Introduction to nondestructive testing

To launch this series of articles, we illustrate the basic flow of an NDT examination and cover the basic terminology used.

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Nondestructive testing (NDT) is an interdisciplinary science that uses many different methods to evaluate the properties of metals, parts, or assemblies without causing damage to the tested part or assembly. Because no damage results from these testing techniques, it is often done as part of the manufacturing process to determine if any flaws are present or created. It is also often used for product evaluation or during failure analysis.

Nondestructive testing relies on a variety of different signals to examine different materials. The most common method, visual inspection, compares the appearance of an object, such as a circuit board or part, against a known standard. This method can be augmented beyond simple visual inspection by an operator by using computerized vision systems, borescopes, microscopes, or other methods to detect any changes in a product. This method can also be used to examine processes, such as welding for proper bead, or weld undercut.

Other methods use sound waves or magnetic fields to determine if any discontinuities exist, still others use X-rays or neutrons to examine for flaws or defects. Some methods use dyes and a carrier to detect leaks or cracks.

There are many applications of NDT in manufacturing and the medical field, as well as in everyday life. For instance, the dyes your mechanic would use in your automotive air conditioning circuit to detect leaks is an example of a simple non-destructive technique. Tapping a watermelon to see if it is ripe is another simple everyday application of an NDT method using sound.

In manufacturing, NDT is used to examine parts to meet specifications such as weld soundness or the presence of overheating due to grinding. NDT can be used to detect the presence of laps or seams, or the presence of inclusions. The soundness of adhesive joints in aerospace is examined using non-destructive testing.

There are numerous different types of NDT techniques used in manufacturing. The most common methods include:

  • Magnetic particle inspection.
  • Ultrasonic inspection.
  • X-ray.
  • Dye penetrant.
  • Eddy current.

However, there are many more NDT methods that are particular to different applications. Each of these methods has its own advantages and limitations. These methods will be discussed in more detail in subsequent articles.

Nomenclature

In the United States, the standard terminology is established by ASTM E-1316 [1] [2]. Some typical [3] terms used in the industry are:

• Discontinuity: “A lack of continuity or cohesion; an intentional or unintentional interruption in the physical structure of the material.” This could be a sharp radius or columnar grain structure in a weld. It could also be porosity or a fatigue crack.

• Indication: “The response or evidence from a nondestructive evaluation.” This is the response from the NDT method. In could be an agglomeration of particles in magnetic particle testing, or evidence of dye from dye penetrant testing. Indications are further broken down into two categories: a relevant indication and a non-relevant indication.

A non-relevant indication is “an NDT indication that is caused by a condition or type of discontinuity that is not rejectable.” This could be noise due to the geometry of the component being examined.

A relevant indication is “an NDT indication that is caused by a condition or type of discontinuity that requires evaluation.” This could be the response due to a large inclusion or the presence of a lap or seam in a forging.

• Interpretation: “The determination of whether indications are relevant or non-relevant.” This is really the crux of the matter and the hardest part of NDT. This is where training is very important. It is not cost effective to call too many false positives or false negatives.

• Flaw: An imperfection or discontinuity that may be detected by NDT but is not necessarily rejectable. This can be naturally occurring or created by the fabrication or manufacturing method.

• Evaluation: “Determination of whether a relevant indication is cause to accept or reject a material or component.”

The hierarchy of these terms is shown in Figure 1.

Figure 1: The basic flow of the NDT examination.

Certification and Training

NDT methods rely heavily on the skill of the inspector. Operators are often certified either internally through their employers or externally through an outside agency such as the American Society of Nondestructive Testing [1]. Depending on the application, a certification program certified by an outside agency is required.

There are different levels of certification. Using the nomenclature of ASNT, these are classified as Levels I, II, and III skill levels.

In Level I, technicians are qualified to only perform certain duties, under close supervision. They follow specific work instructions and procedures. They do not do any interpretation of the data, but only report findings.

Level II technicians or engineers are experienced personnel who can inspect to various standards and codes. They can establish written methods for Level I technicians. Level II personnel can report and interpret results as well as train Level I technicians. Level II technicians or engineers possess a broader and deeper understanding of the testing requirements to the many different standards and have a deeper understanding of the specific test method.

Level III technicians or engineers have a deep knowledge of the method and have many years of experience in the method. These technicians can certify lower level personnel according to accepted standards. These are the technicians who develop and establish methods for Level I technicians to follow.

Many employers use an Employer Based Certification. In this method, each employer specifies the qualifications and requirements for each of the various NDT methods. Typically, the written plan would establish criteria for testing and minimum skill/knowledge required for each level of testing.

Conclusions

In this short article, the basic flow of an NDT examination is illustrated, and the basic terminology used is shown. In future articles, an examination of different NDT methods will be discussed.

Should there be any comments regarding this article, or any suggestions for new articles, please contact the author or the editor. 

References

  1. ASTM, “E1316-20, Standard Terminology for Nondestructive Examinations,” ASTM, Conshohocken, PA, 2020.
  2. “Nondestructive Testing,” Wikipedia, [Online]. Available: en.wikipedia.org/wiki/Nondestructive_testing.
  3. ASM International, Nondestructive Testing and Quality Control, vol. 17, Metals Park, OH: ASM International, 1997.
  4. American Society of Nondestructive Testing, “ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel,” ASNT, Columbus, OH, 2008.