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Air Force Nondestructive Inspection Office (AF-NDI)
Published November 28, 2006
The Air Force Nondestructive Inspection (NDI) Program
, established in 1958, has gone through various enhancements over the years, with the most recent changes incorporated in Air Force Instruction 21-105. Our mission is to provide optimum support to the structural maintenance program, which maintains air and space equipment in a safe, serviceable, and ready condition; and function as Headquarters Air Force focal point for the Air Force Nondestructive Inspection Maintenance Program. We provide support within the Air Force to many organizations including: Major Commands (MAJCOMs), Field Laboratories, System Program Offices (SPO), the Department of Defense (DoD), and Air Force Depots.
is used to detect discontinuities in parts that are conductors of electricity. An eddy current is the circulating electrical current induced in a conductor by an alternating magnetic field. When eddy currents encounter an obstacle, such as a crack, the surrounding currents become distorted. This change is detected on a meter or other type of display.
is used for detecting discontinuities in ferromagnetic parts. The part is magnetized by using an electrical current that induces a magnetic field in the part. A discontinuity, which crosses the magnetic field, creates north and south poles on either side of the defect area. When magnetic particles are applied to the part, the poles attract the particles and an indication of the discontinuity is formed.
is used to detect discontinuities, i.e., cracks, pits, etc., open to the surface on parts made of nonporous materials. This method depends on the ability of the penetrant to enter into a surface discontinuity in the material to which it is applied.
Testing uses ultrasonic vibrations to detect internal defects, delaminations, disbonds, and discontinuities. Ultrasonic Testing (or UT) can be used on most materials and can locate small defects deep into the structure. UT uses a piezoelectric transducer to convert electrical signals to ultrasonic (mechanical) vibrations. A single transducer is used for pulse-echo testing, two transducers can be used for through-transmission or pitch-catch, and multiple transducers can be placed in an array like those used in medical ultrasound. Depending on the "launch" angle, the sound wave can travel straight down into a part, at an angle, or along the surface of the part.
Usually, the part requires little preparation, but knowledge of the internal structure is critical. Typically, an exact replica of the area of interest is made with simulated defects. These are called "calibration standards". It allows the technician to fine tune the UT instrument to the exact inspection parameters. The technician will see what indications are caused by the expected defects, the back surface reflection, and any other non-relevant indications caused by bolt holes, threads, etc.
The most common type of UT instrument is called "A-scan", where the presentation is returned signal amplitude vs. time. Here, the vertical height of the indication represents the size of the defect, and the horizontal position represents the depth of the defect. Other types are the "B-scan", where the signal height represents depth, but the horizontal position is relative to the horizontal position of the defect as the transducer is scanned across the part. The other major type is the "C-scan", or plan view, which appears as a flat picture of the part and different colors are used to represent depth of an indication.
inspection uses the penetrating abilities of electromagnetic radiation to examine the interior of objects. Three prime factors determine the amount of information that radiography can provide about an object:
1) The composition of the object
2) The product of the density and the thickness of the material making up the object.
3)The energy of the x-ray incident upon the object.
Discontinuities within the object can cause localized changes in the first two characteristics above and thus, become detectable