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Eddy Current Testing Inspection

Eddy current testing a modern Non-destructive testing – NDT method that has become one of the prime tool for quality control of products, materials and structures at various stages of manufacturing and in-service inspections. Eddy current inspection can be used to find finest surface and subsurface flaws in any conductive material. Spectrums of quality control engineering applications are existing applying Eddy current principles. Present Eddy current NDT method has applications in Aerospace Component Inspections, Heat exchanger tube inspections and other critical engineering applications.

Eddy current testing and the principles can be used for the following engineering applications:

  • Material sorting – mix up of various grade of materials either chemically different or variation in mechanical properties or metallurgically difference in structures can sorted using Eddy current testing equipment's.
  • Surface and subsurface flaw detection in conducting materials. Flaws could be cracks, laminations, and other discontinuities that can be detrimental to the product performance.
  • Conductivity testing for metals.
  • Accurate measurement of Coating thickness with Eddy current testing. The coating can be conductive or non-conductive with few limitations on the accuracy of the measurements.
  • This NDT inspection method can also be used for thickness testing and measurement of especially low thicknesses of conductive materials where there is a restriction on using Ultrasonic thickness gauging due to component configuration or the quantum of inspection. Eddy current inspections can be either contact or non-contact type tests.
  • Eddy current testing is an effective means of verification of integrity of structures and estimation of corrosion damages for heat exchanger tubes. Routine in-service maintenance inspections using Eddy current techniques yield reliable data for analysis and condition monitoring of plants and structures.

We at 'Speco Alloys Inc' provide reliable and dependable services on Eddy current testing fine tuned to the client needs with our expert team of NDT Level II, III professionals and state of art equipment's & approved NDT testing procedures that meets national and international standards. We can perform Eddy current inspection both in-house and onsite inspections. Contact Speco Alloys Inc for more information on your NDT inspection, training and certification, welding inspection and other quality control needs.

Principle of Eddy Current testing

When a test specimen is brought into proximity to the alternating flux field of an eddy current coil, coil flux causes electrons in the specimen to circulate in a swirling eddy-like pattern; hence the term "eddy currents. "Eddy current behavior depends on the properties of both the flux and the specimen itself. So eddy currents are circular alternating currents caused by a varying magnetic field.

Advantages of Eddy Current Testing

  • Eddy currents are Sensitive to small cracks and other defects
  • Eddy current testing detects surface and near surface defects
  • Eddy current Inspection gives immediate results
  • ET Equipment is very portable
  • This Method can be used for much more than flaw detection
  • Minimum part preparation is required
  • Eddy current Test probe does not need to contact the part
  • Eddy currents Inspects complex shapes and sizes of conductive materials

Limitations of Eddy Current testing

  • Eddy current inspection is suitable for only conductive materials
  • Surface must be accessible to the Eddy current probe/s
  • Skill and training required to carry out eddy current inspection is more extensive than other techniques
  • Surface finish and roughness may interfere with test results
  • Reference standards needed for setup for every application
  • Depth of penetration is limited
  • Flaws such as de-laminations that lie parallel to the probe coil winding and probe scan direction are undetectable and may get detected only using specially designed Eddy current probes for such applications.

Ultrasonic Testing (UT)

It is a non-destructive testing (NDT) method in which beams of high frequency sound waves that are introduced into the material being tested are used to detect surface and sub-surface flaws. The sound waves travel through the materials with some attenuation of energy and are reflected at interfaces. The reflected beam is detected and analysed to define the presence and location of flaws. Ultrasonic waves are almost completely reflected at metal gas interfaces. Partial reflection occurs at metal liquid or metal solid interfaces, with the specific percentage of reflected energy depending mainly on the ratios of certain properties of the matter on opposite sides of the interface.

Cracks, laminations, shrinkage, cavities, bursts, flakes, pores, bonding faults and other discontinuities that can act as metal-gas interfaces can be easily detected. Inclusions and other inhomogenities in the metal being inspected can also detected by causing partial reflection or scattering of the ultrasonic waves, or by producing some other detectable effect on the ultrasonic waves.

Most of the ultrasonic inspection instruments detect flaws by monitoring one or more of the following:

  • Reflection of energy from metal-gas interfaces, metal-liquid interfaces or discontinuities within the metal itself
  • Time of transit of a sound wave through the test piece from the entrance point at the sending (transmitting) transducer to the exit point at the receiving transducer, and
  • Attenuation of the beam of sound waves by absorption and scattering within the test piece.

