1
Nondestructive Testing: An Overview of Techniques and Application for Quality Evaluation

B. Venkatraman1 and Baldev Raj2

1 Indira Gandhi Centre for Atomic Research, Kalpakkam, India

2 National Institute of Advanced Studies, Bengaluru, India

Nondestructive testing (NDT) or nondestructive evaluation (NDE) or nondestructive testing and evaluation (NDT&E) as the name implies is the science and technology of assessing the soundness, acceptability, and fitness for purpose of processes, products, plants, and systems without affecting the functional properties. It is now an inseparable part of modern society. Be it the field of engineering, technology, healthcare, security, research, or heritage, to name a few, NDE is being used right from cradle to end of service to optimize processes, manage quality, predict the life, effect conservation/preservation measures, and limit liability. It is natural to understand from appreciation of this perspective that NDE is a multidisciplinary profession with crosscutting domains where physics, chemistry, materials science, mechanics, electronics, instrumentation, etc. work with synergy to realize end objectives.

Figure 1.1a and b summarizes the history and growth of NDE science and technology. Looking back in the history, it can be observed that while NDT was practiced in ancient times too in a heuristic manner (e.g. potters used tap testing and acoustic emission [AE] to check quality of pots), it was during the seventeenth to nineteenth centuries that the physical basis and science for the NDE methods was laid through the formulations of Maxwell, Faraday, Huygen, Planck, and Kaiser and the discovery of infrared (IR) radiation and X-rays by Herschel and Roentgen, to name a few. The industrialization of economies and the realization that technologies were needed to verify the quality and fitness for purpose of components and processes led to the application of NDT techniques in the late nineteenth and early twentieth centuries. Radiography, penetrant, ultrasonic, and visual testing were the major NDT techniques that were practiced, and NDT was primarily a go/no-go technique for the detection of flaws. This is schematically indicated in Figure 1.1a. The post-World War II with rapid industrialization oversaw extensive applications of NDT in many industries. Many new methods such as infrared thermography (IRT), AE, holography, etc. were developed and applied during this period. In all these cases, NDT was primarily a qualitative method. The advent of fracture mechanics concepts in the 1980s coupled with the liberalization of economies, importance of reduced margins of safety in order to be competitive, and stringency of specifications spurred the development and growth of quantitative NDE. Parallely, innovations in sensor technologies and advances in electronics, instrumentation, computers, robotics, and automation coupled with modeling, simulation, etc. led to miniaturization of NDE and also development of smart and intelligent NDE. This is schematically depicted in Figure 1.1b.

Figure 1.1 (a) and (b) NDT&E: a historical perspective.

Measurements form the heart of inspection and quantitative NDE. By making the right measurements at right points and at right times with quantified uncertainties in the product life cycle, one can ensure fitness for purpose as well as excellence in quality, extended life, and global product competitiveness and customer delight. While NDE has traditionally been applied for defect detection and evaluation during material inspection, manufacturing, fabrication, and in-service inspection, a niche area wherein it has found extensive application is corrosion detection, monitoring, and evaluation.

1.1 Corrosion Damage and NDE

While extensive understanding has been developed in the last few decades about the science of corrosion process and technology to mitigate it, corrosion damage still continues to be a challenging problem in practically all industrial sectors. Corrosion damage progresses with time and can result in leakages and structural failures and, in some cases, can be catastrophic, resulting in loss of human lives. Despite the best efforts at various stages such as selection of materials, proper design, and maintaining appropriate operating environment, corrosion degradation continues to occur and is inevitable. It thus becomes essential to monitor the performance of the installed components for assessing the progress of corrosion degradation and ensuring that it is well within the acceptable limits. It is in this context that the role of NDT&E and quality assurance (QA) becomes crucial.

The role of NDT&E in corrosion damage evaluation is twofold: (i) detection and characterization of the damage and (ii) ensure product quality level in accordance with criterion as set forth by the codes and standards or customer's specifications, thus ensuring the overall safety and reliability and also paving way for remnant life assessment and risk-based analysis. Some of the major advantages of NDE for material inspection include:

1. Ensures product quality and safety and thus fitness for purpose.
2. Provides crucial inputs with respect to flaw dimensions for fracture mechanics-based risk assessment and remnant life prediction.
3. Aids in optimizing the future product design.
4. Ensures reliability and customer satisfaction.
5. Helps in predictive condition management by revealing incipient corrosion damage areas. Preventive measures can then be taken in timely manner, thus preventing costly shutdowns.

As a diagnostic tool for corrosion damage evaluation, a wide range of industries/professions use NDT&E methods: nuclear, aerospace, automotive, chemical, defense, electronics, electrical, fabrication, fertilizers, food processing, marine, medical, metals and nonmetals, petrochemical, power, security, and surface transport, to name a few. It should be emphasized here that, the successful NDE can be achieved only through:

1. Right choice of NDE techniques (single or complementary NDE techniques).
2. Qualified and certified personnel.
3. Calibrated sensors and equipment.
4. Documented procedures with clearly defined evaluation and acceptance criteria based on standards and codes.

Conventional NDE methods - namely, visual testing (VT), liquid penetrant testing (LPT), magnetic particle testing (MPT), radiographic testing (RT), ultrasonic testing (UT), and eddy current testing (ECT) - are primarily used for corrosion detection and corrosion damage evaluation. In the last two decades, advanced techniques such as phased array, digital radiography (DR), Compton backscatter radiography, terahertz (THz) imaging, etc. are being applied for corrosion detection and evaluation. This being an important area, in this book, three chapters are devoted to NDE and corrosion evaluation including corrosion-assisted cracking. In this chapter, we provide a brief overview of the principles, advantages, limitations, and applications of the NDE methods with specific examples. A brief overview of the R&D trends in NDE for corrosion evaluation is also outlined.

1.2 Corrosion: A Brief Overview

As per ISO 8044:2015, corrosion is defined as "Physicochemical interaction (often of an electrochemical nature) between a metal and its environment that results in changes in the properties of the metal, and which may lead to significant impairment of the function of the metal, the environment, or the technical system, of which these form a part." It is to be noted here that while the term "corrosion" applies to the process, the end result is the deterioration or "corrosion damage," which needs to be detected preferably by NDE.

Corrosion of metals and materials is a highly complex phenomenon and can take many different forms (as summarized in Table 1.1). The process of corrosion is affected by several factors. Among these, material type, composition, heat treatment if any, and environmental conditions (including medium and stresses if any) in which the material resides are considered as the major ones. However, the most prominent result of all corrosion types is the loss of thickness and strength of the material, ultimately leading to failure of the component or structure itself. A fundamental understanding of the various types of corrosion is thus essential to evaluate the significance and accordingly arrive at the most appropriate NDE technology and optimum technique for quantitative detection and characterization of corrosion damage. Please refer to Chapter 2 of this book for elaborate description of various corrosion types.

Table 1.1 Corrosion types and characteristics.