ECT EDDY CURRENT STANDARDS (Surface)

5.0 EDDY CURRENT TESTING STANDARDS

The product is passed through or adjacent to an electrical test coil, which has been excited by an alternating current. This induces a flow of eddy currents around the test material or in the case of a sector coil, in the area under the coil. Short, intermittent anomalies or flaws cause a variation in the eddy current pattern, which the instrument detects. The product moves longitudinally through the rotating test probes resulting in a helical search pattern. As the probe passes over a defect, variations in the induced Eddy Current pattern are detected. The minimum flaw length, which can be consistently detected, is a function of the rotary speed of the probe and the throughput speed of the material. Rotary testing is the method of choice for detecting seam type surface defects in non-magnetic and magnetic grades of wire and bar. Detect short surface and some subsurface defects, on or off-line, in magnetic and non-magnetic wire, bar and tube. Inspect welded tube for short ID or OD defects in the weld zone or on the full circumference. Test uniform cross sectional material, including squares, rectangles, hex and round. Inspect small diameter wire or tube for short defects. Check continuity and locate welds in single and multi-conductor insulated wire and cable When testing carbon steel, austenitic stainless and alloy steels that have a permeability higher than 1, it is often necessary to saturate the material with a magnetic field. This has the effect of evening out the permeability variations in the material which would otherwise interfere with the ECT. Saturation coils are used to create the saturation field within which the ECT coil is paced. Detect seam type surface defects in cold drawn wire or cut length bar stock. Inspect in line with continuous wire operations such as wire drawing, parts forming, re-spooling, or straight and cut. Inspect cut lengths, usually off-line. Test parts, such as small shafts and bearings for longitudinal surface defects. Eddy Current technology can be used on ferrous and non-ferrous materials.

5.1. THREE-NOTCH EDDY CURRENT STANDARD

For some special inspections, a calibration/reference block must be made which represents the part to be inspected. This type of block must be made from the same materials and to the same construction blocks as the part to be inspected. The block must contain representative discontinuities in the areas where discontinuities are likely to occur in the real structure.

Eddy current equipment can be used for a variety of applications such as the detection of cracks (discontinuities), measurement of metal thickness, detection of metal thinning due to corrosion and erosion, determination of coating thickness, and the measurement of electrical conductivity and magnetic permeability. Eddy current inspection is an excellent method for detecting surface and near surface defects when the probable defect location and orientation is well known. Defects such as cracks are detected when they disrupt the path of eddy currents and weaken their strength. The images to the right show an eddy current surface probe on the surface of a conductive component. The strength of the eddy currents under the coil of the probe indicated by color. If there is a flaw under the right side of the coil and it can be see that the eddy currents are weaker in this area. Of course, factors such as the type of material, surface finish and condition of the material, the design of the probe, and many other factors can affect the sensitivity of the inspection. Successful detection of surface breaking and near surface cracks requires:

·         Knowledge of probable defect type, position, and orientation

·         Selection of the proper probe. The probe should fit the geometry of the part and the coil must produce eddy currents that will be disrupted by the flaw.

·         Selection of a reasonable probe drives frequency. For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity. For subsurface flaws, lower frequencies are necessary to get the required depth of penetration and this result in less sensitivity. Ferromagnetic or highly conductive materials require the use of an even lower frequency to arrive at some level of penetration.

·         Setup or reference specimens of similar material to the component being inspected and with features that are representative of the defect or condition being inspected for.

The basic steps in performing an inspection with a surface probe are the following:

·         Select and setup the instrument and probe.

·         Select a frequency to produce the desired depth of penetration.

·         Adjust the instrument to obtain an easily recognizable defect response using a calibration standard or setup specimen.

·         Place the inspection probe (coil) on the component surface and null the instrument.

·         Scan the probe over part of the surface in a pattern that will provide complete coverage of the area being inspected. Care must be taken to maintain the same probe-to-surface orientation as probe wobble can affect interpretation of the signal. In some cases, fixtures to help maintain orientation or automated scanners may be required.

·         Monitor the signal for a local change in impedance that will occur as the probe moves over a discontinuity.

The applet below depicts a simple eddy current probe near the surface of a calibration specimen. Move the probe over the surface of the specimen and compare the signal responses from a surface breaking crack with the signals from the calibration notches. The inspection can be made at a couple of different frequencies to get a feel for the effect that frequency has on sensitivity in this application.

