Visual and Optical Testing

Non-destructive testing, NDT, is a very broad group of structural or material inspections, and as the name implies, these inspections do not destroy the material/structure being examined. NDT plays a critical role in assuring that structural components and systems perform their function in a reliable and cost-effective fashion. Because NDT does not permanently alter the article being inspected, it is a highly valuable technique that can save both money and time in product evaluation, troubleshooting, and research. NDT technicians and engineers define and implement tests that locate and characterize material conditions and flaws that might otherwise cause serious accidents, such as planes crashing, reactors failing, trains derailing, pipelines bursting, and various troubling events.

This concept is extended and known as Non-Destructive Evaluation (NDE) when combined with assessing the significance of any defects found. However, they are both terms often used interchangeably. Some testing methods must be conducted in a laboratory setting, and others may be adapted for use in the field. Several commonly employed NDT techniques and their characteristics are described below.

Visual inspection involves using an inspector’s eyes to look for defects, such as scratches, the presence of debris, corrosion, or oxidation. The inspector may also use special tools such as magnifying glasses, mirrors, or borescopes to gain access and inspect the subject area more closely. An extensive fuel inspection program in nuclear power plants (e.g., visual inspections, oxide layer measurements, eddy-current tests of control rods) is carried out underwater and supervised by the regulatory body. So visual testing is usually a part of the Post Irradiation Examination. Visual examinations are also very common in the aircraft industry, where over 80 percent of the inspections done on an aircraft are visual inspections, often used as an initial screening method to detect gross defects and target subsequent testing by other methods.

References:

Materials Science:

U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 2 and 2. January 1993.
William D. Callister, David G. Rethwisch. Materials Science and Engineering: An Introduction 9th Edition, Wiley; 9 edition (December 4, 2013), ISBN-13: 978-1118324578.
Eberhart, Mark (2003). Why Things Break: Understanding the World, by the Way, It Comes Apart. Harmony. ISBN 978-1-4000-4760-4.
Gaskell, David R. (1995). Introduction to the Thermodynamics of Materials (4th ed.). Taylor and Francis Publishing. ISBN 978-1-56032-992-3.
González-Viñas, W. & Mancini, H.L. (2004). An Introduction to Materials Science. Princeton University Press. ISBN 978-0-691-07097-1.
Ashby, Michael; Hugh Shercliff; David Cebon (2007). Materials: engineering, science, processing, and design (1st ed.). Butterworth-Heinemann. ISBN 978-0-7506-8391-3.
J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.

See above:
NDT