There are three principle applications of the eddy current process: crack detection, conductivity testing, and coating thickness measurement.
Crack detection is the most common application as eddy current is a highly effective and cost-efficient method for finding stress-related fractures. It easily reads through paint and other surface coatings without the need to strip or remove them.
Conductivity testing is where eddy current really shines. The eddy current unit measures the conductivity of nonferrous metals based on a system known as the International Annealed Copper Standard (IACS). This ability to detect changes in conductivity allows eddy current technicians to recognize irregularities in heat treatment, work hardening, annealing, and other detrimental metallurgical deviations before they become a problem.
Eddy current is primarily a surface inspection method. The depth of penetration is usually restricted to less than a quarter of an inch. Eddy current density diminishes rapidly with depth making it "iffy" for subsurface work. However, there are specialized low frequency techniques that give this method some subsurface latitude.
5. Ultrasonic testing
Ultrasonic testing principles have been used since the 1930s when its potential was first researched in pre-war Germany.
Ultrasonic testing introduces high frequency sound waves into the test material to detect subsurface discontinuities. Transducers are used to both transmit and receive sound energy in the test article. Test frequencies between 1 and 25 MHz are normally employed. Cracks, laminations, shrinkage cavities, forging bursts, porosity, bonding faults, and other discontinuities with established metal-gas interfaces can easily be detected. Common aircraft applications include: thickness testing of aircraft skin and airframe structural members, crack detection, and corrosion evaluation.
Pulse-echo type test equipment is by far the most commonly used in aviation. In pulse-echo inspection, short bursts of ultrasonic energy are introduced into the test item at regular intervals. If the pulse encounters a reflecting surface (flaw), all or part of the energy is reflected. The amount of energy reflected is a function of the size of the flaw in relation to the size of the incident beam. The direction of the reflected beam (echo) depends on the orientation of the flaw. The reflected sound wave is monitored on the equipment's display. The amount of energy reflected, and the time delay between transmission of the initial pulse and the arrival of the echo are measured.
Some advantages of the ultrasonic testing method include high penetrating capability, high sensitivity and resolution, portability, single surface accessibility, and the immediate interpretation of test results.
There are drawbacks. The successful operation of the test equipment requires experienced technicians and extensive technical knowledge and reference standards are needed to calibrate the equipment and simulate defects. Parts that are roughly finished or geometrically irregular are difficult to inspect. Shallow discontinuities lying immediately beneath the surface may not be detected due to anomalies in sound wave intensity.
The future looks good for nondestructive inspection and aviation. The traditional methods discussed in this article and new technologies will continue to serve the next generation of American aircraft and the latest innovations in materials technology.
Joseph E. Stump is an NDI specialist for General Electric Inspection Services in Rancho Dominguez, CA.
GE Inspection Services
A division of GE Power Systems
1211 Kona Dr.,
Rancho Dominguez, CA 90220
Fax: (310) 635-1646
Mobile: (310) 941-7355
NDT Company Listings:
AEI North America Inc.
(315) 673-0164, www.aeinorthamerica.com
Agfa NDT Inc.
(717) 242-0327, www.agfandt.com
(505) 842-4184, www.cutteraviation.com
(888) 332-3848, www.everestvit.com