Applicability

Ultrasonic testing or inspection (UT) is used for quality control and materials testing in all major industries. This includes Ultrasonic testing of castings, forgings, plates, extruded components, weld joints, electrical and electronic component manufacturing, production of steel, aluminium and titanium, fabrication of structures such as air frames, pressure vessels, ships, bridges, motor vehicles, machinery and jet engines. In service ultrasonic testing for preventive maintenance is used for detecting impending failure of rail road rolling stock axles, press columns, earth-moving equipment, mill rolls, mining equipment and other machines and compo nets. The flaws to be detected include voids, cracks, inclusions, pipe, laminations, bursts and flakes. They may be inherent in the raw materials, may result from fabrication and heat treatment, or may occur in service from fatigue, corrosion or other causes. Ultrasonic testing can also be used to measure thickness of metal sections during manufacturing and maintenance inspections.

Limitations

  • Manual Ultrasonic Flaw detection requires careful attention by experienced technicians
  • Extensive technical knowledge is required for the development of Ultrasonic testing procedures.
  • Parts that are rough, irregular in shape, very small or thin or not homogenous are difficult to be tested
  • Discontinuities that are present in a shallow layer immediately beneath the surface may not be detectable.
  • Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being tested.
  • Reference standards are needed, both for calibrating the equipment

Magnetic Particle Testing (MPT/MT)

Magnetic particle testing or MPT is a non-destructive testing method for locating surface and near surface discontinuities in ferromagnetic materials. It depends for its operation on the fact that when the material or part under test is magnetized, discontinuities that lie in a cause leakage field to the direction of the magnetic field will cause a leakage field to be formed at and above the surface of the part. The presence of this leakage field, and therefore the presence of the discontinuity, is detected by the use of finely divided ferromagnetic particle applied over the surface, some of the particle being gathered and held by the leakage field. This magnetically held collection of particle forms an outline of the discontinuity and generally indicates its location, size, shape and extent. Magnetic particles are applied over a surface as dry particles, or as wet particle in a liquid. Ferromagnetic materials include most of the iron, nickel and cobalt alloys. These materials lose their ferromagnetic properties above a characteristic temperature called the Curie point which is approximately 760◦ C for most of the ferromagnetic material.

Applications

The principal industrial uses of magnetic article testing are final inspection, receiving inspection, in process inspection and quality control, maintenance and overhaul in the transportation industries, plant and machinery maintenance and inspection of large components.

Limitations

Thin coatings of paint and other non-magnetic coverings, such as plating; adversely affect sensitivity of magnetic particle inspection. Other limitations are:

  • Magnetic particle inspection methods will work only on ferromagnetic materials.
  • For best results, the magnetic field must be in a direction that will intercept the principle plane of the discontinuity. Sometimes this requires two or more sequential inspections. With different magnetizations.
  • Demagnetization following magnetic particle testing is often necessary.
  • Post cleaning to remove remnants of the magnetic particle clinging to the surface may be required after testing and demagnetization.
  • Exceedingly large currents sometimes are required for very large parts.
  • Care is necessary to avoid local heating and burning of finished parts or surface at the points of electric contact.
  • Although magnetic particle indications are easily seen, experience and skill in interpreting their significance are needed.

Liquid Penetrant Testing or Dye Penetrant Inspection (LPT/PT/DPI)

Liquid or dye penetrant testing is a non –destructive method for finding discontinuities that are open to the surface of solid and essentially non-porous materials. Indications of flaws can be found regardless of the size, configuration, internal structure, or chemical composition of work piece being tested and regardless of flaw orientation. Liquid penetrant can seep into (and be drawn into) various types of minute surface openings (reportedly, as fine as 4micro inch in width) by capillary action. Because of this, the process is well suited for the detection of all types of surface cracks, laps, porosity, shrinkage areas, lamination and similar discontinuities in casting, forgings, welds and other product forms. Dye penetrant inspection is used extensively for the testing of wrought and cast products of ferrous and non-ferrous metals, powder metallurgy parts, and ceramics and glass objects. In practice, the liquid penetrant inspection process is relatively simple. Equipment generally is simpler and less costly than that for most other NDT methods .When used on ferromagnetic steels, in some instances, the sensitivity of liquid penetrant test is better than that of magnetic particle testing.

