca
        2. Requisitos de Inspeção
      4. Medição da Condutividade
        1. Dimensão e Precisão dos Padrões de Condutividade
        2. Faixa de Condutividade
        3. Estabilidade dos Padrões
        4. Número de Padrões Requeridos
        5. Procedimento de Inspeção
          1. Requisitos do Procedimento de Medição de Condutividade
        6. Contexto e Objetivos
        7. Preparação da Peça
        8. Calibração para Medição de Valores da Condutividade
        9. Calibração para Separação de Mistura de Ligas
        10. Verificação da Calibração
        11. Critério de Aceitação-Rejeição

6 ET INTERPRETATION.

6.1 Flaw Detection. When eddy currents are induced in a metal in the region of a crack or other flaw, the eddy current f low is distorted. The distortion results in a localized decrease in electrical conductivity. In this manner an ET is able to detect f laws

6.1.1 Evaluation of Crack Indications.

6.1.1.1 Acceptance Rejection Criteria. In most cases, the depth of flaws detected by ET cannot be directly measured. In almost all cases, the eddy current signal of the flaw must be compared to the eddy current signal produced by the reference standard. The relationship between response to the standard and the corresponding response to the defect size must be es tablished prior to the test and should be considered an essential part of the setup process. Prior to the start of any test, the in strument setup process SHOULD confirm that the test can be conducted with the required sensitivity. 6.1.1.2 Conditions Affecting Flaw Evaluation. Inspection for cracks, measurement of conductivity, or hardness can of ten be complicated by the surface damage, and manufacturing processes. Included in this category are scratches, gouges, pitting, and metal smearing. Severe damage may require refinishing of the area prior to inspection, inspection at a lower sensi tivity, or selection of another test method.


Figure 4-57. Effect of Discontinuities on Distribution of Eddy Currents

6.1.1.4 Metal Smearing. Flowing of surface metal may result from machining operations, abrasion during service, or by deformation during assembly or disassembly of an aircraft or component. The depth of smearing in nonmagnetic materials and its metallurgical effects will rarely exceed 0.002 to 0.003-inch. At normal crack detection frequencies, the metallurgical changes created by smeared metal may not affect eddy current response. However, metal build-up and depressions associated with the smearing create changes in lift-off. Because the phase angle is displayed, impedance plane analysis instruments will detect flaws even with changes in lift-off. In ferromagnetic steel, eddy current penetration is very shallow and any blem ish of the surface increases the difficulty of inspection.

6.1.1.5 Metal Spacing. The spacing of metal sheets separated by a nonconductive adhesive layer can be successfully measured by using an eddy current frequency for which the thickness of both metal sheets is less than, or equal to three times the corresponding standard depth of penetration.

6.1.1.6 Scratches, Gouges, and Pitting. Scratches, gouges, and pits may result in eddy current signals similar in mag nitude to those from cracks. As test frequencies increase, the sensitivity to scratches tends to increase, because the eddy cur rent field is more concentrated at the surface.

6.1.1.7 Rate of Deflection. Rapidity of response with an impedance plane display instrument is also a means of evaluat ing indications. When traversing a crack, a quick rapid deflection is obtained. Variations in conductivity, gradual thickness changes, out-of-round holes, and variations in edge-to-probe spacing provide a slow, gradual change in measured re sponse. The inspector SHOULD be aware of the rate of change in response from cracks, as contrasted to the rate of signal change from slow changing material properties or test conditions.

6.1.1.8 Estimation of Crack Size. Cracks have the three dimensions of length, width, and depth. All three of these di mensions have an effect on the eddy current response from the flaw. In general, the length of the flaw can be related to the dis tance over which a signal above a specified level is obtained. When the crack is perpendicular to the surface and is less than 2 standard depths of penetration deep, the approximate depth of the crack can be estimated from the eddy current indica tion. With impedance plane analysis instruments the depth can be determined by the phase angle and amplitude of the indication. The width of the crack also influences the magnitude of the indication. With impedance plane analysis instruments, the signal shape, phase, and amplitude can be used to estimate the depth and area of the crack. In general, a crack will be as deep or deeper than indicated by comparing its ET response to the response from the reference EDM notches.

