by Kennedy Smith

In 2001, technicians at the Boeing Co. were asked to examine the Liberty Bell after a man wielding a hammer attacked it. Reportedly, there was little damage done to the historical icon, but the National Parks Service was concerned enough to call for a structural evaluation. Boeing technicians had the task of testing the integrity of the Liberty Bell's structure without doing any further damage to it, a case that clearly called for nondestructive testing.

Boeing's evaluation of the Liberty Bell is just one of the many applications for nondestructive testing, but it's a prime example of why NDT is necessary. Whether it's a 250-year-old historical object or a 2-year-old jet airliner, there are some things you just can't afford to damage while testing for defects.

"Nondestructive testing is any means of examining an object without impairing its life or its future usefulness," explains Betsy Blazar, senior manager of marketing and membership at the American Society for Nondestructive Testing.

There are several nondestructive testing applications used in a variety of industries today. What follows is an overview of several methods, information on some new technologies, and the basics of NDT certification--all provided to help you determine whether NDT is something your company might want to consider.

Industries using NDT

If you think NDT isn't useful for your industry, think again. Because the term NDT encompasses so many different testing methods, it can be found in a host of different industries. Here is just a sampling of industries that employ NDT methods:

Aircraft and aerospace

Power generation and utilities

Chemical and petrochemical

Bridge and building inspection and fabrication

Electronics and microelectronics

General manufacturing

Military

Historical renovation

Typical NDT methods

To more clearly understand how these industries employ nondestructive testing, it's important to examine the most common NDT methods. These include visual and optical inspection, penetrant testing, magnetic particle testing, electromagnetic testing, radiography, ultrasonic testing, acoustic emission testing and leak testing.

Visual and optical testing. This involves looking at an object to detect flaws on its surface. This is a familiar concept for most quality control labs, whether they use microscopes, borescopes, magnifying glasses or other visual inspection devices. However, it's not as simple as it sounds.

"Visual testing can be one of the higher-end inspection methods," says Bill Wiley, NDT instructor at Southeast Community College in Milford, Nebraska. "For example, robotic camera systems have been designed to go into environments where a person can't go, either due to the aggressive nature of the environment or nonaccessibility due to size constraints."

Liquid penetrant testing. With penetrant testing, test objects are coated with a visible or fluorescent dye. A developer is applied, which acts as a blotter, drawing trapped liquid from flaws open to the surface of the part. With visible dyes, color contrast between the dye and the developer allows the tester to see any cracks or other flaws on the surface.

Liquid penetrant testing is fairly inexpensive and, according to Wiley, an easy-to-teach and -learn method. It's ideal for anyone interested in detecting cracks on the surface only. You won't be able to perform a volumetric inspection using this method.

Magnetic particle testing. Most ferromagnetic materials are candidates for this method. The part is subjected to a magnetic field and then either dusted with dry iron particles or covered with a solution containing magnetic particles. Distortion of the magnetic field occurs on surface and near-surface flaws, concentrating the magnetic particles near those imperfections. The pattern of the iron particles produces a visual indication of the flaw.

This type of NDT has some subsurface detection capabilities, possibly up to a quarter of an inch, says Wiley. However, magnetic particle testing is limited in one area in particular: It only works with ferromagnetic materials.

Electromagnetic or eddy current testing. With electromagnetic testing, electrical currents are generated by an induced alternating magnetic field. In conducive materials, the flow of eddy currents is interrupted at imperfections, including dimensional changes or changes in the materials' conducive and permeability properties.

"The industries that most regularly utilize eddy current testing are aircraft and aerospace, power generation and utilities, and chemical and petrochemical," explains Wiley. "The aircraft industry uses it for crack detection and material corrosion assessment. Power generation and chemical plants use it to inspect corrosion, cracking and other damage in the tubing of heat exchangers and steam generators."

Electromagnetic testing is typically a more expensive method than the aforementioned methods--and more difficult to learn and implement.

Radiography. Anyone who's ever broken a bone is familiar with radiographic testing. It involves the use of gamma or X-rays to examine imperfections through a part. A shadowgraph results in a visual map of density changes, cracks and other flaws. Conventional X-ray systems are capable of being brought to a part for cases in which it's not feasible to bring the part to the system.

Innovations in the technology of radiographic testing have brought about digital radiography, real-time radiography and computerized axial thermography, or CAT scans.

"With digital radiography, the film cassette that captures the X-ray image is equipped with microelectronics," Wiley says. From there, the information stored within the cassette can be inserted into a computer for image enhancement, where the technician can manipulate the image to better see the discontinuities within the part.

With real-time radiography, a special electronic sensing device detects radiation as the path goes through a specimen. Then a charged coupled device is connected to the electronics package, where it's converted to an image you can see on a monitor. "The nice thing about real-time radiography is that if you have motion control, you can actually rotate the part while it's underneath the X-ray beam to give you different views," explains Wiley.

Ultrasonic testing. With ultrasonic testing, high-frequency sound waves are directed toward a material to detect changes in the material's properties. A common form of ultrasonic testing is pulse echo, in which sound is transmitted into a test object and reflections, or echoes, from imperfections in the part are returned to a receiver.

Another method is contact testing, which works in the same capacity as an ultrasound test performed on pregnant women. In the case of contact testing, the technician has a transducer in-hand moving across the part to detect flaws.

