NDT - Equipment
Choose the Right NDT Equipment
October 17, 2012
The choice depends on the costs, time, defects and inspection method.
If you’re looking to purchase some new NDT equipment, Mike Hamel has some good news for you. “Very rarely is the wrong equipment chosen for an application,” says Hamel, the president of Test Equipment Distributors (Troy, MI).
Customers have no shortage of options when choosing NDT equipment. Experts say ultrasonic phased array and digital radiography are two of the hot trends in NDT now, but older technology continues to sell. As with any large purchase—from a computer to a washing machine—it helps to consider the available options, price, user friendliness, and time to operate. The article will delve into NDT-specific considerations to make the process as smooth as possible.
Choosing the right equipment is really a matter of choosing the right inspection method. The inspection method depends on the material, the accessibility and many other factors, such as safety and cost. In “Nondestructive Evaluation: Theory, Techniques, and Application,” Peter J. Shull, an associate professor of engineering at Penn State University (Altoona, PA), explains some factors that can influence NDE method selection. He asks: “Is it cost effective to employ NDE? Is speed of inspection a factor? What regulations exist that may dictate not only the NDE method but also the procedure? What environmental issues constrain our choices? Is the test to be performed in the field or in a specialized testing facility?”
The choice may come down to the testing requirements for your application, but the goal is not only to satisfy mandatory specifications. The right equipment can reduce the number of false rejects, as well as provide a higher yield, easier set-up and less downtime, says Donald Budgen, vice president of Magnetic Analysis Corp. (Elmsford, NY), in a paper on the subject. He continues, “Compromising on the initial investment usually results in ongoing excessive operational costs.”
A number of routine inspections are developed with specifications from ASTM International (formerly known as the American Society of Testing and Materials) or SAE International (formerly the Society of Automotive Engineers). But sometimes, especially when testing new materials or alloys, research institutes may help with the task.
The Southwest Research Institute has been working with NDT customers for years to explain how to choose an inspection method. Once a company knows what they want to do, equipment selection follows much more easily. The process of selecting between brands of phased array detectors, for example, provides a targeted goal rather than just knowing you want to solve an inspection problem, says Glenn Light, director, Sensor Systems and NDE Technology at the Southwest Research Institute (San Antonio, TX).
And different industries have varying requirements. Automotive may require fast cycle times whereas that may not be as important in aerospace. Iowa State’s Center for Nondestructive Evaluation (CNDE) sees many different NDT applications, including aerospace. Lisa Brasche, associate director of the Center for Nondestructive Evaluation at Iowa State University (Ames, IA), explains the process of selecting an inspection method: “Basically, the inspection method is determined by the engineering requirements that define the type of anomaly that could lead to failure of the structure or component. Failure could be a fracture or it could be that the item just doesn’t fit the purpose that it was intended for. Typically those requirements are defined by the design engineer based on the intended purpose of the item.”
When someone comes to the scientists and engineers at CNDE with a problem, they consider the suite of methods available. They ask for a description of the application, the geometry involved, the environment, accessibility and other constraints for inspection. From there, they ask what the inspection is looking for in terms of the size, location and orientation of the discontinuity. If it is on the surface, this allows for one set of inspection methods. If it’s an internal defect, some of these options drop out, Brasche says.
In some cases, the customer may know which inspection method they want and CNDE will help design the inspection itself. The customer would provide samples and often they would do a probability of detection study to determine the technique’s effectiveness. CNDE also uses physics-based models to consider a range of factors that could affect inspection. This simulation work allows for testing conditions that it might not be possible to recreate in a real part, especially one that costs thousands of dollars. From there, they might help the company develop training materials for the inspection, an important element because, as Brasche points out, “Eventually there will be an inspector on the floor or in the field using the technique every day.”
The process can break down if the inspector isn’t aware of what to do. Though it may be easy to get the right equipment, the problem comes in when people underestimate the training involved. They don’t realize how much is needed to be certified in the inspection method, or may not realize the safety classes needed for radiography, for example.