6.2 Effect of Scan Rate and Pattern.

6.2.1 Signal Response of Impedance Plane Analysis Instruments. The speed of manual scanning with impedance plane analysis instrumentation does not affect signal response because the system response time is not limited by the response of a meter movement.

6.2.2 Indications on Digital Display or Strip Chart Recorder. The use of a strip chart recorder or digital display for recording indications during manual scanning of fastener holes makes evaluation less subjective. Comparison of rate of deflection from indications in the hole and the reference can be observed at the same time.

6.2.3 Indications with Automatic Bolt-Hole Scanning. Due to the rough surface of many bolt holes, numerous indica tions are obtained from causes other than cracks. Indications should therefore be examined carefully to establish if indica tions could be from cracks or if they are attributable to other causes. Evaluation can be made on the basis of direction of de f lection and rate of deflection.

6.2.4 Indications from Indexing Automatic Scanners. The controlled rate of scanning obtained with the indexing au tomatic scanning (rotational/translational scanners) unit provides additional improvement in ease of evaluation. Because of the small scanning increment (pitch of scanner screw), usually 0.025-inch (40 threads to the inch), any crack of signifi cant size will be detected during at least three consecutive revolutions of the scanner. This should result in three or more evenly spaced indications on the strip chart recorder or digital display. If crack-like indications are observed, inspect the hole visually to determine if the indications are due to obvious deformations such as metal tears or gouges. Gouge indica tions, while cyclic in nature, are generally recognized due to the fact such indications usually appear 180-degrees opposite in phase (or polarity) to crack or slot indications. Additionally, a gouge indication will usually not be as sharply peaked as an indication from a crack or slot. Careful study must be made of such indications to ensure that they do not mask an indication of a crack at the bottom of the gouge.


6.3 Openings, Large Holes, and Cutouts.

6.3.1 Location and Orientation of Cracks. An opening or cutout in a stressed aircraft part serves as a stress riser and a potential source of fatigue cracks and/or stress corrosion cracks. Fatigue cracks initiate at the edges of an opening, hole, or cutout and grow away from the edge at right angles to the direction of stress. Stress corrosion cracking usually occurs in sec tions subject to either an applied or residual tensile stress. The direction of tensile stresses can often be defined by engineering stress analysis or from the history of previous cracking in the part. This application covers openings for doors and accesses in aircraft skins, cutouts at part edges, and attachment holes too large for bolt-hole probes.

6.3.2 Inspection Requirements. If inspection is required only for large cracks (greater than approximately 1/4-inch in length) adequate inspection can usually be performed without special equipment or fixtures. For such cracks, inspection can be performed sufficiently far enough from the edge to avoid interference from edge effects. To detect small cracks, a rela tively constant probe-to-edge distance must be maintained. For maximum reliability, a fixture or probe guide is used to es tablish probe positioning.


6.4 Conductivity Measurement.

6.4.1 Size and Accuracy of Conductivity Standards. For convenience of transportation and storage, conductivity standards are usually kept relatively small. Standards must have sufficient size to prevent edge effects or thickness from hav ing a bearing on conductivity readings. These requirements can be satisfied by requiring length and width to be 1-inch greater than the probe diameter and the thickness greater than 3.5 times the standard depth of penetration at the test instrument fre quency. Standards should be flat, have a surface finish of 63 RMS or better, and be free of any coatings. Standards used for calibrating instruments immediately prior to measuring conductivity SHOULD be accurate within ±0.5% IACS of the nominal value. A second set of standards accurate within 0.35% IACS SHOULD be periodically made available for checking the performance of instruments and field calibration standards. Calibration standards shall be traceable to NIST. Standards are available from manufacturers of eddy current conductivity instruments.

6.4.2 Conductivity Range. The conductivity range of the standards must be within the range of the instrument and cover the range of conductivity values to be measured. The calibration blocks shall have the same change in resistivity with tem perature as the test parts.

