"Making the Grade" Feature Article, August 2004

FEATURE FOCUS: Non-Destructive Evaluation

making the grade

From eddy current to X-ray, testing methods are under study to bring non-destructive evaluation into wider use.

by John DeGaspari, Associate Editor

Productivity earns the bread and butter in the manufacturing business, so plant managers have to push as much product as they can out their doors in the shortest time possible. But goods also have to meet quality standards. After all, customers are more likely to remember bad experiences than good ones, so products that are not up to snuff can taint a company's reputation. And product recalls and make-goods on warranties are expensive.

Among the more sophisticated methods available to manufacturers for keeping their processes in check and minimizing bad products are tests using such phenomena as ultrasound, eddy currents, or X-rays. They can test parts for interior cracks and flaws without breaking anything.

Strides in ultrasonic testing may eventually automate checking spot welds on auto assembly lines. Here, welding robots assemble body-in-white parts at a Ford plant.

Collectively, test methods of this sort are known as non-destructive evaluation, which has long been a standard practice in the power generation, nuclear, and aerospace industries. Automotive suppliers routinely use non-destructive methods to test all their safety-critical parts. Some observers say that non-destructive testing is likely to become more important to the auto industry as it moves to lighter-weight materials.

Non-destructive evaluation is the subject of a good deal of research, which may soon make it easier than ever to integrate testing into assembly lines, and so may take the technology to new corners of industry. Increased sensitivity, higher computing power, and better imaging are improving the quality of information and ease of use. Testing is not limited to the manufacturing cycle. Software simulation tools have been developed to allow design teams to consider part inspection early in the design process.

SMARTER SENSING

R. Bruce Thompson, director of the Center for Non-destructive Evaluation at Iowa State University in Ames, points out that advances in sensing technology have improved understanding of how energy behaves in the microstructure of a material. Overall, sensors are getting steadily better and providing higher-fidelity information, and the presentation of data is becoming increasingly easier for people to interpret.

The center at Iowa State is studying methods of non-destructive testing and developing techniques for using them. It is part of the National Science Foundation's Industry/ University Cooperative Research Center program, a cooperative effort of industry, academia, and the government to support technical research.

Improvements in non-destructive evaluation are providing a varied set of tools, Thompson said. In ultrasonic sensing, for example, there is better understanding of how grain size and defects influence ultrasonic waves, he said.

Phased arrays—in which a piezoelectric vibrating element is divided into many separate elements that can be driven independently—make it possible to create complex patterns that provide information from many angles during a single scan. A similar idea, applied to eddy current—a non-destructive technique that scans a coil over a part and measures the electrical impedance for telltale changes caused by cracks—provides detailed information about a part, Thompson said.

DESIGNING FOR TESTS

One of the developments at Iowa State is a set of modeling tools to help determine if an investigative technique will be adequate. "In the traditional manufacturing process, you figure out what the material is, how resistant the material is to manufacturing, and how to make it," Thompson said. "But typically nobody worries about how to inspect it."

A small start-up company, NDT Technologies, based in Charlottesville, Va., commercializes software for use with non-destructive testing techniques. Joe Gray, X-ray group leader at the Center for Non-Destructive Testing and a co-founder of the company, said modeling tools exist for three non-destructive techniques: ultrasound, eddy current, and X-ray, which are used widely by industry. Work on software for use with magnetic techniques is in progress.

Part geometry always affects inspection techniques and it's a challenge to make sure that you are getting the quality inspection that you need, Gray said. The models give information about waves and currents transmitted through different materials and geometries. The software uses a computer-aided drawing as a starting point, along with the definition of defects and properties of the part that would govern the waves of a particular non-destructive test. Designers could use the models to weigh cost tradeoffs in comparing ways to form a part.

IR thermography checks the adhesive bond of truck bed panel.

A simple, inexpensive casting method, for example, may result in a porous part. When that is the case, a processor has two alternatives: either use a more expensive casting process that puts the part under pressure, or use the less expensive process, but combine it with a non-destructive evaluation technique. Both processes will get good products out the door, but it is not always clear beforehand which is the less expensive course.

