The Importance of Nondestructive Testing

 

Nondestructive testing (NDT) plays a critical role in the safety of the many products, transportation systems, and infrastructure used around the world. The importance of NDT in avoiding catastrophic failures cannot be overstated. Beyond the important aspects of safety, though, NDT measurements themselves provide an essential validation of the quality of a product and add economic value to the manufacturing process, as well as help manufacturers avoid the costly effects of a poor-quality product reaching the consumer.

 

Introduction

NDT is a marvelous technology. Those involved in NDT work with a wide range of physics principles. NDT methods employ the entire electromagnetic spectrum, from radio waves to gamma rays, as well as physical principles based in mechanics, magnetics, fluids, and so on. Those who work in the field of NDT are well versed in many aspects of physics, electronics, mechanics, chemistry, and material science. The application of NDT to validate the fitness-for-service of new products and the continued use of products already in service is a primary thrust of the application of NDT. The NDT measurements themselves are also numeric parameter datasets that serve as a basis for statistical analysis of product quality and consistency, important to manufacturers who value continuous process improvement and control.

Discussion

It was Dilbert who said, “The goal of every engineer is to retire without being blamed for a major catastrophe” (Adams 1993). While a bit tongue in cheek, Dilbert’s insight is of course true. And, the NDT community plays a key role in ensuring that engineers are able to successfully meet that goal.

NDT provides the “eyes” so the structural engineer can see that manufacturing has delivered what was designed. For example, in aerospace, structural engineers and designers have the fundamental responsibility in the creation of the overall product because they provide the critical platform (such as the aircraft, rotorcraft, or spacecraft). The structural and design engineers work together to come up with a configuration that will support the desired performance. Their calculations are based on material properties that are manufactured without discontinuities above the design margins that account for potential subcritical size features. When manufacturing completes the product, the only way that the structural engineers are assured that the aerospace platform is safe is through the NDT results.

It is the NDT inspectors who are able to look into the parts, report on the internal conditions, and validate that there are no features of a size that could critically impact the fitness-for-service of the component or structure.

While ensuring safety by detecting critical-size defects is an essential function of NDT, NDT can also provide other values. When Robert C. McMaster (Figure 1) edited the first NDT Handbook published in 1959, he listed the reasons for applying NDT (McMaster 1959):

• To ensure product reliability

• To prevent accidents and save human lives

• To make a profit for the user (add value)

– To ensure customer satisfaction and to maintain the manufacturer’s “good name”

– To aid in better product design

– To control manufacturing processes

– To lower manufacturing costs

– To maintain a uniform quality level

Product reliability and safety top his list, of course. But he also includes the use of NDT in process control and applying NDT to improve profit, which adds value.

Often, we fall into the mode of thinking of NDT inspectors as “defect chasers” who reject parts. In that role, manufacturing engineers often complain about the impact of NDT on production schedules and costs. Additionally, industrial engineers list NDT as a “nonvalue-added” process step. However, by using the NDT data as a necessary input to the statistical process control tool that tracks performance, NDT becomes an essential value-added process step in the production loop.

Why is inspection such a critical activity in the manufacturing process? Because you don’t inspect in quality; you measure quality. As paraphrased from Lord Kelvin (Figure 2), “If you can’t put a number on it, you don’t know it.” The way we know the quality of the products we create is by taking measurements, and NDT technology provides the tools that can take those measurements. The majority of the time when we apply NDT, we are searching for discontinuities and evaluating indications against acceptance criteria. However, the NDT results contain numerical measurements that can go beyond simply being used as inspections for safety. The numbers can be used to achieve McMaster’s third bullet of adding value to make a profit for the user. Emmanuel Papadakis further lays out the details of the financial merits that can be achieved by employing NDT in process control (Papadakis 2007).

Concepts for using NDT technology as part of manufacturing process control have been around a very long time (Bossi and Moran 1996). But implementation has been difficult. With the development of high-speed inspection technology, huge digital datasets are now gathered and stored. The terms of “big data” analysis and informatics, currently being used in many fields including manufacturing (Lee et al. 2013; Lee et al. 2014; Boulanger et al. 2017), can be applied to NDT data. Useful information can be extracted to improve product quality, reduce product waste, and save money.

Fully automatic X-ray inspection systems with automated defect recognition are running 24/7 on millions of parts, achieving parts inspections in a matter of seconds (Schulenburg 2017). Multiarray ultrasound systems are capable of coverage on composites at over 93 m2/h (1000 ft2/h) (Bossi and Georgeson 2018). Very high-speed automated inspections are possible with visual and eddy current technology at even faster rates. The challenge is to develop valuable numerical extractions from the now-available voluminous NDT data. These extractions can include the material property values and sub-rejectable feature distributions and dimensions. Working with the material manufacturing process and quality engineers, this information can be used to track project trends and see the rewards of continuous process and product improvement.

Summary

NDT involves measurements of product quality. The primary application of NDT is aimed at product safety, with enormous economic benefit in the protection against loss of life and property. Additionally, numerical measurements obtained through NDT are a value-added component to the production when applied as the input to continuous product improvement analysis.

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This article originally appeared in The NDT Technician, Vol. 17, No. 4. Written by Richard H. Bossi, retired Boeing Senior Technical Fellow; email richard.h.bossi@gmail.com.

References

1. Adams, Scott, 1993, “Dilbert,” comic strip, 23 April 1993, available online at http://dilbert.com/strip/1993-04-23.
2. Bossi, Richard H., and Thomas Moran, eds., 1996, “Nondestructive Evaluation for Process Control in Manufacturing,” Proceedings: SPIE – the International Society for Optical Engineering, Vol. 2948, SPIE, Bellingham, WA.
3. Bossi, Richard H., and Gary E. Georgeson, 2018, “Nondestructive Testing of Composites,” Materials Evaluation, Vol. 76, No. 8, pp. 1048–1060.
4. Boulanger, Michele, Wo Chang, Mark Johnson, and T.M. Kubiak, 2017, “The Deal with Big Data,” Quality Progress, Vol. 50, No. 9, pp. 26–33.
5. Lee, J., E. Lapira, B. Bagheri, and H. Kao, 2013, “Recent Advances and Trends in Predictive Manufacturing Systems in Big Data Environment,” Manufacturing Letters, Vol. 1, No. 1, pp. 38–41.
6. Lee, Jay, Behrad Bagheri, and Hung-An Kao, 2014, “Recent Advances and Trends of Cyber-Physical Systems and Big Data Analytics in Industrial Informatics,” in Proceedings of International Conference on Industrial Informatics (INDIN) 2014, Porto Alegre, Brazil.
7. McMaster, Robert, ed., 1959, Nondestructive Testing Handbook, American Society for Nondestructive Testing Inc., Columbus, OH.
8. Papadakis, Emmanuel, 2007, Financial Justification of Nondestructive Testing: Cost of Quality in Manufacturing, CRC Press: Taylor and Francis Group, Boca Raton, FL.
9. Schulenburg, Lennart, 2017, “Increased Process Safety and Efficiency through Automated Defect Recognition (ADR) in X-ray Inspection,” ASNT Annual Conference, Nashville, TN.