Limitations

The major limitation of liquid penetrant testing is that, it can detect only imperfections that are open to the surface. Another factor that may inhibit the effectiveness of liquid penetrant testing is the surface roughness of the object being tested .Rough or porous surfaces are likely to produce false indication.
Liquid penetrant testing depends mainly on a liquid's effectively wetting the surface of a solid work piece of specimen, flowing over the migrating into cavities that are open to the surface. Closely related to wetting ability is the phenomenon of capillary rise or depression.

The liquid penetrant testing requires at least five steps:

  • Surface preparation.
  • Application of penetrant.
  • Removed of Excess penetrant.
  • Application of Developer.
  • Observation and Reporting.

Radiographic testing (RT)

Radiographic testing is an on-destructive testing of components and assemblies that is based on differential absorption of penetrating radiation- either electromagnetic radiation of very short wave-lengths or particulate radiation by the part or test piece being tested. Because of differences in density and variations in thickness of the part, or differences in absorption characteristics caused by variation in composition, different portions of a test piece absorb different amounts of penetrating radiation. Unabsorbed radiation passing through the part can be recorded on film or photosensitive paper, viewed on a florescent screen or monitored by various types of electronic radiation detectors. The term radiography testing usually implies a radiographic process that produces a permanent image on film or paper. Although in a broad sense it refers to all forms of radiographic testing. Neutron radiography refers to radiographic testing using a stream of neutrons rather than electromagnetic radiation.

Uses

Industrial Radiography inspection is used to detect features of a component or assembly that exhibit a difference in thickness or physical density as compared to surrounding material. Large differences are more easily detected than small ones. In general, radiography can detect only those features that have an appreciable thickness in direction parallel to the radiation beam. This means that the ability of the process to detect planar discontinuities such as cracks depends on proper orientation of the test piece during testing. Discontinuities such as voids and inclusions, which have measurable thickness in all directions, can be detected as long as they are not too small in relation to section thickness. In general, features that exhibit a 2% or more difference in absorption compared to the surrounding material can be detected. Radiography is more effective when the flaws are not planar.

Applicability

Radiographic testing is used extensively on castings and elements. Radiography is well suited to the testing of semiconductor devices for cracks, broken wires, unsoldered connections, foreign material and misplaced components. Sensitivity of radiography to various types of flaws depends on many factors, including type of material, type of flaw and product form. Both ferrous alloys can be radio graphed, as can non-metallic materials and composites.

Limitations

Compared to other NDT methods, radiography is expensive. Relatively large capital costs and apace allocations are required for a radiographic laboratory. Field testing of thick sections is a time consuming process. High activity sources require heavy shielding for protection of personnel. Tight cracks in thick sections usually cannot be detected at all, even when properly oriented. Minute discontinuities such as inclusions in wrought material, flakes, micro- porosity and micro-fissures cannot be detected unless they are sufficiently segregated to yield a detectable gross effect. Laminations are impossible to detect with radiography, because of their unfavourable orientation. Laminations do not yield differences in absorption that enable laminated areas to be distinguished from limitation free areas.
It is well known that large doses of X-rays or gamma rays can damage skin and blood cells, can produce blindness and sterility, and in massive doses can cause severe disability or death. Protection of personnel not only those engaged in radiographic work but also those in the vicinity or radiographic testing is of major importance. Safety requirements impose both economic and operational constraints on the use of radiography for testing.

Visual testing (VT)

Visual inspection is one of the most common and most powerful means of non-destructive testing. Visual testing requires adequate illumination of the test surface and proper eye-sight of the tester. To be most effective visual inspection does however, merit special attention because it requires training (knowledge of product and process, anticipated service conditions, acceptance criteria, record keeping, for example) and it has its own range of equipment and instrumentation. It is also a fact that all defects found by other NDT methods ultimately must be substantiated by visual inspection. VT can be classified as direct visual testing, remote visual testing and translucent visual testing. The most common NDT methods MT and PT are indeed simply scientific ways of enhancing the indication to make it more visible. Often the equipment needed is simple for internal inspection, light lens systems such as bore scopes allow remote surfaces to be examined. More sophisticated devices of this nature using fibre optics permit the introduction of the device into very small access holes and channels. Most of these systems provide for the attachment of a camera to permit permanent recording. Speco Alloys Inc material testing facility contains light meters, welding gauges, magnifiers, lenses, other measuring instruments and equipment's for precise control of surface quality. Our NDT inspectors, engineers and technicians are qualified to NDT Level I, II as per written practice prepared according to ASNT recommended practice SNT-TC-1A and in-house ASNT NDT Level IIIs for providing inspection and consulting services.