History of Non-Destructive Testing (NDT)

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This general information is intended as a basic overview of the history of Non-Destructive Testing (NDT) and common test methods. Another popular resource is the American Society for Nondestructive Testing (ASNT): www.asnt.org

Nondestructive Testing, also known as Non-Destructive Evaluation (NDE) and Non-Destructive Inspection (NDI), is a technique of detecting and evaluating flaws in materials or differences in characteristics, without destroying or causing damage to the material.  By contrast, other tests are destructive in nature and render the test part unusable after testing.

NDT goes back hundreds of years, if not longer. Centuries before the term “NDT” was first coined, people used their eyes to look at objects to determine size, shape and the presence of surface imperfections. Today, this is known as visual testing (VT). For certain applications, a cracked piece of wood or rock would pre-maturely fail compared to those without cracks. People also used their ears to listen to differences in the material. Consider a blacksmith listening to the metal as it’s being shaped or the tone of a bell after it was cast. This use of sound waves is today known as Ultrasonic Testing (UT) and Acoustic Emission (AE).

In the late 18th century, Wilhelm Röntgen discovered X-rays. At that time, industry did not yet need this invention, but medicine did. Fast forward to around 1930. Richard Seifert improved the medical equipment and partnered with welding institutes to create what is now known as radiographic testing (RT).

Magnetic particle (MP) crack detection began even earlier than X-ray radiographic testing. In 1868, S.M. Saxby tried to find cracks in gun barrels by magnetic indications. Later in 1917, William Hoke tried the same thing. It wasn’t until 1929 that Victor de Forest and Foster Doane were able to successfully use magnetic particle crack detection in industrial applications. In 1934, they formed a company called Magnaflux.

Also in the early 1900s, the railroad industry used an oil solvent for cleaning, followed by a chalk coating. The chalk coating absorbed oil from cracks, making the cracks easily visible. Today this is known as Liquid Penetrant Testing (PT).

Eddy current testing (ET) has roots dating back to the 1800s. In 1831, Michael Faraday discovered electromagnetic induction: a current produced because of voltage production (electromotive force) due to a changing magnetic field. This either happens when a conductor is placed in a moving magnetic field (when using an AC power source) or when a conductor is constantly moving in a stationary magnetic field. In 1851, French physicist Leon Foucault discovered the phenomenon of eddy currents. In 1879, English scientist David Hughes showed that properties of a coil can change when placed in contact with metals of different conductivity and permeability. In 1933, Professor Friedrich Förster used this technology for measuring conductivity and for sorting mixed-up ferrous components. However, it was not until World War II that these discoveries were put to practical use for industrial material testing. In 1948, Förster founded his own company which continues today. 

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Prior to World War II, sonar, the technique of sending sound waves through water and observing the returning echoes to characterize submerged objects, inspired early ultrasound investigators to explore ways to apply the concept to medical diagnosis. In 1929, Sergei Sokolov studied the use of ultrasonic waves in detecting metal objects. A few years later, Sokolov demonstrated a through-transmission technique for flaw detection in metals. However, the resolution of his experimental devices was poor and could not be used at a practical level. In 1931, Mulhauser, obtained a patent for using ultrasonic waves, using two transducers to detect flaws in solids. In 1940, Dr. Floyd Firestone (University of Michigan) applied for a U.S. patent for the first practical ultrasonic testing method. The patent is granted on April 21, 1942 as U.S. Patent No. 2,280,226, titled "Flaw Detecting Device and Measuring Instrument". In 1942, Donald Sproule filed a patent entitled "the improvements in/ or relating to apparatus for flaw detection and velocity measurement by ultrasonic echo methods". Sproule and Firestone found industrial partners for their instruments: Kelvin-Hughes and Sperry Inc. Later in 1949, two Germans,  Josef Krautkrämer and Karl Deutschl, read a technical paper about the Firestone-Sperry-Reflectoscope. Without knowledge of each other, both started their own developments. Within one year, both companies would present their Ultrasonic testing-flaw-detectors, starting a competition that existed for decades into the future.

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