by Kennedy Smith

In the world of nondestructive testing, there are plenty of technologies to choose from. They include visual and optical inspection, penetrant testing, magnetic particle testing, electromagnetic testing, radiography, acoustic emission testing, leak testing and ultrasonic testing. The common benefit of using any of these methods is that you don’t destroy the material you’re inspecting. With the emergence of advanced computer technology, many of these testing methods have become more popular in industries ranging from petrochemical to automotive. One method that’s receiving much attention is ultrasonic testing.

During the last 10 years, ultrasonic testing has boomed. Experts suggest that a surge in information technology has increased ultrasonic testers’ capabilities while decreasing their price--making ultrasonic testing available to many industries that had previously avoided it. In fact, many experts agree that ultrasonic testing is the up-and-coming nondestructive technology.

History of ultrasonic testing

Most people are already familiar with ultrasonic testing as it relates to medicine, such as women who have ultrasound tests during pregnancy. In a nutshell, ultrasonic testing entails directing high-frequency sound waves toward a material to detect changes in material properties. “A piezoelectric transducer is basically excited with a voltage to generate a sound wave,” explains Dennis Zwigart of StressTel, which manufactures thickness gages, flaw detectors and bolt monitors. “The sound is put into the part and reflected back from something--either the back side of the part or from a flaw--depending on what’s in the material. When it reflects back, the operator can process that signal digitally and determine the thickness of the metal or the flaw that’s inside the metal.”

Ultrasonic testing for nonmedical purposes works in the same capacity as ultrasound and began in the years preceding World War II. With the emergence of sonar (recording echoes from objects submerged in water), scientists were soon testing ways in which sound waves could be used to detect flaws in metals. In the early 1930s, Soviet scientist Sergei Sokolov became the first person to demonstrate through-transmission for flaw detection in metals. Other scientists expanded upon Sokolov’s ideas to create the beginnings of what would become modern ultrasonic testing technology.

During the 1970s, ultrasonic testing had advanced to the point that it could detect very small imperfections within metal. Although this was advantageous for technicians working on flaw detection, manufacturers found themselves rejecting many parts that were once considered passable. Out of this unexpected disadvantage emerged fracture mechanics and other laws to predict fatigue in materials.

Presently, some of the most common applications of ultrasonic testing are in thickness gaging and flaw detection.

Applications and techniques

The list of industries that utilize ultrasonic is long. “We have customers in petrochemical, fossil and nuclear fuels, ship building, mining, automotive, construction, primary metals, aviation and so forth,” notes Zwigart.

Materials that cannot be tested ultrasonically include anything that can’t transmit or allow sound waves. “For example, say I bond two pieces of metal together and want to make sure they’re bonded correctly,” suggests Larry Culbertson of Mitchell Labs, which performs ultrasonic testing for a number of industries. “If there’s an air pocket between the two pieces, the sound waves will not travel through them.”

There are a variety of techniques used in ultrasonic testing. They include:

Pulse-echo. This method uses short pulses of sound that travel through the part to either locate a crack or the back side of the part. It’s suitable for flaw detection or thickness testing. The time it takes for the sound to travel through the part and bounce back is calculated using the simple equation, d = vt/2 where d is the distance from the surface to the discontinuity, v is the velocity of sound waves and t is the round-trip transmit time. The user moves a transducer over the surface of the part, and the tester will record the echoes.

Angle-beam. The angle-beam method has to do with the type of transducer used. An angled sound path can be used to better detect flaws within a material. This is explained further in the next section.

Crack-tip diffraction. This method is used to determine the length of cracks within a material. If an angle beam is scanned over the surface of the part, the lowest point of the crack will appear as the weakest signal because the distance traveled by the sound wave is longer.

Automated scanning. Automated ultrasonic scanning systems are often used with the part and transducer fully immersed in water. This enables consistency of measurements because the coupling agent’s (i.e., water’s) properties remain constant.

Another newer technology is laser ultrasonic inspection, which utilizes laser beams to generate ultrasound and collect signals. Advances in transducer technology have lead to the development of the air-coupled ultrasonic technique. These systems allow sound waves to be transmitted through air and remain strong enough to penetrate the part and return a signal.

Pulser-receivers

The three major components of an ultrasonic tester are the transducer that emits sound, a computer display to show results, and the pulser-receiver, which acts as a kind of translator between the transducer and display. The pulser portion of the pulser-receiver generates electrical pulses, which the transducer converts to sound waves. The receiver portion of the pulser-receiver amplifies the ultrasonic pulses, coming from the part through the transducer.

