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Nondestructive Inspection/Testing (Part Two)

Filed Under: Inspection Concepts and Techniques

Liquid Penetrant Inspection

Penetrant inspection is a nondestructive test for defects open to the surface in parts made of any nonporous material. It is used with equal success on such metals as aluminum, magnesium, brass, copper, cast iron, stainless steel, and titanium. It may also be used on ceramics, plastics, molded rubber, and glass.

Penetrant inspection detects defects, such as surface cracks or porosity. These defects may be caused by fatigue cracks, shrinkage cracks, shrinkage porosity, cold shuts, grinding and heat treat cracks, seams, forging laps, and bursts. Penetrant inspection also indicates a lack of bond between joined metals. The main disadvantage of penetrant inspection is that the defect must be open to the surface in order to let the penetrant get into the defect. For this reason, if the part in question is made of material that is magnetic, the use of magnetic particle inspection is generally recommended.

 

Penetrant inspection uses a penetrating liquid that enters a surface opening and remains there, making it clearly visible to the inspector. It calls for visual examination of the part after it has been processed, increasing the visibility of the defect so that it can be detected. Visibility of the penetrating material is increased by the addition of one or two types of dye: visible or fluorescent.

The visible penetrant kit consists of dye penetrant, dye remover emulsifier, and developer. The fluorescent penetrant inspection kit contains a black light assembly, as well as spray cans of penetrant, cleaner, and developer. The light assembly consists of a power transformer, a flexible power cable, and a hand-held lamp. Due to its size, the lamp may be used in almost any position or location.

The steps for performing a penetrant inspection are:

  1. Clean the metal surface thoroughly.
  2. Apply penetrant.
  3. Remove penetrant with remover emulsifier or cleaner.
  4. Dry the part.
  5. Apply the developer.
  6. Inspect and interpret results.

Interpretation of Results

The success and reliability of a penetrant inspection depends upon the thoroughness that the part was prepared with. Several basic principles applying to penetrant inspection are:

  1. The penetrant must enter the defect in order to form an indication. It is important to allow sufficient time so the penetrant can fill the defect. The defect must be clean and free of contaminating materials so that the penetrant is free to enter.
  2. If all penetrant is washed out of a defect, an indication cannot be formed. During the washing or rinsing operation, prior to development, it is possible that the penetrant is removed from within the defect, as well as from the surface.
  3. Clean cracks are usually easy to detect. Surface openings that are uncontaminated, regardless of how fine, are seldom difficult to detect with the penetrant inspection.
  4. The smaller the defect, the longer the penetrating time. Fine crack-like apertures require a longer penetrating time than defects such as pores.
  5. When the part to be inspected is made of a material susceptible to magnetism, it should be inspected by a magnetic particle inspection method if the equipment is available.
  6. Visible penetrant-type developer, when applied to the surface of a part, dries to a smooth, white coating. As the developer dries, bright red indications appear where there are surface defects. If no red indications appear, there are no surface defects.
  7. When conducting the fluorescent penetrant-type inspection, the defects show up (under black light) as a brilliant yellow-green color and the sound areas appear deep blue-violet.
  8. It is possible to examine an indication of a defect and to determine its cause as well as its extent. Such an appraisal can be made if something is known about the manufacturing processes that the part has been subjected to.
 

The size of the indication, or accumulation of penetrant, shows the extent of the defect and the brilliance is a measure of its depth. Deep cracks hold more penetrant and are broader and more brilliant. Very fine openings can hold only small amounts of penetrants and appear as fine lines. [Figure 10-9]

Figure 10-9. Dye penetrant inspection.
Figure 10-9. Dye penetrant inspection.

False Indications

With the penetrant inspection, there are no false indications in the sense that they occur in the magnetic particle inspection. There are, however, two conditions that may create accumulations of penetrant that are sometimes confused with true surface cracks and discontinuities.

The first condition involves indications caused by poor washing. If all the surface penetrant is not removed in the washing or rinsing operation following the penetrant dwell time, the unremoved penetrant is visible. Evidences of incomplete washing are usually easy to identify since the penetrant is in broad areas rather than in the sharp patterns found with true indications. When accumulations of unwashed penetrant are found on a part, the part must be completely reprocessed. Degreasing is recommended for removal of all traces of the penetrant.