6.4.3 Stability of Standards. Excessively high temperatures and sudden changes in temperature can have damaging metallurgical effects on standards. Aluminum alloys are particularly susceptible to thermal shock. Surfaces of standards can also corrode if exposed to moisture or other hostile environments. Damage due to rough handling can cause erroneous conductivity readings. For these reasons, standards shall be transported and stored in dry, clean, protected areas not subject to excessive temperatures.

6.4.4 Number of Standards Required. A minimum of two calibration blocks with accurately determined conductivity values must be available for calibration of eddy current conductivity meters. When using general purpose instruments, the number of standards may vary from two to several depending on the inspection purpose and the accuracy required.

6.4.5 Inspection Procedures.

6.4.5.1 Conductivity Procedure Requirements. Procedures for conductivity measurement should take into account the varieties of environments and test part conditions which might be encountered. In preparing for conductivity measurement, the following steps should be considered:
  • Background and objectives of the inspection
  • Equipment requirements
  • Part preparation
  • Instrument calibration including calibration standards
  • Conductivity measurement procedures
  • Acceptance/rejection criteria
6.4.6 Background and Objectives. An understanding of the problem that initiates a conductivity measurement require ment enables the inspector to better interpret inspection results and handle unexpected test conditions. The purpose of the test can be separation of mixed or improper alloy, determination of improper heat treatment, and detection of heat or fire dam aged material. The types of material involved and the location of the inspection SHOULD be specifically established. Heat and/or fire damage may be confined to a portion of a part and may vary in the degree of damage. These variables must be con sidered during conductivity measurement.

6.4.7 Part Preparation. As with all types of ET, areas on which conductivity measurement is to be performed must be free of any sharp slivers or foreign material that could damage a probe or cause lift-off changes. Such conditions can be removed with fine emery paper or other approved means. Conductivity measurements can be performed through nonconduc tive coatings that have thicknesses equal to or less than the amount of lift-off adjustment on meter type equipment. Both the thickness and uniformity of the coating thickness and the amount of lift-off adjustment provided should be checked prior to measuring conductivity through nonconductive coatings. If lift-off adjustment cannot be obtained, correction factors can be determined for uniform coatings by establishing the change in conductivity readings caused by the coating and adding this change to each of the measured values. Non-uniform coatings in excess of lift-off adjustment must be removed prior to mea suring conductivity. Excessively rough surfaces SHOULD be smoothed with emery paper to provide a surface finish 250 RMS or better before performing conductivity measurements.

6.4.8 Calibration for Measuring Conductivity Values.

NOTE
See WP 407 00 of TO 33B-1-2 for a procedure for digital conductivity measurement.
  • a. Select a sufficient number of standards to obtain a smooth continuous curve over the range of conductivity to be mea sured. The actual number of samples will depend on the expected range to be measured and the accuracy required.
  • b. Adjust the instrument for lift-off, if applicable, and a standard representing approximately mid-range of the conductivi ties to be measured.
  • c. Determine the meter or scope readings corresponding to each of the intermediate standards and record the conductivity value.
  • d. Note each of the values on a graph with meter or scope readings on the vertical axis and conductivity values on the hori zontal axis. e. Construct a smooth curve through all the points. The curve should increase or decrease smoothly throughout the range with no minimum or maximum values. This curve is used to measure conductivity with the specific instrument and probe.

6.4.9 Calibration for Separation of Mixed Alloys. To calibrate the general purpose instruments for separating two groups of materials with different conductivity, the instrument is set to obtain readings at one end of the scale for one group of material, and the other end of the scale for the second group of material. Lift-off is usually set on a specimen representing the group with the lower value of conductivity or permeability.

6.4.10 Calibration Check. Calibration SHOULD be checked approximately every 10-minutes during continual use and whenever abnormal values are obtained. Whenever an instrument is found to be out of calibration, all measurements per formed since the previous calibration verification SHOULD be rechecked.

6.4.11 Acceptance/Rejection Criteria. Acceptance/rejection criteria can be found in the applicable TO or material specifications. Acceptable conductivity ranges for many aluminum alloys are shown in Table 4-7 in Paragraph 4.8.






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