Transducers

Ultrasonic testing relies on the transducer used to emit sound. The ceramic piezoelectric transducers generate electrical impulses and change them into acoustic energy, and vice versa. “They create mechanical wave forms that go through the part and change the returning pulse echo from mechanical back into electrical so that the scope can read it,” says Culbertson.

There are two major types of transducer: contact and immersion. Contact transducers, as illustrated on page 31, utilize a coupling material, such as water or oil, to smooth rough surfaces and prevent air gaps from manipulating the results.

Immersion transducers don’t come into contact with the component they’re measuring. Instead, they operate in a liquid. The watertight effect eliminates the chance of air pockets affecting results. “We use quite a bit of immersion in the lab,” says

Culbertson. “With a transducer you can’t inspect in the near field. With an immersion transducer, you use water to differentiate the near field and the far field.”

The near field is an area of space in which the sound waves are not uniform. The ultrasonic beam is more uniform in the far field, where the beam is spread out in a pattern originating from the center of the transducer. The variations that occur in the near field eventually change to a smooth and declining amplitude, at which point the far field begins. (See page 32.)

Other transducer types follow:

Dual-element transducer. This transducer uses a pitch-and-catch effect. “It’s got two elements,” explains Culbertson. “One is sending the signal, and one is receiving it.”

Angle-beam transducer. This transducer can be either contact or immersion. Shear wave is anything other than zero degrees. For example, if you’re thickness testing, you go straight into the part. An angle beam will measure at different degrees. “It’s used when you’re looking for defects that are neither parallel nor perpendicular to the part,” notes Culbertson.

Delay line transducer. This contact transducer contains a plastic wedge between the transducer and the part being measured. Basically, it eliminates the near field.

High-frequency transducer. Transducers use frequencies from 0.5 MHz all the way up to 25 MHz--and sometimes up to 50 MHz. The higher the frequency, the more sensitivity.

Normal incidence shear wave transducer. This type of transducer emits shear waves directly into the material without having to use an angle-beam wedge.

Data presentation

There are three ways to display information collected from an ultrasonic tester. They’re known as A-scan, B-scan and C-scan. The A-scan presentation displays the relative amount of energy received on the vertical axis and elapsed time along the horizontal axis. The B-scan display is a cross-sectional view with travel time displayed along the vertical axis and linear position of the transducer displayed along the horizontal axis. C-scan presentations are used with automated data acquisition systems. The C-scan displays information along a plane of the image parallel to the scan pattern of the transducer. Gaps in the scan pattern represent defects within the material.

Popularity of ultrasonic testing

Because of advances in ultrasonic technology during the last 10 years, the method is becoming more popular with manufacturers in all industries. “Nondestructive testing embraces different types of inspection, and the high end of that is X-ray testing,” notes Zwigart. “However, X-ray testing can be expensive. You need highly skilled people, and in some cases, it can even be dangerous. Ultrasonic testing allows manufacturers to use similar techniques without the cost of buying expensive equipment and training technicians.”

Culbertson agrees. “The equipment has become more sophisticated in the last 10 years,” he says. “There are a lot more features in the equipment, and it has become small, lightweight and much less expensive. Ten years ago, you were looking at an instrument that maybe cost $15,000 and it probably weighed 20 pounds. Today ultrasonic flaw detectors are five pounds, very small, with more capabilities than ever before.”

Training

With the growing popularity of ultrasonic testing, opportunities for training have increased as well. “Several universities now offer nondestructive testing,” notes Culbertson. “There are also many NDT courses that can be done by consultants, and through the American Society for Nondestructive Testing.”

Certification in the field of nondestructive testing is done through ASNT. Training and certification through ASNT ensures that the technician is fully equipped to handle most nondestructive tests. NDT Level III examination is ASNT’s main examination, which it has offered since 1976. The organization also offers a Central Certification program, which tests for competency and experience.

Companies also have the option of certifying their own technicians based on tests similar to those of ASNT. Most educational institutions that offer courses in NDT prepare their students for ASNT certification. “Training is usually taught multiple times during the year,” says Zwigart. “People involved in the industry actually have a lot of different places to go for training.” (See page 34.)

Some of the many resources available online include the ASNT Web site at www.asnt.org; the Online Journal of Nondestructive Testing at www.ndt.net; and the NDT Resource Center at www.ndt-ed.org.

About the author

Kennedy Smith is Quality Digest’s associate editor.

Much of the material gathered for this article, including images from which the included tables were adapted, came from the NDT Resource Center, online at www.ndt-ed.org.