"Within the contact testing realm, there's precision thickness measurement, down to thousandths of an inch or less," notes Wiley. "If you want to get even more advanced, you can do ultrasonic data acquisition and imaging."

Acoustic emission testing. Unlike ultrasonic testing, this method involves detecting the sound a flaw itself makes, rather than directing sound at the flaw.

For example, when a tank is pressurized, defects within the tank will basically put out signals of their own. This is known as an acoustic event. "Crack growth is an example," says Wiley. "If you have three or more sensors located in one area, you can triangulate where that defect is located."

Leak testing. An everyday example of leak testing would be immersing a bike tire in water to see where the bubbles come out. Leak testing techniques can be something as simple as a bubble test--in which a soapy film is applied to a part to detect bubbles--to much more advanced techniques, like performing a halogen diode test, which detects the presence of halogen gas that's been passed through a part.

New NDT developments

These methods for nondestructive testing haven't changed, notes Blazar, but the techniques used within these methods are constantly being improved. Alan G. Julier of Agfa NDT Inc./ Krautkramer Ultrasonic Systems notes a trend in ultrasonic NDT toward phased arrays.

"Typically, if a technician wants to examine a part ultrasonically, then the choice of the sensor depends on the size of the defect and thickness of the material," he explains. "Then, having chosen that transducer, we have to mechanically move it over the surface of the object. Change the part's thickness, and you have to change the sensor." Phased arrays, however, allow the technician to make all those changes electronically on one sensor and, in many cases, eliminate the need for sensor movement over the surface. Julier explains that the medical profession has been using this technique for years, but it has just recently caught on as an industrial testing method.

Julier also notes some "fringe" NDT technologies, which can be considered NDT but don't fall into the handful of typical methods. These include thermography, shearography, holography, vibration analysis, strain gaging, ground-penetrating radar and a host of visual and optical methods.

Why consider NDT?

If any of the various NDT methods or fringe technologies sounds like a good candidate for use at your company, consider how it will incorporate into your established testing methods. Nondestructive testing is not something you switch to in replacement of your old testing methods. Rather, it's a type of testing that can be incorporated into the quality control lab. Most of the time, companies only implement NDT when doing so is cost-effective. For example, one manufacturer may make thousands of the same part, so performing destructive testing on one part isn't going to dent the company's budget. On the other hand, a company that makes very expensive equipment--roller coasters for example--simply can't afford not to use NDT.

In cases in which your company is awarded a contract that specifically mandates nondestructive tests, consider outsourcing NDT or employing NDT technicians who are certified to the standards of ASNT.

Certification

In order to perform even the most basic nondestructive test, a technician must be trained, and, in many cases, certified to standards like that of ASNT.

"It's extremely important to be certified," says Blazar of ASNT. "It's the only way that any entity can be assured that their technicians have experience in specific methods and techniques." ASNT's core certification program is the NDT Level III examination, which it has offered since 1976. In the last couple of years, ASNT has offered a Central Certification program, which both tests for competency and looks at the experience of the individual. A list of all individuals certified to ASNT and more information about certification is available at the ASNT Web site, www.asnt.org.

"Central certification can provide a company market advantage," explains Blazar. "Having third-party certified personnel demonstrates commitment to quality and professionalism in NDT. For the individual, it is a professional credential that he or she carries to any employment."

Companies can certify their own NDT technicians, and many do with tests based on the ASNT criteria. In the long run, however, it may be beneficial to get ASNT-certified because it's commonly accepted as the standard third-party certification organization.

Educational institutions that offer courses in nondestructive testing usually train students to become fully prepared for ASNT certification.

Is NDT gaining momentum?

Although about 4,000 people hold ASNT Level III certification and ASNT membership is about 9,000, interest in nondestructive testing is only inching along. In fact, Julier predicts single-digit growth potential for the NDT market, and that's only if there's an upswing in the economy.

"From a sales point of view, the growth has only been about 4 percent during the past five years," Julier notes. "Sadly, NDT is often viewed by many companies in the same manner that they view quality assurance: a necessary evil that has the potential to decrease 'out of the door' shipments. It's generally implemented because the product has to meet critical safety requirements."

Blazar notes, however, that this focus on safety has boosted quality control labs' awareness of NDT. "The world is becoming more cognizant about safety, and because of these heightened concerns, NDT is getting more recognized," she explains.

"Look at the age of some of our infrastructure in this country," she continues. "People are concerned about the integrity of our bridges, our railroads, etc. That's why NDT will keep growing."

An aging icon

Despite its age and previous circumstances, the Liberty Bell has held up remarkably well, as technicians at Boeing discovered. They performed a number of tests to determine if the attack had caused any serious damage. Radiographs of the bell were taken and compared to radiographic images from more than 25 years prior; ultrasonic inspection was used to locate any flaws within the bell; technicians used eddy current testing to map the surface of the bell, determine its conductivity and evaluate its consistency; and florescent penetrant testing was used to check the bell for cracks (not including the obvious one).

Much of the information in this article was provided by the Collaboration for Nondestructive Testing Education's NDT Resource Center, an online NDT information source for students, educators and professionals. For detailed information about NDT, visit the Web site at www.nde-ed.org. Further information about NDT can be found at the American Society for Nondestructive Testing's Web site, www.asnt.org.

About the Author

Kennedy Smith is Quality Digest's assistant editor. Letters to the editor regarding this article may be sent to letters@qualitydigest.com.