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Non-Destructive Testing

Non-destructive testing (NDT) and non-destructive evaluation (NDE) techniques evaluate material properties without impairing the future usefulness of the items being tested. Today, there is a large range of NDT methods available, including:

  • Ultrasonic

  • Radiography

  • Fluoroscopy

  • Microwave

  • Magnetic particle

  • Liquid penetrant

  • Eddy current

  • Holography

The advantages of NDT techniques include the use of automation, 100% product testing and the guarantee of internal soundness. However, some NDT results, like X-ray films or ultrasonic echo wave inspection, are open to interpretation and demand considerable skill on the part of the examiner. Visual Inspection One of the most frequent inspection operations is the visual examination of products, parts, and materials. The color, texture, and appearance of a product gives valuable information if inspected by an alert observer. Lighting and inspector comfort are important factors in visual inspection. In this examination, the human eye is frequently aided by magnifying lenses or other instrumentation. This technique is sometimes called scanning inspection.

 Ultrasonic Testing

The application of high frequency vibration to the testing of materials is a widely used and important non-destructive testing method. Ultrasonic waves are generated in a transducer and transmitted through a material which may contain a defect. A portion of the waves will strike any defect present and be reflected or “echoed” back to a receiving unit, which converts them into a “spike” or “blip” on a screen. Ultrasonic inspection has also been used in the measurement of dimensional thickness. One useful application is the inspection of hollow wall castings, where mechanical measurement would be difficult because of part interference. The ultrasonic testing technique is similar to sonar. Sonic energy is transmitted by waves containing alternate, regularly spaced compressions and refractions. Audible human sound is in the 20 to 20,000 Hertz range. For non-destructive testing purposes, the vibration range is from 200,000 to 25,000,000 Hertz. (Where 1 Hertz = 1 cycle per second).

 Magnetic Particle Testing

Magnetic particle inspection is a non-destructive method of detecting the presence of many types of defects or voids in ferromagnetic metals or alloys. This technique can be used to detect both surface and subsurface defects in any material capable of being magnetized. The first step in magnetic particle testing is to magnetize a part with a high amperage, low voltage electric current. Then fine, steel particles are applied to the surface of the test part. These particles will align themselves with the magnetic field and concentrate at places where magnetic flux lines enter or leave the part. The test part is examined for concentrations of magnetic particles which indicate that discontinuities are present. There are three common methods in which magnetic lines of force can be introduced into a part:

  • Longitudinal inside coil

  • Circular magnetization

And circular magnetization using an internal conductor. The selected method will depend upon the configuration of the part and the orientation of the defects of interest. Alternating current (AC) magnetizes the surface layer and is used to discover surface discontinuities. Direct current (DC) gives a more uniform field intensity over the entire section and provides greater sensitivity for the location of subsurface defects. There are two general categories of magnetic particles (wet or dry), which depend upon the carrying agent used.

 Liquid Penetrant Testing and Eddy Current Testing

Liquid penetrant inspection is a rapid method for detecting open surface defects in both ferrous and nonferrous materials. It may be effectively used on nonporous metallic and non-metallic materials. Tests have shown that penetrants can enter material cracks as small as 3,000 angstroms. The size of dye molecules used in fluorescent penetrant inspection are so small that there may be no surface cracks, too small for penetration. The factors that contribute to the success of liquid penetrant inspection are the ability of a penetrant to carry a dye into a surface defect and the ability of a developer to contrast that defect by capillary attraction.

False POSIUVG results may sometimes confuse an inspector. Irregular surfaces or insufficient penetrant removal may indicate non-existent flaws. Penetrants are not successful in locating internal defects.

Eddy Current Testing

Eddy currents involve the directional flow of electrons under the influence of an electromagnetic field. Non-destructive testing applications require the interaction of eddy currents with a test object. This is achieved by:

  • Measuring the flow of eddy currents in a material having virtually identical conductivity characteristics as the test piece

  • Comparing the eddy current flow in the test piece (which may have defects) with that of the standard

Eddy currents are permitted to flow in a test object by passing an alternating current through a coil placed near the surface of the test object. Eddy currents will be induced to flow in any part that is an electrical conductor. The induced flow of electrons produces secondary electromagnetic field which opposes the primary field produced by the probe coil. This resultant field can be interpreted by electronic instrumentation. Defect size and location cannot be read directly during eddy current testing. This test requires a comparative analysis. Part geometry maybe a limitation in some test applications and a benefit in others. Eddy current methods can be used to check material thickness, alloy composition, the depth of surface treatments, conductivity and other properties.



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