The simulation allows the user to consider the kinds of defects to expect, to evaluate a particular inspection process and its cost, and to compare the results to the cost of using the more expensive casting process without inspection, Gray said.

"You really need to make those decisions before you go down the road of making prototypes," Gray said. "We have made significant progress in developing these kinds of inspection simulations, which allow you to let the inspection of the part be part of the optimization of the process."

U.S. BONDS

The U.S. Department of Energy has a goal of using of lightweight materials in automobiles as an energy-conservation measure. But to introduce new materials successfully into automaking, efficient testing methods for welds and bonds must be developed.

A standard practice in the auto industry today is to test spot welds manually with a pry test, in which a screwdriver-like device is used to check the integrity of the welds. Periodically, whole assemblies are pulled off the assembly line for a teardown, which is labor intensive, time consuming, and expensive.

Deborah Hopkins, a scientist who leads the Industrial Partnerships Group for the Engineering Division at Lawrence Berkeley National Laboratory in California, has been researching non-destructive techniques to test welds and adhesive bonds. Hopkins works in partnership with the Non-Destructive Evaluation Steering Committee of the United States Council for Automotive Research, an organization sponsored by Ford, General Motors, and DaimlerChrysler to foster cooperative research in transportation technology.

Hopkins has been working with adhesive-bonded parts made of composites and aluminum since 1997, and has expanded her research into automated testing of spot welds. Her group is studying phased-array ultrasonic techniques for testing.

Ultrasonic welding of aluminum is demonstrated in a lab.

Hopkins said that phased-array ultrasonics can scan electronically without physically moving the probe, and can be focused to deliver more energy into the part for higher resolution. She added that resolution is also improving as the probes get smaller and the frequencies higher. Hopkins's group is developing a portable system that it plans to test in a manufacturing plant by the end of this year or early next. So far, the group has been applying the phased-array ultrasonic technique to galvanized steel that is used in conventional vehicles, but plans to test the technique with aluminum and high strength steel in the coming year.

Hopkins's group is also working with infrared thermography to characterize defects in welds and gaps in adhesive layers. IR thermography works by sending heat into the piece and watching the thermal response. The thermal technique has some important advantages. It does not need a coupling agent between the sensor head and the part, such as a gel or water column that is required by ultrasonic techniques; it also has a large field of view. On the other hand, interpreting thermal imaging requires special skills.

Still, Hopkins said that thermal imaging has industrial potential for specific applications, particularly processes, such as applying coatings, that use heat. Thermography is particularly attractive for those processes because applying additional heat to the part is unnecessary.

A research group at the University of Windsor in Ontario is also developing ultrasonic techniques to evaluate spot welds. Roman Maev, who holds the university's industrial research chair in applied solid-state physics and materials characterization, has been working on the use of sound waves to look into materials and display the results as an easy-to-read image on a computer screen.

Robotic spot welding is subject to many uncertainties, said Maev. Variations in electrical current can result in welds too large or too small; if a technician neglects to change the copper tip at the end of an electrode, the quality of the weld could be affected. Commercial ultrasonic systems are available to check spot welds, giving the operator a go/no-go signal on an oscilloscope. But, according to Maev, welds with high-strength steel would benefit from more nuanced reporting.

Thermographic image of spot welds potentially may replace teardowns as a way of independently verifying ultrasonic inspection data.

Maev's group claims it has developed a portable ultrasonic imaging device, weighing just seven pounds and operated with a touch screen, that provides the user with an actual image of the size, shape, and internal structure of the weld. The imaging equipment is integrated into the welding tool, and is able to provide an instant read-out of the quality of each weld during production. The device uses a 20 MHz signal, a resolution fine enough to detect cracks as small as 100 micrometers.

Maev's team has built prototypes of the device, which has gone through testing at DaimlerChrysler, where technicians without extensive training successfully operated the test unit during the evaluations. Plans are to manufacture it commercially through a spin-off company.

WHEN AND WHY

Thompson at Iowa State said that he saw a great deal of interest in non-destructive evaluation from the automotive industry 1980s and sees signs of renewed interest today. Even so, non-destructive evaluation is sometimes a tough sell to auto makers.