Eventually, after determining what inspection method is needed, it’s time to shop around. When calling an NDT equipment manufacturer or distributor, you can expect some standard questions: What are you trying to measure? What do your parts look like? What is the turnaround time required? What tolerances do you have? These questions are aimed to figure out what inspection method, or methods, might solve your inspection problem. Hamel of Test Equipment Distributors says that usually his team can determine the right inspection method in a matter of hours, or days at the latest. They might ask for a sample part, such as a chunk of pipeline to be measured, or an example of the defects to be located.
The largest factor in choosing an inspection method is often what type of material the part is made of, says Marcin Sekscinski, applications engineer at Magnaflux (Glenview, IL). “Then you have to take into consideration the dimensions of the part,” he says. “Depending on the size of the part you can do it with a method that can allow you to test larger pieces at a time, if you don’t care about spending more on capital equipment, or measure piece by piece. We have customers in both of those situations.” Automation can be a factor here.
Another factor to consider is electrical power, required with most NDT methods. Sometimes, customers may decide to go with a less powerful unit in order to maintain the same infrastructure. When considering part size, Sekscinski points out that if only three out of 100 parts don’t fit on the unit, it might be worth it to just test those three on the side rather than getting equipment that would fit every part. Though it may be less convenient, it saves customers money. And when dealing with large parts, consider how they will be maneuvered during inspection in the facility’s available space.
Sekscinski says the biggest NDT equipment mistake is “if you’re choosing equipment that has no chance of detecting what you’re looking for. Nowadays doesn’t happen that much.”
One reason this may be less common is that customers see how the equipment works beforehand. Charles Hellier, principal at The Summit Group (Old Lyme, CT) as well as a past national president of the American Society for Nondestructive Testing (ASNT) and Nondestructive Testing Management Association (NDTMA), stresses the importance of getting an equipment demonstration. But, he points out, “The demo is being done by somebody who’s very proficient in the equipment and its operation.” For those just figuring out how to operate the instrument, it can be difficult to pick it up so quickly. “Some of the newer instruments, even after a demo, when you sit down with it for the first time, it’s like learning math all over again,” Hellier says. “The demo is just the first step.”
He recommends a built-in training period included in the purchase order to help with the transition.
As with so many purchases, price is always a factor. Hellier says companies often may buy a less expensive instrument only to find out that it doesn’t have all of the features they need. “When it comes to price, be sure you’re comparing apples to apples,” Hellier says. He suggests negotiating on price if there isn’t a large difference, especially if a bulk order is involved. During the negotiations, Hellier also suggests asking if you can upgrade to a newer instrument model when the next one comes out and receive some kind of credit.
And it may be worth considering that more than one inspection method is necessary. Consider a jet engine blade. It may have gone through a series of inspection methods and have been seen by several types of equipment.
Penn State’s Shull says multiple inspection methods depend on the need for sensitivity and speed. “Testing is either really sensitive, or it’s fast,” Shull says. “You usually don’t get both. If you have the classic problem of finding small flaws in a large area product, you may need to consider a tiered method where you have rapid high area testing but with low sensitivity. Then if you see anything, switch to the more sensitive but slow scanning system.”
While the NDT equipment manufacturers do provide suggestions and guide customers toward products, they stress that they don’t make the decision. The customers must decide for themselves what they need, or make the tradeoff between time to operate and price, for example. Consider if they are inspecting ten parts a day, or more than 1,000.
And sometimes the decision may be made for you, in the case of standards or inspection methods required. The key is not to consider what type of inspection method or equipment you would like to use, but what your customer would like you to use.
And then, use your new purchase wisely. “Even if you pick the proper equipment, a lot of things can go wrong with it,” Sekscinski says. “Just like buying a car, you can still have an accident if you don’t follow the rules of how to use it.”
Though the equipment selection process has gotten more complicated, it also is arguably more interesting. “It’s an exciting time for NDT equipment,” says Hellier. “It’s so much more sophisticated, not like in the old days, with a hammer hitting a shell.”