False indications may also be created where parts press fit to each other. If a wheel is press fit onto a shaft, penetrant shows an indication at the fit line. This is perfectly normal since the two parts are not meant to be welded together. Indications of this type are easy to identify since they are regular in form and shape.

 

Eddy Current Inspection

Electromagnetic analysis is a term describing the broad spectrum of electronic test methods involving the intersection of magnetic fields and circulatory currents. The most widely used technique is the eddy current. Eddy currents are composed of free electrons under the influence of an induced electromagnetic field that are made to “drift” through metal. Eddy current is used to detect surface cracks, pits, subsurface cracks, corrosion on inner surfaces, and to determine alloy and heat-treat condition.

Eddy current is used in aircraft maintenance to inspect jet engine turbine shafts and vanes, wing skins, wheels, bolt holes, and spark plug bores for cracks, heat, or frame damage. Eddy current may also be used in repair of aluminum aircraft damaged by fire or excessive heat. Different meter readings are seen when the same metal is in different hardness states. Readings in the affected area are compared with identical materials in known unaffected areas for comparison. A difference in readings indicates a difference in the hardness state of the affected area. In aircraft manufacturing plants, eddy current is used to inspect castings, stampings, machine parts, forgings, and extrusions. Figure 10-10 shows a technician performing an eddy current inspection on a fan blade.

Figure 10-10. Eddy current inspection.
Figure 10-10. Eddy current inspection.

Basic Principles

When an alternating current (AC) is passed through a coil, it develops a magnetic field around the coil, which in turn induces a voltage of opposite polarity in the coil and opposes the flow of original current. If this coil is placed in such a way that the magnetic field passes through an electrically conducting specimen, eddy currents are induced into the specimen. The eddy currents create their own field that varies the original field’s opposition to the flow of original current. The specimen’s susceptibility to eddy currents determines the current flow through the coil.

The magnitude and phase of this counter field is dependent primarily upon the resistance and permeability of the specimen under consideration and enables us to make a qualitative determination of various physical properties of the test material. The interaction of the eddy current field with the original field results is a power change that can be measured by utilizing electronic circuitry similar to a Wheatstone bridge.

Principles of Operations

Eddy currents are induced in a test article when an AC is applied to a test coil (probe). The AC in the coil induces an alternating magnetic field in the article, causing eddy currents to flow in the article. [Figure 10-11]

Figure 10-11. Generating an eddy current.
Figure 10-11. Generating an eddy current.

Flaws in or thickness changes of the test-piece influence the flow of eddy currents and change the impedance of the coil accordingly. [Figure 10-12] Instruments display the impedance changes either by impedance plane plots or by needle deflection. [Figure 10-13]

Figure 10-12. Detecting an eddy current.
Figure 10-12. Detecting an eddy current.
Figure 10-13. Impedance plane test.
Figure 10-13. Impedance plane test.

The specimen is either placed in or passed through the field of an electromagnetic induction coil, and its effect on the impedance of the coil or on the voltage output of one or more test coils is observed. The process that involves electric fields made to explore a test piece for various conditions involves the transmission of energy through the specimen much like the transmission of x-rays, heat, or ultrasound.

Eddy current inspection can frequently be performed without removing the surface coatings, such as primer, paint, and anodized films. It can be effective in detecting surface and subsurface corrosion, pots, and heat treat condition.

Eddy Current Instruments

A wide variety of eddy current test instruments are available. The eddy current test instrument performs three basic functions: generating, receiving, and displaying. The generating portion of the unit provides an alternating current to the test coil. The receiving section processes the signal from the test coil to the required form and amplitude for display. Instrument outputs or displays consist of a variety of visual, audible, storage, or transfer techniques utilizing meters, video displays, chart recorders, alarms, magnetic tape, computers, and electrical or electronic relays.

A reference standard is required for the calibration of eddy current test equipment. A reference standard is made from the same material as the item is to be tested. A reference standard contains known flaws or cracks and could include items, such as a flat surface notch, a fastener head, a fastener hole, or a countersink hole. Figures 10-14, 10-15, and 10-16 show typical surface cracks, subsurface cracks, and structural corrosion that can be detected with eddy current techniques.

Figure 10-14. Typical surface cracks.
Figure 10-14. Typical surface cracks.
Figure 10-15. Typical subsurface cracks.
Figure 10-15. Typical subsurface cracks.
Figure 10-16. Typical structural corrosion.
Figure 10-16. Typical structural corrosion.

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