Emmanuel Papadakis, a consultant on non-destructive testing, supervised a research and development group at the Ford Motor Co. for 15 years, until 1987. In his view, non-destructive testing has a place in the manufacturing process wherever it can be proved effective in safety-related cases. In other cases, it depends on the cost of testing, the cost of letting bad parts into the field, and the percentage of flawed parts that would be produced over time.

According to George Harmon, non-destructive laboratory coordinator at the DaimlerChrysler Technology Center in Auburn Hills, Mich., safety-critical parts, or inexpensive parts that would cause a great deal of damage if they were to fail, require 100 percent inspection. But non-destructive inspection for the sake of quality can be difficult to justify because there is always a cost.

On the other hand, low-cost inspections, with a payback of three years or less, are more palatable, he said. Resonant frequency, for example, which vibrates a part to check stiffness, is one example. It is fast, automated, and has been used by DaimlerChrysler suppliers to check connecting rods and bearings.

Seven-pound ultrasonic imaging device developed at the University of Windsor for DaimlerChrysler provides an image of the size, shape, and internal structure of spot welds.

Non-destructive evaluation can be a valuable tool for troubleshooting and correcting processing problems. Harmon suggests that non-destructive evaluation techniques can, in many cases, be used to gain control over the process. Ultrasonic sensors placed inside the electrodes of resistant spot welders, for example, can provide real-time signals to keep supplying current until sufficient liquid puddle is produced to make a good weld.

Before his retirement, Gil Chapman was the senior manager of advanced materials at the Liberty and Technical Affairs Group, which focused on advanced concept vehicle development for DaimlerChrysler Corp. In the early 1980s he worked as a researcher at the Ford Motor Co. and developed non-destructive testing techniques to solve quality problems in automotive plants. Chapman developed a low-frequency ultrasonic inspection technique to test the adhesive bonds on Ford's large L-series trucks.

Adhesive bonds between the fenders and hood and their underlying supports were coming apart on a large number of trucks once they went into service. Dust, humidity, and curing problems neutralized the adhesive before the parts could be glued properly. Chapman's technique used a commercially available low-frequency sensor to test if the adhesive made a good bond.

The method was fast, simple, and did not require a liquid couplant to transmit the waves. The manufacturing people liked the approach, which was eventually incorporated at Ford as Test Method FLTM BU 17-1.

Non-destructive testing can help pinpoint problems before too much value is put into a part on a production line, Chapman said. But it also can test a process: "The closer we get to the process itself, the quicker we can identify what the problem is," he said.

Daniel Hutchinson, an engineer with General Motors' Controls, Conveyors, Robotics, and Welding Group, said the company is interested in using non-destructive testing as an in-process monitoring tool and as an audit tool. Reducing the amount of scrap from destructive testing is one of the company's objectives in using non-destructive testing for spot welds, he said. The company uses ultrasonics on its plant floor to check the quality of welds, and increased computational power has led to an increase in testing.

Harmon at DaimlerChrysler also sees non-destructive evaluation as an enabler of new materials. More stringent fuel economy and emissions requirements have stimulated the increasing use of materials such as aluminum, magnesium, and plastic.

"There is a tradeoff between weight and strength," Harmon said. "As you reduce the strength of material or assembly, you need to be that much more concerned about the quality of the individual component."

George Mozurkewich, a technical leader in Ford's Research Laboratories in Dearborn, Mich., said that Ford is involved in a NIST program to develop ultrasonic welding to join aluminum alloys. Resistive welding used for steel does not work as well for aluminum, which requires much more electricity to generate enough heat. In Mozurkewich's view, automation could open the doors to wider use of non-destructive evaluation in the automotive industry.

"I can envision tests on line that are robotically operated, that don't interfere with the smooth flow of production, and that automatically take care of the logistics of data tracking," he said.

Overall, manufacturers are acknowledging the value of selected non-destructive evaluation techniques, and researchers are getting better at adapting non-destructive evaluation techniques to the plant floor through portability, fine resolutions, and smart software.