5.2. AIR FORCE GENERAL PURPOSE / NAVY EDDY CURRENT REFERENCE STANDARD

Inspection Technique Consistent positioning of the probe in relation to edges and interfaces during setup and
scanning should be established to ensure maximum response from flaws with minimum interference from other sources of indications. If conditions are known to exist which may result in false indications or which could mask true indications from flaws, these conditions SHOULD be noted in the procedure and a means of interpreting or evaluating the false indications provided. In performing eddy current inspection of an area, the distance between scans or between measurements must be selected to ensure complete coverage for the minimum size flaw or variation in properties to be detected. In determining maximum distance between scans, consideration must be given to the change in magnitude of flaw response as the probe coil center position increases in distance from the center of the crack.

Scanning Pattern the scanning pattern required for ET is based on the possible initiation site of the crack, the
orientation of the cracks, and the size of the cracks which must be detected. If cracks initiate from an edge in thin material (0.050-inch or so), eddy current inspection is usually limited to a single scan of the edge. For thicker materials, scans might be required on both surfaces adjacent to the edge and one or more scans of the material between the edges. When cracks initiate beneath the heads of non-removable fasteners, the pattern usually consists of a single scan around the protruding head of the fastener to detect cracks growing outward from the hole. If cracks can occur at a variety of positions and orientations, as is possible on flat surfaces, in radii, and on cylindrical surfaces, scanning must be performed in a manner which will assure detection of the smallest cracks required to be found. For these types of inspection areas, the direction of scanning, the number of scans, and the distance between scans SHOULD be specified.

Automatic or Semi-Automatic Equipment Automatic eddy current equipment in conjunction with high speed recorders is capable of operation at extremely high speeds. The upper limits of scanning speed are based on the operating frequency and the sampling rates of the recorder or readout. The principal use for automated eddy current equipment by the military is for the inspection of bolt holes. In this application, rotational speeds of 40-3000 rpm can be obtained by the inspection system.

Use of Recorders or Oscilloscopes The use of recorders or oscilloscopes (CRT type eddy current instruments) permits increasing the speed of manual scanning to the limits imposed by the reaction time of these instruments. Generally, other restrictions related to guiding the probe in the prescribed scanning pattern become the controlling factor when recorders or oscilloscopes are used.

Unless otherwise specified by the weapon system engineering authority, the Air Force general purpose eddy
current standard SHALL be the common standard used to perform ET’s on aluminum components within the Air Force. The standard made to the Navy configuration may be used as a substitute for the Air Force general purpose eddy current standard. When using the Navy standard, calibrate on the long EDM notches for surface inspections and the corner notches in the upper layers for bolt hole inspections unless otherwise directed by a part specific procedure.

Cracks as Reference Standards When an eddy current instrument is setup for detection of cracks, some means must be provided to assure that the sensitivity of the test system is sufficient to detect the smallest required crack size. Ideally, the best standard would be a section of the same material containing a crack of this minimum size. Cracks of
specified sizes are difficult to obtain. With few specimens to choose from, such situations are rare. Fatigue cracks of specified size can be grown under laboratory conditions, but this method is extremely expensive. The length of the crack along the surface and its width at the surface is easily measurable. The depth of the crack is generally unknown and must be approximated from other data. Because of difficulty in obtaining actual cracks for reference standards, a number of other standards may be used. These standards are discussed below.

Artificial Defects for Standards Due to the difficulty of obtaining the types and sizes of real flaws in parts for
use as reference standards; a variety of artificial flaws have been developed to simulate the real flaws. Fatigue cracks have been grown under laboratory conditions, but reproducible sizes in sufficient quantity for standards are impractical. Artificial flaws, such as drilled holes, EDM notches, saw cuts, two surfaces clamped together to simulate a crack, or chemically produced conditions to simulate pits or corrosion, can be produced in a variety of ways. Ideally an artificial flaw will produce an eddy current response identical to the response from a real flaw of the same size, orientation, and location. This ideal is seldom achieved with artificial flaws. Estimation of flaw size from the response to artificial flaws must be based upon correlating previous known flaw sizes with the response from the artificial flaws. To maintain the quality of this correlation, it is necessary to carefully specify the material properties and fabrication process of the artificial defect standard.

5.3. 12-BOLT HOLE EDDY CURRENT REFERENCE STANDARD

12-Hole Eddy Current Bolt hole Standards. Various alloys and notch configurations available. These reference standards are used for Eddy Current Rotary Bolt hole Inspection, and serve as a cost-effective alternative to more complex government bolt hole standard designs